7) September 2012

September 30, 2012

G54.1+0.3, a supernova remnant in Sagitta

SNR G54.1+0.3 is the dusty remnant of a core-collapse supernova explosion of about 17 light years across, located some 20,000 light-years away in the northern constellation Sagitta, the Arrow.

The general picture for a core-collapse supernova – goes something like this. When the nuclear power source at the center of a star is exhausted, the core collapses. In less than a second, a neutron star (or a black hole, if the star is extremely massive) is formed. The formation of a neutron star releases an enormous amount of energy in the form of neutrinos and heat, which reverses the implosion. All but the central neutron star is blown away at speeds in excess of 50 million kilometers per hour as a thermonuclear shock wave races through the now expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst that can be as intense as the light of several billion Suns.

In this composite image of G54.1+0.3 X-rays from Chandra were overlaid on infrared data from Spitzer. X-rays from Chandra are seen in blue, and data from Spitzer in green (shorter wavelength infrared) and red-yellow (longer wavelength infrared).

The dust in G54.1+0.3 is flying past and engulfing a nearby family of stars. Scientists think the stars in the image are part of a stellar cluster in which the supernova exploded. The material ejected in the explosion is now blowing past these stars at high velocities.

The white source near the center of the image is PSR J1930+1852, a dense, rapidly rotating (7 times per second) neutron star, or pulsar, left behind after the supernova explosion. This pulsar, with a very-high-energy gamma-ray emission, is one of the youngest and most energetic pulsars in our galaxy. It generates a pulsar wind nebula, a wind of high-energy particles — seen in blue — that expands into the surrounding environment, illuminating the material ejected in the supernova explosion.

The energetic flow of radiation and particles from the pulsar also created a ring of particles and two jet-like structures (lighter blue).

During the supernova event, the pulsar is highly magnetized and creates an enormous electric field as it rotates. The electric field accelerates particles near the pulsar and produces jets blasting away from the poles, and as a disk of matter and anti-matter flowing away from the equator at high speeds. As the equatorial flow rams into the particles and magnetic fields in the nebula, a shock wave forms. The shock wave boosts the particles to extremely high energies causing them to glow in X-rays and produce a bright ring. The particles stream outward from the ring and the jets to supply the extended nebula.

The infrared shell that surrounds the pulsar wind is made up of gas and dust that condensed out of debris from the supernova. As the cold dust expands into the surroundings, it is heated and lit up by the stars in the cluster so that it is observable in the infrared. The dust closest to the stars is the hottest and is seen to glow in yellow in the image. Some of the dust is also being heated by the expanding pulsar wind as it overtakes the material in the shell.

The unique environment into which this supernova exploded makes it possible for astronomers to observe the condensed dust from the supernova that is usually too cold to emit in the infrared. Without the presence of the stellar cluster, it would not be possible to observe this dust until it becomes energized and heated by a shock wave from the supernova. However, the very action of such shock heating would destroy many of the smaller dust particles. In G54.1+0.3, astronomers are observing pristine dust before any such destruction.

Image Credit: X-ray: NASA/CXC/SAO/T.Temim et al.; Infrared: NASA/JPL-Caltech

September 29, 2012

Centaurus A in infrared

This is the third (and last) day of a series of 3 images of the amazing galaxy Centaurus A. Today an image taken in infrared. (Yesterday an Ultra Deep Field image, and the day before that an image in X-ray, Submillimeter and Optical.data.)

Centaurus A (also designated NGC 5128 and Arp 153) is a starburst galaxy that measures 150.000 × 120.000 light-years, and has a mass of about 1 trillion solar masses. Located in the southern constellation of Centaurus it is, at about 11 million light-years away, the closest active galaxy to Earth, and is part of the M83 group of galaxies. It is moving away from us at 547 kilometers per second.

The galaxy has an active galactic nucleus and a very unusual dust lane. Its bulge is composed mainly of evolved red stars. The dusty disk, however, has been the site of more recent star formation; over 100 star formation regions have been identified in the disk.

Centaurus A is of intermediate type between elliptical and disk (spiral) galaxies: The main body has all characteristics of a large elliptical, but the pronounced dust lane is superimposed well over the center, forming a disk plane around this galaxy. The twisted galaxy is likely the result of a smaller spiral galaxy falling into a much larger galaxy. This collision is also responsible for the intense burst of star formation.

The galaxy is a strong source of radiowaves and is one of the closest radio galaxies to Earth, so its active galactic nucleus has been extensively studied by professional astronomers. The galaxy is also the fifth brightest in the sky, making it an ideal amateur astronomy target, although the galaxy is only visible from low northern latitudes and the southern hemisphere.

A jet that seems to arise from the galaxy’s central supermassive black hole — with a million or so times the mass of the Sun, is responsible for emissions in the X-ray and radio wavelengths. By taking radio observations of the jet separated by a decade, astronomers have determined that the inner parts of the jet are moving at about one half of the speed of light. X-rays are produced farther out as the jet collides with surrounding gases resulting in the creation of highly energetic particles. The radio jets of Centaurus A are over a million light years long.

Centaurus A is shaped like a parallelogram in infrared images. The parallelogram lies along the active galaxy’s central band of dust and stars visible in more familiar optical images.

Astronomers believe that the striking geometric shape represents an approximately edge-on view of the infalling spiral galaxy’s disk in the process of being twisted and warped by the interaction. Ultimately, debris from the ill-fated spiral galaxy should provide fuel for the supermassive black hole lurking at the center of Centaurus A.

Only one supernova has been detected in Centaurus A, the Type Ia supernova SN 1986G, discovered within the dark dust lane of the galaxy. A Type Ia supernova is a result from the violent explosion of a white dwarf star.

Peering deep inside Centaurus A the Spitzer Space Telescope’s penetrating infrared cameras recorded this startling vista in February 2004.

Image Credit: J. Keene (SSC/Caltech) et al., NASA/JPL-Caltech

September 28, 2012

Centaurus A (an Ultra Deep Field Image)

For three days in a row, I am showing you images of the amazing galaxy Centaurus A. Yesterday an image in X-ray, Submillimeter and Optical.data. Today an Ultra Deep Field image, and tomorrow an image taken in infrared.

Centaurus A is shaped like a parallelogram in infrared images. The parallelogram lies along the active galaxy’s central band of dust and stars visible in more familiar optical images.

This Ultra Deep Field Image, the deepest image ever taken of Centaurus A, is taken by the famous astrophotographer Michael Sidonio, www.pbase.com/strongmanmike2002

September 27, 2012

Centaurus A, an unusual starburst galaxy

For three days in a row, I’ll show you images of the amazing galaxy Centaurus A. Today an image in X-ray, Submillimeter and Optical.data. (Tomorrow an Ultra Deep Field image, and the day after that an image taken in infrared.)

Centaurus A is shaped like a parallelogram in infrared images. The parallelogram lies along the active galaxy’s central band of dust and stars visible in more familiar optical images.

