Astronomy Picture of the Day Search Results for "black hole"

APOD: April 12, 1999 - Nearby Spiral Galaxy NGC 4945
Explanation: For such a close galaxy, NGC 4945 is easy to miss. NGC 4945 is a spiral galaxy in the Centaurus Group of galaxies, located only six times further away than the prominent Andromeda Galaxy. The thin disk galaxy is oriented nearly edge-on, however, and shrouded in dark dust. Therefore galaxy-gazers searching the southern constellation of Centaurus need a telescope to see it. The above picture was taken with a large telescope testing a new wide-angle, high-resolution CCD camera. Most of the spots scattered about the frame are foreground stars in our own Galaxy, but some spots are globular clusters orbiting the distant galaxy. NGC 4945 is thought to be quite similar to our own Milky Way Galaxy. X-ray observations reveal, however, that NGC 4945 has an unusual, energetic, Seyfert 2 nucleus that might house a large black hole.

APOD: February 16, 1999 - The Large and Small of M87
Explanation: The small core of elliptical galaxy M87 appears to be energizing its whole galactic neighborhood. Recent images from the Very Large Array (VLA) of radio telescopes indicate that huge bubbles of hot gas not only exist but are still being created. These bubbles measure 200,000 light-years across and surround the entire galaxy. The source creating and feeding the bubbles has been traced to jets pointing back to M87's center, where a supermassive black hole is thought to live. The smallest scale on the above radio-map is 0.2 light-years and imaged by many radio telescopes working together (VLBI). The labeled numbers refer to the wavelength of the radio waves observed. The exact composition of these jets is not known, but thought to contain various subatomic particles.

APOD: January 28, 1999 - The Galactic Center A Radio Mystery
Explanation: Tuning in to the center of our Milky Way galaxy, radio astronomers explore a complex, mysterious place. A premier high resolution view, this startlingly beautiful picture covers a 4x4 degree region around the galactic center. It was constructed from 1 meter wavelength radio data obtained by telescopes of the Very Large Array near Socorro, New Mexico, USA. The galactic center itself is at the edge of the extremely bright object labeled Sagittarius (Sgr) A, suspected of harboring a million solar mass black hole. Along the galactic plane which runs diagonally through the image are tortured clouds of gas energized by hot stars and round-shaped supernova remnants (SNRs) - hallmarks of a violent and energetic cosmic environment. But perhaps most intriguing are the arcs, threads, and filaments which abound in the scene. Their uncertain origins challenge present theories of the dynamics of the galactic center.

APOD: December 4, 1998 - Centaurus A: The Galaxy Deep Inside
Explanation: Deep inside Centaurus A, the closest active galaxy to Earth, lies ... another galaxy! Cen A is a giant elliptical galaxy a mere 10 million light-years distant with a central jumble of stars, dust, and gas that probably hides a massive black hole. This composite combines an optical picture of Cen A with dark lines tracing lobes of radio emission and an infrared image from the ISO satellite (in red). The ISO data maps out the dust in what appears to be a barred spiral galaxy about the size of the prominent nearby spiral M33. The discoverers believe that the giant elliptical's gravity helps this barred spiral galaxy maintain its shape. In turn, material funneled along the spiral's bar fuels the central black hole which powers the elliptical's radio lobes. This apparently intimate association between two distinct and dissimilar galaxies suggests a truly cosmic symbiotic relationship.

APOD: November 28, 1998 - A Lonely Neutron Star
Explanation: How massive can a star get without imploding into a black hole? These limits are being tested by the discovery of a lone neutron star in space. Observations by the Hubble Space Telescope have been combined with previous observations by the X-ray ROSAT observatory and ultraviolet EUVE observatory for the isolated star at the location of the arrow. Astronomers are able to directly infer the star's size from measurements of its unblended brightness, temperature, and an upper limit on the distance. Assuming that the object is a neutron star of typical mass, some previous theories of neutron star structure would have predicted an implosion that would have created a black hole. That this neutron star even exists therefore allows a window to the extreme conditions that exist in the interiors of neutron stars.

APOD: October 23, 1998 - Seyfert Galaxy NGC 7742
Explanation: This might resemble a fried egg you've had for breakfast, but it's actually much larger. In fact, ringed by blue-tinted star forming regions and faintly visible spiral arms, the yolk-yellow center of this face-on spiral galaxy, NGC 7742, is about 3,000 light-years across. About 72 million light-years away in the constellation Pegasus, NGC 7742 is known to be a Seyfert galaxy - a type of active spiral galaxy with a center or nucleus which is very bright at visible wavelengths. Across the spectrum, the tremendous brightness of Seyferts can change over periods of just days to months and galaxies like NGC 7742 are suspected of harboring massive black holes at their cores. This beautiful color picture is courtesy of the newly inaugurated Hubble Space Telescope Heritage Project.

APOD: July 29, 1998 - The High Energy Heart Of The Milky Way
Explanation: These high resolution false color pictures of the Galactic center region in high energy X-ray and gamma-ray light result from a very long exposure of roughly 3,000 hours performed from 1990 to 1997 by the French SIGMA telescope onboard the Russian GRANAT spacecraft. Each image covers a 14x14 degree field which includes most of the central bulge of our Milky Way Galaxy. The X-ray picture (left) reveals a cluster of sources releasing enormous amounts of energy. They are probably binary star systems where matter accretes onto a collapsed object, either a neutron star or a black hole. But according to recent theories, only those binary systems with black holes can radiate above X-ray energies -- in the gamma-ray regime. In that case, the SIGMA sources also shining in the gamma-ray picture (right) betray the presence of accreting stellar black holes! Surprisingly, no high energy source seems to coincide exactly with the Galactic center itself, located near the brightest source at the bottom of both pictures. This indicates that the large black hole thought to be lurking there is unexpectedly quiet at these energies.

