9) November 2012


November 30, 2012

SNR 0509-67.5, a supernova remnant in the LMC

SNR 0509

Image Credit: X-ray: NASA/CXC/SAO/J.Hughes et al, Optical: NASA/ESA/Hubble Heritage Team (STScI/AURA)

SNR 0509-67.5 (SNR 0509 for short) is the remnant of a supernova explosion in the Large Magellanic Cloud, located some 160,000 light-years away in the constellation of Dorado. It is about 23 light-years across and expanding at more than 5,000 kilometers a second, or more than 18 million kilometers an hour.

This bubble is the visible remnant of a Type 1a supernova that occurred about 400 years ago in Earth’s time frame (what means that the light from the explosion reached us about 400 years ago), which was unusually bright and energetic.

A Type Ia supernova is a result from the violent explosion of a white dwarf star (a compact star that has ceased fusion in its core). The white dwarf increases its mass beyond a critical limit by gobbling up matter from a companion star. A runaway nuclear explosion then makes the star suddenly as bright as a whole galaxy, before gradually fading from view.

Astronomers failed to find any remnant of the companion star, however, and concluded that the common scenario of Type Ia supernovae did not apply in this case. The cause of SNR 0509-67.5 can be explained best by two tightly orbiting white dwarf stars spiraling closer and closer until they collided and exploded.

The gaseous envelope formed as the expanding blast wave and ejected material from the supernova tore through the nearby interstellar medium. Ripples in the shell’s surface may be caused either by subtle variations in the density of the ambient interstellar gas, or possibly be driven from the interior by fragments from the initial explosion.

X-ray data show soft green and blue hues of material in the center of the remnant that has been heated to millions of degrees, surrounded by the glowing pink optical shell which shows the ambient gas being shocked by the expanding blast wave from the supernova. Ripples in the shell’s appearance coincide with brighter areas of the X-ray data.

There are no recorded observations of the star exploding. However, researchers have identified light from the supernova that was reflected off of interstellar dust. The light from these echoes travels a longer path than the light that travels straight toward us, and so can be seen hundreds of years after the supernova itself. This delay also allowed the astronomers to measure the spectral signature of the light from the explosion, and were able to deduce it was a Type Ia supernova.

Data from Hubble’s Advanced Camera for Surveys, taken in 2006 with a filter that isolates light from glowing hydrogen were combined with visible-light images of the surrounding star field that were imaged with Hubble’s Wide Field Camera 3 in 2010. These data were then merged with X-ray data from the Chandra X-ray Observatory taken with the Advanced CCD Imaging Spectrometer (ACIS) in 2000 and 2007.


November 29, 2012

Galaxy Triplet NGC 6769-71, an interacting triplet of galaxies in Pavo

NGC 6769, NGC 6970, NGC 6971

Image Credit: ESO

Galaxy Triplet NGC 6769-71 is a gravitational interacting triplet of galaxies, located about 190 million light years away in the southern constellation of Pavo (the Peacock).

Most galaxies are members of clusters of galaxies. In these, they move around among each other in a mostly slow and graceful ballet. But every now and then, two or more of the members may get too close for comfort – the movements become hectic, sometimes indeed dramatic, as when galaxies end up colliding. This image shows an example of such a cosmic tango.

As dramatic and destructive as this may seem, such an interaction event is also an enrichment, a true baby-star boom. A cosmic catastrophe like this one normally results in the formation of many new stars. This is obvious from the blueish nature of the spiral arms in NGC 6769 (upper right) and NGC 6770 (upper left) and the presence of many sites of star forming regions.

The two upper galaxies, NGC 6769 and NGC 6770 are of equal brightness and size, while NGC 6771 (below) is about half as bright and slightly smaller. All three galaxies possess a central bulge of similar brightness. They consist of elderly, reddish stars and that of NGC 6771 is remarkable for its “boxy” shape, a rare occurrence among galaxies.

All three galaxies are barred spiral galaxies: NGC 6769 with very tightly wound spiral arms, while NGC 6770 has two major spiral arms, one of which is rather straight and points towards the outer disk of NGC 6769. NGC 6770 is also peculiar in that it presents two comparatively straight dark lanes and a fainter arc that curves towards the third galaxy, NGC 6771.

Stars and gas have been stripped off NGC 6769 and NGC 6770, starting to form a common envelope around them, in the shape of a Devil’s Mask. There is also a weak hint of a tenuous bridge between NGC 6769 and NGC 6771. All of these features testify to strong gravitational interaction between the three galaxies. The warped appearance of the dust lane in NGC 6771 might also be interpreted as more evidence of interactions.

Moreover, NGC 6769 and NGC 6770 are receding from us at a similar velocity of about 3800 kilometers per second while that of NGC 6771 is slightly larger, 4200 kilometers per second.

NGC 6769 was home to at least two supernovae: SN 1997de and SN 2006ox.

This colour-composite image was obtained on April 1, 2004, the day of the Fifth Anniversary of ESO’s Very Large Telescope. It was taken in the imaging mode of the VIsible Multi-Object Spectrograph on Melipal, one of the four 8.2-m Unit Telescopes of the VLT at the Paranal Observatory (Chile). North is up and East is left.

The text is partly taken from ESO.


November 28, 2012

Henize 70, a superbubble in the LMC

N70, DEM301

Image Credit: FORS Team, 8.2-meter VLT, ESO

Henize 70 (also known as N70 and DEM301) is a bright superbubble of about 300 light-years across within the Large Magellanic Cloud (a satellite galaxy to our Milky Way galaxy), located some 160,000 light-years away in the southern constellation of Dorado.

Superbubbles are large bubbles of interstellar gas, blown by winds from hot, massive stars and supernova explosions, and their interiors are filled with tenuous hot expanding gas.

At the center of Henize 70 is a small group of extremely hot and massive stars. Some of these stars are rapidly losing mass and have stellar winds blowing from their surfaces with velocities that approach 4000 kilometers per second.

Because the lifetimes of massive stars are measured in only tens of millions of years, after one supernova has swept clear a bubble around itself, there isn’t enough time for the interstellar medium to back-fill the cavity before other stars explode in the same region. Each subsequent supernova will rejuvenate the cavity left by the previous ones.

These superbubbles provide astronomers with an excellent opportunity to explore the connection between the life-cycles of stars and the evolution of galaxies.

This image shows a three-colour composite of Henize 70 based on CCD frames obtained with the FORS2 instrument in November 1999.


November 27, 2012

NGC 6384, a barred spiral galaxy in Ophiuchus

NGC 6384

Image Credit: ESA/Hubble & NASA

NGC 6384 is a barred spiral galaxy of some 150,000 light-years across, located some 80 million light-years away in the constellation of Ophiuchus, the Serpent Bearer. It is speeding away from us at roughly 1680 kilometers per second.

The positioning of NGC 6384, not far from the centre of the Milky Way on the sky, means that it is somewhat obscured by our galaxies dust and stars. Combined with the galaxy’s low surface brightness, NGC 6384 is a bit of a challenging target for astrophotographers.

This close-up of the galaxy’s central region is about 70,000 light-years wide. The sharp image shows details in the galaxy’s blue spiral arms and yellowish core. Still, the individual stars seen in the picture are all in the close foreground, well within our own galaxy. The brighter Milky Way stars show noticeable crosses, or diffraction spikes, caused by the telescope itself.

While many stars have already come to the ends of their lives in NGC 6384, in the center, star formation is being fuelled by the galaxy’s bar structure; astronomers think such galactic bars funnel gas inwards, where it accumulates to form new stars.

In 1971, we could witness a Type Ia supernova in NGC 6384, which stood out against the bright foreground stars when one of its stars star exploded.

A supernova occurs when a star explodes in the final phase of its life. A Type Ia supernova is a result from the violent explosion of a white dwarf star (a compact star that has ceased fusion in its core). The white dwarf increases its mass beyond a critical limit by gobbling up matter from a companion star. A runaway nuclear explosion then makes the star suddenly as bright as a whole galaxy, before gradually fading from view.

Supernova explosions are enriching the intergalactic gas with elements like oxygen, iron, and silicon that will be incorporated into new generations of stars and planets.

This finely detailed picture was created from images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. An image taken through a blue filter (coloured blue) was combined with an image taken through a near-infrared filter (coloured red).


