Evidence for Black Holes

14-7 Several binary star systems contain black holes

Black holes are more than fine points of relativity theory: They are real. Their presence has been observed by their effects on the orbits of other stars and on gas and dust near them, and more of them are being located all the time. To find evidence for black holes, we look first to binary star systems.

The technique for detecting black holes formed from collapsing stars is based on the interaction between the black hole and its binary companion. When one star in a close binary becomes a black hole, its gravitational attraction pulls off some of its companion’s atmosphere. However, such black holes have diameters of only a few kilometers, so there is not enough room for all that gas to fall straight in. Rather, the infalling gas swirls into the black hole like water going down a bathtub drain (Figure 14-14). The gas waiting to fall in forms an accretion disk, a disk of gas and dust spiraling in toward the black hole. Magnetic fields in the disk help pull the debris inward. Calculations reveal that this gas is compressed and thereby heated so much from the collisions of its particles that it gives off X-rays (Figure 14-15). Thus, if a visible star has a sufficiently tiny, sufficiently massive X-ray–emitting companion, we have located a black hole. See Guided Discovery: Identifying Stellar-Remnant Black Holes for details on how this is done.

Figure 14-14: Formation of an Accretion Disk Just as the water in this photograph swirls around waiting to get down the drain, the matter pulled toward a black hole spirals inward. Angular momentum of the infalling gas and dust causes them to form an accretion disk around the hole.

Figure 14-15: RIVUXG X-rays Generated by Accretion of Matter Near a Black Hole Stellar-remnant black holes, such as Cygnus X-1, lMC X-3, v404 Cygni, and probably A0620-00, are detected in close binary star systems. This image (of the Cygnus X-1 system) shows how gas from the 30 M_⊙ companion star, HDe 226868, transfers to the black hole, which has at least 11 M_⊙. This process creates an accretion disk. As the gas spirals inward, friction and compression heat it so much that the gas emits X-rays, which astronomers can detect.

To date, in the Milky Way Galaxy alone, at least 10 stellar-remnant black holes in binary star systems have been identified (Table 14-1). There is also growing evidence that black holes can collide with each other or with different types of objects, such as neutron stars.

14-8 Other black holes range in mass up to billions of solar masses

Based on the equations of general relativity, the fate of massive neutron stars led as early as 1939 to the idea of black holes. Since then, calculations have supported at least three other types of black holes:

• 1. Early in the life of the universe, black holes could have formed from the condensation of vast amounts of gas and also from the collisions of stars during the process of galaxy formation, thereby creating supermassive black holes, each with millions or billions of solar masses.
• 2. We saw in Section 12-2 that stars form in clusters. If a cluster forms with enough stars concentrated in a sufficiently small volume of space, collisions between stars in the center of the cluster may lead to the formation of a black hole with between a few hundred and a few thousand solar masses. Such objects are called intermediate-mass black holes.
• 3. Black holes could have formed during the explosive beginning of the universe as tiny amounts of matter were compressed sufficiently to form primordial black holes. These bodies would have masses ranging from a few grams to the mass of a planet.

Supermassive Black Holes

In May 1994, the Hubble Space Telescope obtained compelling evidence for a black hole at the center of the galaxy M87. In the nucleus of M87 is a tiny, bright source of light. Spectra showed that nearby gas and stars are orbiting it extremely rapidly. They can be held in place only if the bright object contains some 3 billion solar masses (Figure 14-16). Given that the source’s size is only slightly larger than the solar system, it can only be a black hole.

Figure 14-16: Supermassive Black Hole The bright region in the center of galaxy M87 has stars and gas held in tight orbits by a black hole. M87’s bright nucleus (center of the region in the white box) is only about the size of the solar system but it pulls on the nearby stars with so much force that astronomers calculate that it is a 3 × 10⁹ M_⊙ black hole. one of the bright jets of gas shooting out perpendicular to the black hole’s accretion disk (discussed in Section 14-9) is visible at the upper right on this image.

GUIDED DISCOVERY: Identifying Stellar-Remnant Black Holes

Shortly after the Uhuru X-ray satellite was launched in the early 1970s, astronomers found a promising black-hole candidate—an X-ray source called Cygnus X-1. This source is highly variable and irregular. Its strong X-ray emission flickers significantly on timescales as short as a hundredth of a second.

