Black Holes

Welcome to the world of the most bizarre ... a star whose gravitational compression has crushed it down to a size of zero volume, infinite density, infinite temperature, and an escape velocity that exceeds the speed of light (even though nothing can move faster than the speed of light). This is a place where all of the laws of Physics break down, and where matter takes on properties we cannot accurately describe. Why, even Quantum Physics struggles to express the conditions that exist inside a Black Hole. I reference Steven Hawking's page at the beginning of this page, just so you can see that the most brilliant mind of our present day struggles to adequately explain the interior of a Black Hole. As we dive into this abyss, let's first begin with a review of the events that lead up to the formation of a Black Hole. Oh ... since there are plenty of students around the country who doubt the existence of such strange objects, I would like to point out that our telescopes have indeed discovered that Black Holes do exist, and are no longer considered science fiction, but science fact. If you want to look at Hollywood's idea of a Black Hole, check out the movie Black Hole (starring Earnest Borgnine) or Event Horizon. Both of these movies are scary, although the latter is much more so than the former, and I have never seen the latter anyway. I chose the darkest background color for this topic, for obvious reasons.

A Black Hole is cosmic body of extremely intense gravity from which nothing, not even light, can escape. A black hole is formed in the final stages of the life of a very massive star. As we have already seen, a star whose mass is less than 1.4 solar masses is destined to become a White Dwarf, approximately the size of Earth, and held up against gravity's inward pressure by degenerate electrons. If the dying star's mass exceeds this Chandrasekhar Limit, it will collapse further, defeating electron degeneracy and crushing the electrons into protons to make nothing but neutrons. The star will collapse under gravity's pressure until the neutrons cannot be physically crushed any closer together, a state called neutron degeneracy. The collapsed core will be the size of a small city. Degenerate neutrons can prevent further collapse up to about 3 solar masses, but above that limiting value, nothing can stop gravity's inward pressure.

A very massive star will live brightly, but for a relatively short time as a main sequence O or B dwarf. After it has exhausted its nuclear fuel in the core, and moved through the Red Supergiant stage, the star will have blown much of its mass away. If the iron dwarf core still has over 3 solar masses at the final stage, the star will explode in a supernova (at which moment the heavy elements of the Periodic Table are forged), and the core will collapse beyond electron and neutron degeneracy ... getting as small as a golf ball, then as small as a pinhead, then as small as an atomic nucleus, and still get smaller, and all the while with a mass in excess of 3 Suns. The crushing weight of constituent matter falling in from all sides compresses the dying star to a point of zero volume and infinite density called the singularity. Details of the structure of a Black Hole are calculated from Albert Einstein's general theory of relativity. The singularity constitutes the center of a Black Hole and is hidden by the object's "surface," the Event Horizon.

Inside the Event Horizon the escape velocity (i.e., the velocity required for matter to escape from the gravitational field of a cosmic object) exceeds the speed of light, so that not even rays of light can escape into space. The radius of the Event Horizon is called the Schwarzschild radius, after the German astronomer Karl Schwarzschild, who in 1916 predicted the existence of collapsed stellar bodies that emit no radiation. The size of the Schwarzschild radius is thought to be proportional to the mass of the collapsing star. For a Black Hole with a mass 10 times as great as that of the Sun, the radius would be 30 km (18.6 miles). Inside the Event Horizon, the laws of Physics break down and properties of Nature cease to exist in a manner describable. So, as you can see from the second diagram, a positive change in the mass of a Black Hole will result in an increase in the radius of the Event Horizon.

Black holes are difficult to observe on account of both their small size and the fact that they emit no light. They can be "observed," however, by the effects of their enormous gravitational fields on nearby matter. For example, if a black hole is a member of a binary star system, matter flowing into it from its companion becomes intensely heated and then radiates X rays copiously before entering the event horizon of the black hole and disappearing forever. Many investigators believe that one of the component stars of the binary X-ray system Cygnus X-1 is a black hole. Discovered in 1971 in the constellation Cygnus, this binary consists of a blue supergiant and an invisible companion star that revolve about one another in a period of 5.6 days. An artist's conception of Cygnus X-1 and an explanation of how an x-ray telescope detects infalling matter onto this object are seen below.

