Mass of Collapsed Star Schwarzschild Radius (solar masses)a (km) 1 ~3 5 15 10 30 20 60 50 150
tum. The Reissner–Nordström black hole (introduced in 1918) has an electric charge but no angular momentum—that is, it is not spinning. In 1963 the New Zealand mathemati- cian ROYPATRICKKERR (b. 1934) applied general relativity
to describe the properties of a rapidly rotating, but uncharged, black hole. Astrophysicists think this model is the most likely “real world” black hole because the massive stars that formed them would have been rotating. One postulated feature of a rotating Kerr black hole is its ringlike structure (the ring singularity) that might give rise to two separate event horizons. Some daring astrophysicists have even sug- gested it might become possible (at least in theory) to travel through the second event horizon and emerge into a new uni- verse or possibly a different part of this universe. The final black hole model has both charge and angular momentum. Called the Kerr-Newman black hole, this theoretical model appeared in 1965. However, astrophysicists currently think that rotating black holes are unlikely to have a significant electric charge, so the uncharged Kerr black hole remains the more favored “real world” candidate model for the stellar black hole.
The British astrophysicist STEPHEN WILLIAM HAWKING
(b. 1942) provided additional insight into the unusual physics of stellar black holes in 1974 when he introduced the concept of Hawking radiation, a postulation suggesting an intimate relationship between gravity, quantum mechanics, and ther- modynamics by which, under special circumstances, black holes can emit thermal radiation and eventually evaporate. Of course, this appears very unusual, since by definition nothing can escape from a black hole. The answer to this apparent paradox is found in the space around the black hole at the event horizon. Quantum mechanics suggests that pairs of virtual particles (each pair containing a particle and its antiparticle) can spontaneously pop up. According to con- temporary nuclear physics, virtual particles are undetectable quantum particles that carry gravity and electromagnetic radiation, including light. Fluctuations of electromagnetic and intense gravitational fields can create pairs of virtual par- ticles. Left to themselves, these pairs (a particle and its antiparticle) will move apart slightly and then come back together to annihilate each other on a very short timescale. However, if the pair of virtual particles is created right along a black hole’s event horizon, as they move apart slightly, these particles may live long enough so that one member of the pair is pulled across the event horizon toward the black hole, while the other particle moves outward and escapes. They cannot get back together to annihilate each other.
This quantum mechanics phenomenon has profound consequences for the physics of a black hole. Without its vir- tual partner, the escaping particle becomes a real particle, and so it appears to an external observer that radiation is actually coming from the black hole. Physicists refer to this emission phenomenon as Hawking radiation. The virtual particle with negative energy that was captured by the black hole con- tributes to the reduction in mass of the singularity. If this happens over a very long period of time, the black hole will simply evaporate.
Hawking radiation gives the appearance that a black hole is emitting radiation like a blackbody with a tempera-
ture inversely proportional to its mass. While still highly speculative, Hawking’s work further suggests that as virtual particles transform into real particles, the process extracts energy from the black hole’s intense gravitational field. Since the transformation consumes more energy than the particles possess, it essentially contributes negative energy to the black hole. As a result, the mass of the singularity decreases, and the black hole eventually evaporates. For example, within this theoretical model, a very small mini–black hole—with a radius of less than 10–10meter and a mass of about 1012kg
(that of a small asteroid)—would have a blackbody tempera- ture of about 1011 K. This mini–black hole would radiate
more intensely and evaporate more quickly than a more mas- sive black hole. In contrast, a black hole of one solar mass (approximately 2 × 1030kg) would last about 1066years.
Another consequence of Hawking’s postulation is the formation of a cloud of real particles and antiparticles just outside the event horizon of a black hole. These particles and antiparticles are each the surviving member of virtual particle pairs that have escaped and transformed into real particles. Their continuous annihilation outside the event horizon in the observable universe forms Hawking radiation.
Current astrophysical evidence that superdense stars, such as white dwarfs and neutron stars, really exist also sup- ports the theoretical postulation that black holes them- selves—representing the ultimate in density—must also exist. But how can scientists detect an object from which nothing, not even light, can escape? Astrophysicists think they may have found indirect ways of detecting black holes. The best currently available techniques depend upon candidate black holes being members of binary star systems. Unlike the Sun, many stars (more than 50 percent) in the Milky Way galaxy are members of a binary system. If one of the stars in a par- ticular binary system has become a black hole, although invisible, it would betray its existence by the gravitational effects it would produce on the observable companion star. Once beyond event horizon, the black hole’s gravitational influence is the same as that exerted by other objects (of equivalent mass) in the “normal” universe. So a black hole’s gravitational effects on its companion would obey Newton’s universal law of gravitation—that is, the mutual gravitational attraction of the two celestial objects is directly proportional to their masses and inversely proportional to the square of the distance between them.
