Betelgeuse, A Red Giant


Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA
This is the first direct image of a star other than the Sun, made with NASA's Hubble Space Telescope. Called Alpha Orionis, or Betelgeuse, it is a red supergiant star marking the shoulder of the winter constellation Orion the Hunter. The image reveals a huge ultraviolet atmosphere with a mysterious hot spot on the stellar behemoth's surface. The enormous bright spot, more than ten times the diameter of Earth, is at least 2,000 Kelvin degrees hotter than the surface of the star.

The image suggests that a totally new physical phenomenon may be affecting the atmospheres of some stars. Follow-up observations will be needed to help astronomers understand whether the spot is linked to oscillations previously detected in the giant star, or whether it moves systematically across the star's surface under the grip of powerful magnetic fields.

The observations were made by Andrea Dupree of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, and Ronald Gilliland of the Space Telescope Science Institute in Baltimore, MD, who announced their discovery today at the 187th meeting of the American Astronomical Society in San Antonio, Texas.

The image was taken in ultraviolet light with the Faint Object Camera on March 3, 1995.

Hubble can resolve the star even though the apparent size is 20,000 times smaller than the width of the full Moon -- roughly equivalent to being able to resolve a car's headlights at a distance of 6,000 miles.






The Life Cycle of Stars


Credit: Wolfgang Brandner (JPL/IPAC), Eva K. Grebel (Univ. Washington), You-Hua Chu (Univ. Illinois Urbana-Champaign), and NASA>
In this stunning picture of the giant galactic nebula NGC 3603, the crisp resolution of NASA's Hubble Space Telescope captures various stages of the life cycle of stars in one single view.

To the upper right of center is the evolved blue supergiant called Sher 25. The star has a unique circumstellar ring of glowing gas that is a galactic twin to the famous ring around the supernova 1987A. The grayish-bluish color of the ring and the bipolar outflows (blobs to the upper right and lower left of the star) indicates the presence of processed (chemically enriched) material.

Near the center of the view is a so-called starburst cluster dominated by young, hot Wolf-Rayet stars and early O-type stars. A torrent of ionizing radiation and fast stellar winds from these massive stars has blown a large cavity around the cluster.

The most spectacular evidence for the interaction of ionizing radiation with cold molecular-hydrogen cloud material are the giant gaseous pillars to the right and lower left of the cluster. These pillars are sculptured by the same physical processes as the famous pillars Hubble photographed in the M16 Eagle Nebula.

Dark clouds at the upper right are so-called Bok globules, which are probably in an earlier stage of star formation.

To the lower left of the cluster are two compact, tadpole-shaped emission nebulae. Similar structures were found by Hubble in Orion, and have been interpreted as gas and dust evaporation from possibly protoplanetary disks (proplyds). The "proplyds" in NGC 3603 are 5 to 10 times larger in size and correspondingly also more massive.

This single view nicely illustrates the entire stellar life cycle of stars, starting with the Bok globules and giant gaseous pillars, followed by circumstellar disks, and progressing to evolved massive stars in the young starburst cluster. The blue supergiant with its ring and bipolar outflow marks the end of the life cycle.

The color difference between the supergiant's bipolar outflow and the diffuse interstellar medium in the giant nebula dramatically visualizes the enrichment in heavy elements due to synthesis of heavier elements within stars.

This true-color picture was taken on March 5, 1999 with the Wide Field Planetary Camera 2.






Generations of Star Formation in the LMC


Image Credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI)
Acknowledgment: Y.-H. Chu (U. Illinois, Urbana-Champaign) and Y. Nazé (U. Liège, Belgium)

NASA's Hubble Space Telescope captures this iridescent tapestry of star birth in a neighboring galaxy in this panoramic view of glowing gas, dark dust clouds, and young, hot stars. The star-forming region, catalogued as N11B, lies in the Large Magellanic Cloud (LMC), located only 160,000 light-years from Earth. With its high resolution, the Hubble Space Telescope is able to view details of star formation in the LMC as easily as ground-based telescopes are able to observe stellar formation within our own Milky Way galaxy. This new Hubble image zooms in on N11B, which is a small subsection within an area of star formation cataloged as N11. N11 is the second largest star-forming region in the LMC. Within the LMC, N11 is surpassed in size and activity only by the immense Tarantula nebula (also known as 30 Doradus.)