Image Credits: X-ray: NASA/CXC/CfA/R.Kraft et al.; Submillimeter: MPIfR/ESO/APEX/A.Weiss et al.; Optical: ESO/WFI

September 26, 2012

NGC 17, the remnant of a galaxy merger in Cetus

NGC 17 (also designated NGC 34) is the remnant of a galaxy merger that lies some 242 million light-years away in the constellation Cetus, the Whale. It is moving away from us at 5881 kilometers per second.

This galaxy features a single nucleus, containing a blue central disk with delicate fine structure in the outer parts, and tidal tails indicative of a merger of two former disk galaxies of unequal mass. At present these galaxies appear to have completed their merger.

The remnant shows clear signs that the recent merger was gas-rich and accompanied by a starburst:
it has a rich system of young star clusters; the blue central disk appears to be young, has a smooth structure and its optical light is dominated by a poststarburst population of approximately 400 million years old; and the center of NGC 17 drives a strong outflow of cool, neutral gas.

NGC 17 seems to have first experienced a galaxy-wide starburst that then shrank to its current central and obscured state. The strong gaseous outflow came last. The galaxy is still gas-rich and can sustain its strong central starburst and present mild central activity for some time to come.

This image was taken by 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)

September 25, 2012

Barnard’s Loop, an emission nebula in Orion

Barnard’s Loop (Sh 2-276) is an emission nebula of some 300 light-years across, located about 1600 light-years away in the constellation of Orion. It is part of the Orion Molecular Cloud Complex which also contains the bright Horsehead and Orion nebulae.

The Loop takes the form of a large arc that envelopes a large part of the Orion Complex and surrounds both Orion’s Belt and Orion’s Sword. A group of hot young stars within the Orion Nebula is believed to be responsible for luminating the loop. As seen from Earth, Barnard’s Loop is covering much of Orion.

This image with a combination of white light and hydrogen alpha light is bringing out the color and detail of Barnard’s Loop. The Orion Nebula (M42) and the Horsehead nebula (Barnard 33) can be seen as well.

Image Credit: Hunter Wilson

September 24, 2012

Mayall II, a globular cluster in Andromeda

Mayall II (also designated G1, M31-G1, NGC-224-G1 or Andromeda’s Cluster) is a massive, slightly elliptical globular star cluster of approximately 42.5 light-years across orbiting the Andromeda Galaxy (M31). It is located about 130,000 light-years from Andromeda’s galactic core and 2.36 million light-years from Earth in the constellation Andromeda, but it is moving toward us at 332 kilometers per second.

It is the brightest globular cluster in the Local Group (a group of about 20 nearby galaxies, including the Milky Way). Mayal II consists of at least 300,000 very old stars — one million according to other sources — with an estimated mass of some 10 million solar masses. The cluster may contain a central, intermediate-mass black hole of about 20,000 solar masses. The age of Mayal II is probably 12 billion years!

A glimpse into the cluster’s finer details allow astronomers to see its fainter helium-burning stars whose temperatures and brightnesses show that this cluster in Andromeda and the oldest Milky Way clusters have approximately the same age. These clusters probably were formed shortly after the beginning of the Universe, providing astronomers with a record of the earliest era of galaxy formation.

Because of the widespread distribution of heavier elements, indicating multiple star generations and a large stellar creation period, many contend that Mayal II is not a true globular cluster, but that it is actually the galactic core that remained of a dwarf galaxy after it was consumed by Andromeda.

Seen from Earth, Mayal II lies almost in the middle of two bright foreground stars.

This colour picture is assembled from separate images taken with the Hubble Space Telescope in visible and near-infrared wavelengths. Image Credit: Michael Rich, Kenneth Mighell, and James D. Neill (Columbia University), Wendy Freedman (Carnegie Observatories), and NASA

September 23, 2012

The Frosty Leo Nebula, a protoplanetary nebula in Leo

The Frosty Leo Nebula (IRAS 09371+1212) is an expanding bipolar protoplanetary nebula, located about 3,000 light-years away in the constellation of Leo. The nebula has acquired its curious name as it has been found to be rich in water in the form of ice grains, and because it is located in the constellation of Leo.

Despite their name, protoplanetary nebulae have nothing to do with planets: they are clouds of dust and gas formed from material shed by an aging central star similar in mass to our Sun. This nebula is particularly noteworthy because it has formed far from the galactic plane, away from interstellar clouds that may block our view.

The intricate shape comprises a spherical halo, a dense disk around the central star, jet-like outflows from this star in the inner regions, lobes and gigantic loops. This complex structure strongly suggests that the formation processes are complex and it has been suggested that there could be a second star, currently unseen, contributing to the shaping of the nebula.

Protoplanetary nebulae like the Frosty Leo Nebula have brief lifespans by astronomical standards and are precursors to the planetary nebula phase, in which radiation from the star will make the nebula’s gas light up brightly. Their rarity makes studying them a priority for astronomers who seek to understand better the evolution of stars.

This image was created from images taken with the High Resolution Channel of Hubble’s Advanced Camera for Surveys.
Image Credit: ESA/Hubble & NASA

September 22, 2012

NGC 3079, a barred spiral galaxy in Ursa Major

NGC 3079 is a barred spiral galaxy of some 70,000 light-years across, located 56.4 million light-years away in the constellation Ursa Major. It is moving away from us at 1116 kilometers per second, possibly together with a companion, the elleptical galaxy NGC 3073.

The disk of NGC 3079 is composed of spectacular star clusters in winding spiral arms and dark dust lanes. The most prominent feature of this galaxy is, however, a lumpy “bubble” of hot gas rising from a cauldron of glowing matter in the very center. The bubbele is some 3000 light-years wide and rises over 3500 light-years above the disk of the galaxy, and is created by fierce “winds” (high-speed streams of particles) released during a burst of star formation. The ongoing winds from hot stars mixed with small bubbles of very hot gas blown by superwinds from supernova explosions.

Superwinds are thought to play a key role in the evolution of galaxies by regulating the formation of new stars, and by dispersing heavy elements to the outer parts of the galaxy and beyond. Astronomers may be seriously underestimating the mass lost in superwinds and therefore their influence within and around the host galaxy.

Gaseous filaments at the top of the bubble are whirling around in a vortex and are being expelled into space at 6 million kilometers per hour. Interestingly, this gas will not reach escape velocity and will rain back down onto the galaxy’s disk where it may collide with gas clouds, compress them, and form a new generation of stars. The two white dots just above the bubble are probably stars in the galaxy.

Observations of the core’s structure indicate that those processes are still active. The models suggest that this outflow began about a million years ago. They occur about every 10 million years. Eventually, all the hot stars will die, and the bubble’s energy source will fade away.

The image was taken by the Hubble Space Telescope.
Image Credit: NASA, Gerald Cecil (University of North Carolina), Sylvain Veilleux (University of Maryland), Joss Bland-Hawthorn (Anglo- Australian Observatory), and Alex Filippenko (University of California at Berkeley)

September 21, 2012

G292.0+1.8, a supernova remnant in Centaurus

G292.0+1.8 is the remnant of an oxygen-rich supernova explosion of some 36 light-years across with an estimated age of about 3,000 years, triggered by the collapse of the core of a massive star. It is located within our Milky Way, about 20,000 light-years away galaxy in the constellation Centaurus.