November 26, 2012

LH 95, a stellar nursery in the LMC

DEM L 252

Image Credit: Hubble Heritage Team, D. Gouliermis (MPI Heidelberg) et al., (STScI/AURA), ESA and NASA

LH 95 is a stellar nursery of about 150 light-years across, located within the Large Magellanic Cloud, some 160,000 light-years away in the southern constellation of Dorado, the Swordfish.

LH 95 is just one of the hundreds of star-forming systems, called associations, located in the Large Magellanic Cloud. This small satellite galaxy of the Milky Way has relatively small amounts of elements heavier than hydrogen, what gives astronomers an insight into star formation in environments different than our Milky Way.

Once massive stars – those with at least 3 times the mass of the Sun – have formed, they generate strong stellar winds and high levels of ultraviolet radiation that ionize the surrounding interstellar gas. The result is a nebula of glowing hydrogen that will expand out into the molecular cloud that originally collapsed to form these stars. The blue haze seen throughout the image around LH 95 is actually part of this bright nebula, known as DEM L 252.

Some dense parts of this star-forming region have not been completely eroded by the stellar winds and can still be seen as dark dusty filaments in the picture. Such dust lanes absorb parts of the blue light from the stars behind them, making them appear redder.

Other parts of the molecular cloud have already contracted to turn into glowing groups of infant stars, the fainter of which have a high tendency to cluster. There are at least two small compact clusters associated with such groups, one to the right, above the centre of the picture and one to the far left. These stellar nurseries host hundreds of infant low-mass stars. Such stars have also been found in the main part of LH 95 amongst its massive bright stellar members.

Usually only the brightest, bluest, most massive stars in a star-forming region are visible, but this image was taken in such high resolution and in such specific colors that many recently formed stars that are more yellow, more dim, and less massive are also discernable. A total of more than 2,500 pre–main sequence stars with masses down to about 0.3 solar masses have been counted in LH 95, thereby giving a detailed picture of what a typical stellar association in the Large Magellanic Cloud looks like.

This image was taken with the Advanced Camera for Surveys onboard the Hubble Space Telescope. It is a composite of two filters that localize visible and infrared light. Because of the color assignments chosen, ionized hydrogen, which is visible within the visible light filter, appears bluish. The choice of color assignment helps to distinguish hot bright blue stars from cooler, less luminous red stars.


November 25, 2012

NGC 4449, a dwarf galaxy in Canes Venatici

NGC 4449

Image Credit & Copyright: Robert Gendler (http://www.robgendlerastropics.com)

NGC 4449 is a dwarf irregular galaxy of some 19,000 light-years across, located about 12.5 million light-years away in the constellation Canes Venatici. It is part of the M94 Group (the Canes Venatici I Group), a galaxy group relatively close to the Local Group (which contains our Milky Way galaxy). It is moving away from us at about 207 kilometers per second.

NGC 4449 is similar in size, shape and brightness, and often compared to, the Large Magellanic Cloud, a satellite galaxy of the Milky Way. Unlike the Large Magellanic Cloud, however, NGC 4449 is considered a starburst galaxy due to its high rate of star formation (twice the one of the LMC).

Its bar consists of a population of stars with an age older than five million years, while the reddish regions in the image are HII regions with embedded ongoing star formation (the reddish emission from heated hydrogen gas is generally a tell-tale sign of active star formation). There are also several large blue star clusters with young, hot and massive stars visible (which will live less than a few hundred million years), some of them near filaments of dust which provide the fuel for future star formation.

Radio wave observations have shown that NGC 4449 is embedded in a huge gaseous halo with a diameter of 14 times the optical one, that show distortions and irregularities likely caused by interactions with nearby galaxies.

A disk like feature and a lopsided arm structure have been found in its halo and are likely the fossil remains of a merger in its remote past. NGC 4449 most likely had an encounter with its irregular companion galaxy, DDO 125 some 500 million years ago.

NGC 4449 also holds the distinction of being the first dwarf galaxy with an identified tidal star stream (not seen in this close-up image) with lots of red giant stars. This star stream represents the remains of a still smaller infalling satellite galaxy, dubbed NGC 4449B, disrupted by gravitational forces and destined to merge with NGC 4449.

This is how galaxies grow: the smaller galaxy is coming in and getting shredded, eventually leaving its stars scattered through the halo of the host galaxy. This particular interaction is called a “stealth merger” because the smaller galaxy NGC 4449B was nearly imperceptible, but has a profound effect on the shape of its partner.

In 2012 astronomers also found a highly flattened globular cluster with two tails of young stars that may be the nucleus of a gas-rich galaxy being disrupted and absorbed by NGC 4449.

With relatively few stars, small galaxies are thought to possess extensive dark matter halos. But since dark matter interacts gravitationally, these observations offer a chance to examine the significant role of dark matter in galactic merger events. These interactions are likely responsible for NGC 4449’s bursts of star formation.

This image is based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA).


November 24, 2012

The Papillon Nebula, a compact H II “blob” in the LMC


Image Credit: M. Heydari-Malayeri (Paris Observatory) et al., ESA and NASA

The Papillon Nebula (N159-5) is a butterfly-shaped High Excitation Blob (HEB) of less than 2 light-years across within the nebula N159, a turbulent star-forming region of more than 150 light-years across. It is located in the Large Magellanic Cloud, about 170,000 light-years away in the constellation of Dorado.

High Excitation Blobs (HEBs) are compact H II regions, a rare class of ionized nebulae in the Magellanic Clouds. They are characterized by high excitation, small size, high density, and large extinction compared to typical Magellanic Cloud H II regions. These objects are tightly linked to the early stages of massive star formation, when the stars begin to hatch from their parental molecular clouds.

This compact ionized “blob” is buried in the center of the maelstrom of glowing gases and dark dust in N159. This image shows unprecedented details of the structure and internal morphology of the Papillon nebula (“Papillon” is French for “butterfly”).

A possible explanation of its bipolar shape is the outflow of gas by strong stellar winds from newborn massive stars (over 10 times the mass of our Sun), hidden in the central absorption zone. Such stars are so hot that their radiation pressure halts the infall of gas and directs it away from the stars in two opposite directions. Presumably, a dense equatorial disk formed by matter still trying to fall in onto the stars focuses the outstreaming matter into the bipolar directions.

It is rather rare that we can see massive stars so early after their birth.

This image was taken on September 5, 1998 with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope.


November 23, 2012

NGC 5907, an edge-on spiral galaxy in Draco

Splinter Galaxy, Draco's Needle and The Knife Edge

This image has been obtained from the Wikimedia Commons.

NGC 5907 (nicknamed Splinter Galaxy, Draco’s Needle and The Knife Edge) is a warped spiral galaxy of about 150,000 light-years across, located some 53.5 million light years away in the northern constellation of Draco, while it is moving away from us at approximately 667 kilometers per second. It is a member of the NGC 5866 Group of galaxies.

This galaxy is seen almost perfectly edge-on from our line of sight. We see a very flat disk, laced with dark dust lanes, that appears extraordinarily elongated.

NGC 5907 has an anomalously low metallicity, meaning that it has very few chemical elements other than hydrogen and helium. It contains few detectable giant stars, being apparently composed almost entirely of dwarf stars. In 1940, a supernova exploded in this galaxy, designated SN 1940A.

NGC 5907 has long been considered a prototypical example of a warped spiral in relative isolation. Then in 2006, an international team of astronomers announced the presence of extended tidal streams of stars surrounding the galaxy what may be the cause for the warp. But, even though NGC 5907 is a member of a galactic group, there are no galaxies near enough to it to be causing an interaction which could account for its streamers of stars.

The arcing tidal streams form tenuous loops (which are not seen in this image) extending more than 150,000 light-years from the narrow, edge-on galaxy. Recorded only in very deep exposures, these streams were likely formed when two galaxies collided.

Previously, researchers had thought that the stellar swirls were formed when a smaller galaxy hit a larger one, but a new study shows that this would have been impossible. In order to produce the observed streams, two roughly equally-sized galaxies crashed into each other 8 or 9 billion years ago. Simulations also showed that the galaxies must have been rich in gas in order to produce the swirls.

The Milky Way is heading for a crash into Andromeda in 4.5 billion years, and the resulting galaxy could have a similar shape. Most large spiral galaxies are thought to have formed in a similar process.