The image illustrates a perfect case of sequential star formation in a nearby galaxy where new star birth is being triggered by previous-generation massive stars. A collection of blue- and white-colored stars near the left of the image are among the most massive stars known anywhere in the universe. The region around the cluster of hot stars in the image is relatively clear of gas, because the stellar winds and radiation from the stars have pushed the gas away. When this gas collides with and compresses surrounding dense clouds, the clouds can collapse under their own gravity and start to form new stars. The cluster of new stars in N11B may have been formed this way, as it is located on the rim of the large, central interstellar bubble of the N11 complex. The stars in N11B are now beginning to clear away their natal cloud, and are carving new bubbles in turn. Yet another new generation of stars is now being born in N11B, inside the dark dust clouds in the center and right-hand side of the Hubble image. This chain of consecutive star birth episodes has been seen in more distant galaxies, but it is shown very clearly in this new Hubble image.

Farther to the right of the image, along the top edge, are several smaller dark clouds of interstellar dust with odd and intriguing shapes. They are seen silhouetted against the glowing interstellar gas. Several of these dark clouds are bright-rimmed because they are illuminated and are being evaporated by radiation from neighboring hot stars.

This image was taken with Hubble's Wide Field Planetary Camera 2 using filters that isolate light emitted by hydrogen and oxygen gas. The science team, led by astronomers You-Hua Chu (University of Illinois) and Yäel Nazé (Universite de Liège, Belgium) are comparing these images of N11B, taken in 1999, with similar regions elsewhere in the LMC. This color composite image was co-produced and is being co-released by the Hubble Heritage Team (STScI) and the Hubble European Space Agency Information Center (HEIC).






Planetary Nurseries Being Torched by Radiation from Hot Star


Credits: NASA, J. Bally (University of Colorado, Boulder, CO), H. Throop (Southwest Research Institute, Boulder, CO), C.R. O'Dell (Vanderbilt University, Nashville, TN)
Planet formation is a hazardous process. These four snapshots, taken by NASA's Hubble Space Telescope, show dust disks around embryonic stars in the Orion Nebula being "blowtorched" by a blistering flood of ultraviolet radiation from the region's brightest star. Within these disks are the seeds of planets. The doomed systems look like hapless comets, with wayward tails of gas boiling off the withering, pancake-shaped disks.

The Frisbee-shaped disks, called protoplanetary disks, are wider than our solar system and reside in the centers of the cocoons of gas. These cocoons were formed from material evaporating off the surface of the disks. Evidence from Hubble's Wide Field and Planetary Camera 2 suggests that dust grains in the disk are already forming larger particles, which range in size from snowflakes to gravel. But these particles may not have time to grow into full-fledged planets because of the relentless "hurricane" of radiation from the nebula's hottest star, called Theta 1 Orionis C.

In the picture at top left, the disk is the green-colored oval near the center. Radiation from the hot star is heating up the disk, causing matter to dissipate, like steam evaporating from the surface of boiling water. A strong "stellar wind," a stream of particles moving at 4,500 to 8,900 miles per hour (7,200 to 14,400 kilometers per hour), is propelling the material away from the disk. The material is glowing because it is being energized by radiation from the hot star.

Located 1,500 light-years away, the Orion Nebula is the nearest "star factory" to Earth. The Hubble pictures were taken Feb. 26, 1998 and Jan. 11, 1999.






A Bow Shock Near A Young Star


Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: C. R. O'Dell (Vanderbilt University)

NASA's Hubble Space Telescope continues to reveal various stunning and intricate treasures that reside within the nearby, intense star-forming region known as the Great Nebula in Orion. One such jewel is the bow shock around the very young star, LL Ori, featured in this Hubble Heritage image.

Named for the crescent-shaped wave made by a ship as it moves through water, a bow shock can be created in space when two streams of gas collide. LL Ori emits a vigorous solar wind, a stream of charged particles moving rapidly outward from the star. Our own Sun has a less energetic version of this wind that is responsible for auroral displays on the Earth.