The pulsar at the center (PSR J1124–5916) is a rapidly rotating neutron star that remained behind after the original, massive star exploded. It is surrounded by outflowing material; a rapidly expanding shell of gas that contains large amounts of elements such as oxygen, neon, magnesium, silicon and sulfur.

Astronomers know that pulsars are formed in supernova explosions, but they are currently unable to identify what types of massive stars must die in order for a pulsar to be born. Astronomers can use the pattern of elements seen in the remnant to make a much closer connection between pulsars and the massive stars from which they form. Embedded in this cloud of multimillion degree gas is a key piece of evidence linking neutron stars and supernovas produced by the collapse of massive stars.

Astronomers believe that an oxygen-rich supernova explosion is triggered by the collapse of the core of a massive star to form a neutron star, releasing tremendous amounts of energy in the process.

G292.0+1.8 is one of three known oxygen-rich supernovas in our Galaxy. These supernovae are of great interest to astronomers because they are one of the primary sources of the heavy elements necessary to form planets and people.

Although considered a “textbook” case of a supernova remnant, the intricate structure shown in this image reveals a few surprises.

Near the center of G292.0+1.8 is the so-called pulsar wind nebula, most easily seen in high energy X-rays. This is the magnetized bubble of high-energy particles that surrounds the pulsar. The narrow, jet-like feature running from north to south in the image is likely parallel to the spin axis of the pulsar.

The pulsar is located slightly below and to the left of the center of G292.0+1.8. Assuming that the pulsar was born at the center of the remnant, it is thought that recoil from the lopsided explosion may have kicked the pulsar in this direction. However, the kick direction and the pulsar spin direction do not appear to be aligned, in contrast to apparent spin-kick alignments seen in some other supernova remnants.

Another key feature of this remnant is the long white line running from left to right across the center called the equatorial belt. This structure is thought to be created when the star – before it died – expelled material from around its equator via winds. The orientation of the equatorial belt suggests the parent star maintained the same spin axis both before and after it exploded.

One puzzling aspect of the image is the lack of evidence for thin filaments of high energy X-ray emission, thought to be an important site for cosmic ray acceleration in supernova remnants. These filaments are seen in other supernova remnants such as Cassiopeia A, Tycho and Kepler. One explanation may be that efficient acceleration occurs primarily in very early stages of supernova remnant evolution, and G292.0+1.8 is too old to show these effects. Casseiopeia A, Tycho and Kepler, with ages of several hundred years, are much younger.

This composite image is obtained in color by the Chandra X-ray Observatory; and in white by optical data from the Digitized Sky Survey.
Image Credit: X-ray: NASA/CXC/Penn State/S.Park et al.; Optical: Pal.Obs. DSS

September 20, 2012

NGC 772, an unbarred spiral galaxy in Aries

NGC 772 (also called Arp 78) is an unbarred spiral galaxy over 100 thousand light-years across (about the same size as our Milky Way Galaxy), located some 130 million light-years away in the constellation Aries. It is moving away from us at 2472 kilometers per second.

It has with a small, very bright, diffuse nucleus, probably a H II nucleus powered by young, massive stars.

Most notable is, however, a prominent elongated outer spiral arm, which has likely arisen due to tidal interactions with the nearby dwarf elliptical galaxy NGC 770 (seen toward the upper right). This unusual long arm shows lots of young blue star clusters. But NGC 772 also possesses many weak, tightly coiled arms which, although well formed, are relatively smooth, indicating only a small current rate of star formation. The relatively smooth multiple arms on the opposite side from the prominent arm are defined primarily by spiral dust lanes.

NGC 772 is surrounded by several satellite galaxies. Faint streams of material seem to connect NGC 772 with these nearby companions. The interacting galaxies NGC 772 and NGC 770 are together cataloged as nr. 78 in Arp’s Atlas of Peculiar Galaxies, a catalog of 338 peculiar galaxies produced by Halton Arp in 1966.

Two supernovae (SN 2003hl and SN 2003iq) have been observed in NGC 772, both of them a Type II supernova.

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 includes numerous faint galaxies in addition to NGC 772 and NGC 770.

Image Credit: Stephen Leshin

September 19, 2012

The Sun, “our star”

The Sun, located in the Orion Arm of the Milky Way galaxy, is the star at the center of the Solar System. It is almost perfectly spherical and has a diameter of about 1,392,684 km, about 109 times that of Earth. Its mass (about 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun’s mass consists of hydrogen, while the rest is mostly helium. The remainder (1.69%, which nonetheless equals 5,628 times the mass of Earth) consists of heavier elements, including oxygen, carbon, neon and iron, among others.

The Sun orbits the center of the Milky Way at a distance of approximately 24,000–26,000 light-years from the galactic center, completing one clockwise orbit, in about 225–250 million years. The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though the distance varies as the Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth’s climate and weather.

The Sun is a so called Population I, or heavy element-rich, star. The Sun was formed about 4.57 billion years ago from the collapse of part of a giant molecular cloud which probably also gave birth to many other stars. One or more supernovae must have occurred near the location where the Sun formed. The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium. These elements have most likely been produced by the nuclear reactions during a supernova.

The Sun is also a yellow dwarf, because its visible radiation is most intense in the yellow-green portion of the spectrum and although its color is white, from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light. Its surface temperature is approximately 5778 K (5505 °C), by contrast, the temperature of its core is 15.7 million K.

The Sun, like most stars, is a main-sequence star, during which nuclear fusion reactions in its core fuse hydrogen into helium. The Sun is about halfway through this stage. Each second, more than four million tonnes of matter are converted into energy within the Sun’s core, producing neutrinos and solar radiation. At this rate, the Sun has so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10 billion years as a main-sequence star.

The Sun does not have enough mass to explode as a supernova. Instead, in about 5 billion years, it will enter a red giant phase. Its outer layers will expand as the hydrogen fuel at the core is consumed and the core will contract and heat up. Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The only object that will remain after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a white dwarf over many billions of years. This stellar evolution scenario is typical of low- to medium-mass stars.

The Sun does not have a definite boundary as rocky planets do, and in its outer parts the density of its gases drops exponentially with increasing distance from its center. All matter in the Sun is in the form of gas and plasma because of its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does near its poles (about 35 days). Nevertheless, it has a well-defined interior structure. The core of the Sun has a density up to about 150 times the density of water and is considered to extend from the center to about 20–25% of the solar radius.

Once regarded by astronomers as a small and relatively insignificant star, the Sun is now thought to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs. The Sun’s hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.

The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a sharp shock front boundary with the interstellar medium. The layers between the photosphere and the heliospere (chromosphere, transition region, and corona) are much hotter than the surface of the Sun. The reason has not been conclusively proven. The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona continuously expands into space forming the solar wind, which fills all the Solar System. The coolest layer of the Sun is a temperature minimum region about 500 km above the surface of the Sun, called the photospere, with a temperature of about 4,100 K.

The Sun is a magnetically active star. It supports a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years around solar maximum. The Sun’s magnetic field, that extends well beyond the Sun itself, leads to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, prominences, and variations in solar wind that carry material through the Solar System. Effects of solar activity on Earth include auroras, and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the Solar System. Solar activity changes the structure of Earth’s outer atmosphere.