This image is created using the 24 inch telescope on Mt. Lemmon, AZ.


November 22, 2012

The Wizard Nebula, a nebula surrounding the star cluster NGC 7380


Image Credit & Copyright: J-P Metsävainio (http://astroanarchy.zenfolio.com)

The Wizard Nebula (Sharpless 142 or SH2-142 for short) is a diffuse nebula surrounding the developing open star cluster NGC 7380. It spans about 140 × 75 light-years and lies within our Milky Way Galaxy, about 7,200 light years away in the constellation of Cepheus. It is moving toward us at 34.13 kilometers per second.

The name Sharpless comes from a catalog of 312 emission nebulae (H II regions). The first edition was published by Stewart Sharpless in 1953 with 142 objects (Sh1) and the second and final version was published in 1959 with 312 objects (Sh2).

This kind of nebulae are the birthplace of stars. They are formed when very diffuse molecular clouds begin to collapse under their own gravity, often due to the influence of a nearby supernova explosion. The cloud collapses and fragments, sometimes forming hundreds of new stars. The newly-formed stars ionize the surrounding gas to produce an emission nebula.

In this case, the stars of NGC 7380 have emerged from its natal cloud some 5 million years or so ago, making it a relatively young cluster. And, although the nebula may last only a few million years, some of the stars being formed may outlive our Sun.

The Wizard Nebula is ionized by the binary star HD215835 (DH Cephei), together with the many young energetic stars within the cluster. They make the nebula that surrounds them glow and their winds and radiation sculpt clouds of gas and dust into the mountainous ridges seen here.

This bright, active star-forming region is part of a much larger molecular cloud in Cepheus, called NGC 7380E, which total mass is estimated to be 6000-15000 solar masses.

This image was created in HST-palette (HST=Hubble Space Telescope) from the emission of ionized elements, R=Sulfur, G=Hydrogen and B=Oxygen.


November 21, 2012

Arp 271, a pair of interacting galaxies in Virgo

NGC 5426 and NGC 5427

Image Credit & Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona (http://www.caelumobservatory.com/index.html)

Arp 271 is a pair of interacting spiral galaxies: NGC 5426 (the more oblique galaxy at left) and NGC 5427 (the nearly face-on spiral galaxy at right). Together they are about 130,000 light-years across and located some 90 million light-years away towards the constellation of Virgo.

The galaxies are in the throes of a slow but disturbing interaction, that began likely a several million years ago. These twin galaxies — which have similar sizes, masses, structures, and shapes with expansive arms and a compact core – appear undisturbed. But the mutual pull of gravity has already begun to alter and distort their visible features.

The gravity between the interacting galaxies has begun to pull some of the outer stars towards the center creating a bridge of stars. This intergalactic bridge acts like a feeding tube, allowing the twins to share gas and dust with one other across the 60,000 light years of space separating them.

Colliding gases caused by the interaction also triggered bursts of star formation in each galaxy. Star-forming, or HII, regions appear as hot pink knots that trace out the spiral patterns in each galaxy. The giant ones in NGC 5426 are more abundant on the side of the galaxy closest to NGC 5427. Starburst activity can also be seen in the galaxy’s connecting bridge.

Likewise, the giant HII regions in NGC 5427’s disk are forming at a high rate and are plentiful. One giant star-forming region at the tip of NGC 5427’s western spiral arm, looks especially large and disturbed, as does the arm itself, which is unusually straight, as if strong tidal forces have broken the arm in two, causing it to bleed starlight.

It is not certain that the interaction between the two galaxies of Arp 271 will end in a collision and ultimately a merging of the two galaxies, although the galaxies have already been affected. This dramatic dance will last for tens of millions of years, creating new stars as a result of the mutual gravitational attraction between the galaxies.

Once thought to be unusual and rare, gravitational interactions between galaxies are now known to be quite common (especially in densely populated galaxy clusters) and are considered to play an important role in galaxy evolution. Most galaxies have probably had at least one major, if not many minor, interactions with other galaxies since the advent of the Big Bang.

Quite possibly, our own Milky Way will undergo a similar collision in about five billion years with the neighbouring Andromeda galaxy, which is now located about 2.6 million light-years away from the Milky Way.


November 20, 2012

NGC 3132, a bright planetary nebula in Vela

the Eight-Burst Nebula, the Southern Ring Nebula

Image Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

NGC 3132 (also known as the Eight-Burst Nebula because of its figure-8 appearance through small telescopes, or the Southern Ring Nebula) is a very bright, asymmetric planetary nebula of approximately 0.4 light-year across, located about 2,000 light-years away in the southern constellation Vela. It is moving away from us at 49 kilometers per second.

Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through early telescopes.

When a star with a mass up to eight times that of the Sun approaches the end of its life, it blows off its outer shells and begins to lose mass. This allows the hot, inner core of the star to radiate strongly, causing this outward-moving cocoon of gas to glow brightly as a planetary nebula.

Over the next several thousand years, the nebula will gradually disperse into space, and then the star will cool and fade away for billions of years as a white dwarf. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.

There are two stars close together — a binary system — in the center of NGC 3132, one of 10th magnitude, the other 16th. It’s the dim star, not the bright one, near the center that caused multiple outbursts and originated the intricate, somewhat concentric structure of the nebula. This hot central star is a white dwarf of about 100,000 K that has now blown off its layers and is making the nebula fluoresce brightly from the emission of its intense ultraviolet radiation.

This expanding cloud of gas is one of the nearest known planetary nebulae. The gases are expanding away from the central star at a speed of about 14.4 kilometers per second. Neither the unusual shape of the surrounding cooler shell nor the structure and placements of the cool filamentary dust lanes running across NGC 3132 are well understood.

This image is taken with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope using three different color filters. North is to the bottom left hand corner of this image.

November 19, 2012

NGC 1309, a spiral galaxy in Eridanus

NGC 1309

Image Credit: NASA, ESA, The Hubble Heritage Team, (STScI/AURA) and A. Riess (STScI)

NGC 1309 is a spiral galaxy of about 75,000 light-years across, located approximately 120 million light-years away in the constellation Eridanus, and is one of over 200 members of the Eridanus Group of galaxies. It is running away from us at 2136 kilometers per second.

Bright blue areas of star formation pepper the moderately wound spiral arms, while dark dust lanes follow the spiral structure into a yellowish central nucleus of older-population stars. The image is complemented by myriad far-off background galaxies.

NGC 1309 was home to a Type Ia supernova, SN 2002fk, whose light reached Earth in September 2002.

A supernova occurs when a star explodes in the final phase of its life when it used up almost all of its main fuel. A Type Ia supernova is a result from the violent explosion of a white dwarf star. This white dwarf was accreting matter from its companion in a binary star system. When the white dwarf collected enough mass and was no longer able to support itself, the star detonated. The exploding star can become billions of times as bright as the Sun before gradually fading from view. At its maximum brightness, it may outshine an entire galaxy for several weeks.

This category of supernovae produces consistent peak luminosity. The stability of this luminosity allows astronomers to use these supernovae as standard candles, to calibrate distance measures in the Universe. By comparing nearby Type Ia supernovae to more distant ones, they can determine not only that the Universe is expanding, but that this expansion is accelerating.

This composite image is taken by the Hubble Space Telescope during August and September 2005.

November 18, 2012

The Southern Crab Nebula, a planetary nebula in Centaurus


Image Credit: Romano Corradi, Instituto de Astrofisica de Canarias, Tenerife, Spain; Mario Livio, Space Telescope Science Institute, Baltimore, Md.; Ulisse Munari, Osservatorio Astronomico di Padova-Asiago, Italy; HugoSchwarz, Nordic Optical Telescope, Canarias, Spain; and NASA/ESA

The Southern Crab Nebula (He2-104) is a bipolar planetary nebula of several light-years long, located about 7000 light-years away in the southern constellation of Centaurus. It is dubbed the “Southern Crab Nebula” because, from Earth, it looks like the body and legs of a crab. The adjective “southern” distinguishes it from the Crab Nebula, which is in the northern sky. The nebula is approaching us at 143.6 kilometers per second.

Images taken with Earth-based telescopes show only the larger, hourglass-shaped nebula, while this Hubble image also reveals a small, bright nebula embedded in the center of the larger one.

Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through early telescopes.

When a star with a mass up to eight times that of the Sun approaches the end of its life, it blows off its outer shells and begins to lose mass. This allows the hot, inner core of the star to radiate strongly, causing this outward-moving cocoon of gas to glow brightly as a planetary nebula. Over the next several thousand years, this nebula will gradually disperse into space.

At the center of the Southern Crab lies a pair of aging stars, each with a mass roughly equal to that of the Sun. They cannot be seen in this image because they are buried in the glow of the tiny, central nebula). One of them is a red giant, a bloated star that is exhausting its nuclear fuel and is shedding its outer layers in a powerful stellar wind. Its companion is a hot, white dwarf, a stellar zombie of a burned-out star. Because it is far away from its partner, it could take as much as 100 years for the two to orbit around each other.

This odd duo is called a symbiotic system. The red giant star turns out to be a Mira variable, a pulsating star that is dumping material into an accretion disk surrounding the white dwarf and then spirals onto its hot surface. Gas continues to build up on the surface until it sparks an eruption, blowing material into space.

This explosive event may have happened twice in the Southern Crab. Astronomers speculate that the hourglass-shaped nebulae represent two separate outbursts that occurred several thousand years apart. The jets of material in the lower left and upper right corners may have been accelerated by the white dwarf’s accretion disk and probably are part of the older eruption.

It is thought that when enough mass keeps falling onto the white dwarf, this might happen again, and will create another expanding hourglass-shaped nebula.

November 17, 2012

Maffei 2, a barred spiral galaxy in Cassiopeia


Image Credit: NASA/JPL-Caltech/UCLA

Maffei 2 (also known as UGCA 39) is a, heavily obscured, intermediate barred spiral galaxy of about 15,000 light-years across, located near the Galactic Plane, some 9.8 million light-years away in the constellation Cassiopeia, far behind — but seemingly next to — the Heart Nebula (IC 1805). It is a member of the IC 342/Maffei Group, the group of galaxies that is the closest to the Local Group. This galaxy is approaching us at roughly 17 kilometers per second.

It lies in the “Zone of Avoidance”, the area of the sky that is obscured by interstellar dust, gas and stars in our own Milky Way, and is about 99.5 percent obscured by these dust clouds. As a result Maffei 2, is barely detectable at optical wavelengths, and is, therefore, discovered from its infrared emission only in 1968 by Paolo Maffei, together with its elliptical neighbour Maffei 1.

This infrared image from the Spitzer Space Telescope penetrates the dust to reveal the galaxy in all its glory. It clearly shows the unusual structure of Maffei 2, with a strong central bar and prominent dusty, asymmetric spiral arms. It contains billions of stars, and also harbors a burst of star formation in its very core (one of the nearest examples of this process). These structural asymmetries, as well as its nuclear starburst, are possibly caused by an ongoing merger with a small satellite companion galaxy.

November 16, 2012

Messier 15, a globular cluster in Pegasus

Image Credit: ESA/Hubble & NASA

Messier 15 (also designated NGC 7078) is a globular cluster of about 175 light-years across, located some 33,600 light-years away in the constellation Pegasus (The Flying Horse). Over 12 billion years old, it is one of the oldest known globular clusters, but is rich and bright despite its age. The cluster is approaching us at roughly 107 kilometers per second.

A globular cluster is a spherical conglomeration of old stars that formed together from the same cloud of gas, found in the outer reaches of the Milky Way in a region known as the halo and orbiting the Galactic Centre.

Home to over 100,000 stars, Messier 15 has a total luminosity of 360,000 times that of the Sun; its brightest stars have a luminosity of 1,000 times that of our Sun. It contains a large number of variable stars (112) and pulsars (9), including one double neutron star system. It is also the first globular cluster known to harbour a planetary nebula, and it is still one of only four globulars known to do so. The planetary nebula, called Pease 1, can be seen in this image as a small blue blob to the lower left of the globular’s core.

Messier 15 is perhaps the densest of all globulars in our Milky Way galaxy. The cluster has undergone a process of contraction called “core collapse”, in which gravitational interactions between stars led to many members of the cluster migrating towards the center, which is quite common in the dynamical evolution of globulars (about 20% of the globular clusters have undergone a core collapse).

This central core is extremely small compared to the cluster, only about 1.4 light-years, and half of the stars of the cluster are packed in a central sphere with a radius of just 10 light-years. It is still unclear if the central core of M15 is packed so dense simply because of the mutual gravitational interaction of the stars it is made of, or if it houses a central medium-mass black hole. In that case it would be among the nearest and better observable to us, being only little more remote than the Galactic Center and much less obscured by interstellar matter.

Messier 15 can be seen with the naked eye under very good conditions and can be observed with binoculars or a small telescope, appearing as a fuzzy star. Telescopes with a larger aperture (at least 6 in./150 mm diameter) will start to reveal individual stars, the brightest of which are of magnitude +12.6.

This image was created from images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys.

November 15, 2012

IC 59 and IC 63, two arc-shaped nebulae in Cassiopeia

IC 59 & IC 63

Image Credit & Copyright: Ken Crawford, Rancho Del Sol Observatory (http://www.imagingdeepsky.com)

IC 59 (left) and IC 63 (right) are a combination of faint, arc-shaped emission and reflection nebulae, located about 600 light-years away in the constellation Cassiopeia. Together they are approximately 10 light-years across.

The bluish glow shinning down from the top comes from the intense radiation of the bright, hot star Gamma Cassiopeia that is located only 3 to 4 light-years from the nebulae, and which may also have shed this nebulous material into the space around it. The edges of the nebulae glow brightly from this intense radiation that is slowly evaporating and lighting up these flowing shapes of gas and dust.

Gamma Cassiopeia is with a radius of 14 times our Sun, 55,000 times more luminous, 19 times more massive, and rotates at about 300 kilometers per hour, or 150 times more rapidly than the Sun. It is known as an eruptive blue-white subgiant variable star. (Eruptive variable stars vary in brightness because of violent processes and flares in their coronae and chromospheres.)

This star is an erratic variable that reached a maximum brightness in 1937, but then unexpectedly dropped in surface temperature from 12,000°K to 8500°K. It is encircled by a surrounding gaseous disk of material thrown off by its rapid rotation, that radiates the emissions. Mass loss is apparently related to the brightness variations.

IC 63 — the brighter of the two and slightly closer to Gamma Cassiopeia than IC 59 — is a combination of an emission and reflection nebula. Unlike a reflection nebula which appears blue, the glowing hydrogen gas appears red. IC 59 is primary a refection nebula, showing much less red hydrogen, and is appearing blue of dust reflected starlight that is passing through it.

November 14, 2012

The Triangulum Galaxy, a spiral galaxy in the Local Group

Image Credit & Copyright: Robert Gendler (http://www.robgendlerastropics.com)

The Triangulum Galaxy (catalogued as Messier 33 or NGC 598) is a spiral galaxy, located approximately 3 million light-years away in the constellation Triangulum. With a diameter of about 50,000 light years (half the diameter of the Milky Way), the Triangulum galaxy is the third largest member of the Local Group (a group of galaxies which also contains the Milky Way, Andromeda and about 30 other smaller galaxies). However, the faintest outlayers seem to reach more far out, so that the true diameter may be some 82 million light-years. It is approaching our Galaxy at 24 kilometers per second.

Triangulum may be home to 40 billion stars, compared to 400 billion for the Milky Way, and 1 trillion (1000 billion) stars for Andromeda. The mass of the Triangulum Galaxy has been estimated between 10 and 40 billion solar masses.

The disk-shaped galaxy, surrounded by a faint halo, has prominent loosely wound arms of gas and dust that spiral out from the nucleus, and dark dust lanes. There is no bulge at the nucleus. The inner part of the galaxy has two luminous spiral arms, along with multiple spurs that connect the inner to the outer spiral features. The main arms are designated IN (north) and IS (south).

Although Triangulum is classified as unbarred, there may be a weak bar-like structure about the galactic nucleus. The nucleus of this galaxy is an H II (star forming) region, and it contains an ultraluminous X-ray source, which is the most luminous source of X-rays in the Local Group. However, the nucleus does not appear to contain a supermassive black hole.