The material in the fast wind from LL Ori collides with slow-moving gas evaporating away from the center of the Orion Nebula, which is located to the lower right in this Heritage image. The surface where the two winds collide is the crescent-shaped bow shock seen in the image.

Unlike a water wave made by a ship, this interstellar bow shock is a three-dimensional structure. The filamentary emission has a very distinct boundary on the side facing away from LL Ori, but is diffuse on the side closest to the star, a characteristic common to many bow shocks.

A second, fainter bow shock can be seen around a star near the upper right-hand corner of the Heritage image. Astronomers have identified numerous shock fronts in this complex star-forming region and are using this data to understand the many complex phenomena associated with the birth of stars.

This image was taken in February 1995 as part of the Hubble Orion Nebula mosaic. A close visitor in our Milky Way galaxy, the nebula is only 1,500 light-years from Earth. The filters used in this color composite represent oxygen, nitrogen, and hydrogen emissions.






Young Stars in Cosmic Dance


Credit: NASA and B. Reipurth (CASA, Univ. of Colorado)
This composite image (left), made with two cameras aboard NASA's Hubble Space Telescope, shows a pair of 12 light-year-long jets of gas blasted into space from a young system of three stars. The jet is seen in visible light, and its dusty disk and stars are seen in infrared light. These stars are located near a huge torus, or donut, of gas and dust from which they formed. This torus is tilted edge-on and can be seen as a dark bar near the bottom of the picture.

Apparently, a gravitational brawl among the stars occurred a few thousand years ago and kicked out one member (on the left edge of the bright blob above the disk). As a result, the two other stars were joined together as a tight binary pair and flew off in the opposite direction, and appear as a red blob below the disk.

The huge jet comes from one of the stars in this tight binary pair. The star spews out streams of gas in opposite directions, like water from a garden hose. It is not a smooth flow, but rather happens episodically, creating lumps of gas that fly across space at over one million miles per hour. These gaseous cannonballs catch up with and "rear-end" slower moving blobs, creating a pattern that resembles a string of Christmas lights embedded in the jet.

The visible-light image (upper part) was taken with Hubble's Wide Field Planetary Camera 2 in November, 1998, and the infrared image (lower part) by Hubble's Near Infrared Camera and Multi-Object Spectrometer in March, 1998. The disk and associated stars are embedded in a large dark cloud and are only visible at infrared wavelengths.

The research team consists of Bo Reipurth, Ka Chun Yu and John Bally from the University of Colorado; Steve Heathcote from Cerro Tololo Inter-American Observatory, and Luis Felipe Rodriguez from the Universidad Nacional Autonoma de Mexico (UNAM).






Views of Three Stellar Jets


Credit: STScI and NASA
These Hubble Space Telescope views of gaseous jets from three newly forming stars show a new level of detail in the star formation process, and are helping to solve decade-old questions about the secrets of star birth. They are examples of "Herbig-Haro objects" which are formed when young stars eject jets of material back into interstellar space. The jets are a common "exhaust product" of the dynamics of star formation and are blasted away from a disk of gas and dust falling onto an embryonic star.

[upper left] - This view of a protostellar object called HH 30 reveals an edge-on disk of dust encircling a newly forming star. Light from the forming star illuminates the top and bottom surfaces of the disk, making them visible, while the star itself is hidden behind the densest parts of the disk. The reddish jet emanates from the inner region of the disk, and possibly directly from the star itself. Hubble's detailed view shows, for the first time, that the jet expands for several billion miles from the star, but then stays confined to a narrow beam. The protostar is 450 light-years away in the constellation Taurus.
Credit: C. Burrows (STScI & ESA), the WFPC 2 Investigation Definition Team, and NASA

[upper right] - This view of a different and more distant jet in object HH 34 shows a remarkable beaded structure. Once thought to be a hydrodynamic effect (similar to shock diamonds in a jet aircraft exhaust), this structure is actually produced by a machine-gun-like blast of "bullets" of dense gas ejected from the star at speeds of one-half million miles per hour. This structure suggests the star goes through episodic "fits" of construction where chunks of material fall onto the star from a surrounding disk. The protostar is 1,500 light-years away and in the vicinity of the Orion Nebula, a nearby star birth region.
Credit: J. Hester (Arizona State University), the WFPC 2 Investigation Definition Team, and NASA

