6) August 2012

August 31, 2012

UGC 8335, an interacting pair of galaxies in Ursa Major

UGC 8335 (also designated Arp 238) is a strongly interacting pair of spiral galaxies, locted about 425 million light-years away in the northeast of the constellation Ursa Major. Their shapes have been severely disturbed by tidal interaction between the two galaxies.

It is the 238th galaxy in Arp’s Atlas of Peculiar Galaxies, a catalog of 338 peculiar galaxies produced by Halton Arp, which was originally published in 1966. His purpose for the atlas was to jump start research into these peculiar galaxies in the hope it would lead to better understanding of how galaxies behave and evolve.

The interaction has united the galaxies via a bridge of material and has yanked two strongly curved tails of gas and stars from the outer parts of their bodies. Both galaxies show dust lanes in their centers.

There is a third arm from the top galaxy’s core. Only 2 from the bottom galaxy’s core. This third arm is not in the same plane as the others but is headed mostly toward us rather at near right angles to the plane of the other arms. This could be pure illusion but the stars of it are clearly crossing the main disk turning it blue when it otherwise would be rather red in color. The stars of this arm are bluer than the other arms.

It also appears that this third arm is really a continuation of an arm of the bottom galaxy seen in front of the top galaxy. Could the odd stub from the nucleus of the bottom galaxy be part of the arm of the top galaxy? Hard to tell.

UGC 8335 is also one of the strongest MASER emitters. MASER is an acronym for Microwave Amplification by Stimulated Emission of Radiation. In MASER emission some molecules, such as the OH radical and the water molecule H2O, absorb energy either from collisions with other molecules or from stellar radiation and convert this energy into strong emission lines at radio wavelengths. The resulting strength of such emission lines is millions of times stronger than expected in normal interstellar clouds.

On 25 April 2012 a Type II supernova (SN 2012by) has been identified within ARP 238, in the arm of the bottom galaxy.

This image — oriented south up — is taken by the Hubble Space Telescope.
Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia/NRAO/Stony Brook University)

August 30, 2012

N49, an asymmetric supernova remnant in the LMC

N49 (also designated LMC N 49 or DEM L 190) is a supernova remnant of about 75 light-years across, located within the Large Magellanic Cloud (LMC), some 160,000 light-years away in the southern constellation Dorado. N49 is the brightest supernova remnant in the Large Magellanic Cloud. It has million-degree gas in its center but cooler gas at the outer parts, between 8,000 and 300,000 degrees.

The supernova remnant N 49 is only a few thousand years old. However, because of its distance, it takes the light of the stellar explosion 160,000 years to reach Earth. So, by our Earth calendars the blast actually occurred over 160,000 years ago.

A massive dying star produced a strong wind that cleared a low density bubble around it. When the star exhausted its supply of hydrogen, it exploded, sending a shock wave through the interstellar gas. The shock wave has now encountered the shell of dense gas at the edge of the bubble, what slows the shock wave to 100 – 300 kilometers per second.

The core of the original star, which lies deep within this cloud of gases, is a neutron star that is spinning at the blinding speed of one revolution every eight seconds. Its magnetic field is about a quadrillion (a thousand times a trillion) times more powerful than that of the Earth, putting it in the rare category of “magnetars”. This magnetar hurtles through the supernova debris cloud at over 1,200 kilometers per second.

On March 5, 1979, this magnetar displayed a historic gamma-ray burst episode. Gamma rays have a million or more times the energy of visible light photons. The neutron star in N 49 has had several subsequent gamma-ray emissions, and is now recognized as a “soft gamma-ray repeater.” These objects are a peculiar class of stars producing gamma rays that are less energetic than those emitted by most gamma-ray bursters.

The neutron star in N 49 is also emitting X-rays, whose energies are slightly less than that of soft gamma rays. High-resolution X-ray satellites have resolved a point source near the center of N 49, the likely X-ray counterpart of the soft gamma-ray repeater.

The delicate filaments and knots throughout the supernova remnant — also visible in X-ray — are sheets of debris from the stellar explosion. This filamentary material will eventually be recycled into building new generations of stars in the Large Magellanic Cloud. Our own Sun and planets are constructed from similar debris of supernovae that exploded in the Milky Way billions of years ago.

The unique filamentary structure has long set N49 apart from other well understood supernova remnants, as most supernova remnants appear roughly circular in visible light. This supernova remnant is expanding into a denser region to the southeast, which causes its asymmetrical appearance.

This image is a color representation of data taken in July 2000, with Hubble’s Wide Field Planetary Camera 2. Color filters were used to sample light emitted by sulphur, oxygen and hydrogen.

Image Credit: Hubble Heritage Team (STScI / AURA), Y. Chu (UIUC) et al., NASA

August 29, 2012

Messier 95, a barred spiral galaxy in Leo

Messier 95 (also known as NGC 3351) is a barred spiral galaxy of about 75,000 light-years across, located some 38 million light-years away in the constellation of Leo, which is receding from us at a speed of some 778 kilometers per second. It is a member of the Leo I or M96 group, a group of galaxies in the constellation Leo which also contains M96, M105 and a number of fainter galaxies.

Messier 95, which contains billions of stars, has a pronounced spiral structure with nearly circular arms before they spread out. The arms are traced by dark dust lanes, young open clusters of bright blue stars, and telltale pinkish star forming regions.

Another striking feature of Messier 95 is a glowing yellowish core surrounded by a bright star-forming ring, with a diameter of almost 2000 light-years, where a large proportion of the galaxy’s star formation takes place. Its brightness is likely due to bursts of star formation.

On 16 March 2012 a Type II supernova, designated as SN 2012aw, was discovered in Messier 95. In this image it can be seen as the very bright star in a spiral arm on the sout-east.

A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum. Type II supernovae are mainly observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies.

Image Credit & Copyright: Adam Block, Mt. Lemmon SkyCenter, University of Arizona

August 28, 2012

Uranus, the seventh planet from the Sun

Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is a gas giant, but, together with Neptune astronomers sometimes place them in a separate category called “ice giants” because they contain more “ices” such as water, ammonia and methane, along with traces of hydrocarbons, than Jupiter and Saturn. Like the other giant planets, Uranus has a ring system and numerous moons. Both ice giants likely formed closer to the Sun than their present positions, and moved outwards after formation.

Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km (about 20 AU). The intensity of sunlight on Uranus is about 1/400 that on Earth. Because of discrepancies between the predicted and observed orbits, which were most likely due to the gravitational tug of an unseen planet, Neptune was discovered.

The rotational period of the interior of Uranus is 17 hours, 14 minutes, clockwise (retrograde). As on all giant planets, its upper atmosphere experiences very strong winds in the direction of rotation. At some latitudes, such as about two-thirds of the way from the equator to the south pole, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.

Uranus’s mass is roughly 14.5 times that of the Earth, making it the least massive of the giant planets. Its diameter is slightly larger than Neptune’s at roughly four times Earth’s. Uranus is the second least dense planet, after Saturn. This indicates that it is made primarily of various ices, such as water, ammonia, and methane. The total mass of ice in Uranus’s interior is between 9.3 and 13.5 Earth masses. Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5 Earth masses. The remainder of the non-ice mass is accounted for by rocky material.

Uranus consists of three layers: a rocky (silicate/iron-nickel) core in the center, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope. The core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20% of Uranus’s; the mantle comprises the bulk of the planet, with around 13.4 Earth masses, while the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20% of Uranus’s radius.

Uranus’s core has a temperature of about 5000 K. The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles. This fluid is sometimes called a water–ammonia ocean. The bulk compositions of Uranus and Neptune are very different from those of Jupiter and Saturn, with ice dominating over gases.

The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers. It has equatorial and polar radii of 25 559 ± 4 and 24 973 ± 20 km, respectively.

Uranus’s internal heat appears markedly lower than that of the other giant planets. One of the hypotheses suggests that when Uranus was hit by a supermassive impactor, which caused it to expel most of its primordial heat, it was left with a depleted core temperature. Another hypothesis is that some form of barrier exists in Uranus’s upper layers which prevents the core’s heat from reaching the surface.