APOD: June 22, 1998 - The Doomed Dust Disk of NGC 7052
Explanation: What created the dust disk in the center of NGC 7052, and what keeps it spinning? Although the disk might appear as a relatively tame "hubcap in space", the unusual center of elliptical galaxy NGC 7052 is probably the remnant of a titanic collision between galaxies. What's more, the disk's spin indicates the tremendous gravity of a massive central black hole. Analysis of this recently released photo by the Hubble Space Telescope indicates that the disk is thousands of light-years across, rotates faster than 100 kilometers per second, at a distance of 150 light-years from the center, and contains more mass than a million Suns. The theorized central black hole is thought to be yet 100 times more massive, and may swallow the entire disk in the next few million years.

APOD: May 22, 1998 - The Center of Centaurus A
Explanation: A fantastic jumble of young blue star clusters, gigantic glowing gas clouds, and imposing dark dust lanes surrounds the central region of the active galaxy Centaurus A. This mosaic of Hubble Space Telescope images taken in blue, green, and red light has been processed to present a natural color picture of this cosmic maelstrom. Infrared images from the Hubble have also shown that hidden at the center of this activity are what seem to be disks of matter spiraling into a black hole with a billion times the mass of the Sun! Centaurus A itself is apparently the result of a collision of two galaxies and the left over debris is steadily being consumed by the black hole. Astronomers believe that such black hole "central engines" generate the radio, X-ray, and gamma-ray energy radiated by Centaurus A and other active galaxies. But for an active galaxy Centaurus A is close, a mere 10 million light-years away, and is a relatively convenient laboratory for exploring these powerful sources of energy.

APOD: May 7, 1998 - A Powerful Gamma Ray Burst
Explanation: Gamma-ray bursts are thought to be the most powerful explosions in the Universe, yet the cause of these high-energy flashes remains a mystery. Blindingly bright for space-based gamma-ray detectors the burst sources are so faint at visible wavelengths that large telescopes and sensitive cameras are required to search for them. The faint optical flash from a relatively intense gamma-ray burst detected on December 14th of last year seems to have originated in the galaxy indicated in this Hubble Space Telescope image - taken months after the burst had faded from view. Astronomers have recently announced that this galaxy's spectrum, recorded using the large Keck telescope atop Hawaii's Mauna Kea, indicates that it lies at a distance of about 12 billion light-years. The energy required to produce the observed flash of gamma-rays from this distance would be staggering! Some estimates suggest that in a few seconds the burster released the equivalent energy of several hundred supernovae (exploding stars). The eruption of such a large amount of energy in such a short time is so extreme that even exotic theoretical models of the bursters are being challenged. Could the bursts be caused by the cataclysmic merger of neutron stars with black holes ... or something as yet unknown?

APOD: April 20, 1998 - Name This Satellite
Explanation: Can you name this satellite? In December, NASA's third Great Observatory is planned for launch. The two NASA Great Observatories currently in orbit are the Hubble Space Telescope and the Compton Gamma-Ray Observatory, both now named for famous scientists. But after whom should the Advanced X-ray Astrophysics Facility (AXAF) be named? If your submitted suggestion conforms with contest rules and is chosen, you will have named the most powerful X-ray satellite ever built, and may even win a prize. AXAF is the size of a bus, has strange mirrors polished to atomic smoothness, and will produce X-ray images five times clearer of objects twice as faint as any previous X-ray satellite. This should allow AXAF the ability to see X-rays emitted near small black holes, from distant active galaxies, and inside huge clusters of galaxies. Astronomers now hope for an uneventful launch, routine operations, and spectacular discoveries.

APOD: March 21, 1998 - The Gamma Ray Sky
Explanation: What if you could "see" gamma rays? If you could, the sky would seem to be filled with a shimmering high-energy glow from the most exotic and mysterious objects in the Universe. In the early 1990s NASA's orbiting Compton Observatory, produced this premier vista of the entire sky in gamma rays - photons with more than 40 million times the energy of visible light. The diffuse gamma-ray glow from the plane of our Milky Way Galaxy runs horizontally through the false color image. The brightest spots in the galactic plane (right of center) are pulsars - spinning magnetized neutron stars formed in the violent crucibles of stellar explosions. Above and below the plane, quasars, believed to be powered by supermassive black holes, produce gamma-ray beacons at the edges of the universe. The nature of many of the fainter sources remains unknown.

APOD: February 23, 1998 - M104: The Sombrero Galaxy
Explanation: What's going on in the center of this spiral galaxy? Named the Sombrero Galaxy for its hat-like resemblance, M104 features a prominent dust lane and a bright halo of stars and globular clusters. Something truly energetic is going on in the Sombrero's center, as it not only appears bright in visible light, but glows prodigiously in X-ray light as well. This X-ray emission coupled with unusually high central stellar speeds cause many astronomers to speculate that a black hole lies at the Sombrero's center - a black hole possibly a billion times the mass of our Sun.

APOD: January 17, 1998 - At The Core Of M15
Explanation: Densely packed stars in the core of the globular cluster M15 are shown in this Hubble Space Telescope (HST) image. The star colors roughly indicate their temperatures - hot stars appear blue, cooler stars look reddish-orange. The region visible here is only about 1.6 light-years across, compared to the 4.3 light-year distance to our own Sun's nearest neighbor. Imagine the night sky viewed from a planet orbiting a star near this cluster's center! M15 has long been recognized as one of the densest clusters of stars in our galaxy outside of the galactic center itself. Even the unprecedented resolving power of the HST cameras could not separate the individual stars in its innermost regions. However, this HST image reveals that the density of stars continues to rise toward the cluster's core, suggesting that a sudden, runaway collapse due to the gravitational attraction of many closely packed stars or a single central massive object, perhaps a black hole, could account for the core's extreme density.