[bottom] - This view of a three trillion mile-long jet called HH 47 reveals a very complicated jet pattern that indicates the star (hidden inside a dust cloud near the left edge of the image) might be wobbling, possibly caused by the gravitational pull of a companion star. Hubble's detailed view shows that the jet has burrowed a cavity through the dense gas cloud and now travels at high speed into interstellar space. Shock waves form when the jet collides with interstellar gas, causing the jet to glow. The white filaments on the left reflect light from the obscured newborn star. The HH 47 system is 1,500 light-years away, and lies at the edge of the Gum Nebula, possibly an ancient supernova remnant which can be seen from Earth's southern hemisphere.
Credit: J. Morse/STScI, and NASA

The scale in the bottom left corner of each picture represents 93 billion miles, or 1,000 times the distance between Earth and the Sun. All images were taken with the Wide Field Planetary Camera 2 in visible light. The HH designation stands for "Herbig-Haro" object -- the name for bright patches of nebulosity which appear to be moving away from associated protostars, in honor of astronomers George Herbig and Guillermo Haro, who did much of the early work in this area in the 1950's.






A Wider View of the HH 34 Region


Credit: ESO
This object has a remarkable, very complicated appearance that includes two opposite jets that ram into the surrounding interstellar matter. This structure is produced by a machine-gun-like blast of "bullets" of dense gas ejected from the star at high velocities (approaching 250 km/sec). This seems to indicate that the star experiences episodic "outbursts" when large chunks of material fall onto it from a surrounding disk.

HH 34 is located at a distance of approx. 1,500 light-years, near the famous Orion Nebula, one of the most productive star birth regions. Note also the enigmatic "waterfall" to the upper left, a feature that is still unexplained.






HH 30's Dynamic Disk and Jets


Credits: NASA, Alan Watson (Universidad Nacional Autonoma de Mexico), Karl Stapelfeldt (Jet Propulsion Laboratory), John Krist and Chris Burrows (European Space Agency/Space Telescope Science Institute)
These images of HH 30 show changes over only a five-year period in the disk and jets of this newborn star, which is about half a million years old. The pictures were taken between 1995 and 2000 with the Wide Field and Planetary Camera 2 aboard NASA's Hubble Space Telescope. Astronomers are interested in the disk because it is probably similar to the one from which the Sun and the planets in our solar system formed.

Hubble reveals an edge-on disk (located at the bottom of the images), which appears as a flattened cloud of dust split into two halves by a dark lane. The disk blocks light from the central star. All that is visible is the reflection of the star's light by dust above and below the plane of the disk. The disk's diameter is 450 astronomical units (one astronomical unit equals the Earth-Sun distance). Shadows billions of miles in size can be seen moving across the disk. In 1995 and 2000, the left and right sides of the disk were about the same brightness, but in 1998 the right side was brighter. These patterns may be caused by bright spots on the star or variations in the disk near the star. The dust cloud near the top of these frames is illuminated by the star and reflects changes in its brightness.

The star's magnetic field plays a major role in forming the jets (located above and below the disk), which look like streams of water from a fire hose. The powerful magnetic field creates the jets by channeling gas from the disk along the magnetic poles above and below the star. The gaps between the compact knots of gas seen in the jet above the disk indicate that this is a sporadic process. By tracking the motion of these knots over time, astronomers have measured the jet's speed at between 200,000 to 600,000 miles per hour (160,000 and 960,000 kilometers per hour). Oddly, the jet below the disk is moving twice as fast as the one above it.






HH 32


Credit: NASA/The Hubble Heritage Team (AURA/STScI)
HH 32 is is about 1,000 light-years from Earth and radiation from the bright central star has already cleared much of the dust out of the central region, thus exposing the star to direct view.