Although there is no well-defined solid surface within Uranus’s interior, the outermost part of Uranus’s gaseous envelope is called its atmosphere. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C). It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds.

The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of −300 and 50 km; the stratosphere, spanning altitudes between 50 and 4000 km; and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface. There is no mesosphere.

The composition of the Uranian atmosphere is different from the rest of the planet, consisting mainly of molecular hydrogen and helium. The third most abundant constituent is methane. Methane possesses prominent absorption bands in the visible and near-infrared making Uranus aquamarine or cyan in color. The abundances of less volatile compounds such as ammonia, water and hydrogen sulfide in the deep atmosphere are poorly known. Spectroscopy has uncovered traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets.

The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude. It is believed to possess a highly complex cloud structure. The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes.

The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature around 800 to 850 K. The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere. The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.

Uranus has a complicated planetary ring system. The rings are composed of extremely dark particles, which vary in size from micrometers to a fraction of a meter. Thirteen distinct rings are presently known, the brightest being the ε ring. All except two rings of Uranus are extremely narrow—they are usually a few kilometres wide. The rings are probably quite young; the dynamics considerations indicate that they did not form with Uranus. The matter in the rings may once have been part of a moon (or moons) that was shattered by high-speed impacts.

The magnetic field is peculiar, both because it does not originate from the planet’s geometric center, and because it is tilted at 59° from the axis of rotation what results in a highly asymmetric magnetosphere. Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants. One hypothesis is that, unlike the magnetic fields of the other planets, which are generated within their cores, the ice giants’ magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean.

Uranus’s magnetosphere contains charged particles: protons and electrons with small amounts of hydrogen molecular ions. No heavier ions have been detected. Many of these particles probably derive from the hot atmospheric corona. The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps. The particle population is also high enough to cause darkening or space weathering of the moon’s surfaces and rings. Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles. Uranus’s aurorae seem to be insignificant for the energy balance of the planetary thermosphere.

At ultraviolet and visible wavelengths, Uranus’s atmosphere is remarkably bland in comparison to the other gas giants. One proposed explanation for this dearth of features is that Uranus’s internal heat appears markedly lower than that of the other giant planets. The lowest temperature recorded in Uranus’s tropopause is 49 K, making Uranus the coldest planet in the Solar System.

The visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands. A narrow band is the brightest large feature on the visible surface of the planet. It is called a southern “collar”. The cap and collar are thought to be a dense region of methane clouds. Besides the large-scale banded structure, there are clouds, although there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter. Some small clouds live for hours, while at least one southern cloud may have persisted since Voyager flyby in 1986.

At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from −100 to −50 m/s. Wind speeds increase with the distance from the equator. Closer to the poles, the winds shift to a prograde direction, flowing with the planet’s rotation. Windspeeds at −40° latitude range from 150 to 200 m/s. In the northern hemisphere maximum speeds as high as 240 m/s are observed near +50 degrees of latitude.

Physical seasonal changes are happening in Uranus. Near the summer and winter solstices, Uranus’s hemispheres lie alternately either in full glare of the Sun’s rays or facing deep space. The brightening of the sunlit hemisphere is thought to result from the local thickening of the methane clouds and haze layers located in the troposphere. Other changes in the southern polar region can be explained by changes in the lower cloud layers.

Uranus has 27 known moons; the five main moons are Miranda, Ariel, Umbriel, Titania and Oberon. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. Its north and south poles therefore lie where most other planets have their equators. This gives it seasonal changes completely unlike those of the other major planets. Each pole gets around 42 years of continuous sunlight, followed by 42 years of darkness. Near the time of the equinoxes, the Sun faces the equator of Uranus giving a period of day-night cycles similar to those seen on most of the other planets. Uranus reached its most recent equinox on December 7, 2007.

One result of this axis orientation is that, on average during the year, the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Nevertheless, Uranus is hotter at its equator than at its poles. The underlying mechanism which causes this is unknown. The reason for Uranus’s unusual axial tilt is also not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus, causing the skewed orientation.

Image Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory

August 27, 2012

“Pillars of Creation”, elephant trunks in the Eagle Nebula

The famous “Pillars of Creation” are so called elephant trunks — massive pillars of dense gas and dust that are also incubators for new stars – located within the Eagle Nebula (Messier 16), about 7,000 light-years away in the constellation Serpens. They are so named because the gas and dust are in the process of forming, or creating, new stars.

The leftmost pillar is about four light-years in length from base to tip. The finger-like protrusions at the top of the clouds are larger than our Solar System.

These dense pillars protrude from the interior wall of a dark molecular cloud like stalagmites from the floor of a cavern. The pillars, which have survived longer than their surroundings, are composed of cool molecular hydrogen and dust that are being eroded by the ultraviolet light of hot, massive newborn stars. This process is called “photoevaporation.” The ultraviolet light is also responsible for illuminating the convoluted surfaces of the columns and the streamers of gas boiling away from their surfaces.

As the pillars are slowly eroded away, small globules of even denser gas buried within the pillars are uncovered. These globules have been dubbed “EGGs.” EGGs is an acronym for “Evaporating Gaseous Globules,” but it is also a word that describes what these objects are. Forming inside at least some of the EGGs are embryonic stars — stars that abruptly stop growing when the EGGs are uncovered and they are separated from the larger reservoir of gas from which they were drawing mass. Eventually, the stars themselves emerge from the EGGs as the EGGs themselves succumb to photoevaporation.

Spitzer Space Telescope took images of the pillars next to a giant cloud of glowing dust scorched by the heat of a supernova. One group of astronomers think the supernova’s shock wave destroyed the pillars about 6,000 years ago. But because the Eagle Nebula is located some 7,000 light years away, this destruction is not yet visible on Earth, but should be visible in the next 1000 years. So, what astronomers see now is evidence of the supernova just before its destructive shock wave reached the pillars.

However, this interpretation of the hot dust has been disputed by an astronomer uninvolved in the Spitzer observations, who argues that a supernova should have resulted in stronger radio and x-ray radiation than has been observed, and that winds from massive stars could instead have heated the dust. If this is the case, the Pillars of Creation will undergo a more gradual erosion.

This image was taken by the Hubble Telescope on April 1, 1995. It is composed of 32 different images from four separate cameras in the Wide Field and Planetary Camera 2 on board Hubble.
The photograph was made with light emitted by different elements in the cloud and appears as a different colour in the composite image: green for hydrogen, red for singly ionized sulfur and blue for double-ionized oxygen atoms.

Image Credit: NASA, ESA, STScI, Jeff Hester and Paul Scowen, at the time both of Arizona State University

August 26, 2012

I Zwicky 18, a dwarf irregular galaxy in Ursa Major

I Zwicky 18 is a very luminous dwarf irregular galaxy, much smaller than our Milky Way, located about 59 million-light years away in the constellation Ursa Major. It is receding from us at a speed of 751 kilometers per second.

The galaxy has just a few old stars, but lots of very young stars and a few starburst regions. Spectroscopy shows that its stars are composed almost entirely of hydrogen and helium — the main ingredients created in the Big Bang — with heavier elements almost completely absent.

This youthful appearance resembles galaxies typically found only in the early Universe. The faint, older stars within this galaxy, however, suggest its star formation started at least one billion years ago and possibly as much as ten billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies in this region: more than 10 billions of years ago.

I Zwicky 18 may still be creating Population III stars — extremely massive and hot stars with virtually no surface metals, except for a small quantity of metals formed in the Big Bang, normally formed in the early Universe.

The concentrated bluish-white knots embedded in the heart of the galaxy are two major starburst regions where stars are forming at a furious rate. The wispy blue filaments surrounding the central starburst regions – which extend 16 times farther out from the center than the stars — are bubbles of gas that have been blown away by stellar winds and supernovae explosions from a previous generation of hot, young stars. This gas is now heated by intense ultraviolet radiation unleashed by a new generation of hot, young stars and emits at least 1/3 of the total light seen from the galaxy, and could account for as much as half the light emitted.