APOD: December 6, 1997 - A Quasar Portrait Gallery
Explanation: QUASARs (QUASi-stellAR objects) lie near the edge of the observable Universe. Discovered in 1963, astronomers were astounded that such objects could be visible across billions of light-years, as this implies they must emit prodigious amounts of energy. Where does the energy come from? Many believe the quasar's central engine is a giant black hole fueled by tremendous amounts of infalling gas, dust, and stars. This gallery of quasar portraits from the Hubble Space Telescope offers a look at their local neighborhoods: the quasars themselves appear as the bright star-like objects with diffraction spikes. The images in the center and right hand columns reveal quasars associated with disrupted colliding and merging galaxies which should provide plenty of debris to feed a hungry black hole.

APOD: December 2, 1997 - Micro-Quasar GRS1915 Puffs
Explanation: On the far side of our Galaxy, gas clouds explode away from a small black hole. This might seem peculiar, as black holes are supposed to attract matter. But material falling toward a black hole collides and heats up, creating an environment similar to a quasar that is far from stable. In the above time-lapse sequence, micro-quasar GRS1915 expels bubbles of hot gas in spectacular jets. These computer enhanced radio images show one plasma bubble coming almost directly toward us at 90 percent the speed of light, and another moving away. Each of the four frames marks the passage of one day. Originally detected on October 29th, these bubbles have now faded from view.

APOD: November 24, 1997 - Jet Near Light Speed
Explanation: Einstein's Special Theory of Relativity says that nothing can travel faster than the speed of light. Jets of protons and electrons that shoot away from objects such as quasars and black holes appear to travel at speeds approaching this maximum speed, though. Such jets carry tremendous energy and can ram straight through interstellar material. In the above frame from a computer simulation, a jet traveling only 98 percent of light speed rams and mixes with interstellar material. Even higher energy jets might well explain the structure seen around Cygnus A.

APOD: November 7, 1997 - Evidence for Frame Dragging Black Holes
Explanation: Gravity can do more than floor you. According to recent measurements of a star system thought to contain a black hole, it can spin you too. This effect, called frame-dragging, is most prominent near massive, fast spinning objects. Now, a team led by W. Cui (MIT) has used the orbiting Rossi X-ray Timing Explorer to search for it near a system thought to contain a black hole. Cui's team claim that matter in this system gets caught up and spun around the black hole at just the rate expected from frame-dragging. Such discoveries help scientists better understand gravity itself.

APOD: October 19, 1997 - The Heart Of NGC 4261
Explanation: What evil lurks in the hearts of galaxies? This Hubble Space Telescope picture of the center of the nearby elliptical galaxy NGC 4261 tells one dramatic tale. The gas and dust in this disk are swirling into what is almost certainly a massive black hole. The disk is probably what remains of a smaller galaxy that fell in hundreds of millions of years ago. Collisions like this may be a common way of creating such active galactic nuclei as quasars. Strangely, the center of this fiery whirlpool is offset from the exact center of the galaxy - for a reason that for now remains an astronomical mystery.

APOD: September 26, 1997 - A Lonely Neutron Star
Explanation: How massive can a star get without imploding into a black hole? These limits are being tested by the discovery of a lone neutron star in space. Observations by the Hubble Space Telescope released Wednesday, have been combined with previous observations by the X-ray ROSAT observatory and ultraviolet EUVE observatory for the isolated star at the location of the arrow. Astronomers are able to directly infer the star's size from measurements of its unblended brightness, temperature, and an upper limit on the distance. Assuming that the object is a neutron star of typical mass, some previous theories of neutron star structure would have predicted an implosion that would have created a black hole. That this neutron star even exists therefore allows a window to the extreme conditions that exist in the interiors of neutron stars.

APOD: September 12, 1997 - The Center of NGC 6251 is Glowing
Explanation: Something is lighting up the center of galaxy NGC 6251. Leading speculation holds that it is a large black hole not shrouded by gas and dust typically found near the center of a galaxy. Observations taken with the Hubble Space Telescope and released earlier this week indicate a new perspective on the strange beasts that rule the centers of galaxies: a bright central object that is illuminating a surrounding material disk, shown in blue. The lack of reflection from the upper part of the disk indicates that this disk is warped in shape. Although not visible in the above composite image, a huge plasma jet streams out from the central object, perpendicular to the warped disk.

APOD: June 17, 1997 - Arp 220: Spirals in Collision
Explanation: Arp 220 is the brightest object in the local universe. But why does it shine so brightly? Arp 220 was cataloged as a peculiar galaxy in the 1960s. In the late 1980s, it was discovered to be an ultraluminous infrared galaxy and headed a list compiled from observations with the now-defunct IRAS satellite. New observations with the Hubble Space Telescope are quite revealing. Photos by NICMOS in the infrared taken in April and released just last week now better resolve the two colliding spiral galaxies at the center of Arp 220. A result of this spiral collision are fantastic knots of new star formation visible as the bright spots on the above photograph. Below the "half-moon" shaped knot on the right is a massive disk of dust possibly hiding a dying spiral's central black hole. The bright knot to the left is the center of the other broken spiral galaxy. The galaxy cores are about 1200 light years apart and are orbiting each other.

APOD: June 13, 1997 - Streaming From A Black Hole
Explanation: Glowing gas clouds are streaming from the core of galaxy NGC4151 at hundreds of thousands of miles per hour. A powerful tool, the Hubble Space Telescope's new STIS instrument, makes it possible to map out the cloud velocities - producing this false color "velocity map" for the central regions of NGC4151. The horizontal line is light from the intensely bright region near the galaxy nucleus. Emission at two wavelengths characteristic of Oxygen atoms in the gas clouds is visible along this line. Below the line the emission is displaced to the left, indicating motion toward us (blue shift); above the displacement is to the right indicating a receding motion (red shift). Where do the clouds come from? As evidence mounts, the widely accepted explanation for energetic nuclear activity in galaxies is based on material spiraling into a central black hole with over a million times the mass of our sun. The rotating disk of interstellar debris which develops is thought to blast out high velocity jets along the axis of the disk. Do all galaxies contain supermassive black holes?