The timescale of the flickers tells us the largest size a body can have: If some parts of an X-ray source are brightening while others are dimming, then the source’s total emission will be more or less continuous. It won’t flicker. For Cygnus X-1 to flicker as much as it does, the brightening and dimming it undergoes must be synchronized. For this to happen, the brightening process must start somewhere and then quickly travel all across Cygnus X-1 to stimulate the brightening everywhere and then the darkening everywhere in each pulse. However, this signal to brighten or darken cannot propagate faster than the speed of light. Because light travels 3 × 10⁸ km/s, it travels 3000 km in the “flicker time” of a hundredth of a second. This value means that for all of Cygnus X-1 to brighten and then to darken simultaneously, it must be no more than about 3000 km across, smaller in diameter than Earth. If it were larger, then it would take longer than a hundredth of a second to go from bright to dark, so the light it emits could not be synchronized.

Cygnus X-1 occasionally emits radio radiation, and, in 1971, radio astronomers succeeded in associating Cygnus X-1 with the visible star HDE 226868 (Figure GD 14-1), a B0 supergiant with a surface temperature of about 31,000 K. Because such stars do not emit significant amounts of X-rays, HDE 226868 alone cannot be the Cygnus X-1 X-ray source. Spectroscopic observations soon showed that the lines in the spectrum of HDE 226868 shift back and forth within a period of 5.6 days. This behavior is characteristic of a single-line spectroscopic binary (see Section 11-12), and HDE 226868’s companion is too dim to produce its own set of spectral lines. The clear implication is that HDE 226868 and Cygnus X-1 are the two components of a binary star system.

The B0 supergiant HDE 226868’s mass is estimated at about 30 M_⊙, like other B0 supergiants. As a result, Cygnus X-1 must have at least 11 M_⊙; otherwise, it would not exert enough gravitational pull to make the B0 star wobble by the amount deduced from the periodic Doppler shift of its spectral lines. Cygnus X-1 cannot be a white dwarf or a neutron star, because its mass is too large for either of these objects. The only remaining possibility is that it must be a fully collapsed star—a black hole.

In the early 1980s, a similar binary system was identified in the nearby galaxy called the Large Magellanic Cloud. The X-ray source, called LMC X-3, exhibits rapid fluctuations, just like those of Cygnus X-1. LMC X-3 orbits a B3 main-sequence star every 1.7 days. From its orbital data, astronomers conclude that the mass of LMC X-3 is about 10 M_⊙, which would make it a black hole.

Another black-hole candidate is a spectroscopic binary in the constellation Monoceros that contains the flickering X-ray source A0620-00. The visible companion of A0620-00 is an orange-colored dwarf star. The low-mass, main-sequence star of spectral type K orbits the X-ray source every 7.75 hours. From orbital data, astronomers conclude that the mass of A0620-00 must be greater than 3 M_⊙, probably about 5 M_⊙.

HDE 226868 This star is the visual companion of the X-ray source Cygnus X-1. This binary system is located about 8000 light-years from earth and contains a black hole of at least 11 M_⊙ in orbit with HDe 226868, a B0 blue supergiant star. The photograph was taken with the 200-in. telescope at palomar observatory on palomar Mountain, north of San Diego. The slightly dimmer star above is an optical double that is not part of the binary system.

Since 1994, black holes in the centers of many galaxies have been identified by their X-ray emissions and gravitational effects on surrounding gas and stars. For example, a distinct, frisbee-shaped disk of gas and dust around the supermassive black hole in NGC 7052 was seen by the Hubble Space Telescope in 1998 (Figure 14-17). A black hole containing about 4 million solar masses has even been found at the center of our Milky Way, only 26,000 light-years from Earth. Indeed, evidence for central, supermassive black holes has been found in most of the several dozen nearest galaxies. To date, over 2.5 million supermassive black holes have been identified and astronomers are coming to the conclusion that they exist in the centers of most, if not all, large galaxies. We discuss galaxies in the next two chapters.

Figure 14-17: Disk of Gas and Dust Around a Supermassive Black Hole (a) Swirling around a 3 × 10⁸ M_⊙ black hole in the center of the galaxy NGC 7052, this disk of gas and dust is 3700 ly across. The gas is cascading into the black hole, which will consume it over the next few billion years. The black hole appears bright because of light emitted by the hot, accreting gas outside its event horizon. NGC 7052 is 191 million ly from earth in the constellation vulpecula. (b) This drawing shows how the gases spiraling inward in an accretion disk heat up as they approach the black hole. Color coding follows Wien’s law: red (coolest), followed by orange, yellow, green, blue, and violet (hottest).