Besides the bluish-white young stars, white-reddish stars also are visible in I Zwicky 18. These stars may be as old as 10 billion years.

The distance of I Zwicky 18 has been measured by observing Cepheid variable stars within the galaxy. These massive flashing stars pulse in a regular rhythm. The timing of their pulsations is directly related to their brightness. Astronomers determined the observed brightness of three Cepheids and compared it with the actual brightness predicted by theoretical models. These models were calculated specifically for I Zwicky 18’s deficiency in heavy elements, indicating the galaxy’s stars formed before these elements were abundant in the Universe. The Cepheid distance also was validated through the observed brightness of the brightest red stars older than one billion years.

The galaxy’s larger-than-expected distance may explain why astronomers have had difficulty detecting older, fainter stars within the galaxy. The faint, old stars in I Zwicky 18 are almost at the limit of Hubble’s resolution and sensitivity.

The galaxy’s primordial makeup suggests that its rate of star formation has been much lower than that of other galaxies of similar age. However, it remains a mystery why I Zwicky 18 formed so few stars in the past, and why it is forming so many new stars right now. Possibly the trigger for this recent episode of bright star formation is the changing gravitational influence of I Zwicky 18’s smaller companion galaxy, not visible in this image.

The reddish extended objects surrounding I Zwicky 18 are ancient, fully formed galaxies of different shapes that are much farther away.

Image Credit: NASA, ESA, and A. Aloisi (Space Telescope Science Institute and ESA)

August 25, 2012

NGC 2023, a reflection nebula in Orion

NGC 2023 (also designated LBN 954) is a diffuse reflection nebula of some 4 light-years across, located 1467.7 light-years away on the south of the Orion B molecular cloud, very close to the Horsehead Nebula (Barnard 33), in the constellation Orion.

This nebula is actually surrounding, and powered by the large blue-white B type star HD 37903, which is the most luminous member of a cluster of young stars. B type stars typically have a surface temperature somewhere in the range of 11,000 – 25,000 Kelvin. In comparison the Sun has a temperature of 5,000 Kelvin.

NGC 2023, one of the largest and brightest sources of fluorescent molecular hydrogen, is a region where star forming is taking place actively.

Image Credit: ESO/J. Emerson/VISTA/Cambridge Astronomical Survey Unit

August 24, 2012

NGC 1448, a spiral galaxy in Horologium

NGC 1448 (also designated NGC 1457 and ESO 249-16) is a spiral galaxy located about 60 million light-years away in the constellation Horologium. It has a prominent disk of young and very bright stars surrounding its small, shining core. The galaxy is receding from us with 1168 kilometers per second.

NGC 1448 has recently been a prolific factory of supernovae, the dramatic explosions that mark the death of stars: after a first one observed in this galaxy in 1983 (SN 1983S), two more have been discovered during the past decade.

Visible as a red dot inside the disc, in the upper right part of the image, is the supernova observed in 2003 (Type II supernova SN 2003hn), whereas another one, detected in 2001 (Type Ia supernova SN 2001el), can be noticed as a tiny blue dot in the central part of the image, just below the galaxy’s core. If captured at the peak of the explosion, a supernova might be as bright as the whole galaxy that hosts it.

A Type Ia supernova is a result from the violent explosion of a white dwarf star. This category of supernovae produces consistent peak luminosity. The stability of this luminosity allows these supernovae to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance.

A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum. Type II supernovae are mainly observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies.

This image was obtained using the 8.2-metre telescopes of ESO’s Very Large Telescope. It combines exposures taken between July 2002 and the end of November 2003.

Image Credit: ESO

August 23, 2012

Jupiter’s moon Ganymede, the largest moon in the Solar System

Jupiter’s moon Ganymede is, with a diameter of 5,268 km the largest moon in the Solar System, even larger than the planet Mercury. It also has the highest mass of all moons, with 2.025 times the mass of the Earth’s Moon. It is the seventh moon outward from Jupiter and one of the four so called Galilean moons (the other three are: Io, Europa and Callisto – all discovered by Galileo Galilei in the year 1610.)

Ganymede orbits Jupiter at a distance of 1,070,400 km and completes a revolution every seven days and three hours. Like most known moons, Ganymede is tidally locked, with one side of the moon always facing toward the planet. Ganymede participates in orbital resonances with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times.

Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain’s disrupted geology was likely the result of tectonic activity brought about by tidal heating.

Ganymede is composed of approximately equal amounts of silicate rock and water ice, with additional volatile ices such as ammonia. Water ice seems to be ubiquitous on the surface. The brighter, grooved regions have a more icy composition than the dark regions. Besides water, analysis has revealed several other materials on Ganymede’s surface, like carbon- and sulfur dioxide and various organic compounds. Especially the dark regions contain clays with organic materials.

The large craters on Ganymede have almost no vertical relief and are quite flat. They lack central depressions common to craters. This is probably due to slow and gradual adjustment to the soft icy surface. These large phantom craters range from 50 to 400 km in diameter. Both bright and dark rays of ejecta exist around Ganymede’s craters — rays tend to be bright from craters in the light, grooved terrain and dark from the dark cratered.

A saltwater ocean, which could potentially also host life, is believed to exist nearly 200 km below Ganymede’s surface, sandwiched between layers of ice. The magnesium- and sodium sulfates that were also found, likely originate from this subsurface ocean. Just how deep this ocean is, and whether it exists in pockets or as a continuous band around the moon, are questions the JUICE- team hopes to answer. (JUICE is the next planned mission to the Jovian moons.)

Ganymede appears to be fully differentiated, consisting of an iron-rich, liquid core, silicate mantle and an outer, very thick (maybe 800 km thick) ice mantle which might contain some rock mixed in. The existence of the iron-rich core provides a natural explanation for the intrinsic magnetic field of Ganymede.

The convection in the liquid iron, which has high electrical conductivity, is the most reasonable model of magnetic field generation. Ganymede is the only moon in the Solar System known to possess a magnetosphere, although the meager magnetosphere is buried within Jupiter’s much larger magnetic field. But, this field is powerful enough to generate an aurora, like Earth’s.

Ganymede also has polar caps which extend to 40° latitude, likely composed of water frost. The caps’ formation is due to the migration of water to higher latitudes and bombardment of the ice by plasma. The presence of a magnetic field on Ganymede results in more intense charged particle bombardment of its surface in the unprotected polar regions; sputtering then leads to redistribution of water molecules, with frost migrating to locally colder areas within the polar terrain.

Ganymede has a tenuous oxygen atmosphere that includes O, O2, and possibly O3 (ozone). But, the atmosphere is far too thin to support life as we know it. Another minor constituent of the Ganymedian atmosphere is atomic hydrogen.

Ganymede probably formed by an accretion in Jupiter’s disk of gas and dust surrounding Jupiter after its formation. The accretion of Ganymede probably took about 10,000 years, much shorter than the 100,000 years estimated for Callisto. Ganymede formed closer to Jupiter then Callisto, where the disk was denser, which explains its shorter formation timescale.

Several probes (Pioneer 10 and 11, Voyager 1 and 2, and Galileo) flying by or orbiting Jupiter have explored Ganymede. The next planned mission to the Jovian system is the Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. After flybys of the other three Galilean moons, the probe is planned to enter orbit around Ganymede in 2032.

Image Credit: NASA/JPL

August 22, 2012

NGC 1760, a large emission nebula in the Large Magellanic Cloud

NGC 1760 (also designated LHA 120-N 11, informally N11) is an emission nebula, or actually a complex ring of emission nebulae connected by glowing filaments over 1000 light-years across and located about 160,000 light-years away within the Large Magellanic Cloud, in the constellation of Dorado. It is one of the most active star formation regions in the nearby Universe.