Many young stars, like the central object in HH 32, are surrounded by disks of gas and dust that form as additional material is attracted gravitationally from the surrounding nebula. Material in the disk gradually spirals in toward the star and eventually some of it accretes onto the star, increasing its mass. A fraction of the gas, however, is ejected perpendicularly to the disk at speeds near 200 miles per second, and forms two oppositely directed jets. These 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 (red) and sulfur ions (blue). 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 because it is partially obscured by dust surrounding the star.






Light from Mysterious Erupting Star Reverberates Through Space


Credit: NASA, ESA and H.E. Bond (STScI)
In January 2002, a dull star in an obscure constellation suddenly became 600,000 times more luminous than our Sun, temporarily making it the brightest star in our Milky Way galaxy.

The mysterious star has long since faded back to obscurity, but observations by NASA's Hubble Space Telescope of a phenomenon called a "light echo" have uncovered remarkable new features. These details promise to provide astronomers with a CAT-scan-like probe of the three-dimensional structure of shells of dust surrounding an aging star.

Light from a stellar explosion echoing off circumstellar dust in our Milky Way galaxy was last seen in 1936, long before Hubble was available to study the tidal wave of light and reveal the netherworld of dusty black interstellar space.

The outburst of V838 Mon was somewhat similar to that of a nova, a more common stellar outburst. A typical nova is a normal star that dumps hydrogen onto a compact white-dwarf companion star. The hydrogen piles up until it spontaneously explodes by nuclear fusion — like a titanic hydrogen bomb. This exposes a searing stellar core, which has a temperature of hundreds of thousands of degrees Fahrenheit.

By contrast, however, V838 Mon did not expel its outer layers. Instead, it grew enormously in size, with its surface temperature dropping to temperatures not much hotter than a light bulb. This behavior of ballooning to an immense size, but not losing its outer layers, is very unusual and completely unlike an ordinary nova explosion. The star is so unique it may represent a transitory stage in a star's evolution that is rarely seen. The star has some similarities to highly unstable aging stars called eruptive variables, which suddenly and unpredictably increase in brightness.

The circular light-echo feature has now expanded to twice the angular size of Jupiter on the sky. Astronomers expect it to continue expanding as reflected light from farther out in the dust envelope finally arrives at Earth. Bond predicts that the echo will be observable for the rest of this decade.






The Heart of a Dying Star


Credit: Rodger Thompson, Marcia Rieke, Glenn Schneider, Dean Hines (University of Arizona); Raghvendra Sahai (Jet Propulsion Laboratory); NICMOS Instrument Definition Team; and NASA
The Egg Nebula, also known as CRL 2688, is shown on the left as it appears in visible light with the Hubble Space Telescope's Wide Field and Planetary Camera 2 (WFPC2) and on the right as it appears in infrared light with Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS). Since infrared light is invisible to humans, the NICMOS image has been assigned colors to distinguish different wavelengths: blue corresponds to starlight reflected by dust particles, and red corresponds to heat radiation emitted by hot molecular hydrogen.

Objects like the Egg Nebula are helping astronomers understand how stars like our Sun expel carbon and nitrogen -- elements crucial for life -- into space. Studies on the Egg Nebula show that these dying stars eject matter at high speeds along a preferred axis and may even have multiple jet-like outflows. The signature of the collision between this fast-moving material and the slower outflowing shells is the glow of hydrogen molecules captured in the NICMOS image.

The distance between the tip of each jet is approximately 200 times the diameter of our solar system (out to Pluto's orbit).






An Old Star Gives Up the Ghost


Credit: NASA and The Hubble Heritage Team (STScI/AURA)
NASA's Hubble Space Telescope has recently [2003] obtained images of the planetary nebula NGC 6369. This object is known to amateur astronomers as the "Little Ghost Nebula," because it appears as a small, ghostly cloud surrounding the faint, dying central star. NGC 6369 lies in the direction of the constellation Ophiuchus, at a distance estimated to be between about 2,000 and 5,000 light-years from Earth.

When a star with a mass similar to that of our own Sun nears the end of its lifetime, it expands in size to become a red giant. The red-giant stage ends when the star expels its outer layers into space, producing a faintly glowing nebula. Astronomers call such an object a planetary nebula, because its round shape resembles that of a planet when viewed with a small telescope.