The most immediately visible features of the Sun are usually its sunspots, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures. Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field causes strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. The largest sunspots can be tens of thousands of kilometers across.

The number of sunspots is not constant, but varies over an 11-year cycle known as the solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun. The solar cycle has a great influence on space weather, and a significant influence on the Earth’s climate since the Sun’s luminosity has a direct relationship with magnetic activity. Solar activity minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. A recent theory claims that there are magnetic instabilities in the core of the Sun that cause fluctuations with periods of either 41,000 or 100,000 years. These could provide an explanation of the ice ages.

Image Credit: NASA/ESA

September 18, 2012

RCW 49, a diffuse nebula in Centaurus

RCW 49 (also designated GUM 29) is a diffuse nebula of about 350 light-years across, located 13,700 light years away in the southern constellation Centaurus. It is a dark and dusty stellar nursery that contains more than 2,200 stars.

Because many of the stars in RCW 49 are deeply embedded in plumes of dust, they cannot be seen at visible wavelengths. When viewed with Spitzer’s infrared eyes, however, RCW 49 becomes transparent.

This image highlights the nebula’s older stars (blue stars in center), its gas filaments (green) and dusty tendrils (pink). But it also uncovers more than 300 newborn stars, speckled throughout the cosmic dust clouds what shows that star formation is taking place throughout the nebula. Astronomers are interested in further studying these newfound proto-stars because they offer a fresh look at star formation in our galaxy.

RCW 49 is one of the most luminous and massive H II regions (a low-density cloud of partially ionized gas in which star formation has recently taken place) in our Milky Way galaxy. At its center lies the Westerlund 2 compact star cluster, which contains some of the hottest, brightest, and most massive stars known. The age of the W2 cluster is estimated to be 23 million years. The estimated stellar mass of the cluster is about 30,000 solar masses.

The infrared data indicate the likely presence of protoplanetary disks around some of the infant stars, among the faintest and farthest potential planet-forming disks ever observed. Such exciting results give further support to the idea that planet-forming disks are a natural part of a star’s evolution.

This image is taken with the infrared array camera on NASA’s Spitzer Space Telescope.
Image Credit: E. Churchwell (University of Wisconsin), NASA/JPL-Caltech

September 17, 2012

NGC 1808, a barred spiral galaxy in Columba

NGC 1808 is a barred spiral galaxy of some 35,000 light-years across, located about 40 million light-years away in the southern constellation Columba. NGC 1808 is undergoing so much star formation it has been deemed a starburst galaxy. The galaxy is moving away from us at 995 kilometers per second.

NGC 1808 is distinguished by a peculiar and complex nucleus, an unusually warped disk, and strange flows of hydrogen gas out from the central regions. The galaxy’s center is the hotbed of vigorous star formation.

The starburst must be at least 50 million years old, and can be no older than 100 million years old. Star formation has been rapid and continuous. Without an influx of fresh molecular gas into the central region, the star forming activity can only be maintained at this rate for another 6 to 20 million years.

NGC 1808 is called a barred spiral galaxy because of the straight lines of star formation on both sides of the bright nucleus. This star formation may have been triggered by the rotation of the bar, or by matter which is streaming along the bar towards the nuclear region (feeding the starburst). Filaments of dust are being ejected from the core into the galactic halo by massive stars that have exploded as supernovae in the starburst region.

The outer spiral arms of the galaxy are warped with respect to the inner arms (which display a prominent dark dust lane). This is evidence that NGC 1808 may have had a tidal interaction with the nearby galaxy NGC 1792. Such an interaction could have created the bar morphology, and hurled gas towards the nucleus of NGC 1808, igniting the exceptional burst of star formation seen there.

Stars are often born in compact clusters within starbursts, while dense gas and dust heavily often obscures these starburst region. In NGC 1808 are star formation regions in the bar and also many young star clusters in the nucleus of NGC 1808. The nucleus of the galaxy show two maxima. Either the galaxy has two nuclei (from a previous merger), or one of the dusty filaments happens to bisect the nucleus along our line of sight.

NGC 1808 is also a prominent radio source. The radio emission is produced by supernova remnants, of which supernova 1993af has been observed.

Image Credit: Steve Mazlin, Jack Harvey, Rick Gilbert, and Daniel Verschatse (SSRO/PROMPT/CTIO)

September 16, 2012

SN E0102, the remains of an exploded star in the SMC

Supernova remnant 1E 0102.2-7219 (E0102 for short) is the spectacular remains of an exploded star, located about 190,000 light-years away within the Small Megallanic Cloud, in the constellation Tucana. The expanding multimillion degree remnant is about 30 light-years across and contains more than a billion times the oxygen contained in the Earth’s ocean and atmosphere.

This debris of an exploded star was created when a star that was much more massive than the Sun exploded, an event that would have been visible from the Southern Hemisphere of the Earth over 1000 years ago.

We see the remnant as it was about 190,000 years ago, around a thousand years after the explosion occurred. The star exploded outward at speeds in excess of 20 million kilometers per hour (12 million mph) and collided with surrounding gas. This collision produced two shock waves, one traveling outward, and the other rebounding back into the material ejected by the explosion.

The gas has been heated to millions of degrees Celsius by the rebounding, or reverse shock wave. The X-ray data from NASA’s Chandra X-ray Observatory show that this gas is rich in oxygen and neon. These elements were created by nuclear reactions inside the star and hurled into space by the supernova.

E0102 has a hotter oxygen-rich outer ring surrounding a cooler, denser inner ring. The two prominent filaments of oxygen gas and hot dust have “cooled” to about 30,000 degree Celsius. Most of this dust is located in the central region. Even if all of the hot dust was formed in the explosion, the estimated mass of dust is at least 100 times lower than what is predicted by theoretical models.

Images such as these, taken with different types of telescopes, give astronomers a much more complete picture of supernova explosions, like measuring the energy of the matter as it expands into the galaxy. These images also provide unprecedented details about the creation and dispersal of heavy elements necessary to form planets like Earth.

In this image of E0102, the lowest-energy X-rays are colored orange, the intermediate range of X-rays is cyan, and the highest-energy X-rays are blue. An optical image from the Hubble Space Telescope (in red, green and blue) shows additional structure in the remnant and also reveals foreground stars in the field.

Image Credit: X-ray: (NASA/CXC/MIT/D.Dewey et al. & NASA/CXC/SAO/J.DePasquale); Optical: (NASA/STScI)

September 15, 2012

Hickson Compact Group 40, an ensemble of galaxies in Hydra

Hickson Compact Group 40 (also known as Arp 321 or VV 116) is an ensemble of seven galaxies — five of which are clearly seen — located about 300 million light-years away in the constellation of Hydra.

From top to bottom, the 5 prominent galaxies in Hickson 40 are a spiral, an elliptical, two more spirals, and a lenticular. They clearly appear to be touching each other. The elliptical galaxy and two of the spiral galaxies show some levels of nuclear activity.

