Have Scientists Really Found Black Holes?

IT SEEMS like science fiction—once-bright stars becoming invisible, crushed by their own gravitational force, with nothing, not even light, escaping their grasp. Many astronomers believe that such black holes may be commonplace in the universe. Would you like to know more about them? The story begins in the beautiful northern constellation called Cygnus, meaning “the Swan.”

Cygnus X-1—A Black Hole?

Since the 1960’s, astronomers have been interested in a certain area of the constellation Cygnus. Orbiting observatories launched above Earth’s atmosphere detected a powerful source of X rays coming from this area, dubbed Cygnus X-1.

Scientists have long known that the hotter an object is, the more energy it gives off at shorter, more energetic electromagnetic wavelengths. If you heat a piece of iron in a very hot furnace, at first it will glow red and then yellow and white as the iron gets hotter. In that way, stars are like iron bars. Relatively cool stars, about 3,000 K, are reddish in color, while a yellow star, like the Sun, has a surface temperature closer to 6,000 K.a However, you would have to heat stellar gas to millions of kelvins to get the X-ray radiation coming from Cygnus X-1. No star has a surface temperature like that.

At the location of Cygnus X-1, astronomers have found a star with a surface temperature estimated at 30,000 K—very hot, indeed, but not nearly hot enough to account for the X rays. This star, cataloged as HDE 226868, is estimated to be about 30 times as massive as the Sun and 6,000 light-years away from Earth. This supergiant has a companion, and the two are whirling around each other in an orbital waltz every 5.6 days. Scientists calculate that the companion is only a few million miles from HDE 226868. According to some sources, this companion is about ten times as massive as the Sun. But there is something very strange about this companion—it is invisible. No normal star that big should be invisible at such a distance from Earth. An object that massive that appears to give off X rays but not visible light is a good candidate for being a black hole, say scientists.

A Trip to a Black Hole

Imagine that you could travel to Cygnus X-1. Assuming it is indeed a black hole, what you would see might well look like the illustration on page 17. The big star is HDE 226868. While this star is millions of miles in diameter, the black hole may be about 40 miles [about 60 kilometers] in diameter. The tiny black dot in the center of the whirlpool of glowing gas is the event horizon, or surface, of the black hole. It is not a solid surface, however, but more like a shadow. It is the edge of the region in which the gravity around the black hole is so strong that not even light can escape. Many scientists think that inside the horizon, in the center of the black hole, is a point of zero volume and infinite density, known as a singularity, into which all the matter in the black hole has disappeared.

The black hole is draining the companion star’s outer layers of gas. The gas from the star forms a glowing pancake as the gas spirals faster and faster and becomes heated by friction around the black hole. This disk of superheated gas produces X rays just outside the black hole, as the gas is accelerated to fantastic speeds by the intense gravity. Of course, once the gas falls into the black hole, no more X rays—or anything else—can escape.

Cygnus X-1 is a spectacular sight, but don’t get too close! Not only are the X rays deadly but so is its gravity. On Earth, a slight difference in the force of gravity exists between your head and your feet while you are standing. This difference creates a tiny pull that cannot be felt. However, at Cygnus X-1, that small difference is multiplied 150 billion times, creating a force that would actually stretch your body, as if invisible hands were pulling your feet one way and your head another!

What Would Make a Black Hole?

PRESENT scientific understanding is that stars shine because of a ceaseless struggle between gravity and nuclear forces. Without gravity to squeeze the gas deep inside the star, nuclear fusion could not take place. On the other hand, without nuclear fusion to resist the pull of gravity, some very strange things can happen to stars.

Scientists believe that when stars about the size of our sun exhaust their nuclear fuel of hydrogen and helium, gravity squeezes them down to hot cinders about the size of the earth, called white dwarfs. A white dwarf may contain as much mass as the sun, but its mass is crammed into a space a million times smaller.

You can think of ordinary matter as mostly empty space, with almost all the mass of each atom located in a tiny nucleus surrounded by a much larger cloud of electrons. But inside a white dwarf, gravity squeezes the electron cloud into a tiny fraction of its previous volume, shrinking the star to the size of a planet. For stars about the size of our sun, at this point there is a standoff between gravity and forces possessed by the electrons, preventing any further compression.

But what of stars heavier than the sun, with more gravity? For stars more than 1.4 times as massive as the sun, the force of gravity is so great that the electron cloud is squeezed out of existence. The protons and electrons then combine into neutrons. The neutrons resist further squeezing, provided the gravity is not too strong. Instead of a white dwarf the size of a planet, the result is a neutron star the size of a small asteroid. Neutron stars consist of the densest known material in the universe.

What, though, if the gravity is further increased? Scientists believe that in stars about three times the mass of the sun, the gravity is too strong for the neutrons to withstand. No form of matter known to physicists can resist the cumulative force of all this gravity. It seems that the asteroid-size ball of neutrons would get squeezed not just into a smaller ball but into nothing, into a point called a singularity, or some other as yet undescribed theoretical entity. The star would apparently disappear, leaving behind only its gravity and a black hole where it used to be. The black hole would form a gravitational shadow in place of the former star. It would be a region in which gravity was so strong that nothing—not even light—could escape.