BLACK HOLES
| Black Hole1 | The image on the left, taken with Hubble's Wide Field
Planetary and Camera 2 shows the core of the galaxy M84 where a suspected black hole
dwells. Astronomers mapped the motions of gas in the grip of the black hole's powerful
gravitational pull by aligning the Space Telescope Imaging Spectrograph's (STIS's)
spectroscopic slit across the nucleus in a single exposure. The STIS data on the right shows the rotational motion of stars and gas along the slit. The change in wavelength records whether an object is moving toward or away from the observer. The larger the excursion from the centerline -- as seen as a green and yellow picture element (pixels) along the center strip, the greater the rotational velocity. If no black hole were present, the line would be nearly vertical across the scan. Instead, STIS's detector found the S-shape at the center of this scan, indicating a rapidly swirling disk of trapped material encircling the black hole. Along the S-shape from top to bottom, velocities skyrocket as seen in the rapid, dramatic swing to the left (blueshifted or approaching gas), then the region in the center simultaneously records the enormous speeds of the gas both approaching and receding for orbits in the immediate vicinity of the black hole, and then an equivalent swing from the right, back to the center line. STIS measures a velocity of 880,000 miles per hour (400 kilometers per second) within 26 light-years of the galaxy's center, where the black hole dwells. This motion allowed astronomers to calculate that the black hole contains at least 300 million solar masses. (Just as the mass of our Sun can be calculated from the orbital radii and speeds of the planets.) M84 is located in the Virgo Cluster of galaxies, 50 million light-years from Earth. |
| Black Hole2 | Resembling a gigantic hubcap in space, a 3,700
light-year-diameter dust disk encircles a 300 million solar-mass black hole in the center
of the elliptical galaxy NGC 7052. The disk, possibly a remnant of an ancient galaxy
collision, will be swallowed up by the black hole in several billion years. Because the front end of the disk eclipses more stars than the back, it appears darker. Also, because dust absorbs blue light more effectively than red light, the disk is redder than the rest of the galaxy (this same phenomenon causes the Sun to appear red when it sets in a smoggy afternoon). Hubble's Faint Object Spectrograph (replaced by the STIS spectrograph in 1997) was used to observe hydrogen and nitrogen emission lines from gas in the disk. Hubble measurements show that the disk rotates like an enormous carousel, 341,000 miles per hour (155 kilometers per second) at 186 light-years from the center. The rotation velocity provides a direct measure of the gravitational force acting on the gas by the black hole. Though 300 million times the mass of our Sun, the black hole is still only 0.05 per cent of the total mass of the NGC 7052 galaxy. Despite its size, the disk is 100 times less massive than the black hole. Still, it contains enough raw material to make three million sun-like stars. The bright spot in the center of the disk is the combined light of stars that have crowded around the black hole due to its strong gravitational pull. This stellar concentration matches theoretical models linking stellar density to a central black hole's mass. NGC 7052 is a strong source of radio emission and has two oppositely directed `jets' emanating from the nucleus. (The jets are streams of energetic electrons moving in a strong magnetic field and unleashing radio energy). Because the jets in NGC 7052 are not perpendicular to the disk, it may indicate that the black hole and the dust disk in NGC 7052 do not have a common origin. One possibility is that the dust was acquired from a collision with a small neighboring galaxy, after the black hole had already formed. NGC 7052 is located in the constellation of Vulpecula, 191 million light-years from Earth. |
| Black Hole3 | A supermassive black hole residing in the hub of the nearby
galaxy, NGC 4438, has blown a huge bubble of hot gas into space. Known as a peculiar
galaxy because of its unusual shape, NGC 4438 is in the Virgo
Cluster, 50 million light-years from Earth. These NASA Hubble Space Telescope images of the galaxy's central region clearly show one of the bubbles rising from a dark band of dust. The other bubble, emanating from below the dust band, is barely visible, appearing as dim red blobs in the close-up picture of the galaxy's hub (the colorful picture at right). The background image represents a wider view of the galaxy, with the central region defined by the white box. These extremely hot bubbles are caused by the black hole's interaction with material swirling around it in an accretion disk (the white region below the bright bubble). Some of this material is spewed from the disk in opposite directions. Acting like high-powered garden hoses, these twin jets of matter sweep out material in their paths. The jets eventually slam into a wall of dense, slow-moving gas, which is traveling at less than 223,000 mph (360,000 kph). The collision produces the glowing material. The bubbles will continue to expand and will eventually dissipate. Compared with the life of the galaxy, this bubble-blowing phase is a short-lived event. The bubble is much brighter on one side of the galaxy's center because the jet smashed into a denser amount of gas. The brighter bubble is 800 light-years tall and 800 light-years across. Both pictures were taken March 24, 1999 with the Wide Field and Planetary Camera 2. False colors were used to enhance the details of the bubbles. The red regions in the picture denote the hot gas. |
| Black Hole4 | Joseph F. Dolan, of NASA's Goddard Space Flight
Center in Greenbelt, MD, observed pulses of ultraviolet light from clumps
of hot gas fade and then disappear as they swirled around a massive,
compact object called Cygnus XR-1. This activity is just as would have
been expected if the hot gas had fallen into a black hole. The discovery
comes from a detailed statistical analysis of a 1992 observation of one of
the first black holes ever discovered, Cygnus XR-1, which lies 6,000
light-years from Earth in the summer constellation Cygnus the Swan.
Dynamical models predict that gas from Cygnus XR-1's companion star continuously falls into the black hole. The gas can't directly fall in, but instead swirls into a flattened pancake called an accretion disk. The viscosity in the accretion disk causes the gas to spiral down toward the event horizon. About 1,000 miles above the event horizon (in the case of stellar-mass black holes) the disk vanishes because gas can no longer maintain a stable orbit. This is due to the dragging of space-time by the black hole's intense gravitational field. Instead, blobs of hot gas break off from the inner rim of the disk, like icebergs off an ice shelf. The blob then spirals down to the event horizon. Because of gravitational effects on light near the black hole, the blob appears to pulsate as it makes thousands of orbits around the black hole every second. When it falls inside the accretion disk, the light quickly stretches to longer and longer wavelengths because of the distortion of space-time by the black hole's intense gravity. This movie is an artist's depiction of the events mentioned above. |
| Black Hole5 | Evidence for the existence of a massive black hole at the center of the Milky Way! |
| Black Hole6 | Some artistic representations of black holes. |