Triangulum appears to be divided into two distinct components with a different star formation history between the inner disk (within a radius of 30 million light-years) and the outer disk and halo, and may be explained by a scenario of “inside-out” galaxy formation. This occurs when gas is accumulated at large radii later in a galaxy’s life space, while the gas at the core becomes exhausted. The result is a decrease in the average age of stars with increasing radius from the galaxy core.

The northern main spiral arm contains four large HII regions, while the southern arm has greater concentrations of young, hot stars. The estimated rate of supernova explosions in the Triangulum Galaxy is one supernova explosion every 147 years, on average. As of 2008, a total of 100 supernova remnants have been identified in the Triangulum Galaxy, and a majority of the remnants lie in the southern half of the spiral galaxy. Similar asymmetries exist for highly luminous concentrations of massive stars. The center of the distribution of these features is offset about two arc minutes to the southwest.

About 54 globular clusters have been identified in this galaxy, but the actual number may be 122 or more. The confirmed clusters may be several billion years younger than globular clusters in the Milky Way, and cluster formation appears to have increased during the past 100 million years. This increase is correlated with an inflow of gas into the center of the galaxy.

At least 112 variables have been discovered in Triangulum, including 4 novae and about 25 Cepheids. In 2007, a black hole about 15.7 times the mass of the Sun was detected in the galaxy. The black hole, named M33 X-7, orbits a companion star which it eclipses every 3.5 days. It is the largest stellar mass black hole known.

Triangulum’s brightest and largest H II region is NGC 604, a diffuse emission nebula containing ionized hydrogen. As seen from Earth, NGC 604 is located northeast of the galaxy’s central core. It is one of the largest H II regions known, with a diameter of nearly 1500 light-years and a spectrum similar to that of the Orion Nebula.

NGC 604, may have undergone a discrete outburst of star formation about three million years ago. Astronomers counted 200 young hot massive stars (of 15 to 60 solar masses) which have recently formed here. This nebula is the second most luminous HII region within the Local Group of galaxies.

Triangulum is also home to the smaller H II regions NGC 588, 592 and 595. Other prominent HII regions in the galaxy include IC 132, IC 133 and IK 53.

One of the small Local Group member galaxies, the Pisces Dwarf (LGS 3), located a little more than 2 million light-years away, lies at a distance of 913 million light-years from both Triangulum and Andromeda. This means it could be a satellite galaxy of either galaxies. The Pisces Dwarf has a core radius of 483 light-years and 26 million solar masses.

Triangulum itself is a remote but gravitationally bound companion of the Andromeda galaxy. The distance of Triangulum from Andromeda is about 750,000 light-years. Triangulum is moving towards the Andromeda Galaxy and a clumpy stream of hydrogen gas and stars is linking Triangulum with Andromeda, what suggests that a past interaction between these two galaxies took place between 2-8 billion years ago, and a more violent encounter 2.5 billion years in the future.

The fate of the Triangulum Galaxy is unclear, but seems to be linked to its larger neighbor Andromeda. Suggested future scenarios for Triangulum range from being torn apart and absorbed by Andromeda fueling it with hydrogen to form new stars or losing all of its gas—thus, the ability to form new stars—to participating in the collision between the Milky Way and Andromeda, most likely ending orbiting the merger remnant of the latter two galaxies and fusing with it much later. Two other possibilities are a collision with the Milky Way before Andromeda arrives or an ejection out of the Local Group.

The Triangulum Galaxy is one of the most distant objects that can be viewed with the naked eye – under exceptionally good viewing conditions with no light pollution. Through a small telescope, the galaxy looks like a diffuse patch about the same size as the full Moon.

November 13, 2012

SXP 1062, a pulsar in a supernova remnant

SXP 1062

Image Credit: X-ray: NASA/CXC/Univ.Potsdam/L.Oskinova et al & ESA/XMM-Newton; Optical: AURA/NOAO/CTIO/Univ.Potsdam/L.Oskinova et al

SXP 1062 is a young pulsar within a supernova remnant of some 744 light years across, located in the Wing of the Small Magellanic Cloud (a small satellite galaxy to our Milky Way), about 180,000 light-years away in the southern constellation of Tucana.

The Wing of the Small Magellanic Cloud is a peripheral region of this small galaxy. The Wing is part of the tidal feature that connects the Small Magellanic Cloud to its neighbour, the Large Magellanic Cloud.

The bubble-shaped feature on the right-hand side of the image is the supernova remnant that encloses the pulsar (the bright white source in the center). The diffuse blue glow at the center represents X-ray emission from both the pulsar and the hot gas that fills the remnant of the supernova.

Optical images show that SXP 1062 is part of a binary system, and that it accretes mass from this stellar companion, a massive, hot, blue star. On the left side in this image (seen in optical light) is a spectacular formation of gas and dust in a star-forming region. (Can someone tell me its name?)

A supernova occurs when a star explodes at the end of its life. After some supernova explosions, when the star collapses and becomes so dense that protons and electrons squish together to form neutrons, there remains a small, ultra-dense neutron star. Rapidly rotating, highly magnetized neutron stars are called pulsars. (After other supernova explosions, a black hole may be left behind.) The explosion throws a large, roughly spherical cloud of dust and hot gas into space surrounding the neutron star (or black hole). When this slams into the existing interstellar medium, it heats up so much it glows in X-rays.

Since supernova remnants shine only for a few tens of thousands of years before dispersing into the interstellar medium, not many pulsars have been detected while still embedded in their expanding shell. This is the first clear example of such a pair in the Small Magellanic Cloud.

SXP 1062 is rotating unusually slowly — about once every 18 minutes, in an extremely long period — 1,062 seconds. (In contrast, some pulsars are found to revolve multiple times per second, including most newly born pulsars.) This relatively leisurely pace of SXP 1062 makes it one of the slowest rotating pulsars in the Small Magellanic Cloud

Since pulsars slow down as they age, SXP 1062’s sluggish rotation seems to imply an advanced age, in contrast to the fairly young age of the supernova remnant (between 10,000 and 40,000 years old) that surrounds it. This means that the pulsar is very young, from an astronomical perspective, since it was presumably formed in the same explosion that produced the supernova remnant. Therefore, assuming that it was born with rapid spin, it is a mystery why SXP 1062 has been able to slow down by so much, so quickly.

This false-colour image combines the X-ray view, based on data from XMM-Newton (blue), with optical data from NOAO’s Cerro Tololo Inter-American Observatory (CTIO), obtained using two special filters that reveal the glow of oxygen (green) and hydrogen (red).

November 12, 2012

NGC 4414, a dusty spiral galaxy in Coma Berenices

UGC 7539

Image Credit: Hubble Heritage Team (AURA/STScI/NASA/ESA)

NGC 4414 (also known as UGC 7539) is a massive unbarred spiral galaxy of some 56,000 light-years in diameter, located about 62.3 million light-years away in the constellation Coma Berenices, while it is moving away from us at approximately 716 kilometers per second. It is a member of the Coma I group of galaxies, very close to the Virgo Cluster.

NGC 4414 is a typical example of a flocculent galaxy, what means it is patchy, with discontinuous spiral arms (the opposite of a grand design spiral galaxy, which has well-defined spiral arms).

The central regions of this galaxy contain primarily older, yellow and red stars. The outer spiral arms are considerably bluer due to the ongoing formation of young, blue stars — the brightest of which can be seen individually — between which is what appears to be a Luminous Blue Variable. The arms are also very rich in clouds of interstellar dust, seen as dark patches and streaks silhouetted against the starlight.

NGC 4414 is quite isolated in space, without signs of past interactions with other galaxies. The galaxy exhibits a high gas density and therefore is very rich in gas, both neutral hydrogen and molecular hydrogen, of which the neutral hydrogen extends far beyond its optical disk. Despite the high gas density, NGC 4414 is not a starburst galaxy, a fact that could be related to its isolation.

In 1974 a supernova, SN 1974G, was observed which is the only supernova in this galaxy to be recorded so far. But, strangely enough, there is no direct evidence for a supermassive black hole in the center of NGC 4414.

Based on careful brightness measurements of the many Cepheid variable stars in NGC 4414, astronomers were able to make an accurate determination of its distance. Along with similarly determined distances to other nearby galaxies, NGC 4414 contributes to astronomers’ overall knowledge of the expansion rate of the Cosmos, and helps them determine the age of the Universe.