APOD: May 16, 1997 - Signed, "A Black Hole"
Explanation: This artistic image is actually the signature of a supermassive black hole in the center of distant galaxy M84 - based on data recently recorded by Hubble's new Space Telescope Imaging Spectrograph (STIS). Very near black holes the force of gravity is so strong that even light can not escape ... but the presence of a black hole can also be revealed by watching matter fall into it. In fact, material spiraling into a black hole would find its speed increasing at a drastic rate. These extreme velocity increases provide a "signature" of the black hole's presence. STIS relies on the Doppler effect to measure gas velocity rapidly increasing to nearly 240 miles per second within 26 light years of the center of M84, a galaxy in the Virgo Cluster about 50 million light years away. The STIS data show that radiation from approaching gas, shifted to blue wavelengths left of the centerline, is suddenly redshifted to the right of center indicating a rapidly rotating disk of material near the galactic nucleus. The resulting sharp S-shape is effectively the signature of a black hole estimated to contain at least 300 million solar masses. Do all galaxies have central black holes?

APOD: May 1, 1997 - A Galactic Cloud of Antimatter
Explanation: The center of our Milky Way Galaxy is full of surprises. Its latest spectacular is a mysterious cloud glowing in gamma rays produced by annihilating antimatter particles! Star Trek fans are all too familiar with the consequences of mixing matter (electrons) and antimatter (positrons) - the particles catastrophically annihilate converting their masses to energy according to Einstein's famous E=mc². Positron/electron annihilation energy is emitted as gamma rays with photon energies of 511,000 electron volts. Searching for these high energy photons, the OSSE instrument onboard NASA's orbiting Compton Gamma Ray Observatory has recently produced this map of the Galactic Center (GC) region. As anticipated, it shows annihilation gamma rays as a bright spot at the GC with fainter horizontal emission from the galactic plane. Astoundingly, it also reveals a large and unexpected cloud of annihilation radiation, probably about 4,000 light years across, extending nearly 3,500 light years above the GC. What could have created this cloud? Associated with no previously known object, it seems to imply that a fountain of antimatter positrons streams from the GC. Present guesses about the source of the positrons include the violent and exotic environments surrounding starbirth, neutron star collisions, and black holes at the GC. Are there other such clouds in our Galaxy?

APOD: April 13, 1997 - Jets from SS433
Explanation: SS433 is one of the most exotic star systems known. Its unremarkable name stems from its inclusion in a catalog of stars which emit radiation characteristic of atomic hydrogen. Its very remarkable behavior stems from a compact object, a black hole or neutron star, which has produced an accretion disk with jets. As illustrated in this artist's vision of the SS433 system based on observational data, a massive, hot star (left) is locked in a mutual orbit with a compact object. Material transfers from the massive star into an accretion disk surrounding the compact object blasting out two jets of ionized gas in opposite directions - at about 1/4 the speed of light! Radiation from the jet tilted toward the observer is blueshifted, while radiation from the jet tilted away is redshifted. The binary system itself completes an orbit in about 13 days while the jets precess (wobble like a top) with a period of about 164 days. Are the jets from SS433 related to those from black holes at the centers of galaxies?

APOD: April 5, 1997 - A Black Hole in M87?
Explanation: The center of nearby giant galaxy M87 is a dense and violent place. In this 1994 photograph by the Hubble Space Telescope, a disk of hot gas was found to be orbiting at the center of this massive elliptical galaxy. The disk is evident at the lower left of the picture. The rotation speed of gas in this disk indicates the mass of the object the gas is orbiting, while the size of the disk indicates an approximate volume of the central object. These observations yield a central density so high that the only hypothesized object that could live there is a black hole. The picture also shows a highly energetic jet emanating from the central object like a cosmic blowtorch. The jet is composed of fast moving charged particles and has broken into knots as small as 10 light years across.

APOD: February 22, 1997 - The Gamma Ray Sky
Explanation: What if you could "see" gamma rays? If you could, the sky would seem to be filled with a shimmering high-energy glow from the most exotic and mysterious objects in the Universe. In the early 1990s NASA's orbiting Compton Observatory, produced this premier vista of the entire sky in gamma rays - photons with more than 40 million times the energy of visible light. The diffuse gamma-ray glow from the plane of our Milky Way Galaxy runs horizontally through the false color image. The brightest spots in the galactic plane (left of center) are pulsars - spinning magnetized neutron stars formed in the violent crucibles of stellar explosions. Above and below the plane, quasars, believed to be powered by supermassive black holes, produce gamma-ray beacons at the edges of the universe. The nature of many of the fainter sources remains unknown.

APOD: February 8, 1997 - M104: The Sombrero Galaxy
Explanation: The famous Sombrero galaxy (M104) is a bright nearby spiral galaxy. The prominent dust lane and halo of stars and globular clusters give this galaxy its name. Something very energetic is going on in the Sombrero's center, as much X-ray light has been detected from it. This X-ray emission coupled with unusually high central stellar velocities cause many astronomers to speculate that a black hole lies at the Sombrero's center - a black hole a billion times the mass of our Sun.

APOD: January 21, 1997 - Journey to the Center of the Galaxy
Explanation: In Jules Verne's science fiction classic A Journey to the Center of the Earth, Professor Hardwigg and his fellow explorers encounter many strange and exciting wonders. What wonders lie at the center of our Galaxy? Astronomers now know of some of the bizarre objects which exist there, like vast dust clouds, bright young stars, swirling rings of gas, and possibly even a large black hole. Much of the Galactic center region is shielded from our view in visible light by the intervening dust and gas. But it can be explored using other forms of electromagnetic radiation, like radio, infrared, X-rays, and gamma rays. This beautiful high resolution image of the Galactic center region in infrared light was made by the SPIRIT III telescope onboard the Midcourse Space Experiment. The center itself appears as a bright spot near the middle of the roughly 1x3 degree field of view, the plane of the Galaxy is vertical, and the north galactic pole is towards the right. The picture is in false color - starlight appears blue while dust is greenish grey, tending to red in the cooler areas.