Earlier in this book we studied how tidal effects of one body on another occur in several situations, including the Earth–Moon system and the heating of Io and Europa by Jupiter. In 2004, astronomers observed several supermassive black holes in other galaxies absorbing parts of passing stars. These black holes created such high tides on those stars that the stars were pulled apart. Some of each star’s mass was then drawn into the black hole. These discoveries were made because infalling gas from the stars was rapidly heated to millions of kelvins, causing the gas to emit a burst of X-rays that was observed by orbiting telescopes. Subsequent observations also reveal that supermassive black holes can consume vast quantities of interstellar gas, sometimes stripping enough gas from a galaxy to prevent it from forming new generations of stars.

As we will explore in more detail in Chapter 18, galaxies were created from condensing gas and stars in the early universe. Our technology is now providing us with observations of that epoch of the universe, and the formation process of supermassive black holes in the centers of galaxies is now being studied. These black holes apparently formed, in part, from the gas that was condensing to create galaxies and, in part, from the collisions of stars and black holes that came into existence shortly after the beginning of time. Observational evidence indicates that the formation of supermassive black holes is still ongoing. We will explore this further in Chapter 16.

Intermediate-Mass Black Holes

At the end of the twentieth century, astronomers began discovering objects that appear to be black holes with between 10² and 10⁵ M_⊙. Again, these objects were identified as potential black holes by the intensity and spectra of the X-rays that they emit and by the orbits of stars around them. For example, near the center of the galaxy M82, astronomers found what appears to be a 500–1000 M_⊙ black hole (Figure 14-18), while in the galaxy NGC 1313, astronomers have observed what appears to be two intermediate-mass black holes, each with 200–500 M_⊙. Most intermediate-mass black holes are observed in globular clusters.

Figure 14-18: An Intermediate-Mass Black Hole M82 is an unusual galaxy in the constellation Ursa Major. The inset shows an image of the central region of M82 from the Chandra X-ray observatory. The bright, compact X-ray source shown varies in its light output over a period of months. The properties of this source suggest that it is a black hole of roughly 500 M_⊙.

In support of the belief that these objects are black holes, computer simulations of stars in the crowded regions, where such objects are found, show that black holes in this mass range can form as a result of collisions and close tidal interactions between stars. Do not fear—such a black hole will not develop near Earth. There have to be at least a million times more stars per volume than there are in our stellar neighborhood for such frequent stellar collisions or near misses to occur.

Primordial Black Holes

Even more exotic black holes may have formed along with the universe itself. The British astrophysicist Stephen Hawking (1942–) has proposed that the Big Bang explosion from which most astronomers believe the universe emerged may have been chaotic and powerful enough to have compressed tiny knots of matter into primordial black holes. Their masses may have ranged from a few grams to greater than the mass of Earth. Astronomers have not yet observed evidence of primordial black holes, although that does not mean they do not necessarily exist—just that we cannot yet detect them.

14-9 Black holes and neutron stars in binary systems often create jets of gas

We saw in Section 13-11 how rotating beams of radiation are emitted by neutron stars. Similarly, many neutron stars and black holes emit steady (non-rotating) pairs of gas jets shooting out in opposite directions.

Consider a black hole (neutron stars behave analogously) in a binary system. If the companion star is still fusing material, then its outer layers can be pulled off, as discussed in Section 12-15. The infall of this matter toward the compact companion is too rapid for all of the mass to enter the event horizon immediately. The resulting accretion disk around the black hole is the key to explaining the presence of the jets.

As the disk mass spirals down toward the event horizon, this gas is compressed into a smaller volume. Such compression heats the gas, which, in turn, causes its pressure to increase. As a result of the increased pressure, the gas expands, forming a doughnut-shaped region around the black hole (Figure 14-19). As the gas starts the final plunge toward the black hole, its temperature skyrockets to tens or even hundreds of millions of kelvins. At such temperatures, the pressure is so great that much of the infalling gas expands and, finding little resistance perpendicular to the plane of the accretion disk, it squirts out as two jets (Figure 14-19). These jets are prevented from spreading out by a magnetic field created as the hot gases orbit in the accretion disk and by pressure from surrounding gas. Astronomers have measured the speed of this gas to be as fast as 32 × 10⁶ km/hr or 3 % of the speed of light.

Figure 14-19: Jets Created by a Black Hole in a Binary System Some of the matter spiraling inward in the accretion disk around a black hole is superheated and redirected outward to produce two powerful jets of particles that travel at close to the speed of light. The companion star is off to one side of this drawing.

Whereas neutron stars and stellar-mass black holes create jets as a result of being in binary star systems, supermassive black holes have so much gravitational attraction that they pull huge quantities of nearby interstellar gas and dust into orbit around themselves without needing a binary companion. Figure 14-17a shows such a disk, and Figure 14-16 shows one of the jets emitted by the supermassive black hole in M87. We will explore more about jets created by black holes in Chapter 17.