The Hubble photograph of NGC 6369, captured with the Wide Field Planetary Camera 2 (WFPC2) in February 2002, reveals remarkable details of the ejection process that are not visible from ground-based telescopes because of the blurring produced by the Earth's atmosphere.

The remnant stellar core in the center is now sending out a flood of ultraviolet (UV) light into the surrounding gas. The prominent blue-green ring, nearly a light-year in diameter, marks the location where the energetic UV light has stripped electrons off of atoms in the gas. This process is called ionization. In the redder gas at larger distances from the star, where the UV light is less intense, the ionization process is less advanced. Even farther outside the main body of the nebula, one can see fainter wisps of gas that were lost from the star at the beginning of the ejection process.

The color image has been produced by combining WFPC2 pictures taken through filters that isolate light emitted by three different chemical elements with different degrees of ionization. The doughnut-shaped blue-green ring represents light from ionized oxygen atoms that have lost two electrons (blue) and from hydrogen atoms that have lost their single electrons (green). Red marks emission from nitrogen atoms that have lost only one electron.

Our own Sun may eject a similar nebula, but not for another 5 billion years. The gas will expand away from the star at about 15 miles per second, dissipating into interstellar space after some 10,000 years. After that, the remnant stellar ember in the center will gradually cool off for billions of years as a tiny white dwarf star, and eventually wink out.






Supernova 1987A Ring Blazes Back to Life


Credit: NASA, Peter Challis and Robert Kirshner (Harvard-Smithsonian Center for Astrophysics), Peter Garnavich (University of Notre Dame), and the SINS Collaboration
[Left] This NASA Hubble Space Telescope Wide Field and Planetary Camera 2 image shows the glowing gas ring around supernova 1987A, as seen on February 2, 2000. The gas, excited by light from the explosion, has been fading for a decade, but parts of it are now being heated by the collision of an invisible shockwave from the supernova explosion.

[Right] Image processing is used to emphasize four new bright knots of superheated gas discovered in the February 2 Hubble observations. The brightest knot, at the far right, was seen in 1997. Astronomers have been waiting several years to see more of the ring light-up as the supernova shockwave smashes into it. This is the first definitive sign of the full onset of a dramatic and violent collision which will continue over the next few years, rejuvenating SN1987A as a powerful source of X-ray and radio emissions.

Both images were made in visual light. Computer image processing techniques were used to enhance details in the ring.






Supernova Remnant E0102-72
X-ray (blue), Optical (green), and Radio (red)


Credit: X-ray (NASA/CXC/SAO); Optical (NASA/HST); Radio: (CSIRO/ATNF/ATCA)
E0102-72 is the remnant of a star that exploded in a nearby galaxy known as the Small Magellanic Cloud. The galaxy is approximately 190,000 light years from Earth, so 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 hr (12 million mph) and collided with surrounding gas. This collision produced two shock waves, or cosmic sonic booms­ one traveling outward, and the other rebounding back into the material ejected by the explosion.

The radio image was made using the Australia Telescope Compact Array. The radio waves are due to extremely high energy electrons spiraling around magnetic field lines in the gas and trace the outward moving shock wave.

The Chandra X-ray image, shown in blue, shows gas that has been heated to millions of degrees Celsius by the rebounding, or reverse shock wave. The X-ray data 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.

The Hubble Space Telescope optical image shows dense clumps of oxygen gas that have "cooled" to about 30,000 degree Celsius.

Images such as these, taken with different types of telescopes, give astronomers a much more complete picture of supernova explosions. They can map how the elements necessary for life are dispersed, and measure the energy of the matter as it expands into the galaxy.






Chandra Maps Vital Elements in Supernova


Credit: NASA/CXC/SAO/Rutgers/J.Hughes
The red, green, and blue regions in this Chandra X-ray image of the supernova remnant Cassiopeia A show where the intensity of low, medium, and high energy X rays, respectively, is greatest. The red material on the left outer edge is enriched in iron, whereas the bright greenish white region on the lower left is enriched in silicon and sulfur. In the blue region on the right edge, low and medium energy X rays have been filtered out by a cloud of dust and gas in the remnant.