Single, isolated galaxies are rather rare in the Universe. They tend to form groups or clusters. A system with two galaxies is called a binary galaxy, a system containing more than two but less than several dozen galaxies is called a group (like the Local Group of Galaxies, which houses over 30 galaxies including our Milky Way, Andromeda, and the Magellanic Clouds). A big system containing more than this is called a cluster.

There are groups of galaxies in which the members are in so small a space that they appear to be touching each other. These are called compact groups of galaxies. Hickson Compact Group 40 is one of 100 compact galaxy groups catalogued by the Canadian astronomer Paul Hickson.

Many galaxies which are located so close to each other are gravitational interacting and either slowly merging to form 1 or 2 giant galaxies, or pulling each other apart. Evidence of tidal interaction as a result of mutual gravitational attraction is actually seen in all 3 spiral galaxies in this group. The lenticular galaxy also shows evidence of interaction at its nucleus. We may be observing such a merger here.

Compact groups offer a window into what commonly happened in the Universe’s formative years when large galaxies were created from smaller building blocks. This isolated group of galaxies provides an interesting laboratory for studying the effects of close proximity and interaction on the evolution of galaxies. For instance, how these factors influence the presence of active galactic nuclei, and the relationship between galaxy interaction, activity, and morphology.

Two blueish white dots in the image are stars in our own Galaxy. Small reddish objects are galaxies located billions of light years away. They appear redder than the members of Hickson Compact Group 40 because of the Doppler effect caused by the expansion of the Universe.

Image Credit: CISCO, Subaru 8.3-m Telescope, NAOJ

September 14, 2012

IRAS 10082-5647, a baby star in Vela

IRAS 10082-5647 is a baby star surrounded by a reflection nebula, located in the constellation Vela. Its reflected light is giving the interstellar cloud of gas and dust a pearly glow.

At just a few million years old, the star is a youngster that has yet to begin fusing hydrogen in its core. The star is in its so called pre-main sequence phase, where it will spend around 1% of its life. IRAS 10082-5647 is still embedded in the envelope of gas and dust of which it was born.

For now the star is heating itself via gravitational collapse; as stellar material falls in on itself, the core becomes denser and builds up immense pressure. Eventually the star’s core will get dense enough for fusion to begin, and IRAS 10082-5647 will enter what’s called the main sequence phase of its life, where it will spend around 80% of its life creating energy by burning hydrogen in its core.

This images was taken by the Advanced Camera for Surveys aboard the Hubble Space Telescope.
Image Credit: ESA/Hubble & NASA

September 13, 2012

NGC 1073, a barred spiral galaxy in Cetus

NGC 1073 is a barred spiral galaxy of about 80,000 light-years across that lies some 55 million light-years away in the constellation of Cetus (The Sea Monster). It is moving away from us at 1208 kilometers per second. Our own Milky Way galaxy is a similar barred spiral, and the study of galaxies such as NGC 1073 helps astronomers learn more about our celestial home.

NGC 1073 has a prominent long bright bar of stars across the center, and a bright active nucleus that likely houses a supermassive black hole. Also visible in this image are dark filamentary dust lanes, young clusters of bright blue stars, and red emission nebulas of glowing hydrogen gas.

There are spiral galaxies with and without a central bar. Central bars, made of dense lines of stars at the galaxies’ centers, are thought to form as gravity causes density waves that push gas inward, supplying material for new stars. This inflow of gas can also feed the hungry giant black holes in the centers of most such galaxies.

The bars might form as galaxies age, in part because very distant galaxies dating from the Universe’s early days tend not to have them. In fact, about one-fifth of the spiral galaxies from early Universe contain bars, while more than two-thirds of spirals seen today have them. Adding to this idea is the fact that bars are more often found in galaxies full of older, redder stars, and less often in galaxies with bluer, younger stars.

This image also reveals an odd, rough ring-like structure around the galaxy that is the result of recent star formation. A bright X-ray source known as IXO 5 is located inside the ring and is most likely a binary system containing a star and black hole locked in orbit around each other.

NGC 1073 is also well-known due to its line-of-sight towards three brilliant quasars (two of which are visible at the bottom right), which are incredibly bright sources of light coming from billions of light-years away. This brightness is caused by matter heating up and falling into supermassive black holes at the heart of galaxies. The chance alignment through NGC 1073 make them look like they are part of the galaxy, while they are in fact some of the most distant objects observable in the Universe.

At the top of the image are several objects with a reddish hue, each of which is a distant galaxy lurking far beyond NGC 1073.

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

September 12, 2012

The Running Chicken Nebula, an emission nebula in Centaurus

IC 2944/2948 (also known as the Running Chicken Nebula and the Lambda Centauri Nebula) is an emission nebula and home to the new open star cluster Collinder 249, born from the cloud 7.9 million years ago. It is 70 – 77 light years across and located around 5,800 – 6,500 light-years away in the constellation of Centaurus, and appears to surround the bright star Lambda Centauri. But, in fact, Lambda Centauri is much closer to Earth than the Running Chicken Nebula and has nothing to do with the nebula at all. The strange nickname comes from the bird-like shape of its brightest region.

Within this nebula are hot newborn stars that formed from clouds of hydrogen gas which shine brightly with ultraviolet light. The hottest members of the cluster produce enough ultraviolet radiation and strong winds to both ionize and excavate the cloud and making the nebula glow red, typical of star-forming regions.

Aside from the glowing gas, another sign of star formation in the Running Chicken is the series of opaque black clumps silhouetted against the red background in part of this image. These are so called Bok globules, which are usually active star formation regions. They appear dark as they absorb the light from the luminous background. Observations of these dark Bok globules using infrared telescopes, which are able to see through the dust that normally blocks visible light, have indeed revealed that stars are forming within many of them.

The most prominent collection of Bok globules in this image is known as Thackeray’s Globules, after the South African astronomer who discovered them in the 1950s. These are visible among a group of bright stars in the upper right part of the image.

If the stars cocooned in Thackeray’s Globules are still gestating, then the stars of cluster Collinder 249 are their older siblings. Still young in stellar terms, at just a few million years old, these stars shine brightly, and their ultraviolet radiation provides much of the energy that lights up the nebula. These glowing nebulae are relatively short-lived in astronomical terms (typically a few million years), meaning that the Running Chicken Nebula will eventually fade away as it loses both its gas and its supply of ultraviolet radiation.

This image was taken with the Wide Field Imager on the MPG/ESO 2.2-metre telescope.
Image Credit: ESO

September 11, 2012

Neptune, the eighth and farthest planet from the Sun

Neptune, named for the Roman god of the sea, is the eighth and farthest planet from the Sun in our Solar System. The average distance between Neptune and the Sun is 4.50 billion km (about 30.1 times the Earth–Sun distance), and it completes an orbit on average every 164.79 years. From the Earth, Neptune goes through apparent retrograde motion every 367 days. Neptune is the fourth-largest planet by diameter and the third largest by mass. Neptune radiates about 2.61 times as much energy as it receives from the Sun (Uranus only radiates 1.1 times as much).