APOD: January 15, 1997 - Black Hole Signature From Advective Disks
Explanation: What does a black hole look like? If alone, a black hole would indeed appear quite black, but many black hole candidates are part of binary star systems. So how does a black hole binary system look different from a neutron star binary system? The above drawings indicate it is difficult to tell! Recent theoretical work, however, has provided a new way to tell them apart: advective accretion flows (ADAFs). A black hole system so equipped would appear much darker than a similar neutron star system. The difference is caused by the hot gas from the ADAF disk falling through the event horizon of the black hole and disappearing - gas that would have emitted much light were the central object only a neutron star. Recent observations of the soft X-ray transient V404 Cyg has yielded a spectrum much like an ADAF onto a black hole - and perhaps brighter than allowable from an ADAF onto a neutron star.

APOD: January 14, 1997 - Black Holes and Galactic Centers
Explanation: Do all galaxies have black holes at their centers? Although not even a single galaxy has yet been proven to have a central black hole, the list of candidates has increased yet again. Recent results by astronomers using the Hubble Space Telescope now indicate that most - and possibly even all - large galaxies may harbor one of these dense beasts. In all the galaxies studied, star speeds continue to increase closer the very center. This in itself indicates a center millions of times more massive than our Sun is needed to contain the stars. This mass when combined with the limiting size make the case for the central black holes. Will we ever know for sure?

APOD: January 5, 1997 - Too Close to a Black Hole
Explanation: What would you see if you went right up to a black hole? Above are two computer generated pictures highlighting how strange things would look. On the left is a normal star field containing the constellation Orion. Notice the three stars of nearly equal brightness that make up Orion's Belt. On the right is the same star field but this time with a black hole superposed in the center of the frame. The black hole has such strong gravity that light is noticeably bent towards it - causing some very unusual visual distortion. In the distorted frame, every star in the normal frame has at least two bright images - one on each side of the black hole. In fact, near the black hole, you can see the whole sky - light from every direction is bent around and comes back to you. Black holes are thought to be the densest state of matter, and there is indirect evidence for their presence in stellar binary systems and the centers of globular clusters, galaxies, and quasars.

APOD: January 4, 1997 - A Star Where Photons Orbit
Explanation: The above computer animated picture depicts how a very compact star would look to a nearby observer. The star pictured is actually more compact that any known except a black hole, so it is only hypothetical. The observer is situated at the photon sphere, where photons can orbit in a circle. To help the viewer better visualize the great distortions created by gravity, a map of the Earth was projected onto the star, and a map of the familiar night sky was projected above. From here one can either look down and see several duplicate images of the entire surface of the star, look up and see several duplicate images of the entire night sky, or look along the photon sphere and see the back of one's own head.

APOD: November 25, 1996 - A Quasar Portrait Gallery
Explanation: QUASARs (QUASi-stellAR objects) lie near the edge of the observable Universe. Discovered in 1963, astronomers were astounded - to be visible at such extreme distances of billions of light-years they must emit prodigious amounts of energy. Where does the energy come from? Many believe the quasar's central engine is a giant black hole fueled by tremendous amounts of infalling gas, dust, and stars. This recently released gallery of quasar portraits from the Hubble Space Telescope offers a look at their local neighborhoods: the quasars themselves appear as the bright star-like objects with diffraction spikes. The images in the center and right hand columns reveal quasars associated with disrupted colliding and merging galaxies which should provide plenty of debris to feed a hungry black hole. Yet, in the left hand column a quasar is seen at the center of an otherwise normal looking spiral (above) and elliptical galaxy. Whatever the secret of the quasar's energy, all these sites must provide fuel for its central engine.

APOD: November 1, 1996 - Spiral Galaxy NGC 3628 Edge On
Explanation: This is what a spiral galaxy looks like sideways. This view of NGC 3628 nearly resembles our own Milky Way Galaxy, which is also known to be a spiral. The dark band across the center is absorbed starlight caused by the galaxy's own interstellar dust. NGC 3628 is the faintest member of the Leo Triplet, a group of galaxies dominated by M65 and M66. The Leo Triplet lies about 35 million light years distant. The center of NGC 3628 emits variable X-ray radiation perhaps indicating the presence of a massive black hole.

APOD: October 11, 1996 - The Double Nucleus of M31
Explanation: The center of M31 is twice as unusual as previously thought. In 1991 the Planetary Camera then onboard the Hubble Space Telescope pointed toward the center of our Milky Way's closest major galactic neighbor: Andromeda (M31). To everyone's surprise, M31's nucleus showed a double structure. The nuclear hot-spots are quite close together when considering Galactic distances: M31 is about 150,000 light years across while the above shows only the central 30 light-years. Subsequent ground-based observations have led to speculation that indeed two nuclei exist, are moving with respect to each other, that one nucleus is slowly tidally disrupting the other, and that one nucleus may be the remains of smaller galaxy "eaten" by M31. The nuclei of many galaxies, including M31, are known to be quite violent places, and the existence of massive black holes are frequently postulated to explain them.

APOD: September 11, 1996 - In the Center of Spiral M77
Explanation: What is happening in the center of nearby spiral galaxy M77? To find out, astronomers used the Hubble Space Telescope to peer deep into the dusty chaos of this active galactic nucleus in 1994. They found a network of filamentary gas and opaque dust that provides only clues as to what central monster had left this mess. Due to the presence of hot ionized gas clouds near the core, changes in brightness that can take less than a week, and the ultraviolet halo surrounding the whole galaxy, the leading hypothesis is that a supermassive black hole lies at the center of this Seyfert Type 2 galaxy. Also known as NGC 1068, this galaxy lies only about 50 million light years distant and is visible with only a small telescope.