Astrophysicists have also speculated that a substantial part of the energy of matter spiraling into a black hole is con- verted by collision, compression, and heating into X-rays and gamma rays that display certain spectral characteristics. X- ray and gamma radiations emanate from the material as it is pulled toward the black hole. However, once the captured material crosses the black hole’s event horizon, this telltale radiation cannot escape.
Suspected black holes in binary star systems exhibit this type of prominent material capture effect. Astronomers have discovered several black hole candidates using space-based astronomical observatories (such as the Chandra X-Ray
Observatory). One very promising candidate is called
Cygnus X-1, an invisible object in the constellation Cygnus (the Swan). The notation Cygnus X-1 means that it is the
91 black hole 91
first X-ray source discovered in Cygnus. X-rays from the invisible object have characteristics like those expected from materials spiraling toward a black hole. This material is apparently being pulled from the black hole’s binary compan- ion, a large star of about 30 solar masses. Based on the sus- pected black hole’s gravitational effects on its visible stellar companion, the black hole’s mass has been estimated to be about six solar masses. In time, the giant visible companion might itself collapse into a neutron star or a black hole, or else it might be completely devoured by its black hole com- panion. This form of stellar cannibalism would also signifi- cantly enlarge the existing black hole’s event horizon.
In 1963 the Dutch-American astronomer MAARTEN
SCHMIDT (b. 1929) was analyzing observations of a “star”
named 3C 273. He had very confusing optical and radio data. What he and his colleagues had discovered was the quasar. Today astronomers know that the quasar is a type of active galactic nucleus (AGN) in the heart of a normal galaxy. AGN galaxies have hyperactive cores and are much brighter than normal galaxies. The AGNs emit energy equiv- alent to converting the entire mass of the Sun into pure ener- gy every few years. Energy is emitted in all regions of the electromagnetic spectrum, from low-energy radio waves all the way to much higher-energy X-rays and gamma rays. Fur- thermore, the energy output of these AGN can vary on short time scales (hours and days), suggesting that the source is very compact. Stars by themselves, powered by nuclear fusion, cannot generate such levels of energy. Even the impressive supernovae explosions are insufficient. So astro- physicists puzzled over what physical processes could pro- duce the power of more than 100 Milky Way galaxies and do it within a region of space only a few light-years across.
To further compound this cosmic mystery, some of these AGN galaxies have extraordinary jets of material rushing out of their cores that stretch far into space—up to 100 to 1,000
times the diameter of the galaxy. After considering all types of energetic processes, including the simultaneous explosions of thousands of supernovae, most astrophysicists now think that supermassive black holes represent the most plausible answer. Although nothing emerges from a black hole, matter falling into one can release tremendous quantities of energy just before it crosses the event horizon. For example, the region just outside the even horizon will glow in X-rays and gamma rays, the most energetic forms of electromagnetic radiation.
Matter captured by the gravity of a black hole will even- tually settle into a disk around the black hole. Scientists call the inner region of swirling, superheated material an accre- tion disk. Stellar black holes can have accretion disks if they have a nearby companion star. Material from the companion will be drawn into orbit around the black hole, thereby form- ing an accretion disk. The diameter of the accretion disk depends on the mass of the black hole. The more massive the black hole, the larger the accretion disk. The accretion disk of a stellar black hole will stretch out only a few hundred or thousand kilometers from the center. However, the accretion disk of a supermassive black hole is much bigger and becomes solar system–sized.
Perhaps the most spectacular accretion disks exist in active galaxies that probably contain supermassive black holes. The Hubble Space Telescope (HST) has provided astronomers strong evidence for this assumption. For exam- ple, the galaxy NGC 4261 is an elliptical galaxy whose core contains an unexpected large disk of dust and gas. Astronomers think that a black hole may lurk within the cen- tral region of this galaxy. Radio observations of this galactic core have also revealed jets of material ejected from the cen- ter of this disk. This provides corroborating evidence for the existence of a very large black hole.
Exactly what creates and controls the flow of matter out these jets is still not clearly understood. It is almost as if black holes are messy eaters, consuming matter but spewing out leftovers. Most likely, the jets have something to do with the rotation and/or the magnetic fields of the black hole. Whatev- er the actual cause, most astronomers believe that only black holes are capable of producing such spectacular and “outra- geous” behavior.