Neptune is an intermediate body between Earth and the larger gas giants: it is 17 times the mass of Earth but just 1/19th that of Jupiter. The planet’s surface gravity is only surpassed by Jupiter. Neptune’s equatorial radius of 24764 km is nearly four times that of the Earth. Neptune and Uranus are often considered a sub-class of gas giant termed “ice giants”, due to their higher concentrations of “ices” such as water, ammonia and methane relative to Jupiter and Saturn.

The interior of Neptune is primarily composed of ices and rock. The core of Neptune is composed of iron, nickel and silicates, with a mass about 1.2 times that of the Earth. The pressure at the centre is 7 Mbar, millions of times more than that on the surface of the Earth, and the temperature may be 5,400 K.

The mantle reaches temperatures of 2,000 K to 5,000 K and is rich in water, ammonia and methane. As is customary in planetary science, this mixture is referred to as icy even though it is a hot, highly dense fluid. This fluid is sometimes called a water-ammonia ocean.

Its atmosphere is composed primarily of hydrogen and helium, along with “ices” and traces of hydrocarbons and possibly nitrogen. It forms about 5% to 10% of its mass and extends perhaps 10% to 20% of the way towards the core. Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere.

At high altitudes, Neptune’s atmosphere is 80% hydrogen and 19% helium. A trace amount of methane is also present. Absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue.

For reasons that remain obscure, the planet’s thermosphere is at an anomalously high temperature of about 750 K. The planet is too far from the Sun for this heat to be generated by ultraviolet radiation. The thermosphere contains traces of carbon dioxide and water, which may have been deposited from external sources such as meteorites and dust.

Neptune’s magnetic field is strongly tilted relative to its rotational axis at about 13,500 km from the planet’s physical centre. Neptune’s bow shock, where the magnetosphere begins to slow the solar wind, occurs at a distance of 34.9 times the radius of the planet. The tail of the magnetosphere extends out to at least 72 times the radius of Neptune, and very likely much farther.

Neptune’s atmosphere is notable for its active and visible weather patterns. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 kilometres per hour (1,300 mph). For example, in 1989, the planet’s southern hemisphere possessed a Great Dark Spot, an anti-cyclonic storm system spanning 13000×6600 km, was discovered, comparable to the Great Red Spot on Jupiter. Some five years later Great Dark Spot was gone. Instead, a new storm similar to the Great Dark Spot was found in the planet’s northern hemisphere.

More storms have also been discovered, like the Scooter ( a white cloud group farther south than the Great Dark Spot which is moving faster than the Great Dark Spot) and the Small Dark Spot which is a southern cyclonic storm. Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. The long orbital period of Neptune results in seasons lasting forty years. The cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo, due to seasonal changes. The long orbital period of Neptune results in seasons lasting forty years.

Because of its great distance from the Sun, Neptune’s outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching −218 °C (55 K). Temperatures at the planet’s centre are approximately 5,400 K (5,000 °C).

In 2007 it was discovered that the upper troposphere of Neptune’s south pole was about 10 °C warmer than the rest of Neptune. The relative “hot spot” is due to Neptune’s axial tilt, which has exposed the south pole to the Sun for the last quarter of Neptune’s year, or roughly 40 Earth years.

Neptune has 13 known moons. The largest by far, comprising more than 99.5% of the mass in orbit around Neptune and the only one massive enough to be spheroidal, is Triton. It has a planetary ring system as well, though one much less substantial than that of Saturn. The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue. The rings have a clumpy structure, which may be due to the gravitational interaction with small moons in orbit near them. Neptune’s rings are much more unstable than previously thought. Images show considerable decay in the rings.

Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.

Image Credit: I just don’t know; can somebody help me with this?

September 10, 2012

The Pleiades, the famous open star cluster in Taurus

The Pleiades, or Seven Sisters (Messier 45 or M45) is a young and very bright open star cluster located between 391 and 456 light-years away in the northern constellation of Taurus, what makes it one of the nearest star clusters to Earth. The cluster core has a diameter of about 16 light-years while the total diameter is about 86 light-years. The total mass of the Pleiades is about 800 solar masses.

The cluster, which contains over 1,000 members, is dominated by hot blue and extremely luminous stars, several of which can be seen with the naked eye depending on local observing conditions. The nine brightest stars of the Pleiades are named for the seven daughters of “father” Atlas and “mother” Pleione: Alcyone, Asterope (a double star, also called Sterope), Electra, Maia, Merope, Taygeta and Celaeno.

The cluster also contains many brown dwarfs, which are objects with less than about 8% of the Sun’s mass, not heavy enough for nuclear fusion reactions to start in their cores and become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass. Astronomers have made great efforts to find and analyse brown dwarfs in the Pleiades and other young clusters, because they are still relatively bright and observable, while brown dwarfs in older clusters have faded and are much more difficult to study.

Under ideal observing conditions, some hint of nebulosity may also be seen around the cluster. This is a reflection nebula, caused by dust reflecting the blue light of the hot, young stars. It was formerly thought that the dust was left over from the formation of the cluster, but at the age of about 100 million years, almost all the dust originally present would have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.

Like most open clusters, the Pleiades will not stay gravitationally bound forever, as some component stars will be ejected after close encounters and others will be stripped by tidal gravitational fields. Calculations suggest that the cluster will take about 250 million years to disperse, with gravitational interactions with giant molecular clouds and the spiral arms of our galaxy also hastening its demise.

Image Credit: NASA

September 9, 2012

Abell 39, a spherical planetary nebula in Hercules

Abell 39 is an almost perfectly spherical planetary nebula with a low surface brightness. It is located some 6.800 light-years away in the constellation of Hercules and about 4,600 light-years above the Galactic plane — the plane in which the majority of the Milky Way galaxy’s mass lies. With a diameter of about 5 light-years it is one of the largest known spheres.

Planetary nebulae have nothing to do with planets except that to early astronomers these round objects looked like the planets Uranus and Neptune. Planetary nebula are the last stage of life for stars like our Sun. After billions of years, stars reach a point where there is little hydrogen gas to burn. To help convert their stellar furnaces to burn other elements such as helium, the star balloons in size to become a red giant. Eventually, however, the star collapses back on itself. This increases the temperature at its core and most of the stars material is catapulted into space, forming a bubble around the star. Then the star cools down to become a white dwarf.

Abell 39 is also formed as a once Sun-like star expelled its outer atmosphere over a period of thousands of years. Still visible, the nebula’s central star is evolving into a hot white dwarf.

The mass of the central star is estimated to be about 0.61 sun masses with the material in the planetary nebula comprising an additional 0.6 sun masses. The reason why the central star is slightly off center, by 0.1 light-year, is currently unknown.

Abell 39 has a nearly uniform shell. However, one side of the nebula is 50% more luminous than the other. Additionally, irregularities in the surface brightness are seen across the face of the shell. The source of the asymmetry is not known but it could be related to the offset of the central star.

The thickness of the spherical shell is about a third of a light-year.There is a faint halo that extends beyond the bright rim. Abell 39 has been expanding for about 22,100 years, based on its radius and an assumed expansion velocity between 32 and 37 kilometers per second.

Its shape has allowed astronomers to accurately estimate how much relative material in the nebula is absorbing and emitting light. Observations indicate that Abell 39 contains only about half the abundance of oxygen found in the Sun.