APOD: September 10, 1996 - M77: Spiral with a Strange Glow
Explanation: Why is M77 surrounded by an ultraviolet glow? M77, also called NGC 1068, appears at first sight to be a relatively normal barred spiral galaxy. But when photographed in the ultraviolet (UV), as shown above in false color, the galaxy sports an ultraviolet halo - shown as violet in the photograph. The blue spiral structure closer to the picture's center indicates normal ultraviolet emission from bright young stars that have recently formed there. Astronomers now hypothesize that the outer glow arises from UV light emitted from the galaxy's active center and reflected to us from clouds of gas and dust. These same gas and dust clouds obscure the active center of this Seyfert galaxy - where an ultramassive black hole is thought to live.

APOD: August 24, 1996 - Why is QSO 1229+204 so Bright?
Explanation: What causes the center of this barred spiral galaxy to light up brighter than almost anything in the universe? The quasar there is a good fraction of the way across our observable universe but appears so bright that astronomers had to use the high resolving power of the Hubble Space Telescope (HST) just to see the host galaxy. HST then resolved something very interesting. Not only was QSO 1229+204 at the core of an unusual barred spiral galaxy, but this galaxy was in the process of colliding with a dwarf galaxy. Gas from this collision quite possibly fuels a supermassive black hole causing QSO 1229+204 to shine so brightly.

APOD: August 18, 1996 - A Milestone Quasar
Explanation: Here is a rather typical quasar. But since quasars are so unusual it is quite atypical of most familiar objects. Of the two bright objects in the center of this photo, the quasar is on the left. The bright image to quasar's right is a star, the faint object just above the quasar is an elliptical galaxy, with an apparently interacting pair of spiral galaxies near the top. Quasars appear as unresolved points of light, as do stars, and hence quasars were thought to be a type of star until the 1960s. We now know that the brightest quasars lie far across the visible universe from us, and include the most distant objects known. Quasars may occupy the centers of galaxies and may even be much brighter than their host galaxies. In fact, the centers of many nearby galaxies have similarities to quasars - including the center of our own Milky Way Galaxy. The exact mechanism responsible for a quasar's extreme brightness is unknown, but thought to involve supermassive black holes. This picture represents a milestone for the six-year-old Hubble Space Telescope as it was picture number 100,000, taken on June 22, 1996.

APOD: August 4, 1996 - NGC 3393: A Super Spiral?
Explanation: A bird? A plane? No, but pictured here is something physically much larger, flying much higher, and moving much faster than either of these. It is, in fact, a Seyfert type 2 spiral galaxy. The "S" is actually a lane of stars, gas and dust circling the core. Designated NGC 3393, the bright core makes this galaxy a Seyfert and the infrared glow of central dust help distinguish it as "type 2." Seyfert galaxies have extremely energetic nuclei similar to more powerful quasars. Seyferts are thought to have black holes in their centers. Most of the lines and small spots in this image are due to cosmic rays striking the imager and are unrelated to structure in the galaxy.

APOD: May 29, 1996 - The COMPTEL Gamma-Ray Sky
Explanation: This premier gamma-ray view of the sky was produced by the COMPTEL instrument onboard NASA's orbiting Compton Gamma Ray Observatory. The entire sky is seen projected on a coordinate system centered on our Milky Way Galaxy with the plane of the Galaxy running across the middle of the picture. Gamma-ray intensity is represented by a false color map - low (blue) to high (white). COMPTEL's sensitivity to gamma-rays which have over 1 million times the energy of visible light photons reveals the locations of some of the Galaxy's most exotic objects. The brightest source, the Crab pulsar, is located near the plane of the Galaxy on the far right. Moving along the plane from the Crab, more than halfway toward the galactic center, another bright gamma-ray source, the Vela pulsar, appears. The galactic center itself, along with the famous black hole candidate Cygnus X-1 (near the plane, halfway from the center to the left edge) are also seen as bright sources. Both above and below the plane, spots of gamma-ray emission due to distant active galaxies are also visible.

APOD: May 15, 1996 - The Milky Way Near the Northern Cross
Explanation: This beautiful image of the sky near the bright star Deneb (just above center) reveals the stars, nebulae, and dark clouds along the plane of our Milky Way Galaxy as seen from the northern hemisphere (near Columbia Missouri, USA). Just below Deneb lies the suggestively shaped North American emission nebula. Deneb is the brightest star in the constellation Cygnus, located in the tail of this celestial swan. Cygnus contains the asterism known as the Northern Cross and marks one side of the "Great Rift" in the Milky Way, a series of dark obscuring dust clouds which stretches on through the constellation Sagittarius. Deneb defines the top of the Northern Cross while the body of the cross extends past the upper right corner of the picture. Cygnus also harbors the most famous candidate for a black hole in our galaxy, Cygnus X-1.

APOD: May 5, 1996 - Planet Near a Galaxy Core
Explanation: What would the night sky look like if you lived on a planet near the center of a galaxy? Now imagine that this galaxy houses a black hole billions of times more massive than a star. From this spectacular vantage point, the sky might look like the above illustration. This drawing is based on recent observations of the center of NGC 4261, made by the Hubble Space Telescope. Results indicate that a disk of dust 800-light years wide surrounds the black hole. The hypothetical planet depicted above lies within this disk. The black hole itself heats gas to white-hot temperatures, generating light that is reddened when reflected off the dust. Jets shoot off from the poles of the black hole, perpendicular to the disk. However, friction with the dust and gas would cause planets near the black hole to spiral in and disappear forever. NASA has recently announced a new initiative to search for Earth-like planets in our Galaxy.