It is one of the largest and most spectacular star-forming regions within the Large Magellanic Cloud (LMC), the largest satellite galaxy of our Milky Way. In fact, it is the second largest, only surpassed in the size and activity by the Tarantula nebula (or 30 Doradus), located at the opposite side of the LMC. The dramatic and colorful features visible in the nebula are the telltale signs of star formation.

A leading hypothesis for the formation of NGC 1760 is that several successive generations of stars, each of which formed further away from the center of the nebula than the last, have created shells of gas and dust. These shells were blown away from the newborn stars in the turmoil of their energetic birth and early life, creating the ring- and bean-like shapes so prominent in this image.

In NGC 1760 altogether, three generations of stars can be found. ‘Grandmother’ stars that have carved a large superbubble, leading to the birth of the cluster of massive bright blue-white ‘mother’ stars (NGC 1761) in the center. These in turn gives birth to new star ‘babies’ inside the dark globules.

NGC 1761 (also designated LH9) is composed of about 50 massive hot young stars that emit intense ultraviolet radiation that has eroded a large hole in their surroundings. These stars are among the most massive stars known anywhere in the Universe. The bright region just above center is N11B, another explosive domain where stars are being formed even today.

Although the Large Magellanic Cloud is much smaller than our own Milky Way, it is an active star-forming galaxy. Studying these stellar nurseries helps astronomers understand a lot more about how stars are born and their ultimate development and lifespan.

Image Credit: Robert Gendler and Ryan M. Hannahoe

August 21, 2012

ESO 77-14, a pair of dancing interacting galaxies

ESO 77-14 (also designated ESO 077-IG014 and AM 2317-692) is a pair of similar-sized interacting spiral galaxies, located about 550 million light-years away in the constellation Indus. The pair is moving away from us at a speed of over 12,000 kilometers per second.

Interacting galaxies are found throughout the Universe, sometimes dramatic collisions that trigger bursts of star formation, on other occasions as stealthy mergers that form new galaxies. ESO 77-14 belongs to the first category.

The two galaxies, which are spinning each other around in a celestial dance, have linked arms; a bridge of gas and dust connects the two like siamese twins. The gravitational interaction is also causing the disruption of their main bodies. The galaxy on the right has a long, bluish arm while its companion has a shorter, redder arm. The dust lanes between the two galaxy centers show the extent of the distortion to the originally flat disks that have been pulled into three-dimensional shapes.

Galaxy mergers, which were more common in the early Universe than they are today, are thought to be one of the main driving forces for cosmic evolution, turning on quasars, sparking frenetic star births and explosive stellar deaths.

Even apparently isolated galaxies will show signs in their internal structure that they have experienced one or more mergers in their past.

This image was created using the Hubble Space Telescope.
Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

August 20, 2012

The Horsehead Nebula, a dark molecular cloud in Orion

The Horsehead Nebula (Barnard 33) is a diffuse dark nebula located about 1500 light-years away in the constellation Orion, just to the south of the easternmost star in Orion’s Belt, Alnitak. The dark cloud of dust and gas is part of a region in the vast and complex Orion Nebula where star formation is taking place right now.

It is one of the most identifiable nebulae because of the shape of its swirling cloud of dark dust and gas, which is similar to that of a horse’s head when viewed from Earth. The dark molecular cloud is visible only because its obscuring dust is silhouetted against the bright red emission nebula IC 434. The Horsehead is a fascinating, active, and complex neighborhood.

The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright, hot blue star Sigma Orionis. The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead’s neck casts a shadow to the left. The dark gas extends below the more obvious horse’s head into a shapeless blob beneath it.

Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the Horsehead Nebula’s base are young stars just in the process of forming. The visible heart of the nebula emerges from the gaseous complex to serve as an active site of the formation of “low-mass” stars.

Stellar nurseries can contain over 100 known organic and inorganic gases as well of dust consisting of large and complex organic molecules. The region of the Orion Nebula containing the Horsehead is a stellar nursery. The darkness of the massive nebula is not explained by the dust and gas, but by the complex blocking the light of stars behind it. The heavy concentrations of dust in the Horsehead Nebula results in alternating sections of nearly complete opacity and transparency.

As a cloud core emerging from its parental cloud, and as an active site of low-mass star formation, the Horsehead is revealing the intricate interrelations between gas, dust, and the light from hot stars. Small red spots in the base of the Horsehead betray the presence of hidden protostars, and red streaks near the yellowish nebula are Herbig-Haro objects, which are jets of material ejected from protostars.

The Horsehead Nebula is rotating.

The radio waves are Doppler shifted as different parts of the nebula move toward or away from us, producing either a blueshift or a redshift. The horse’s nose is turning toward us and part of the mane is turning away–as if the horse were trying to look in our direction. Furthermore, most of the horse’s neck shares the same spin: the left side is approaching and the right side is receding.

Astronomers estimate that most of the neck will take about 4 million years to spin once–if the nebula survives that long. However, radiation from Sigma Orionis, hits and erodes the top of the nebula. As a result, astronomers think the Horsehead, which is about half a million years old, will endure for only another 5 million years. That’s enough time to let the nebula pirouette once before its demise.

The Horsehead’s rotation has sculpted the gas and dust into its unique shape. The rotation has caused a centrifugal force that’s flung what is now the horse’s nose, on the left, and the mane, on the right, away from the neck, thereby giving the nebula its beautiful appearance. Without this rotation, the Horsehead Nebula probably wouldn’t look like a horse.

Image Credit: Canada-France-Hawaii Telescope/Coelum

August 19, 2012

Messier 13, a globular cluster in Hercules

Messier 13 (also known as NGC 6205, The Great Globular Cluster in Hercules and the Hercules Globular Cluster) is a globular (star) cluster of about 145 light-years in diameter, located some 25,100 light-years away in our Milky Way galaxy, in the constellation Hercules. It is one of the most prominent and best known globulars of the Northern hemisphere and contains over 300,000 stars, the brightest of which is the variable cepheid star V2. Its total luminosity is 300 thousand times the Sun.

Globular clusters are gravitationally bound concentrations of ancient stars (up to a million such stars) which form a nearly spherical system. They orbit the galactic center along highly elliptical paths, and on average one revolution takes 300 million years. Recent estimates indicate that about 150 globulars exist in the Milky Way (the Andromeda Galaxy has been estimated to contain approximately 500 globular clusters).

Messier 13 is composed of some of the oldest stars in the Universe. They are estimated to be about 14 billion years old. Born before the Mily Way’s stars had a chance to create metals and distribute them in star-forming regions, Messier 13’s iron content relative to hydrogen is just 5 percent that of the Sun.

The density of stars near the core of Messier 13 is so great (about 500 times more concentrated than in the solar neighbourhood) that the visible-light and near-infrared images all become a bright central blur.

When the Hubble Space Telescope was pointed towards Messier 13 they found 15 blue straggler stars (unusually hot and bright stars) and 10 other possibles. Blue stragglers are so named because they seemingly lag behind in the aging process, appearing younger than the population from which they formed. Real blue stragglers should exhibit signs of lithium, carbon and oxygen depletion, which are chemical signatures of past mass transfer from less massive and older stars. These stars appear to be bluer and so younger than most other, apparently older, stars.

Messier 13 was selected in 1974 as target for one of the first radio messages addressed to possible extra-terrestrial intelligent races, and sent by the big radio telescope of the Arecibo Observatory. The reason was that M13 was a large collection of stars with a high star density, so, the chances of a life harboring planet with intelligent life forms, were believed to be higher. However, more recent studies suggest that planets are very rare in the dense environments of globular clusters.

Image Credit: Mike van den Berg

August 18, 2012

IC 4601, a reflection nebula in Scorpius

IC 4601 is a reflection nebula which is located about 420 light-years away in Scorpius. It is part of a larger interstellar cloud of dust and gas –where new stars are being born — known as the Horsehead Nebula (IC 4592). In fact IC 4601 lies near the horse’s ear.

These types of nebulae are called “reflection,” because they are reflecting the light of nearby stars. The characteristic blue hue of reflection nebulae is caused by the tendency of interstellar dust to more strongly scatter blue starlight.