Background galaxies are visible near the nebula, and some can be seen through the translucent nebula.

Image Credit: WIYN/NOAO/NSF

September 8, 2012

The Needle Galaxy, an edge-on spiral galaxy in Coma Berenices

The Needle Galaxy (NGC 4565) is a large barred spiral galaxy that spans over 100,000 light-years, which is located only about 40 million light-years away in the northern constellation Coma Berenices (Berenice’s Hair). This bright galaxy is receding from us at some 1230 kilometers per second.

NGC 4565 is known as the Needle Galaxy for its narrow profile, and is a famous and excellent example of an edge-on spiral galaxy. In fact, some consider the Needle galaxy to be a prominent celestial masterpiece Messier missed. If our own galaxy was viewed from this perspective, the Milky Way would appear very much like the Needle Galaxy.

The Needle Galaxy has a flat, thin disk structure that shows warping at it’s edges due to an ancient interaction with a passing galaxy. Obscuring lanes of gas and dust lace the galaxy’s disk and are reddening the light emanating from the star-filled interior. Its bulging central core is dominated by older yellowish stars.

Studying galaxies like the Needle Galaxy helps astronomers learn more about our own Milky Way. It is relatively close by, and being seen edge-on makes it a particularly useful object for comparative study. The Needle galaxy reveals several features that scientists also find in the Milky Way. Both are spiral galaxies with dark lanes of interstellar dust that are blocking some of the light pouring out of their galactic cores.

Many background galaxies are also visible in this image, giving full meaning to their nickname of “island universes”.

Image Credit: Howard Trottier

September 7, 2012

DR22, a star-forming cloud in Cygnus

DR22 is a star-forming cloud with gas and dust with a young star cluster in its heart, located about 4,500 light-years away from Earth, at the edge of the Cygnus-X complex (one of the richest known regions of star formation in our Galaxy) in the heart of the constellation Cygnus, the Swan. DR22 shows evidence of recent massive star formation and is bursting with new stars.

After blowing away their natal material the young stars in the star cluster (which are less than a million years old) emit winds and harsh ultraviolet light that sculpt the remnant cloud into fantastic shapes and blow out a cavity in the middle. Astronomers are not sure when that activity suppresses future star formation by disruption, and when it facilitates star formation through compression.

This infrared image is one of the first to be taken during Spitzer’s warm mission — a new phase that began after the telescope, which operated for more than five-and-a-half years, ran out of liquid coolant. The picture was snapped at Spitzer’s still-quite-chilly temperature of 30 Kelvin (about minus 406 Fahrenheit).

Spitzer’s infrared eyes can both see through and see dust, giving it a unique view into star-forming nests. The blue areas in this image are dusty clouds, and the orange is mainly hot gas.

Image Credit: NASA/JPL-Caltech

September 6, 2012

Arp 81, an interacting pair of galaxies in Draco

Arp 81 (also designated UGC 11175 and VV 247) is a strongly interacting pair of spiral galaxies, consisting of NGC 6621 (right) and NGC 6622 (left) seen about 100 million years after their closest approach. The galaxy pair is located about 280 million light-years away in the constellation Draco and is receding from us at 6329 kilometers per second.

Arp 81 is the 81st galaxy in Arp’s Atlas of Peculiar Galaxies, a catalog of 338 peculiar galaxies produced by Halton Arp in 1966.

NGC 6621, the larger of the two, is a very disturbed spiral galaxy. The encounter created twisted streams of gas and dust, and also pulled a long tail out of NGC 6621, stretching for some 200,000 light-years that has now been wrapped behind its body.

The collision has also triggered extensive star formation between the two galaxies. Clusters of stars can form quickly after a strong enough perturbation, and Arp 81 has indeed a very rich collection of young massive star clusters (an even richer collection of star clusters than the well know Antennae galaxies). The galaxy pair is classified as a starburst system and is as such a strong far-infrared and radio source.

The galaxies are in their late interaction phase and are destined to merge into one large galaxy in the distant future, making repeated approaches until they finally coalesce.

This color composite image is taken by the Hubble Space Telescope.
Image Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and W. Keel (University of Alabama)

September 5, 2012

The Lemon Slice Nebula, a planetary nebula in Camelopardalis

The Lemon Slice Nebula (IC 3568) is a relatively young planetary nebula with a diameter of only about 0.4 light-years, and growing at the rate of some dozen kilometers per second. It is located about 4,500 light-years away from Earth in the constellation of Camelopardalis, the Giraffe (just 7.5 degrees from Polaris).

Planetary nebula have nothing to do with planets except that to early astronomers these round objects looked like the planets Uranus and Neptune. Planetary nebula are the last stage of life for stars like our Sun. After billions of years, stars reach a point where there is little hydrogen gas to burn. To help convert their stellar furnaces to burn other elements such as helium, the star balloons in size to become a red giant. Eventually, however, the star collapses back on itself. This increases the temperature at its core and most of the stars material is catapulted into space, forming a bubble around the star. Then the star cools down to become a white dwarf. This doesn’t happen all at once but in stages.

The Lemon Slice Nebula is one of the most simple nebulae known, with an almost perfectly spherical shape. The central star is a very hot and bright Red Giant, and can be seen as a red-orange hue in a small telescope. The nucleus is surrounded by very bright matter, apparently perfectly round, and fading out a little at the edges. A smooth, faint halo of interstellar dust surrounds the bright region.

Many planetary nebulae show bi-polar, twin-lobed, structures, or are obvious double ring affairs. But IC 3568, however, reveals just a faint structure. The bright inner region resolves into a complex structure with linear features pointing away from the central star, making the nebula look like the inside of a lemon, which is reinforced through false color. Deep imagery reveals an inner shell about 0.2 light-years.

Brightening at the edges, the Lemon Slice looks more like a shell, the whole lemon as it were. The star, with a temperature estimated from two methods at 57,000 Kelvin and still heating (at a luminosity a couple thousand times that of the Sun), has recently made the transition to becoming hot enough to marginally doubly-ionize the nebula’s helium.

Image Credit: Howard Bond ( Space Telescope Science Institute), Robin Ciardullo (Pennsylvania State University) and NASA/ESA

September 4, 2012

NGC 1187, an impressive spiral galaxy in Eridanus

NGC 1187 is an impressive spiral galaxy that looks to be relatively young. It is located about 60 million light-years away in the constellation of Eridanus (The River), and is receding from us at 1393 kilometers per second.

NGC 1187 is seen almost face-on, which gives us a good view of its spiral structure. About half a dozen prominent spiral arms can be seen, each containing large amounts of gas and dust. The bluish features in the spiral arms indicate the presence of young stars born out of clouds of interstellar gas.

Looking towards the central regions, we see the bulge of the galaxy glowing yellow. This part of the galaxy is mostly made up of old stars, gas and dust. Unlike other spiral galaxies, which feature a round bulge, NGC 1187 boasts a subtle central bar structure. The latter is not large enough to classify the galaxy as a barred spiral. Such bar features are thought to act as mechanisms that channel gas from the spiral arms to the centre, enhancing star formation there.

