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Explanation: Officially the Chandra Deep Field - South, this picture represents the deepest ever x-ray image of the Universe. One million seconds of accumulated exposure time with the orbiting Chandra X-ray. Observatory went in to its making. Concentrating on a single, otherwise unremarkable patch of sky in the constellation Fornax, this x-ray image corresponds to the visible light Hubble Deep Field - South released in 1998. Chandra's view, color coded with low energies in red, medium in green, and high-energy x-rays in blue, shows many faint sources of relatively high-energy x-rays. These are likely active galaxies feeding supermassive central black holes and large clusters of galaxies at distances of up to 12 billion light-years. The stunning picture supports astronomers' ideas of a youthful universe in which massive black holes were much more dominant than at present.
APOD: 2001 January
19 - Black Holes Are Black Q: Why are black holes black? A: Because they have an event horizon. The event horizon is that one-way boundary predicted by general relativity beyond which nothing, not even light, can return. X-ray astronomers using the space-based Chandra Observatory now believe they have direct evidence for event horizons - therefore black holes - in binary star systems which can be detected in x-ray light. These binaries, sometimes called x-ray novae, are known to consist of relatively normal stars dumping material on to massive, compact companions. As illustrated, the material swirls toward the companion in an accretion disk which itself glows in x-rays. If the compact companion is a neutron star (right), the material ultimately smashes into the solid surface and glows even more brightly in high energy x-rays. But if it is indeed a black hole with a defining event horizon, then the x-ray hot material approaches the speed of light as it swirls past the surface of no return and is lost from view. Recent work describes observations of two classes of x-ray binaries, one class 100 times fainter than the other. The results imply the presence of an event horizon in the fainter class which causes the extreme difference in x-ray brightness.
APOD: 2000 December
20 - Sgr A: Fast Stars Near the Galactic Center Why are these stars moving so fast? Shown above is a time-lapse movie in infrared light detailing how stars in the central light-year of our Galaxy have moved over the past eight years. The yellow mark at the image center represents the location of a peculiar radio source named Sgr A*. If these fast stars are held to the Galactic Center by gravity, then the central object exerting this gravity must be both compact and massive. Analysis of the stellar motions indicates that over one million times the mass of our Sun is somehow confined to a region less than a fifth of a light-year across. Astronomers interpret these observations as strong evidence that the center of our Galaxy is home to a very massive black hole.
APOD: 2000 December
10 - Too Close to a Black Hole What would you see if you went right up to a black hole? Above are two computer generated images 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 distortions. 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.
Explanation: Powerful forces are at play in the nearby Circinus Galaxy. Hot gas, colored pink, is being ejected out of the spiral galaxy from the central region. Much of Circinus' tumultuous gas, however, is concentrated in two rings. The outer ring, located about 700 light-years from the center, appears mostly red and is home to tremendous bursts of star formation. A previously unseen inner ring, inside the green disk above, is visible only 130 light years from the center on this recently released, representative color image taken by the Hubble Space Telescope. At the very center is an active galactic nucleus, where matter glows brightly before likely spiraling into a massive black hole. Although only 15 million light years distant, the Circinus Galaxy went unnoticed until 25 years ago because it is so obscured by material in the plane of our own Galaxy. The galaxy can be seen with a small telescope, however, in the constellation of Circinus.
Explanation: Amazingly detailed, this false-color x-ray image is centered on the galaxy Cygnus A. Recorded by the orbiting Chandra Observatory, Cygnus A is seen here as a spectacular high energy x-ray source. But it is actually more famous at the low energy end of the electromagnetic spectrum as one of the brightest celestial radio sources. Merely 700 million light-years distant, Cygnus A is the closest powerful radio galaxy and the false-color radio image (inset right) shows remarkable similarity to Chandra's x-ray view. Central in both pictures, the center of Cygnus A shines brightly while emission extends 300,000 light-years to either side along the same axis. Near light speed jets of atomic particles produced by a massive central black hole are believed to cause the emission. In fact, the x-ray image reveals "hot spots" suggestive of the locations where the particle jets are stopped in surrounding cooler, denser gas. The x-ray image also shows that the jets have cleared out a huge cavity in the surrounding gas. Bright swaths of emission within the cavity likely indicate x-ray hot material ... swirling toward the central black hole.
Explanation: Early on, x-ray satellites revealed a surprising cosmic background glow of x-rays and astronomers have struggled to understand its origin. Now, peering through a hole in the obscuring gas and dust of our own Milky Way Galaxy, the powerful orbiting XMM-Newton telescope has recorded this deep image of the x-ray sky, resolving some of the mysterious background into many faint individual sources. The tantalizing image is color-coded, with red representing relatively low energy x-rays, photons with 500 or so times the energy of visible light. Green and blue colors correspond to increasingly energetic x-rays with up to about 10,000 times visible light energies. Notably, the faint sources tend to be green and blue, showing x-ray characteristics of huge amounts of material falling into massive black holes in very distant galaxies. Do massive black holes reside in the hearts of all large galaxies? The XMM-Newton results add to the growing consensus that they do and that, from across the universe, x-rays produced as matter feeds these black holes account for the cosmic x-ray background.
Explanation: This haunting image from the orbiting Chandra Observatory reveals the Perseus Cluster of Galaxies in x-rays, photons with a thousand or more times the energy of visible light. Three hundred twenty million light-years distant, the Perseus Cluster contains thousands of galaxies, but none of them are seen here. Instead of mere galaxies, a fifty million degree cloud of intracluster gas, itself more massive than all the cluster's galaxies combined, dominates the x-ray view. From this angle, voids and bright knots in the x-ray hot gas cloud lend it a very suggestive appearance. Like eyes in a skull, two dark bubbles flank a bright central source of x-ray emission. A third elongated bubble (at about 5 o'clock) forms a toothless mouth. The bright x-ray source is likely a supermassive black hole at the cluster center with the bubbles blown by explosions of energetic particles ejected from the black hole and expanding into the immense gas cloud. Fittingly, the dark spot forming the skull's "nose" is an x-ray shadow ... the shadow of a large galaxy inexorably falling into the cluster center. Over a hundred thousand light-years across, the Perseus Cluster's x-ray skull is a bit larger than skulls you may see tonight.
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. If We Can't See Them, How Do We Know They're There?
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Since
black holes are small (only a few to a few tens of kilometers in size),
and light that would allow us to see them cannot escape, a black hole
floating alone in space would be hard, if not impossible, to see. For
instance, the photograph above shows the optical companion star to the
(invisible) black hole candidate Cyg X-1. However, if a black hole passes
through a cloud of interstellar matter, or is close to another "normal"
star, the black hole can accrete matter into itself.As the matter falls
or is pulled towardsthe black hole, it gainskinetic energy, heats up and
is squeezed by tidal forces. The heating ionizes the atoms, and when the
atoms reach a few million degrees Kelvin, they emit X-rays. The X-rays
are sent off into space before the matter crosses the Schwarzschild radius
and crashes into the singularity. Thus we can see this X-ray emission.