Chapter 13: Star Birth and Death

Different trajectories an object can get based on the initial velocity. A = The object falls towards the ground (in this case the Earth). B = The object gets into a stationary orbit around the Earth (orbital speed/velocity). C = The object has too high velocity to go into a stationary orbit at the same altitude, but too low velocity to leave the Earth's field of gravity and will eventually fall into an orbit at a higher altitude. D = The object has high enough velocity to leave the Earth's field of gravity (escape velocity).

The nature of a black hole can be understood in terms of the idea of escape velocity. Imagine that the Sun has somehow been compressed into a black hole of 1 solar mass. A rocket passing at a great distance would experience the same gravity field as a rocket at a great distance from the Sun. At 1 A.U. from the black hole, for example, the velocity needed to escape into interstellar space would be 42 kilometers per second, the same as the speed needed to leave the Earth's orbit. You can see that the gravity far from a black hole is not very severe. It is not true that a black hole acts like a cosmic "vacuum cleaner," sucking up everything around it. But as we get much closer to the black hole, the escape velocity increases. Larger speeds are needed to escape the stronger and stronger gravity. At a distance of 3 kilometers, the speed needed to escape would be the speed of light. Since we know of nothing that can travel faster than light, nothing can escape this region.

Simulated view of a black hole in front of the Large Magellanic Cloud. The ratio between the black hole Schwarzschild radius and the observer distance to it is 1:9. Of note is the gravitational lensing effect known as an Einstein ring, which produces a set of two fairly bright and large but highly distorted images of the Cloud as compared to its actual angular size.

The radius corresponding to the event horizon is called the Schwarzschild radius, after the astronomer who was the first to solve Einstein's equations of general relativity for a collapsed object. How is a black hole produced? An object of any mass can become a black hole if it is sufficiently compressed. However, black holes were predicted to exist as a consequence of stellar evolution. Any star that ends its life with a core mass of 3 solar masses or more will become a black hole because no known force in nature can prevent its collapse within its event horizon.

NASA StarChild image of Stephen Hawking.

What lies within the event horizon of a black hole? Nobody really knows. The event horizon is not a physical barrier, just an information barrier. Einstein's theory predicts that matter will keep collapsing gravitationally until it has shrunk to a point of zero volume and infinite density! This endpoint is called a singularity and it cannot be adequately described using general relativity. Black holes are not entirely black. In the 1970s, English physicist Stephen Hawking calculated that black holes could create subatomic particles near their event horizons and slowly radiate away their energy, or "evaporate." This so-called Hawking radiation is expected to be dramatic for microscopic black holes, but barely noticeable for solar-sized black holes. Far more important is the fact that any material falling toward the event horizon will be subject to enormous gravitational forces. The friction and heating of material that falls in will be released in the form of X-rays. Therefore, a black hole may be a source of energy due to the death spasms of matter falling into it.

Artist impression of a binary system with an accretion disk around a black hole being fed by material from the companion star.

Suppose a black hole is orbiting around an evolved star that has expanded into a giant state and is shedding mass. Some of the expanding gas would fall toward the black hole at relativistic speeds. Because this gas would, on average, have some angular momentum around the star, rather than falling directly toward it, it would form a disk of gas spiraling inward toward the black hole. This disk is called an accretion disk. Its gas would be extremely hot because it would be constantly hit by new gas streaming in from the other star. Because of the high temperature, the disk would radiate at short ultraviolet or X-ray wavelengths.

Chandra image of Cygnus X-1.

The best evidence for a black hole would be a massive, high-temperature X-ray source orbiting another normal star. There are currently about two dozen excellent candidates and another dozen or so plausible candidates. One such object is Cygnus X-1, a binary system composed of a supergiant star we can see orbiting an unseen companion. The companion object has been calculated to have a mass of at least 5 solar masses, and the system is one of the brightest X-ray sources in the sky. It's one of the strongest candidates for a black hole among several dozen binary systems with good orbital mass constraints. The argument for a black hole in these systems has three steps. First, the X-ray emission must be consistent with accretion onto a compact object. Second, the calculation of the orbit of the binary system must lead to a mass for the compact object that exceeds 3 solar masses. Third, it is assumed that general relativity is the correct theory of gravitation and that neutron stars with masses above 3 solar masses cannot exist. Is the evidence convincing? All of the observations have uncertainties attached and the interpretation is indirect and circumstantial. Whereas the evidence for the existence of neutron stars is convincing, the evidence for stellar black holes is strong but not overwhelming. Black holes are the most exotic member of the stellar "zoo," and astronomers continue to work to prove their existence beyond doubt.

Even with this modest sample of black holes, astronomers can extrapolate to an estimate of about 10 million black holes in the Milky Way galaxy, and a staggering 40 quintillion or 4 x 10¹⁹ in the universe. These exotic objects make up 1% of the normal matter in the universe.