Scientists using the Hubble Space Telescope have also discovered a 3,700 light-year-diameter dust disk encircling a suspected 300-million-solar mass black hole in the center of the elliptical galaxy NGC 7052, located in the constellation Vulpecula about 191 million light-years from Earth. This disk is thought to be the remnant of an ancient galaxy collision, and it will be swallowed up by the giant black hole in several billion years. Hubble Space Telescope measurements have shown that the disk rotates rapidly at 155 km/s at a distance of 186 light-years from the center. The speed of rotation pro- vides scientists a direct measure of the gravitational forces acting on the gas due to the presence of a suspected super- massive black hole. Though 300 million times the mass of the Sun, this suspected black hole is still only about 0.05 percent of the total mass of the NGC 7052 galaxy. The bright spot in the center of the giant dust disk is the combined light of stars that have been crowded around the black hole by its strong
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92 black hole
This is an artist’s visualization of a doomed star being torn apart after it wandered too close to a supermassive black hole in galaxy RX J1242-11. As it neared the enormous gravity of the black hole, the star was stretched by tidal forces until it was torn apart. Before being consumed by the black hole, inflowing gas from the doomed star was heated to millions of degrees Celsius, creating one of the most extreme X-ray outbursts ever observed from the center of a galaxy. (Courtesy of NASA)
gravitational pull. This stellar concentration appears to match the theoretical astrophysical models that link stellar density to a central black hole’s mass.
In the 1990s X-ray data from the German-American
Roentgen Satellite (ROSAT) and the Japanese-American Advanced Satellite for Cosmology and Astrophysics (ASCA)
suggested that a mid-mass black hole might exist in the galaxy M82. This observation was confirmed in September 2000, when astronomers compared high-resolution Chandra
X-Ray Observatory images with optical, radio, and infrared
maps of the region. Scientists now think such black holes must be the results of black hole mergers, since they are far too massive to have been formed from the death of a single star. Sometimes called the “missing link” black holes, these medium-sized black holes fill the gap in the observed (candi- date) black hole masses between stellar and supermassive. The M82 “missing link” is not in the absolute center of the galaxy, where all supermassive black holes are suspected to reside, but it is fairly close to it.
blastoff
The moment a rocket or aerospace vehicle rises from its launch pad under full thrust.See alsoLIFTOFF.
blazar
A variable extragalactic object (possibly the high- speed jet from an active galactic nucleus as viewed on end) that exhibits very dynamic, sometimes violent, behavior.See alsoBL LAC OBJECT.
bl lac object
(bl lacertae) A class of extragalactic objects thought to be the active centers of faint elliptical galaxies that vary considerably in brightness over very short periods of time (typically hours, days, or weeks). Scientists further spec- ulate that a very high-speed (relativistic) jet is emerging from such an object straight toward an observer on Earth.blockhouse
(block house) A reinforced-concrete structure, often built partially underground, that provides launch crew personnel and their countdown processing and monitoring equipment protection against blast, heat, and possibly an abort explosion during the launch of a rocket. Modern launch control centers are more elaborate facilities that are usually located much farther away from launch pads than the blockhouse structures used during the 1950s and 1960s.blowdown system
In rocket engineering, a closed propel- lant/pressurant system that decays in ullage pressure level as the liquid propellant in a tank is consumed and ullage volume increases.blue giant
A massive, very high-luminosity star with a sur- face temperature of about 30,000 K that has exhausted all its hydrogen thermonuclear fuel and left the main sequence por- tion of the Hertzsprung-Russell diagram.See alsoSTAR.
blueshift
When a celestial object (such as a distant galaxy) approaches an observer at high velocity, the electromagnetic radiation it emits in the visible portion of the spectrumappears shifted toward the blue (higher-frequency, shorter- wavelength) region. Compare withREDSHIFT.
See also DOPPLER SHIFT.
Bode, Johann Elert
(1747–1826) German Astronomer Johann Bode was the astronomer who publicized an empirical formula that approximated the average distance to the Sun of each of the six planets known in 1772. This formula, often called BODE’SLAW, is only a convenient mathematical relation-ship and does not describe a physical principle or natural phe- nomenon. Furthermore, Bode’s empirical formula was actually discovered in 1766 by JOHANN DANIEL TITIUS (1729–96).
Bode popularized this relationship. As a result, he often (incor- rectly) receives credit for its development. This empirical rela- tionship is, therefore, more properly called the Titus-Bode law in recognition of the efforts of both 18th-century German astronomers. In 1801 Bode published Uranographia, a com- prehensive listing of more than 17,000 stars and nebulas.