Black Hole Evidence

Lost in a black hole? Or just lost in thought? It is all too easy to start thinking of black holes as purely theoretical constructs, fantasies, mind games. Can we actually see a black hole?

Star Words

Time dilation is the apparent slowing of time (as perceived by an outside observer) as an object approaches the event horizon of a black hole, or moves at very high relative velocities, approaching the speed of light.

Star Words

Time dilation is the apparent slowing of time (as perceived by an outside observer) as an object approaches the event horizon of a black hole, or moves at very high relative velocities, approaching the speed of light.

Star Words

A singularity is the infinitely dense remnant of a massive core collapse.

Star Words

A singularity is the infinitely dense remnant of a massive core collapse.

Not directly, but we can certainly see its effects. Black holes are like the monsters in old (and new) movies. Before we ever see the monster itself, we see its footprints and its claw marks on the trees, the muddy trail that leads back to the swamp. What are the muddy trails of black holes?

In our own galaxy, there is a bright source of x-rays in the constellation Cygnus (the Swan) known as Cygnus X-1. It is the binary companion of a B star, and the x-rays from it flare up and fade quickly, indicating that it is very small in radius. Remember that we mentioned the x-rays are often emitted in the neighborhood of a stellar-remnant black hole. In addition, the x-ray source has no visible radiation and a mass (inferred from the orbit of its companion) of about 10 solar masses. In this case circumstantial evidence may be enough to convict.

The black hole in Cygnus is what is called a stellar remnant black hole. In theory, black holes can have much higher masses—masses far greater than a stellar core. These are called supermassive black holes and are found in the cores of galaxies. The Hubble Space Telescope image reproduced here is about as close as we have gotten to a supermassive black hole. Released on May 25, 1994, the image shows a whirlpool of hot (10,000 K) gas swirling at the center of an elliptical galaxy (see Chapter 22, "A Galaxy of Galaxies") known as M87, 50 million light-years from us in the direction of the constellation Virgo.

A Hubble Space

Telescope image of a I .

hole at the center of galaxy M87, 50 million I Vv'; ■ ■ ■ ■'i light-years from the earth. I y. ' I

Using the Hubble's Faint Object Spectrograph, astronomers Holland Ford and Richard Harms were able to measure how light from the gas is redshifted and blueshifted as one side of the 60-light-year-radius disk of gas spins toward us and the other away from us. The radius at which gas is spinning, and the velocity of its rotation tell us how much matter must be within that radius. With the high resolution of the Hubble Space Telescope and radio telescopes like the Very Long Baseline Array (VLBA), astronomers can trace the rotation of gas to smaller and smaller distances from the center. Doing so, they find that even at very small radii, the gas is rotating at velocities that indicate something very massive still lies within that radius.

At the heart of M87 and other galaxies, indications are strong that supermassive black holes, perhaps with the mass of many billions of suns, lie in residence.

The Least You Need to Know

V A neutron star is one possible remnant of a massive star that has exploded as a supernova.

V Black holes are the other possible remnant of collapsed massive star cores. If the core has a mass greater than three solar masses, the collapse cannot stop and a black hole is born.

V If you don't get too close to a black hole, its effects (in terms of gravity) are no different than a star of the same mass. They are not giant galactic "vacuum cleaners."

V According to general relativity, black holes, stars and anything with mass distorts the space around it. This distortion can influence the path of both particles and light.

V Because light cannot escape from them, we can't see black holes, but we can indirectly observe evidence that their mass is present.

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