This composite image is obtained with the Hubble Space Telescope in 1995 and 1999, using the Wide Field Planetary Camera 2 through three different color filters.

November 11, 2012

Herbig-Haro 32, a bright Herbig–Haro object in Aquila

HH 32

Image Credit: NASA and The Hubble Heritage Team (AURA/STScI)

Herbig-Haro 32 (HH 32) is a bright Herbig–Haro object located some 960 light-years away in the constellation of Aquila.

A Herbig-Haro object is a small bright nebula in a star-forming region, created when jets of material — at speeds of several hundred kilometres per second — from a newborn star collide with the interstellar medium. The bipolar jets plow into the surrounding nebula, producing strong shock waves that heat the gas and cause it to glow in the light of hydrogen atoms (green) and sulfur ions (blue), several other atoms and ions, and sometimes radiation from the exciting star that is reflected by the surrounding gas (red).

This glow makes it a Herbig-Haro object, in honor of astronomers George Herbig and Guillermo Haro. However, many HH objects don’t show a clearly jet-like morphology and appear as systems of a few scattered, emitting knots. All known Herbig-Haro objects have been found within the boundaries of dark clouds and are strong sources of infrared.

They are transient phenomena, lasting not more than a few thousand years. They can evolve visibly over a few years, as parts of them fade while others brighten as they collide with clumpy material in the interstellar medium.

HH 32 is an excellent example of a Herbig-Haro object, although rather old. The intense wind of material blowing from the bright, young central star (called AS 353A) has already cleared much of the dust out of the central region, thus exposing the star to direct view.

We can still see interstellar gas and dust around the young star, and two oppositely directed jets coming from this star. The end of the optical jet is some 12,000 AU (12,000 times the distance between Earth end the Sun) from the central star. The total length of the bipolar outflow is about 0.54 light-years.

The jet on the top side, whose furthest extent is about 0.2 light-year from the star, is pointed more nearly in our direction, while the opposite jet on the bottom lies on the far side of the star and is fainter either because it is partially obscured by dust surrounding the star or because there is much less material in front of the star.

November 10, 2012

Holmberg II, a dwarf irregular galaxy in Ursa Major

Arp 268, UGC 4305

Image Credit: NASA & ESA

Holmberg II (also known as Arp 268 and UGC 4305) is a very bright dwarf irregular galaxy located only about 9.8 million light-years away in the constellation Ursa Major. It is a member of the M81 Group of galaxies, and one of the few that isn’t distracted by gravity from other nearby galaxies.

This small galaxy is a patchwork of dense star-forming regions and extensive barren areas with less material, which can stretch across thousands of light-years.

Holmberg II is dominated by giant bubbles of glowing gas – the largest about 5,500 light-years wide – which are regions of old star formation. The cavities are blown by high-mass stars (as these stars form in dense regions of gas and dust, they expel strong stellar winds that blow away the surrounding material) and of gas by the shock waves produced in supernovae (the violent explosions that mark the end of the lives of massive stars).

As a dwarf galaxy, it has neither the spiral arms of galaxies like the Milky Way nor the dense nucleus of an elliptical galaxy of which the gravitational pull would destroy the fragile bubbles. This makes Holmberg II a gentle haven where these fragile structures can hold their shape.

New star birth is also taking place, but not in the same areas as the bubbles because these are drained now of gas or dust. The star formation regions in Holmberg II appear as massive, disorganized patches filled with hundreds of young, blue stars, that occupy a relatively large fraction of the disk. One region in particular has almost as many young stars as the famous Tarantula Nebula in the Large Magellanic Cloud.

Holmberg II is the perfect example of the “champagne” model of starbirth – where new stars create even newer ones. It works like this: when a bubble is created by stellar winds, it moves outwards until it reaches the edge of the molecular cloud that spawned it. At the exterior edge, dust and gas have been compressed and form a nodule similar to a blister. Here another new star forms.. and triggers again… and triggers again… similar to the chain reaction which happens when you open a bottle of champagne.

The galaxy also hosts an ultraluminous X-ray source in the middle of three gas bubbles in the top right of the image. There are competing theories as to what causes this powerful radiation — one intriguing possibility is an intermediate-mass black hole which is pulling in material from its surroundings.

Holmberg II enables astronomers to study star birth in an environment that isn’t disturbed by density waves (as happens in larger galaxies such as the Milky Way) or by deformation caused by the pull of another galaxy, and that is conveniently close.

This image was captured by the NASA/ESA Hubble Space Telescope. It is a composite of visible and near-infrared exposures taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys.

November 9, 2012

NGC 1491, an emission nebula in Perseus


Image Credit & Copyright: Ken Crawford, Rancho Del Sol Observatory (http://www.imagingdeepsky.com)

NGC 1491 (also designated SH2-206 and LBN 704) is a bright emission nebula and HII region, located on the edge of a vast cloud region of neutral gas, about 10,700 light-years away in the Perseus arm of our Milky Way Galaxy in the constellation Perseus.

HII regions are well known for being places where new stars are born, and are created when ultraviolet radiation from hot stars ionizes the surrounding gas, causing it to glow in visible light. The surrounding dust is also heated by this radiation, so we also see it glow in infrared light.

The blue 11.22 magnitude star, BD +50 ° 886, is illuminating the nebula while its strong stellar wind is “blowing” a bubble in the gas that immediately surrounds it. The intense radiation from the star is also eroding the gas clouds surrounding it.

The entire nebula is quite irregular with a subtle bite cut out of the nebulosity from the east side that creates a darker hollow, and a high surface brightness region — as seen in this image — preceding the star. A faint, elongated haze extends from this patch to the northeast past the star giving an elongated appearance.

This narrowband image of the brighter part of NGC 1491 was processed in the standard Hubble palette.

November 8, 2012

The Antennae Galaxies, a pair of merging galaxies in Corvus

NGC 4038 & NGC 4039

Image Credit & Copyright: Robert Gendler (http://www.robgendlerastropics.com/

The Antennae Galaxies [NGC 4038 (right in this image) and NGC 4039] are a pair of merging galaxies of some 61,000 light-years across (without the tidal tails), located about 45 million light-years away within the NGC 4038 group of galaxies in the southern constellation Corvus (and also visible very low on the southern horizon in the Northern Hemisphere). The Antennae are among the closest known merging galaxies, while they are moving away from us at about 1642 kilometers per second.

The Antennae are in the midst of a galactic collision. About 1.2 billion years ago, the Antennae were two separate galaxies. NGC 4039, the larger of the two before they collided, was an unbarred spiral galaxy and NGC 4038 was a barred spiral galaxy. 900 million years ago, the Antennae began to approach one another, and 600 million years ago, the Antennae passed through each other. 300 million years ago, the Antennae’s stars began to be released from both galaxies. Today the two tidal tails of ejected stars, gas and dust extend out almost 500,000 light years from their respective centers, making them resemble the antennae of an insect, hence the name of the galaxy pair.

Within 400 million years, the Antennae’s nuclei will collide and become a single core with stars, gas, and dust around it. Observations and simulations of colliding galaxies suggest that the Antennae Galaxies will eventually form one giant elliptical galaxy. They give us a preview of what may happen when our Milky Way collides with the neighboring Andromeda galaxy in several billion years.

The galaxies are criss-crossed by filaments of dark brown dust, and dotted by over a thousand bright star clusters, containing tens of thousands of massive, hot, young stars — the result of a burst of star formation triggered by the collision. The brightest and most compact of them are called super star clusters.

Only about 10% of the newly formed super star clusters in the Antennae will live to see their ten millionth birthday. The vast majority of the super star clusters formed during this interaction will disperse, with the individual stars becoming part of the smooth background of the galaxy. It is however believed that about a hundred of the most massive clusters will survive to form regular globular clusters.

The most massive of the massive young stars have already sped through their evolution in a few million years and exploded as supernovae. Two supernovae have been discovered in the Antennae: SN 2004GT and SN 2007sr. The two small, thin lines in this image pinpoint the location of SN 2007sr, a Type Ia supernova that occurred on December 18th, 2007.

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.