If a collapsed star is a neutron star with a solid surface, vast amounts of energy must be released when in-falling material strikes that surface. In contrast, if the accreting object is a black hole, only a small amount of energy can escape before it crosses the event horizon and vanishes forever.

Scientists using the Hubble Space Telescope took an entirely different approach and observed pulses of ultraviolet light from clumps of hot gas fade and then disappear as they swirled around a massive, compact object called Cygnus X-1, long believed to have a candidate black hole.

Hubble, measuring fluctuations in ultraviolet light from gas trapped in orbit and around the black hole found two examples of a so-called "dying pulse train," the rapidly decaying, precisely sequential lashes of light from a hot blob of gas spiraling into the black hole. Without an event horizon, the blob of gas would have brightened as it crashed onto the surface of the accreting body. One event had six decaying pulses; the other had seven pulses. The results are consistent with what astronomers would expect to see if matter were really falling into a black hole.

(This is such an interesting comparison, for SS Cygni is a White Dwarf star accreting material from a companion, and these blobs of material brighten at they crash onto the White Dwarf surface, and do so in periodic fashion as seen earlier in the White Dwarf page, and also on the front of my t-shirt from the AAVSO.)

Just within the past 2 years, astronomers with the Hubble Space Telescope have been following the movement of a Rogue Black Hole. Apparently a binary star system contains a Black Hole that is slowly accreting material from its less-evolved companion. The system perhaps formed within a globular cluster and was kicked out of its nursery, probably by the explosive force generated when the larger mass star went Supernova. After a 7 billion year journey through the Milky Way, this system is now passing the Sun ... but at a safe distance. Scientists are able to track the path of this pair of stars because it leaves a signature disruption of the galactic dust in its wake.









Black Hole at the Heart of the Milky Way Galaxy

For quite a while, astronomers predicted the presence of a supermassive Black Hole in the heart of the Milky Way Galaxy, but its presence has been difficult to demonstrate. The shifting wavelengths of light by the gravitational influence of this object, as well as the strong radio emissions were regarded as strong supportive evidence, but more was needed. Finally, that evidence was discovered. The ESO image below shows the point that astronomers believe is the exact center of the Milky Way Galaxy.

Then Dr. Andrea Ghez turned the Keck Telescopes at that point, and with the aid of Adaptive Optics, she followed the movement of a dozen stars that were very close to the galactic center. NOVA created a really nice presentation: Monster of the Milky Way. Click on Monster to see the first 9:38 of the video on YouTube. But when you click on the image below-left, you will see a YouTube vide of the motion of the stars around an invisible object.

In the last 1990s and early years of this century, Dr. Andrea Ghez, at UCLA, painstakingly photographed the stars at the galactic core. Previous attempts to study this area of the Milky Way had been limited by distance and the fuzziness of our atmosphere. But with Adaptive Optics, Ghez was able to take really sharp photographs of the stars, and do so over a series of days, weeks, and years. What she observed was the orbital patterns of these stars around an invisible point. The video clip clearly shows these orbits, and the tremendous speed of the stars as they orbit the invisible point is the best evidence yet of a supermassive Black Hole whose mass appears to be around 4 billion Suns worth of matter. Astronomers theorize that the Andromeda Galaxy might have a Black Hole of 20 billion solar masses in its core. The Earth is in no danger of beinig swallowed by the Milky Way's Black Hole core, but there is a different danger.