IC 4601 is illuminated by the intense radiation of the stars present in its vicinity, among which the most brilliant star HD 147010, and the two stars of a binary system known as HD 147013, which are all blue giants.

The dust of IC 4601 contains the heavy elements that planets are made of, and plays a major role in the creation of new stars. There probably are baby stars wrapped in these blankets of dust.

Image Credit: Giovanni Benintende

August 17, 2012

Messier 77, a huge spiral galaxy in Cetus

Messier 77 (also designated NGC 1068 and Arp 37) is a very bright barred spiral galaxy of about 170,000 light-years in diameter and an estimated mass of nearly 1 billion sunmasses, what makes it one of the biggest galaxies of Messier’s catalog. The galaxy lies some 47 million light-years away in the constellation Cetus, and is receding from us at a speed of about 1137 kilometers per second.

Messier 77 is among the brightest and most nearby active galaxies. It looks like a rather normal, barred spiral galaxy, but its core, however, is very luminous, not only in optical, but also in ultraviolet and X-ray light. It is an example of a Seyfert II galaxy with an active galactic nucleus (AGN) — one with an expanding core of starbirth. It is obscured from view by astronomical dust at visible wavelengths.

Messier 77 has broad structured spiral arms with, besides obscuring lanes of gas and dust, lots of regions with massive star formation. There are also many evolved yellow stars like our own Sun in the outer regions. The galaxy has very broad emission lines, indicating that giant gas clouds are rapidly moving out of this galaxy’s core, at several hundreds of kilometers per second.

At the center of Messier 77 astronomers found an infrared source of less than 12 light-years in diameter what appears to be a supermassive black hole with a mass equivalent to about 100 million times the mass of our Sun, which accounts for the nuclear activity in this galaxy.

Messier 77’s nucleus has an innermost, comparatively “hot” cloud of dust, heated to about 500°C and with a diameter of about 3 light-years. It is surrounded by a cooler, dusty region, with a temperature of about 50°C, measuring 11 light-years across and about 7 light-years thick. This is most likely the predicted central, disk-shaped cloud that rotates around the black hole.

This image was taken using the Advanced Camera for Surveys on the Hubble Space Telescope and processed by André van der Hoeven.

August 16, 2012

Barnard 3, a cosmic wreath

Barnard 3 (also designated IRAS Ring G159.6-18.5 and nicknamed the Wreath Nebula) is an emission nebula and H II region only about 1,000 light-years away in the Perseus molecular cloud complex within our Milky Way galaxy, near the boundary between the constellations of Perseus and Taurus. Interstellar clouds like these are stellar nurseries, where baby stars are being born.

The green ring is made of tiny particles of warm dust whose composition is very similar to smog found here on Earth. The red cloud in the center is most likely made of dust that is more metallic and cooler than the surrounding regions.

HD 278942, the bright star in the middle of the red cloud which is brighter and hotter than our Sun, is so luminous that it is the likely cause of the surrounding ring’s glow. In fact its powerful stellar winds carved out a gigantic cavity in the nebula, creating a bubble of about 25 light years in diameter. The bright greenish-yellow region left of the center is similar to the rest of the green “wreath” material, only more dense.

In this star-forming nebula new stars are being born throughout the dusty region, while the bluish-white stars scattered throughout are located both in front of, and behind, the nebula.

The colors used in this image represent specific wavelengths of infrared light. Blue and cyan (blue-green) represent light emitted at wavelengths of 3.4 and 4.6 microns, which is predominantly from stars. Green and red represent light from 12 and 22 microns, respectively, which is mostly emitted by dust.

This false-color image was taken with the Wide-field Infrared Survey Explorer (WISE).
Image Credit: NASA/JPL-Caltech/WISE Team

August 15, 2012

NGC 6240, a pair of merging galaxies in Ophiuchus

NGC 6240 (also designated VV 617 and IC 4625) is pair of merging galaxies about 400 million light-years away in the constellation Ophiuchus. The pair is moving away from us with a speed of about 7339 km/sec.

This image shows them in a rare, short-lived phase of their evolution just before they merge into a single, larger galaxy. The prolonged, violent collision which began about 30 million years ago, has resulted in this butterfly- or lobster-shaped galaxy with two distinct active galactic nuclei.

The merging process triggered dramatic star formation, sparked numerous supernova explosions and spew forth tidal tails of stars, gas, and dust. NGC 6240 is therefor a prime example of a “starburst” galaxy in which stars are forming, evolving, and exploding at an exceptionally rapid rate due to a relatively recent merger. The collision also created huge amounts of heat — turning NGC 6240 into an active ultraluminous infrared galaxy.

Radio, infrared, and optical observations led to the discovery of two orbiting supermassive black holes, about 3,000 light-years apart, which will drift toward one another and eventually merge together into one even larger supermassive black hole. This detection of a binary black hole supports the idea that black holes grow to enormous masses in the centers of galaxies by merging with other black holes.

Because of the distance and the finite speed of light, the predicted merger of supermassive black holes in NGC 6240 has likely already occurred long ago. But we won’t know about it for some tens to hundred millions of years.

Image Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

 

August 14, 2012

DR 6 nebula, a “Galactic Ghoul” in Cygnus

DR 6 nebula (The Galactic Ghoul) is a star forming cloud of gas and dust located some 3,900 light-years away in the constellation Cygnus. The center (or “nose”) of the nebula is roughly 3.5 light-years long and contains a cluster of about 10 massive newborn stars, ranging in size from 10 to 20 times the mass of our Sun.

The DR 6 nebula is nicknamed the “Galactic Ghoul” because of its resemblance to a ghoul; astronomers have described it as “some sort of freakish space face,” emphasizing the cavities in the cloud that look like hollow eyes and a screaming mouth.

These large cavities are the result of the strong stellar winds and energetic light emanating from the bright young stars in the nebula’s central “nose”. The green material remaining in the eyes and mouth is comprised of gas, while the red regions and tendrils beyond make up the dusty cloud that originally gave birth to the young stars.

Within the nebula’s central bar, a second generation of stars is in the process of forming. These stars, in turn, will sculpt their stellar nursery, and ultimately affect the birth of yet another generation of stars.

This image in four infrared colors is taken by NASA’s Spitzer Space Telescope.
Image Credit: NASA/JPL-Caltech/S.Carey (Caltech)

August 13, 2012

Jupiter, the largest planet in the Solar System

Jupiter is the fifth planet from the Sun and, with a diameter of 142,984 km at its equator, it is the largest planet in the Solar System. Jupiter has a mass of one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Its density is the second highest of the gas giant planets, but lower than any of the four terrestrial planets. Jupiter is classified as a gas giant, along with Saturn, Uranus and Neptune – these four planets are sometimes referred to as the Jovian or outer planets.

The average distance between Jupiter and the Sun is 778 million km (about 5.2 times the average distance from the Earth to the Sun) and it completes an orbit every 11.86 years.
The axial tilt of Jupiter is relatively small. As a result this planet does not experience significant seasonal changes.

Jupiter’s rotation is the fastest of all the Solar System’s planets, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an Earth-based amateur telescope.

Although Jupiter would need to be about 75 times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter. Despite this, Jupiter still radiates more heat than it receives from the Sun.

Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. Beyond this basic outline, there is still considerable uncertainty. The core is often described as rocky, but its detailed composition is unknown. The core may also be entirely absent, as gravitational measurements are not yet precise enough to rule that possibility out entirely.

Jupiter is perpetually covered with clouds, located in the tropopause, arranged into bands of different latitudes, known as tropical regions. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 100 m/s (360 km/h) are common in zonal jets.

The cloud layer is only about 50 km deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region. There may also be a thin layer of water clouds underlying the ammonia layer, as evidenced by flashes of lightning.These electrical discharges can be up to a thousand times as powerful as lightning on the Earth.