Supernova explosions are enriching the intergalactic gas with elements like oxygen, iron, and silicon that will be incorporated into new generations of stars and planets. In the Antennae, there are also rich deposits of neon, magnesium, and silicon discovered, which are among the elements that form the building blocks for habitable planets.

Although galaxy mergers today are not common, it is believed that in the past they were an important channel of galaxy evolution. Therefore understanding the (physics of) galaxy mergers is a very important task for astrophysicists and cosmologists.

November 7, 2012

Vela Supernova Remnant

Vela Supernova Remnant

Image Credit: NASA/The Hubble Heritage Team (STScI/AURA)

The Vela Supernova Remnant (R762-0019) is a huge supernova remnant around 100 light-years across, located about 800 light-years away in the southern constellation Vela, what makes it one of the closest known supernova remnants to Earth. The stellar explosion occurred approximately 11,000-12,300 years ago. This was direct observational proof that supernovae can form neutron stars.

The Vela supernova remnant includes the Pencil Nebula (NGC 2736) — and also overlaps the Puppis Supernova Remnant, which is four times more distant. Both the Puppis and Vela remnants are among the largest and brightest features in the X-ray sky.

A supernova occurs when a star explodes in the final phase of its life. The exploding star can become billions of times as bright as the Sun before gradually fading from view. At its maximum brightness, the exploded star may outshine an entire galaxy.

After some supernova explosions, there remains a small, dense star composed mainly of neutrons. Such a star is called a neutron star. Rapidly rotating, highly magnetized neutron stars are called pulsars. (After other supernova explosions, a black hole may be left behind.) The explosion throws a large, roughly spherical cloud of dust and hot gas into space surrounding the neutron star (or black hole). When this slams into the existing interstellar medium, it heats up so much it glows in X-rays

The star in Vela brightened by about 100 million times to rival the Moon as the brightest object in the night sky. Near the centre of the expanding nebula is the Vela pulsar (PSR J0835-4510), a faint, rapidly-spinning neutron star only a few kilometres in diameter, the remnant of the star that exploded. This tiny, massive object spins about 11 times a second.

The Vela supernova explosion was roughly spherical. Non-uniformity of the interstellar medium causes Vela’s appearance to be irregular. The outer layers of the star crashed into the interstellar medium, driving a shock wave that is still visible today. The above image captures much of that filamentary and gigantic shock in visible light.

This close-up image was taken with the Hubble Space Telescope. The little arrow shows the place of the Vela pulsar.

November 6, 2012

Comet Hartley 2, a small periodic comet


Image Credit: NASA/JPL-Caltech/UMD

Comet Hartley 2 (officially designated as 103P/Hartley) is a small periodic comet of about 1.2 to 1.6 kilometres (0.75 to 0.99 mi) in diameter, with an orbital period of 6.46 years. Its perihelion is near the Earth’s orbit at 1.05 AU from the Sun. The comet spins around one axis, but also tumbles around a different axis. It was discovered by Malcolm Hartley on March 15, 1986.

The comet passed within 0.12 AU (18 million km; 11 million mi) of Earth on 20 October 2010, only eight days before coming to perihelion (closest approach to the Sun) on 28 October 2010. Hartley 2 is estimated to come back to perihelion around 20 April 2017.

Hartley 2 is 2.25 kilometers (1.40 mi) long, and “peanut shaped”. Its nucleus is highly elongated and rotates over an 18-hour period. The nucleus has a radius of about 0.57 kilometers (0.35 mi) and a low albedo. The mass of the comet is estimated to be about 300 megatonnes (300,000,000,000 kg).

The comet has a smooth, relatively inactive middle region, or waist. On its rougher ends, the comet’s surface contains glittering, blocky objects that are about 50 meters (165 feet) high and 80 meters (260 feet) wide. These objects appear to be two to three times more reflective than the surface average.

Hartley 2 is a hyperactive comet, spewing out more water than other comets its size. When warmed by the Sun, dry ice (frozen carbon dioxide) deep in the comet’s body turns to gas jetting off the comet and dragging water ice with it. The comet should only be able to survive up to another 100 apparitions (around 700 years) at its current rate of mass loss.

Several jets of material are being ejected from the dark side of the comet, rather than the sunlit side. The rays coming off the rough ends consist of hundreds of tons of fluffy ice and dust chunks – the largest particles are of golf ball to basketball-size – and they are ejected by jets of carbon dioxide. The CO2 ice within the comet must be primordial, dating from the beginnings of the Solar System.

The strong activity in water release and carbon dioxide-powered jets didn’t occur equally in the different regions of the comet. The jets were seen at the ends of the comet, with most occurring at the small end. In the waist of the comet, water was released as vapor with very little carbon dioxide or ice.

Some of the dust, icy chunks, and other material coming off the ends of the comet are moving slowly enough to be captured by even the weak gravity of the comet. This material then falls back into the lowest point—the waist.

Despite its close passage by Earth’s orbit, the comet is not yet a known source of meteor showers. However, that could change. Dust trails from the recent returns of 103P/Hartley 2 move in and out of Earth’s orbit, and the 1979-dust trail is expected to hit in 2062 and 2068.

Hartley 2 was the target of a flyby of the Deep Impact spacecraft, as part of the EPOXI mission, on 4 November 2010, with closest approach of 694 kilometers (431 mi) of Hartley 2. This encounter was the fifth time a spacecraft from planet Earth has imaged a comet close-up.

November 5, 2012

NGC 2997, a grand design spiral galaxy in Antlia

NGC 2997

Image Credit & Copyright: Robert Gendler (http://www.robgendlerastropics.com/)

NGC 2997 is a bright, unbarred grand-design spiral galaxy, located about 40 million light-years away in the southern constellation Antlia (the Air pump). It is the brightest member of a group of galaxies of the same name in the Local Supercluster of galaxies along with the Local Group.

NGC 2997 contains hundreds of billions of stars and is thought to have a mass of about 100 billion times that of our Sun, but is probably less massive than our own Milky Way Galaxy. The galaxy is speeding away from us at about 1085 kilometers per second.

With a disk that is inclined 45 degrees to our line of sight, NGC 2997 has an oval appearance. The inner disk is covered with dust lanes, silhouetted against the central part of the galaxy, which shows a high surface brightness.

Like all grand design spirals, NGC 2997 has prominent and well-defined spiral arms, which appear to originate in the yellow nucleus. These sprawling arms are peppered with bright red blobs of ionized hydrogen which are regions of star formation, where the bright blue stars are born that generate most of the light in the arms of the galaxy.

Its small nucleus, that most likely hosts a supermassive black hole, also shows an interesting structure and concentrates an older population of yellowish stars. The nucleus is surrounded by a chain of hot giant clouds of ionized hydrogen.

November 4, 2012

The Spirograph Nebula, a planetary nebula in Lepus

IC 418

Image Credit: NASA and The Hubble Heritage Team (STScI/AURA). Acknowledgement: R. Sahai (JPL) et al.

The Spirograph Nebula (IC 418) is a bright planetary nebula of approximately 0.3 light-year across, located in the Milky Way Galaxy, some 2,000 light-years away in the constellation of Lepus. The nebula is moving away from us at about 62 kilometers per second.

The name derives from the strange, not well understood, texture of the nebula which resembles a pattern which can be created using the Spirograph, a toy which produces geometric patterns on paper.

Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through early telescopes.

A planetary nebula represents the final stage in the evolution of a star similar to our Sun. Only a few thousand years ago, the star at the center of the Spirograph Nebula was a red giant, but then ejected its outer layers into space to form this expanding nebula. The stellar remnant at the center is the hot core of the red giant. Over the next several thousand years, the nebula will gradually disperse into space, and then the star will cool and fade away for billions of years as a white dwarf. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.

Glowing like a multi-faceted jewel, the nebula’s luminosity is around 1500 times that of the Sun. The small, slightly elongated nebula consist of a filigree pattern and an inner ring. The star is off-center within the inner ring. The outer ring is subtly bi-polar, rather reminiscent of the Ring Nebula in Lyra, perhaps showing us what the Ring used to look like.

The patterns of the Spirograph are perhaps related to chaotic winds from the variable central star, which changes brightness unpredictably in just a few hours. The light from the central core excites surrounding atoms in the nebula causing them to glow. Still, the temperature of the central star is only about 35,000 K, what makes the star so “cool” that only about 75 percent of the nebular helium is ionized.