The drawing below is an artist's idea of what a Quasar looks like. Astronomers first saw these superbright lights at the most distant reaches of the Universe ... glowing far more brightly that previously imagined. They called the objects Quasistellar Objects, or "Quasars" for short. Until recently, astronomers did not understand them, but now the mystery seems to be solved. Gigantic spiral galaxies (and no other forms of galaxy) appear to have supermassive Black Holes in their cores that are ravenously swallowing entire stars. As the star is swallowed, huge jets of X-ray energy is thrown out from the Black Hole. The energy of these jets is truly stupendous. Now astronomers believe that the early galaxies had active Black Hole cores that were consuming stars during the early turbulent years of the galaxy. Eventually, the central region of the galaxy would be emptied of stars and gas, and the Quasar would shut off. This is why only the most distant galaxies exhibit active bursts ... because these galaxies are also relatively young. The older galaxies that are nearer to us have cleaned out their central regions and are now quiescent. But ... what would happen if a rogue star were to pass too close to the galactic core? Would the galaxy reignite its Quasar jets? What would happen if those jets swept past us?

Other Black Hole types

Some black holes apparently have nonstellar origins. Various astronomers have speculated that large volumes of interstellar gas collect and collapse into supermassive black holes at the centers of quasars and peculiar galaxies (e.g., galactic systems that appear to be exploding). A mass of gas falling rapidly into a black hole is estimated to give off more than 100 times as much energy as is released by the identical amount of mass through nuclear fusion. Accordingly, the collapse of millions or billions of solar masses of interstellar gas under gravitational force into a large black hole would account for the enormous energy output of quasars and certain galactic systems. In 1994 the Hubble Space Telescope provided conclusive evidence for the existence of a supermassive black hole at the center of the M87 galaxy. It has a mass equal to two to three billion Suns but is no larger than the solar system. The black hole's existence can be strongly inferred from its energetic effects on an envelope of gas swirling around it at extremely high velocities. In 1998, the Hubble Space Telescope discovered another Black Hole in the center of galaxy NGC 7052, with an estimated mass of 300 million Suns. Both massive Black Hole object locations are shown in the HST images below ... M87 to the left, and NGC 7052 to the right.

One of these most powerful sources of evidence for the existence of these supermassive black holes comes from the gravitational influence of the tremendous object's mass upon the radiation that is streaming toward Earth. It is possible to look at the light heading toward Earth from a galactic core Black Hole and compare it to the light moving in the opposite direction from the same source. The amount of shifting of the light energy is the evidence of a huge solar mass object that occupies a very small space, and the only way an object of such mass can occupy such a small space is if it were a Black Hole. A Hubble Space Telescope image of this light shifting from a galactic core Black Hole in M84 is seen below.

The existence of another kind of nonstellar black hole has been proposed by the British astrophysicist Stephen Hawking. According to Hawking's theory, numerous tiny primordial black holes, possibly with a mass equal to that of an asteroid or less, might have been created during the big bang, a state of extremely high temperatures and density in which the universe is thought to have originated roughly 10 billion years ago. These so-called mini black holes, unlike the more massive variety, lose mass over time and disappear. Subatomic particles such as protons and their antiparticles (i.e., antiprotons) may be created very near a mini black hole. If a proton and an antiproton escape its gravitational attraction, they annihilate each other and in so doing generate energy ... energy that they in effect drain from the black hole. If this process is repeated again and again, the black hole evaporates, having lost all of its energy and thereby its mass, since these are equivalent

A ravenous black hole

This is a Hubble Space Telescope image of an 800-light-year-wide spiral-shaped disk of dust fueling a massive black hole in the center of galaxy NGC 4261, located 100 million light-years away in the direction of the constellation Virgo. By measuring the speed of gas swirling around the black hole, astronomers calculate that the object at the center of the disk is 1.2 billion times the mass of our sun.










Now, to see what happens if we take a virtual trip to a Black Hole, please move forward to the Black Hole Thought Experiment.

Some other interesting websites that tell more about the story of Black Holes are found below.

UCSC Black Holes Website

Cambridge Site for Black Holes

If this material has not grabbed your attention, then go back to the Smallest Stars Introduction, Star Introduction, or the Syllabus ... of just take a nap and hurry back here. Perhaps you feel ready to take the Star Quiz!

| Home | Course Information | Assignments | Teacher Bio | Course Units | Syllabus || Links |