NGC 1187 looks tranquil and unchanging, but it has hosted two supernovae explosions since 1982. In October 1982, the first supernova, SN 1982R, was discovered and in 2007 the other one, called SN 2007Y which astronomers could study in detail. In this annotated image the Type Ib supernova SN 2007Y can be seen, long after the time of maximum brightness, near the bottom of the image.

A supernova is a violent stellar explosion, resulting from the death of either a massive star or a white dwarf in a binary system. Supernovae are amongst the most energetic events in the Universe and are so bright that they often briefly outshine an entire galaxy before fading from view over several weeks or months. During this short period a supernova can radiate as much energy as the Sun is expected to emit over its entire life span.

Type Ib supernovae are categories of stellar explosions that are caused by the core collapse of massive stars. These stars have shed (or been stripped of) their outer envelope of hydrogen. This type is usually referred to as stripped core-collapse supernova.

Most spirals have supernovae in them about every three centuries, so two supernovae within 30 years was a bit unusual. The rate is statistical though, so you might get two close together, or a long stretch without one. The last one in our Milky Way was about 170 years ago, and the last known before that was 400 years ago.

NGC 1187 is a gas-rich galaxy, and is forming lots of stars. That might lead to a higher-than-normal supernova rate, since that means more high-mass stars are being born, only to explode a few million years later. Both of the recent supernovae in NGC 1187 were caused by the core collapse of a high-mass star – so maybe this does play into it.

Around the outside of the galaxy many much fainter and more distant galaxies can also be seen. Some even shine right through the disc of NGC 1187 itself. Their mostly reddish hues contrast with the pale blue star clusters of the much closer galaxy.

This picture was taken with ESO’s Very Large Telescope at the Paranal Observatory in Chile.
Image Credit: ESO

September 3, 2012

NGC 6210, a planetary nebula in Hercules

NGC 6210 is small but fairly bright planetary nebula, located about 6,500 light-years away in the constellation of Hercules. The entire nebula measures 1.6 light-years while the inner shell is about 0.5 light-years in diameter. The nebula is moving away from us at 14 kilometers per second. The colors in this image are caused by ionized oxygen.

Planetary nebulae have nothing to do with planets, the term for this class of objects originated because when viewed through a small telescope in the 16th century, these objects appeared to be clouds (nebulae) that were similar in appearance to Uranus, which was just discovered. The name has not been changed, even though planetary nebulae are now known to be completely unrelated to the planets of the Solar System.

NGC 6210 is the last gasp of a star slightly less massive than our Sun at the final stage of its life cycle. The multiple shells of material ejected by the dying star form a superposition of structures with different degrees of symmetry, giving NGC 6210 its odd shape.

The life of a star ends when the fuel available to its thermonuclear engine runs out. The estimated lifetime for a Sun-like star is some ten billion years. When the star is about to expire, it becomes unstable and ejects its outer layers, forming a planetary nebula and leaving behind a tiny, but very hot, remnant, known as white dwarf. This compact object, here visible at the centre of the image, cools down and fades very slowly. Stellar evolution theory predicts that our Sun will experience the same fate as NGC 6210 in about five billion years.

This sharp image shows the inner region of this planetary nebula in unprecedented detail, where the central star is surrounded by a thin, bluish bubble that reveals a delicate filamentary structure. This bubble is superposed onto an asymmetric, reddish gas formation where holes, filaments and pillars are clearly visible.

At least two bipolar jets of material flung off by the central star can be seen in the image. The jets are thought to be driven by a “fast wind” – material propelled by radiation from the hot central star. Research suggests that the jets occur episodically and also that the various shells are not of the same age. The central star likely ejected its outer layers in several periods, reshaping the nebula created by the fast stellar winds several times.

This picture was created from images taken with Hubble’s Wide Field Planetary Camera 2.
Image Credit: ESA/Hubble and NASA

September 2, 2012

IC 10, a dwarf galaxy in Cassiopeia

IC 10 is an irregular dwarf galaxy with an H II nucleus of about 5,000 light-years across, located some 2.2 million light-years away in the northern constellation Cassiopeia and it is approaching the Milky Way at approximately 350 kilometers per second. IC 10 is an outlying member of the Local Group of galaxies and belongs to the M31 subgroup.

Compared to other Local Group galaxies, IC 10 has a large population of newly formed stars that are massive and intrinsically very bright, including a luminous X-ray binary star system that contains a black hole. Although considerably smaller the Small Magellanic Cloud (SMC) IC 10 has a similar luminosity.

IC 10 is the only known starburst galaxy in the Local Group of galaxies, and compared to both of the Magellanic Clouds it has many more Wolf-Rayet stars. The green emissions are from older stars while the red filaments are H-alpha regions with active star formations.

Its higher metallicity compared to the SMC suggests that star formation activity has continued for a longer time period. The evolutionary status of the Wolf-Rayet stars suggests that they all formed in a relatively short timespan. Currently the galaxy produces stars at the rate of 0.04–0.08 solar masses per year, which means that the gas supply in the galaxy can last for only a few billion years longer.

The galaxy has a huge envelope of hydrogen gas, which is far larger than the apparent size of the galaxy in visible light. IC 10 is also unusual in the respect that the visible part of the galaxy seems to rotate in a different direction than the outer envelope.

Astronomers have found the most massive known stellar black hole within IC 10. It is orbited by a companion star, which passes in front of the black hole, periodically blocking the hole’s X-rays. By observing the periodic dimming, scientists were able to determine the orbit of the companion, and the mass of the two bodies.

With a mass of 24-33 times the Sun, the black hole smashes all known records for stellar black holes. These black holes form during the death of a star. Usually, it is expected that a dying star will throw off much of its mass before a black hole forms. How this hole managed to retain so much mass is something of a mystery.

One theory involves a possible scarcity of heavy elements in the star from which the hole was born. Heavy elements get more of a push on their electrons from radiation pressure inside a star. The outgoing light literally carries away some of the mass. If the star did not have much of the heavy elements, the light would not have been able to push out much mass, and such a heavy black hole could form. Because the entire galaxy is lacking in heavy elements, this theory definitely fits the observations.

Despite its closeness, the galaxy is rather difficult to study because it lies near the plane of the Milky Way and is therefore heavily obscured by interstellar matter.

Image Credit: The Survey Team of the Lowell Observatory

September 1, 2012

The Monkey (Head) Nebula, a bright emission nebula in Orion

The Monkey (Head) Nebula (designated NGC 2174) is a very bright H II emission nebula located about 6,400 light-years away in the constellation Orion, close to the border with the Gemini constellation. The cloud of dust and gas surrounds the open cluster of young stars called NGC 2175.

As the star cluster heats up the nebula — composed mostly of hydrogen gas — the dust begins to glow. Strong winds and radiation from the massive newly formed stars blow away the nearby material of the dark stellar nurseries in which they formed, creating a shell-like structure.

The dark columns seen on the right side of the nebula are clouds of dust that are silhouetted against the glowing gas of the nebula. These opaque clouds of dust and gas are the birth place of many new stars.

Image Credit: Don Goldman, Ph.D. (Astrodon)

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