The small size of the nebula is consistent with the low star temperature of early evolution, as the star is still heating and the nebula growing, albeit at a low expansion rate of 12 kilometers per second.

This image was obtained with the Hubble Space Telescope. It is shown in a false-color representation, based on Wide Field Planetary Camera 2 exposures taken in February and September, 1999 through filters that isolate light from various chemical elements. Red shows emission from ionized nitrogen (the coolest gas in the nebula, located furthest from the hot nucleus), green shows emission from hydrogen, and blue traces the emission from ionized oxygen (the hottest gas, closest to the central star).

November 3, 2012

ESO 243-49, a large spiral galaxy in Phoenix


Image Credit: NASA, ESA and S. Farrell (University of Sydney)

ESO 243-49 is a large edge-on spiral galaxy, located about 290 million light-years away from Earth in the southern constellation Phoenix. It is speeding away from us at about 6639 kilometers per second.

Towards the edge of the galaxy, above the galactic plane, astronomers discovered in 2009 one of the first intermediate-mass black holes ever found, probably devouring a companion star. Known as HLX-1 (Hyper-Luminous X-ray source 1), the black hole, surrounded by a swirling disk of hot gas, has an estimated weight of about 20,000 solar masses. The black hole may be encircled by a massive cluster of young, blue stars of about 250 light-years across.

Such young clusters of stars are commonly found inside galaxies like the host galaxy, but not at their edges, as found with HLX-1. One possible scenario is that the HLX-1 black hole was the central black hole in a dwarf galaxy. The larger host galaxy may then have captured the dwarf. In this conjecture, most of the dwarf’s stars would have been stripped away through the collision between the galaxies. At the same time, new young stars would have formed in the encounter. The interaction that compressed the gas around the black hole would then have also triggered star formation.

The possible star cluster may be less than 200 million years old. This means that the bulk of the stars formed following the dwarf’s collision with the larger galaxy. The age of the stars tells how long ago the two galaxies crashed into each other.

The stars within the cluster can’t be resolved individually because the galaxy is too far away. Their presence is inferred from the color and brightness of the light coming from the black hole’s location. The brightness and color is consistent with other clusters of stars seen in other galaxies, but some of the light may be coming from the gaseous disk around the black hole.

HLX-1 brightens and fades over a period of roughly 367 days. Its X-ray brightness increases dramatically in about 10 days, falls slowly over the next 100-200 days, then stays low until the next flare-up.

A possible explanation is that the accretion disk is fed by a star in an elongated orbit. When the star comes closest, the black hole’s gravity pulls gas from its surface. The gas funnels into the accretion disk, heating the disk and causing it to emit a burst of X-rays. Over the next few months, the black hole ingests some of the gas, so the disk fades. HLX-1 may get sucked into the center of ESO 243-49, feeding its monstrous black hole eventually.

The discovery of the possible star cluster and the black hole has important implications for understanding the evolution of black holes and galaxies, because it had not been clear whether middleweight black holes formed inside ancient star clusters or by repeated mergers of small galaxies. This finding not only suggests they grew by mergers, but it also hints they will keep merging. That could explain another mystery – how supermassive black holes got so big.

This image is created by using NASA’s Hubble Space Telescope. The circle identifies a unique X-ray source that pinpoints the black hole.

November 2, 2012

NGC 1579, a reflection and emission nebula in Perseus

The Northern Trifid or Trifid of the North

Image Credit & Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona (http://www.caelumobservatory.com/index.html)

NGC 1579 (also designated Sh2-222) is a dusty star forming region providing contrasting emission and reflection nebulae in the same field. It is some 3 light-years across, and located about 2,100 light-years away in the northern constellation Perseus. It resembles the better known Trifid Nebula (Messier 20), located in the southern sky. That’s why this nebula is often called “the Northern Trifid” or “Trifid of the North”.

This vast, dark cloud of dust and gas is one of the interstellar nurseries breeding new stars to light our Milky Way galaxy. NGC 1579 is lit it by an embedded, extremely young, massive star named LkHα 101, which is several times more massive the Sun and a strong emitter of the characteristic red hydrogen alpha light. This star lies within a small, young star cluster, together with dozens of sibling stars that are also newly formed.

The central region of NGC 1579 is glowing red of hydrogen gas excited by the hot stars whereas the blue regions are reflected starlight. Prominent dark dust lanes extend throughout the nebula. In fact, obscuring dust, pervasive in NGC 1579, is drastically dimming the visible light from the massive, young, hot stars still embedded in the cosmic cloud.

November 1, 2012

Saturn’s moon Rhea


Image Credit: NASA/JPL/Space Science Institute

Rhea is, at 1,528 kilometers (949 miles) across, the second-largest moon of Saturn and the ninth largest moon in the Solar System. At a distance of 527,040 km (327,490 miles), Rhea orbits Saturn in a nearly perfect circle in 4 days 12 hours and 25 minutes, turning once on its axis during that time. The moon is tidally locked with its parent, what means that one side always faces toward Saturn.

It is an icy body with a density of 1.233 times that of liquid water (it is only the tenth-most-massive moon in the Solar System) what indicates that it is made of about 25% rock and 75% water ice. Probably Rhea lacks a rocky core. Thus, it is thought that this moon is composed of a homogenous mixture of ice and rock — a frozen dirty snowball. The temperature on Rhea is 99 K (−174 °C) in direct sunlight and between 73 K (−200 °C) and 53 K (−220 °C) in the shade. Rhea could sustain an internal liquid water ocean through heating by radioactive decay.

Rhea has a rather typical heavily cratered surface, with the exceptions of a few large fractures, caused by extensive tectonism, on the trailing hemisphere (the side facing away from the direction of motion along Rhea’s orbit) and a very faint “line” of material at Rhea’s equator that may have been deposited by material deorbiting from its rings (yes, Rhea might have a tenuous ring system).

The moon has two very large impact basins, which are about 400 and 500 km across. They are both covered in impact craters, indicating they are quite ancient. The more northerly and less degraded of the two is called Tirawa; the other one Mamaldi. There is also a 48 km-diameter impact crater that is prominent because of an extended system of bright rays. This crater, called Inktomi, is nicknamed “The Splat”, and may be one of the youngest craters on the inner moons of Saturn.

Its surface can be divided into two geologically different areas based on crater density; the first area contains craters which are larger than 40 km in diameter, whereas the second area, in parts of the polar and equatorial regions, has only craters under that size. This difference may indicate there was a major resurfacing event some time in Rhea’s history. However, it would have been long ago because there are few young craters with rays extending away from them (as on Earth’s Moon), and the average age of the plains is thought to be around four billion years old.

The leading hemisphere is heavily cratered and uniformly bright. The craters lack the high relief features seen on the Moon and Mercury. On the trailing hemisphere there is a network of bright streaks on a dark background with lengths of tens to hundreds of kilometers, often cutting through plains and craters. These wispy areas are subsidence fractures that make canyons (some of them several hundred meters high). The walls of those canyons are bright because darker material falls off them, exposing fresh bright water ice. These fracture cliffs show Rhea may have been tectonically active in its past, thus, in fact the streaks are tectonically formed ice cliffs.

Rhea has a thin atmosphere, more properly called an exosphere (where the atmosphere merges into space), consisting of oxygen and carbon dioxide in proportion of roughly 5 to 2. The main source of oxygen is water ice at its surface. The source of the carbon dioxide is less clear, but it may be related to oxidation of the organics present in ice or to outgassing of the moon’s interior.

On March 6, 2008, NASA announced that as Cassini passed by Rhea it discovered a tenuous ring system. This would mark the first discovery of rings about a moon. Unfortunately, a careful search later on failed to find any evidence of rings. Astronomers still don’t agree about how to explain the data which suggested their existence.

The first images of Rhea were obtained by Voyager 1 & 2 spacecraft in 1980–1981. After that, there were four close targeted fly-bys by the Cassini orbiter: at a distance of 500 km in November 2005, at 5,750 km in August 2007, at 100 km in March 2010 and at 69 km in January 2011. Rhea has been also imaged many times from long to moderate distances by the orbiter.

This image is Cassini view of Rhea’s anti-Saturnian hemisphere, showing the moon’s two largest impact basins (Mamaldi above and left of center, and adjacent Tirawa to its upper right).

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