APOD: March 6, 1996 - Jets From SS433
Explanation: SS433 is one of the most exotic star systems known to astronomers. Its unremarkable name stems from its inclusion in a catalog of stars which emit radiation characteristic of atomic hydrogen. Its very remarkable behavior stems from a compact object, a black hole or neutron star, which has produced an accretion disk with jets. As illustrated in this artist's vision of the SS433 system based on observational data, a massive, hot star (left) is locked in a mutual orbit with a compact object. Material transfers from the massive star into an accretion disk surrounding the compact object blasting out two jets of ionized gas in opposite directions - at about 1/4 the speed of light! Radiation from the jet tilted toward the observer is blueshifted, while radiation from the jet tilted away is redshifted. The binary system itself completes an orbit in about 13 days while the jets precess (wobble like a top) with a period of about 164 days. Are the jets from SS433 related to those from black holes at the centers of galaxies?

APOD: March 5, 1996 - A Black Hole in M87's Center?
Explanation: The center of nearby giant galaxy M87 is a dense and violent place. In this 1994 photograph by the Hubble Space Telescope, a disk of hot gas was found to be orbiting at the center of this massive elliptical galaxy. The disk is evident on the lower left of the above photograph. The rotation speed of gas in this disk indicates the mass of the object the gas is orbiting, while the size of the disk indicates an approximate volume of the central object. These observations yield a central density so high that the only hypothesized object that could live there is a black hole. The picture also shows a highly energetic jet emanating from the central object. The jet is composed of fast moving charged particles and has broken into knots as small as 10 light years across.

APOD: January 3, 1996 - The X-ray Timing Explorer
Explanation: Launched Saturday on a Delta rocket, the X-ray Timing Explorer (XTE) will watch the sky for rapid changes in X-rays. XTE carries three separate X-ray telescopes. The Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE) will provide the best timing information in the widest X-ray energy range yet available. They will observe stellar systems that contain black holes, neutron stars, and white dwarfs as well as study the X-ray properties of the centers of active galaxies. XTE's All Sky Monitor (ASM) will scan the sky every 90 minutes to find new X-ray transients and track the variability of old ones. XTE has a planned life time of two years.

Tomorrow's picture: Symbiotic Star System R Aquarii

APOD: January 2, 1996 - The X-Ray Sky
Explanation: What if you could see X-rays? If you could, the night sky would be a strange and unfamiliar place. X-rays are about 1,000 times more energetic than visible light photons and are produced in violent and high temperature astrophysical environments. Instead of the familiar steady stars, the sky would seem to be filled with exotic binary star systems composed of white dwarfs, neutron stars, and black holes, along with flare stars, X-ray bursters, pulsars, supernova remnants and active galaxies. This X-ray image of the entire sky was constructed with Skyview, using data from the first High Energy Astronomy Observatory (HEAO 1), and plotted in a coordinate system centered on the galactic center with the north galactic pole at the top. Sources near the galactic center are seen to dominate in this false color map which shows regions of highest X-ray intensity in yellow. Astronomers' ability to observe the sky at X-ray energies will be greatly enhanced by the recently launched X-ray Timing Explorer (XTE) satellite.

Tomorrow's picture: The X-ray Timing Explorer

APOD: December 30, 1995 - LMC X-1: A Black Hole Candidate
Explanation: The strongest source of X-rays in the Large Magellanic Cloud originates from an unusually energetic binary star system. This strong source, dubbed LMC X-1, is thought to be a normal and compact star orbiting each other. Gas stripped of the normal star falls onto the compact star, heats up, and emits X-rays. The X-rays shining from the system knock electrons off atoms for light years around, causing some atoms to glow noticeably in X-rays when the electrons re-combine. Motion in the binary system indicates the compact star is probably a black hole, since its high mass - roughly five times that of our Sun - should be enough to cause even a neutron star to implode.

Tomorrow's picture: The X-ray Sources of M31

APOD: December 26, 1995 - Accretion Disk Binary System
Explanation: Our Sun is unusual in that it is alone - most stars occur in multiple or binary systems. In a binary system, the higher mass star will evolve faster and will eventually become a compact object - either a white dwarf star, a neutron star, or black hole. When the lower mass star later evolves into an expansion phase, it may be so close to the compact star that its outer atmosphere actually falls onto the compact star. Such is the case diagrammed above. Here gas from a blue giant star is shown being stripped away into an accretion disk around its compact binary companion. Gas in the accretion disk swirls around, heats up, and eventually falls onto the compact star. Extreme conditions frequently occur on the surface of the compact star as gas falls in, many times causing detectable X-rays, gamma-rays, or even cataclysmic novae explosions. Studying the extreme conditions in these systems tells us about the inner properties of ordinary matter around us.

Tomorrow's picture: Nova Cygni 1992

APOD: December 5, 1995 - The Swirling Center of NGC 4261
Explanation: What evil lurks in the hearts of galaxies? The above picture by the Hubble Space Telescope of the center of the nearby galaxy NGC 4261 tells us one dramatic tale. Here gas and dust are seen swirling near this elliptical galaxy's center into what is almost certainly a massive black hole. The disk is probably what remains of a smaller galaxy that fell in hundreds of millions of years ago. Collisions like this may be a common way of creating such active galactic nuclei as quasars. Strangely, the center of this fiery whirlpool is offset from the exact center of the galaxy - for a reason that for now remains an astronomical mystery.

Tomorrow's picture: 24 Hours from Jupiter

APOD: November 29, 1995 - Releasing Compton
Explanation: Named for Nobel laureate physicist Arthur Holly Compton, the Compton Gamma Ray Observatory (CGRO) Satellite was launched in April of 1991 aboard the Space Shuttle Atlantis. CGRO's mission is to explore the Universe at gamma-ray energies. The massive space based observatory is seen here held upright in the shuttle payload bay behind smiling astronaut Jerry Ross. Ross and his colleague Jay Apt have just finished a successful, unplanned spacewalk to free a jammed antenna prior to releasing CGRO into orbit. CGRO has been operating successfully since, providing the first all-sky survey at gamma-ray energies along with exciting new observations of the sun, quasars, pulsars, supernova, black holes, and gamma-ray bursts.