The image was made with the Advanced CCD Imaging Spectrometer (ACIS).






A Bright Supernova in the Nearby Galaxy NGC 2403


Credit: NASA, ESA, A.V. Filippenko (University of California, Berkeley), P. Challis (Harvard-Smithsonian Center for Astrophysics), et al.
The explosion of a massive star blazes with the light of 200 million Suns in this NASA Hubble Space Telescope image. The arrow at top right points to the stellar blast, called a supernova. The supernova is so bright in this image that it easily could be mistaken for a foreground star in our Milky Way Galaxy. And yet, this supernova, called SN 2004dj, resides far beyond our galaxy. Its home is in the outskirts of NGC 2403, a galaxy located 11 million light-years from Earth. Although the supernova is far from Earth, it is the closest stellar explosion discovered in more than a decade.

The star that became SN 2004dj may have been about 15 times as massive as the Sun, and only about 14 million years old. (Massive stars live much shorter lives than the Sun; they have more fuel to "burn" through nuclear fusion, but they use it up at a disproportionately faster rate.) A team of astronomers led by Jesus Maiz of the Space Telescope Science Institute discovered that the supernova was part of a compact cluster of stars known as Sandage 96, whose total mass is about 24,000 times the mass of the Sun. Many such clusters — the blue regions — as well as looser associations of massive stars, can be seen in this image. The large number of massive stars in NGC 2403 leads to a high supernova rate. Two other supernovae have been seen in this galaxy during the past half-century.

The heart of NGC 2403 is the glowing region at lower left. Sprinkled across the region are pink areas of star birth. The myriad of faint stars visible in the Hubble image belong to NGC 2403, but the handful of very bright stars in the image belong to our own Milky Way Galaxy and are only a few hundred to a few thousand light-years away. This image was taken on Aug. 17, two weeks after an amateur astronomer discovered the supernova.

Japanese amateur astronomer Koichi Itagaki discovered the supernova on July 31, 2004, with a small telescope. Additional observations soon showed that it is a "Type II supernova," resulting from the explosion of a massive, hydrogen-rich star at the end of its life. The cataclysm probably occurred when the evolved star's central core, consisting of iron, suddenly collapsed to form an extremely dense object called a neutron star. The surrounding layers of gas bounced off the neutron star and also gained energy from the flood of ghostly "neutrinos" (tiny, almost non-interacting particles) that may have been released, thereby violently expelling these layers.

This explosion is ejecting heavy chemical elements, generated by nuclear reactions inside the star, into the cosmos. Like other Type II supernovae, this exploding star is providing the raw material for future generations of stars and planets. Elements on Earth such as oxygen, calcium, iron, and gold came long ago from exploding stars such as this one.

Astronomers will continue to study SN 2004dj over the next few years, as it slowly fades from view, in order to gain a better understanding of how certain types of stars explode and what kinds of chemical elements they eject into space.

This color-composite photograph was obtained by combining images through several filters taken with the Wide Field Camera of the Advanced Camera for Surveys. The colors in the image highlight important features in the galaxy. Hot, young stars are blue. Older stars and dense dust lanes near the heart of the galaxy are red. The hydrogen-rich, star-forming regions are pink. The dense concentration of older stars in the galaxy's central bulge is yellow.

In addition to the visible-light image shown here, ultraviolet images and spectra are being obtained with Hubble's Advanced Camera for Surveys. Astronomers are also using ground-based telescopes to study the supernova.






Bare Neutron Star Streaking Across Space


Credit: NASA and F. M. Walter (State University of New York at Stony Brook)
It's as big as Manhattan Island, is 10 trillion times denser than steel and is hurtling our way at speeds over 100 times faster than a supersonic jet. An alien spaceship? No, it's a runaway neutron star, called RX J185635-3754, forged in a stellar explosion that would have been visible to our distant ancestors in 1 million B.C.

Precise observations made with NASA's Hubble telescope confirm that the interstellar interloper turns out to be the closest neutron star ever seen. Now located 200 light-years away in the southern constellation Corona Australis, it will swing by Earth at a safe distance of 170 light-years in about 300,000 years. A light-year is the distance traveled by light in a full year (about 6 trillion miles).