The best known feature of Jupiter is the Great Red Spot, a persistent anticyclonic storm that is large enough to contain two or three planets of Earth’s diameter, located 22° south of the equator. It is known to have been in existence since at least 1831, and possibly since 1665. Mathematical models suggest that the storm is stable and may be a permanent feature of the planet. The oval object rotates counterclockwise, with a period of about six days. The maximum altitude of this storm is about 8 km above the surrounding cloudtops.

In 2000, an atmospheric feature formed in the southern hemisphere that is similar in appearance to the Great Red Spot, but smaller. This was created when several smaller, white oval-shaped storms merged to form a single feature.The merged feature was named Oval BA, and has been nicknamed Red Spot Junior. It has since increased in intensity and changed color from white to red.

Jupiter is primarily composed of gaseous and liquid matter; most of its mass is composed of hydrogen with a quarter being helium with a small amount of heavier elements. The atmospheric proportions of hydrogen and helium are very close to the theoretical composition of the primordial solar nebula. Abundances of heavier inert gases in Jupiter’s atmosphere are about two to three times that of the Sun.

Surrounding Jupiter is a faint planetary ring system made of dust, rather than ice – as with Saturn’s rings – and a powerful magnetosphere (14 times as strong as the Earth’s). There are also at least 66 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury.

Jupiter has been explored on several occasions by robotic spacecraft: the Pioneer and Voyager flyby missions and later the Galileo orbiter. The most recent probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007.

This processed color image of Jupiter was produced from a Voyager image captured in 1979. The colors have been enhanced to bring out detail.
Image Credit: NASA/JPL/U.S. Geological Survey

August 12, 2012

NGC 3259, a bright spiral galaxy in Ursa Major

NGC 3259 (also designated UGC 5717) is a bright barred spiral galaxy located approximately 110 million light-years away from Earth in the constellation Ursa Major.

The bright core of this galaxy is inhabited by a supermassive black hole that is consuming anything around it. This galactic feast emits intense radiation across the whole electromagnetic spectrum, including visible light, what explains the high luminosity of the galaxy’s core.

The well formed spiral arms of NGC 3259 have abundant dark lanes of dust and gas which are dotted with numerous bright blue spots. These spots are rich globular clusters full of bright, young, hot stars. These new stars are blue – which gives the entire galaxy a blue complexion.

The galaxy has a small companion named LEDA 213712 (visible to the left of the image), a much smaller galaxy that is most likely orbiting NGC 3259, like the Magellanic Clouds are orbiting around our own Milky Way Galaxy.

In the background, numerous distant galaxies can be seen, easily identifiable by their elliptical shapes. They are visible here mainly in infrared light, which is shown in red in this image.

This image was taken by the NASA/ESA Hubble Space Telescope.
Image Credit: ESA/Hubble & NASA

August 11, 2012

IRAS 19024+0044, a protoplanetary nebula in Aquila

IRAS 19024+0044 is a protoplanetary nebula located about 11,000 light-years away in the constellation Aquila, resembling a dragonfly or starfish. The central Sun-like star, which is nearing the end of its life, is surrounded by an asymmetric cloud of gas and dust.

Astronomers study protoplanetary nebulae because they offer a glimpse at how stars like our own Sun end their lives. When a Sun-like stars runs out of the hydrogen fuel that powers fusion in its core, it sheds its outer layers into space, creating a beautiful and often intricately shaped cloud of gas and dust around it. At first, this cloud of gas and dust only reflects light from its parent star. Eventually the cool gas and dust is heated and ionized by the intense ultraviolet radiation streaming from the now exposed core of the star, which has now become a white dwarf. The glowing nebula around the white dwarf star has become a fully fledged planetary nebula.

Hubble captured IRAS 19204+0044 in a rare and short-lived period of its life. It has clearly five blue elongated lobes that extend away from the central star. The central region shows two dark bands southwest and northeast of the center, and a very faint, diffuse halo surrounds the lobes. Astronomers don’t know for sure what causes the lobes to be uneven. Changing jets or explosive ejections of matter from the star are a couple of possibilities.

IRAS 19024 +0044 is blue because the light component of this color from the star more easily disperse the gas and dust in the rays of red and orange. This is similar to what happens to sunlight in the Earth’s atmosphere, giving the sky its distinctive shade of blue.

This composite picture was created from images taken by the Hubble Space Telescope.
Image Credit: ESA/Hubble, NASA and R. Sahai

August 10, 2012

Coddington’s Nebula, a dwarf irregular galaxy in Ursa Major

Coddington’s Nebula (IC 2574) is a faint dwarf irregular galaxy which is receding from us at a speed of just 55 km/sec. It is about 50 thousand light-years across and located some 12 million light-years away in the northern constellation Ursa Major. IC 2574 is a member of the Messier 81 Group of galaxies, named after the most prominent galaxy in its midst, the big spiral galaxy Messier 81.

These dwarf irregular galaxies are thought to resemble some of the earliest that formed in the Universe. Dwarf irregular galaxies thus serve as useful “living fossils” for studying the evolution of more complex galaxy types such as our own Milky Way, with its central bar and spiral arms.

Pinkish bubbles blown by supernova explosions abound in IC 2574. The colour of these shells comes from hydrogen gas irradiated by newborn stars. The formation of the stars was triggered by stellar winds and supernova explosions spewing material into the galaxy’s interstellar medium and triggering further star formation. The expanding (25km/sec) shells in IC 2574 are of particular interest to astronomers as they reveal how supernova-driven explosions ignite round after round of star formation.

IC 2574 has had several stellar baby booms. A new study suggests one burst of star formation occurred about 100 million years ago in the central region of the galaxy, while a later burst took place less than 10 million years ago within 13,000 and 26,000 light years from the galaxy’s centre.

A years-long study of radio radiation emitted by cold hydrogen gas, and the motions of gas and stars in IC 2574 show that most of the mass is not in the form of visible stars and clouds of gas, but in the “dark matter” that pervades all known galaxies and clusters of galaxies.

Image Credit: Martin Winder and Dietmar Hager

August 9, 2012

Hickson Compact Group 31, a group of interacting galaxies

Hickson Compact Group 31 (also known as NGC 1741 Group) is a group of interacting galaxies consisting of eight galaxies: the Wolf-Rayet galaxy NGC 1741 (actually two colliding dwarf galaxies) and its irregular dwarf companions. The entire group spans about 150 thousand light-years, and is located about 166 million light-years away in the constellation Eridanus. The distorted galaxies are quickly producing massive, hot, young stars that are pumping out ultraviolet radiation, heating up surrounding gas clouds, and causing them to glow.

Compact groups are small systems of galaxies, with each member apparently in close proximity to the other members. They are presumed to be physically associated and often show morphological peculiarities indicative of gravitational interaction. Hickson Compact Group 31 is one of 100 compact galaxy groups catalogued by the Canadian astronomer Paul Hickson. Compact groups offer a window into what commonly happened in the Universe’s formative years when large galaxies were created from smaller building blocks.

The slowly merging galaxies of HCG 31 waited billions of years to come together, forming thousands of new star clusters. The oldest stars in a few of its ancient globular clusters are about 10 billion years old, while the brightest clusters, hefty groups each holding at least 100,000 stars, are less than 10 million years old. The entire system is rich in hydrogen gas, the stuff of which stars are made.

The bright, distorted object at middle, left, is NGC 1741, consisting of NGC 1741A and 1741B. The numerous bluish star clusters have formed in the streamers of debris pulled from the galaxies and at the site of their head-on collision. The cigar-shaped object above the galaxy duo is another member of the group. A bridge of star clusters connects the trio. A longer rope of bright star clusters points to a fourth member of the group, at lower right. The bright object in the center is a foreground star.

The interacting galaxies of Hickson Compact Group 31 will continue to destroy each other, millions of stars will form and explode, and thousands of nebulae will form and dissipate before the dust settles and the galaxies will form one large elliptical galaxy about one billion years from now.

Such encounters between dwarf galaxies are normally seen billions of light-years away and therefore occurred billions of years ago. But the galaxies of Hickson Compact Group 31 are relatively nearby, what makes them easier to study. The relatively young and bright star clusters within HCG 31, allows astronomers to calculate the clusters’ age, trace the star-formation history, and determine that the galaxies are undergoing the final stages of galaxy assembly.