APOD: November 27, 1995 - Too Close to a Black Hole
Explanation: What would you see if you went right up to a black hole? Above are two computer generated pictures highlighting how strange things would look. On the left is a normal star field containing the constellation Orion. Notice the three stars of nearly equal brightness that make up Orion's Belt. On the right is the same star field but this time with a black hole superposed in the center of the frame. The black hole has such strong gravity that light is noticeably bent towards it - causing some very unusual visual distortion. In the distorted frame, every star in the normal frame has at least two bright images - one on each side of the black hole. In fact, near the black hole, you can see the whole sky - light from every direction is bent around and comes back to you. Black holes are thought to be the densest state of matter, and there is indirect evidence for their presence in stellar binary systems and the centers of globular clusters, galaxies, and quasars.

Tomorrow's picture: Shadow at the Lunar South Pole

APOD: November 26, 1995 - A Star Where Photons Orbit
Explanation: The above computer animated picture depicts how a very compact star would look to a nearby observer. The star pictured is actually more compact that any known except a black hole, so it is only hypothetical. The observer is situated at the photon sphere, where photons can orbit in a circle. To help the viewer better visualize the great distortions created by gravity, a map of the Earth was projected onto the star, and a map of the familiar night sky was projected above. From here one can either look down and see several duplicate images of the entire surface of the star, look up and see several duplicate images of the entire night sky, or look along the photon sphere and see the back of one's own head.

Tomorrow's picture: Too Near a Black Hole

APOD: November 20, 1995 - At the Core of M15
Explanation: Densely packed stars in the core of the globular cluster M15 are shown in this Hubble Space Telescope (HST) image taken in April of 1994. The stars revealed are contained in an area 1.6 light years across and their colors roughly indicate their temperatures - hot stars appear blue, cooler stars look reddish-orange. M15 has long been recognized as one of the densest cluster of stars in our galaxy outside of the galactic center itself. Even the unprecedented resolving power of the HST cameras could not separate the individual stars in its innermost regions. However, this HST image reveals that the density of stars continues to rise toward the cluster's core, suggesting that a sudden, runaway collapse due to the gravitational attraction of many closely packed stars or a single central massive object, perhaps a black hole, could account for the core's extreme density.

APOD: November 9, 1995 - M104: The Sombrero Galaxy
Explanation: The famous Sombrero galaxy (M104) is a bright nearby spiral galaxy. The prominent dust lane and halo of stars and globular clusters give this galaxy its name. Something very energetic is going on in the Sombrero's center, as much X-ray light has been detected from it. This X-ray emission coupled with unusually high central stellar velocities cause many astronomers to speculate that a black hole lies at the Sombrero's center - a black hole a billion times the mass of our Sun. This image was taken in blue light by the 0.9 meter telescope at Kitt Peak National Observatory.

APOD: October 27, 1995 - The Tarantula and the Supernova
Explanation: In this close-up of the Large Magellanic Cloud, the spidery looking nebula on the left is fittingly known as as the Tarantula nebula. It is an emission nebula surrounding a cluster of hot, young stars called the 30 Doradus super cluster. This cluster may contain the most massive stars known (about 50 times the mass of the Sun). Such massive stars put out more than 100 times as much energy as our Sun. The bright "star" (lower right) is actually Supernova 1987a and is a harbinger of things to come for the stars within the Tarantula. Massive stars burn their nuclear fuel at drastically enhanced rates to support their high energy output. As a result their lives last only a few million years compared to the Sun's few billions of years. They end in a spectacular death explosion, a supernova, like the star which exploded in 1987 as seen above. Supernovae may leave behind imploded stellar cores which form neutron stars or black holes.

APOD: October 22, 1995 - A Quasar-Galaxy Collision?
Explanation: In 1963 astronomers were astounded to discover that certain faint, star-like objects have very large redshifts. The large redshifts imply that these objects, now known as quasars (QUASi-stellAR objects), lie near the edge of the observable Universe. To be visible at such extreme distances of billions of light years, they must emit tremendous amounts of energy. Where does the energy come from? In the most widely accepted model, a quasar is the bright nucleus of an active galaxy powered by a central, supermassive black hole. This Hubble Space Telescope image shows a quasar known as PKS 2349 (the star-like object near the center) and a galaxy (surrounding fuzzy patch), but the quasar is not at the galaxy's center! In fact, the galaxy and quasar seem to be colliding or merging. This and other recent HST observations suggest that astronomers' standard ideas about quasars may be wrong.

APOD: September 1, 1995 - Subrahmanyan Chandrasekhar 1910-1995
Explanation: On August 21, 1995 one of the greatest astrophysicists of modern times passed away. Subrahmanyan Chandrasekhar was a creative, prolific genius whose ability to combine mathematical precision with physical insight changed humanity's view of stellar physics. His most famous discovery was that not all stars end up as white dwarf stars, but those retaining mass above a certain limit - today known as "Chandrasekhar's limit," undergo further collapse. His detailed mathematical papers and books on a wide variety of astrophysical subjects, including, for example, black holes, are classic references for research at every level. Obituaries are available from the University of Chicago Press and Reuters News Service, and a WWW page has been set-up to record personal memories.

Tomorrow's picture: Hot Gas and Dark Matter

APOD: August 10, 1995 - The Orbiting Hubble Space Telescope
Explanation: The Hubble Space Telescope (HST) is the largest orbiting public optical telescope in history. Its 2.4 meter diameter reflecting mirror and its perch above Earth's atmosphere allow it to create exceptionally sharp images. Originally launched in 1990, HST optics were repaired to their intended accuracy in 1993 by the first of several regular servicing missions. Astronomers using HST continue to make numerous monumental scientific discoveries, including new estimates of the age of our universe, previously unknown galaxies, evidence of massive black holes in the centers of galaxies, previously unknown moons, and a better understanding of physical processes in our universe.