Because it is the closest neutron star ever seen and its distance has been well established by Hubble, astronomers can compare stellar theories against a variety of its physical properties such as size, inherent brightness, and true age.

Since the object has no companion star that would affect its appearance, this discovery will allow future astronomers to more easily confirm stellar theories. The results are being presented today at the 2000 meeting of the American Astronomical Society's High Energy Astrophysics Division (HEAD) in Honolulu, HI.

"The scientific importance of this object lies in the fact that the neutron star is isolated," says Frederick M. Walter of the State University of New York (SUNY), in Stony Brook, NY. "It appears to be hot, not because it is accreting hydrogen gas as it moves through space, but because it is still young and cooling off. Since we know its approximate age, we can test how fast neutron stars cool off. Because this is the closest and brightest of the few known isolated neutron stars, it is the easiest to study and is an excellent test bed for nuclear astrophysical theories."

The neutron star's wayward trajectory was caught in three Hubble snapshots taken in 1996 and 1999. The three Hubble images show that the star moves across the sky with a characteristic apparent "wobble" (a reflection of the Earth's own orbital motion, an effect called parallax), which is expected of an object located about 200 light-years away.

In addition, the observations reveal that the neutron star is streaking across the sky from west to east at a rate of 1/3 of an arc second per year. (An arc second is a unit of angular measure. There are 3,600 arc seconds in a degree and 360 degrees in a full circle.) In 5,400 years, RX J185635-3754 travels a distance equal to the diameter of the Moon. Although this apparent motion may seem slow, it is actually one of the fastest moving stars in the sky. The fastest, Barnard's star, moves 10 arc seconds each year). The apparent motion, combined with the distance, means that the neutron star is moving at a speed of about 240,000 miles per hour (389,000 kilometers per hour).

This neutron star may be approaching from a grouping of young stars in the constellation Scorpius. About 1 million years ago, a massive star in a binary star system exploded as a supernova, releasing its companion star, an ultra-hot, blue star now known as Zeta Ophiuchus, which is also zooming away from the region. Because 1 million years ago the neutron star and Zeta Ophiuchus were in about the same location in space, the neutron star may be the remnant of the original binary companion of Zeta Ophiuchus, the star which exploded.

The runaway neutron star was first reported in 1992, when astronomers detected a very bright source of X-ray emission with the Roentgen Satellite (ROSAT). Because it was not seen in optical light and appeared to be within 500 light-years of the Earth, Walter and S.J. Wolk (Stony Brook) and R. Neuhaeuser (Max-Plack-Institut fuer Extraterrestrische Physik) surmised that it was likely to be a neutron star, a hot, dense stellar corpse with a six-mile radius.

Four years later, Stony Brook astronomers Walter and L.D. Matthews reported the optical identification of the star using the Hubble telescope. The object is very faint (26th magnitude or about 20 billion times fainter than the bright star Vega), and has a blue color. The blue color indicates that the object is hot, as expected from the bright X-ray emission. The temperature is about 1 million degrees Fahrenheit (600,000 degrees Kelvin). In September 2000, images taken with the European Southern Observatory's Very Large Telescope showed a small, cone-shaped "bowshock" in front of the neutron star, created as the star plowed through interstellar space.

The Hubble results have been accepted for publication in the Astrophysical Journal.






The Changing Rate of Star Formation


Credit: NASA
This graph traces the history of the rate of star formation over the past 12 billion years from shortly after the birth of the universe to the present.

The graph is based on observations of distant galaxies made by Hubble Space Telescope and ground-based observatories. Hubble shows a steep rise in star formation rate that happened shortly after the big bang. The ground-based data show the precipitous decline in star formation rate started from about 9 billion years ago to present. No observations exist yet to fill in the gap between these two data sets, where the slopes would joint to form a peak of starbirth activity. This will be an area of research by Hubble in the future.

The vertical axis gives values for the mean rate of star formation as multiples of today's rate (unit value 1). Data suggest this rate may have been as high as 15 times today's value. The horizontal axis shows time in billions of years, from the big bang to present.