This composite image was composed from observations made by the Hubble Space Telescope’s Advanced Camera for Surveys, NASA’s Spitzer Space Telescope, and the Galaxy Evolution Explorer (GALEX).
Image Credit: NASA, ESA, S. Gallagher (The University of Western Ontario), and J. English (University of Manitoba)

August 8, 2012

IC 4634, an S-shaped planetary nebula in Ophiuchus

IC 4634 is a planetary nebula located over 7500 light-years away from Earth in the constellation of Ophiuchus, the Serpent Holder. It has an S-shaped structure in which the inner shell is expanding with 20 km/s.

The two shining, S-shaped ejections are from a dying central star. This star bloated as it aged and launched its outer layers off into space. The star’s very hot, exposed core has since beamed intense ultraviolet radiation at these lost shells of gas, making them glow in rich colours.

The end of a star’s life is anything but peaceful. Once the fuel source of hydrogen and helium run out for a star like our Sun, it swells to enormous size and becomes a red giant. During this process, the star puffs off bubbles of gas and forms a planetary nebula. If the star is spinning, as seen with IC 4634, symmetrical rings of material are thrown off. All that is left behind is the hot core of the dying star called a white dwarf.

Apparently, IC 4634 has experienced several episodes of symmetric ejections, with the outer S-shaped feature being related to an earlier ejection, and the outermost arclike string of knots being the relic of a still much earlier ejection. There is tantalizing evidence that the action of these collimated outflows has also taken part in the shaping of the innermost shell and inner S-shaped arc of IC 4634. The result is remarkably symmetric on each side of the central star.

On the basis of composition measurements, IC 4634 is classified as a metal-poor planetary nebula with a low-mass central star; its metallicity appears to be lower than in the Sun or the average planetary nebula. The most likely temperature of the central star in IC 4634 appears to be about 55,000 K, with a mass of about 0.55 Solar Mass. The compact size of IC 4634, its high surface brightness, and remarkable morphology make it a worthwhile object for study.

Planetary nebulae like IC 4634 are not related to planets at all. Astronomers call such an object a planetary nebula, because its round shape resembles that of the distant planets Uranus and Neptune when viewed with a small telescope.

Planetary nebulae fade gradually over tens of thousands of years. The hot, remnant stellar core will eventually cool off for billions of years as a white dwarf. Our own Sun will undergo a similar process, so, planetary nebulae could well offer a glimpse of the future that awaits our own Sun in about five billion years.

The image is taken with Hubble Space Telescope’s Wide Field Planetary Camera 2 (WFPC2).
Image Credit: ESA/Hubble and NASA

August 7, 2012

Leo I Dwarf Galaxy, a remote satellite of our Milky Way galaxy

Leo I Dwarf Galaxy (also designated PGC 29488 and UGC 5470) is a dwarf spheroidal galaxy with a diameter of approximately 2000 light-years and a mass between 15 to 30 million times the mass of the Sun. It is located about 820,000 light-years away in the constellation Leo and is running away from us at a speed of about 285 km/s.

Leo I is a member of the Local Group of galaxies and is thought to be the most distant of the eleven known small satellite galaxies orbiting our Milky Way galaxy. It has been suggested that Leo I is in fact a tidal debris stream in the outer halo of the Milky Way, although this hypothesis has not been confirmed. It seems to be certain that the galaxy does not rotate. No globular clusters have been found in the galaxy.

Most of the stars in this dwarf elliptical are far older than the Sun, and have only 1% of the Sun’s already minor amounts of elements heavier than hydrogen and helium; but rather surprisingly, Leo I has quite a few stars of sufficient brightness to limit their lifetimes to only a billion or three years, implying that was an episode of star formation (accounting for 70% to 80% of its population) extending from around 6 to 2 billion years ago, most likely as a result of gravitational interactions with our galaxy.

There is no significant evidence of any stars that are more than 10 billion years old. About one billion years ago, star formation in Leo I appears to have dropped suddenly to an almost negligible rate, although some low-level activity may have continued until 200-500 million years ago. Therefore it may be the youngest dwarf satellite of the Milky Way. In addition, the galaxy may be embedded in a cloud of ionized gas with a mass similar to that of the whole galaxy.

Because of its proximity to Regulus — the brightest star in the constellation Leo and one of the brightest stars in the night sky — Leo I is sometimes called the Regulus Dwarf. Scattered light from Regulus makes studying the galaxy more difficult, and extremely difficult to view visually despite its considerably high total visual brightness.

This image is a composite of two images, with more detail in the top image, as indicated by a diagonal ‘line’ running across the lower part of the image.
Image Credit: WikiSky (snapshot based on Sloan Digital Sky Survey)

August 6, 2012

The Dragonfish Nebula, a massive emission nebula in the Southern Cross

The Dragonfish Nebula (G298), named for its resemblance to a terrifyingly toothy deep-sea fish, is a massive emission nebula and star-forming region of about of 450 light-years across, located some 30,000 light-years away toward the constellation of the Southern Cross (also known as the Crux constellation). It may have a total mass exceeding 100,000 times the Sun’s mass, and may contain millions of stars.

This turbulent region is home to some of the most luminous massive stars in our Milky Way galaxy. The strong stellar winds of the young and massive stars have blown a bubble in the gas and dust, carving out a shell of more than 100 light-years across (seen in lower, central part of image). This shell forms the “toothy mouth” of the Dragonfish, and the two bright spots make it up its beady eyes.

The infrared light in this region is coming from the gas and dust that are being heated up by the unseen central cluster of massive stars. The bright spots along the shell, including the “eyes,” are possible smaller regions of newly formed stars, triggered by the compression of the gas and dust by winds from the central, massive stars.

Hidden in its gaping maw the Dragonfish may contain the Milky Way’s most massive cluster of young stars. In 2010 the first hint of this cluster was found in the form of a big cloud of ionised gas (the Dragonfish). Its microwave emissions led to the suspicion that radiation from massive stars nearby had ionised the gas. A little later a knot of 400 massive stars in the cloud’s heart have been identified. The cluster probably contains many more stars too small and dim to see.

The surrounding cloud of ionised gas is producing more microwaves than clouds around other star clusters in our galaxy. That suggests the Dragonfish contains the brightest and most massive young cluster discovered so far, with a total mass in the range of 100,000 times the mass of the sun. Because it is located nearby in our own galaxy, it can be studied in great detail.

Due its distance and location there’s a vast amount of interstellar material (like gas and dust) between us and the Dragonfish, absorbing its light, so in optical light it’s essentially invisible. But infrared light can pierce that fog, and the image above was taken using NASA’s Spitzer Space Telescope, designed to look in the infrared.

Image Credit: NASA/JPL-Caltech/GLIMPSE Team/M Rahman/Univ. of Toronto

August 5, 2012

NGC 7049, an unusual galaxy in Indus

NGC 7049 is a giant galaxy on the border between spiral and elliptical galaxies that spans about 150,000 light-years. It is located about 100 million light-years away from Earth in the southern constellation of Indus.

NGC 7049 is the “brightest” galaxy of the Indus Triplet of galaxies (NGC 7029, NGC 7041, NGC 7049), and its structure might have arisen from several recent galaxy collisions. Bright Cluster Galaxies are among the most massive galaxies in the universe and are also the oldest. They provide astronomers the opportunity of studying the many globular clusters contained within them.

Globular clusters are very dense and compact groupings of a few hundreds of thousands of stars bound together by gravity. They contain some of the first stars to be produced in a galaxy. NGC 7049 has far fewer such clusters than other similar giant galaxies in very big, rich groups. This indicates to astronomers how the surrounding environment influenced the formation of galaxy halos in the early Universe.

The globular clusters in NGC 7049 are seen as the sprinkling of small faint points of light in the galaxy’s halo. The halo – the ghostly region of diffuse light surrounding the galaxy – is composed of myriads of individual stars and provides a luminous background to the remarkable swirling ring of dust lanes surrounding NGC 7049’s core.

NGC 7049’s striking appearance is primarily due to this unusually prominent dust ring, seen mostly in silhouette. The opaque ring is much darker than the millions of bright stars glowing behind it. Generally these dust lanes are seen in much younger galaxies with active star forming regions. Not visible in this image is an unusual central polar ring of gas circling out of the plane near the galaxy’s center.

The image was taken by the Advanced Camera for Surveys on the Hubble Space Telescope, which is optimised to hunt for galaxies and galaxy clusters in the remote and ancient Universe, at a time when our cosmos was very young.

Image Credit: NASA, ESA and W. Harris (McMaster University)

August 4, 2012

NGC 7822, an emission nebula in Cepheus

NGC 7822 (also designated Sharpless 171, SH 2-171) is a young irregular emission nebula and star forming region of about 40 light-years across, located some 3,300 light-years away at the edge of a giant molecular cloud toward the northern constellation Cepheus. It is home to the young star cluster Berkeley 59, whose stars are just a few million years old. A supernova remnant associated with NGC 7822, indicates that a massive star has already exploded.

Berkeley 59 includes one of the hottest stars discovered in the vicinity of our Sun, namely BD+66 1673, which is an eclipsing binary system consisting of a very bright star that exhibits a surface temperature of nearly 45000 K and a luminosity ~100000 times that of the Sun.

Cosmic pillars of cold molecular gas and clouds of dark dust – often called elephant trunks – lie within NGC 7822. Powering the nebular glow are the young, hot stars of the Berkeley 59 cluster, whose powerful winds and radiation also sculpt and erode the dense pillar shapes. Stars could still be forming inside the pillars by gravitational collapse, but as the pillars are eroded away, any forming stars will ultimately be cut off from their reservoir of star stuff.

Image Credit: Neil Fleming

August 3, 2012

Gale Crater on Mars

Gale Crater is a crater on Mars near the northwestern part of the Aeolis quadrangle. It is 154 km (96 mi) in diameter and believed to be about 3.5 to 3.8 billion years old.

Aeolis Palus is the plain between the northern wall of Gale Crater and the northern foothills of Aeolis Mons (also known as “Mount Sharp”), an enormous central mound of debris, rising 5.5 km (18,000 ft) above the northern crater floor and 4.5 km (15,000 ft) above the southern crater floor – slightly taller than the southern rim of the crater itself. The mound is composed of layered material and may have been laid down over a period of around 2 billion years.

The origin of this mound, actually consisting of several distinct smaller hills, is not known with certainty, but research suggests it is the eroded remnant of sedimentary layers that once filled the crater completely, possibly originally deposited on a lakebed. However, there is debate around this issue.

Observations of possible cross-bedded strata on the upper mound suggest aeolian processes, like dust or volcanic ash blown in by the wind, but the origin of the lower mound layers remains ambiguous. Numerous channels eroded into the flanks of the crater’s central mound could give access to the layers for study. In close-up images polygons are visible on the crater floor, indicating contraction due to water loss, cooling, or some other process.

NASA’s Mars rover, Curiosity, which was launched 26 November 2011, will explore Aeolis Mons after the landing on Aeolis Palus in Gale Crater within a few days, on 5/6 August 2012, depending on the time-zone you live in. Gale is also one of four prospective sites for ESA’s ExoMars.

This oblique, southward-looking view of Gale crater shows the landing site and the mound of layered rocks that NASA’s Mars Science Laboratory, Curiosity, will investigate. The landing site is in the smooth area in front of the mound (marked by a yellow ellipse, which is 20 km [12.4 mi] by 25 km [15.5 mi]).

The landing site contains material washed down from the wall of the crater, which will provide scientists with the opportunity to investigate the rocks that form the bedrock in this area. The landing ellipse also contains a rock type that is very dense and very bright colored; it is unlike any rock type previously investigated on Mars. It may be an ancient playa lake deposit, and it will likely be the mission’s first target in checking for the presence of organic molecules.

The area of top scientific interest for Curiosity is at the base of the mound, just at the edge of the landing ellipse. Here, orbiting instruments have detected signatures of both clay minerals and sulfate salts. Scientists studying Mars have several important hypotheses about how these minerals reflect changes in the Martian environment, particularly changes in the amount of water on the surface of Mars. Curiosity will use its full instrument suite to study these minerals and how they formed to give us insights into those ancient Martian environments. These rocks are also a prime target in checking for organic molecules.

More information on:

• Curiosity’s mission: http://annesastronomynews.com/lifes-molecules-could-lie-within-reach-of-curiosity/
• Curosity’s landing: http://annesastronomynews.com/curiosity-nears-daring-landing-on-mars/
• Planet Mars: http://annesastronomynews.com/photo-gallery-ii/the-solar-system/mars-has-the-highest-mountain-and-the-largest-canyon-valles-marineris-which-is-3000-km-long-spans-600-km-across-and-is-8-km-deep-in-the-solar-system-the-ironoxide-on-its-surface-gives-it-a-re/

Image Credit: NASA/JPL-Caltech/ASU/UA 

August 2, 2012

NGC 1376, a beautiful snowflake in Eridanus

NGC 1376 is a snowflake-shaped spiral galaxy located more than 180 million light-years away in the constellation Eridanus. It is moving away from us at a speed of 4152 km/s.

Concentrated along the spiral arms of NGC 1376, bright blue knots of glowing gas highlight areas of active star formation. These regions show an excess of light at ultraviolet (UV) wavelengths because they contain brilliant clusters of hot, newborn stars that are emitting UV light.

The less intense, red areas near the core and between the arms consist mainly of older stars. The reddish dust lanes are colder, denser regions where interstellar clouds may collapse to form new stars. Visually intermingled between the spiral arms is a sprinkling of reddish background galaxies.

The galaxy was home to supernovae SN 1990go, SN 2003lo and SN 2011dx, of which supernova SN 1990go rivaled the brightness of the entire nucleus from ground-based telescopes for several weeks in 1990.

This bluish-white galaxy belongs to a class of spirals that are seen nearly face on from our line of sight. This orientation aids astronomers in studying details and features of the galaxy from an unobscured vantage point.

Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
The image was taken by a team led by Rodger Thompson Rodger Thompson (University of Arizona)

August 1, 2012

G299.2-2.9, a supernova remnant in Musca

G299.2-2.9 is a middle-aged supernova remnant with a size of 85.5 x 52.25 light-years, located about 16,000 light-years from Earth in our Milky Way galaxy, toward the constellation Musca. Evidence points to G299.2-2.9 being the glowing remains of a Type Ia supernova, where a white dwarf has grown sufficiently massive to cause a thermonuclear explosion.

Because, at an age of about 4,500 years, it is older than most supernova remnants, G299.2-2.9 gives a look at how the remnants evolve over time. It also provides a probe of the Type Ia supernova explosion that produced this structure.

This image shows clearly the bright knots, shell, and diffuse emission extending beyond the bright shell, as well as the faint inner region. X-ray emission from this inner region reveals relatively large amounts of iron and silicon, as expected for a remnant of a Type Ia supernova.

The outer shell of the remnant is complex, with at least a double shell structure. Typically, such a complex outer shell is associated with a star that has exploded into space where gas and dust are not uniformly distributed.

Since most theories to explain Type Ia supernovae assume they go off in a uniform environment, detailed studies of this complicated outer shell should help astronomers improve their understanding of the environments where these explosions occur.

It is very important to understand the details of Type Ia explosions because astronomers use them as cosmic distance markers to measure the accelerated expansion of the universe and to study dark energy. The discovery of the accelerated expansion in the late 1990s led to the award of the 2011 Nobel Prize in Physics to Adam Riess, Brian Schmidt, and Saul Perlmutter.

This composite image shows G299.2-2.9 in X-ray light in gold, that has been overlaid on an infrared image.
Image Credits: X-ray: NASA/CXC/U.Texas/S. Park et al, ROSAT; Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF