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Issue 3.11

The Astrophysics Spectator

June 7, 2006

Continuing with black holes in this issue of The Astrophysics Spectator, a page is added to the “General Relativity” path that describes why we believe very distant galaxies with brilliant cores harbor massive black holes. A new commentary is also added this issue that explains why theorists are at times right to disregard observations that conflict with their theories.

Among our black hole candidates, the most brilliant are those that we believe power the active nuclei of very distant galaxies. These Active Galactic Nuclei (AGN) can radiate as much as 100 trillion times the energy of the Sun as visible and ultraviolet light, x-rays, and gamma-rays. Their power output can change in only a day, which motivates the hypothesis that black holes are their engines. The basic theory for the AGN is that gas falls onto a massive black hole, releasing gravitational potential energy that is radiated away to us. In some instances, the variability seen in the electromagnetic spectrum gives us a measure of a black hole candidate's mass. Typical masses found by observers are in the 10 to 100 million solar mass range.

Despite being the brightest and most numerous of our black hole candidates, the AGN black hole candidates tell us nothing about general relativity. The theory for the AGN relies only on the existence of a massive and compact object, because the only feature required of the black hole is its deep gravitational potential. The many unusual characteristics of a black hole, such as its event horizon and its ability to cause light to orbit it, play no role in replicating the behavior of the AGN.

Next Issue: I am taking a month off from writing, so the next issue of The Astrophysics Spectator is planned for release on July 18. This time off should help me get the site back on schedule.

Jim Brainerd


Estranged Theory. The theoretical astrophysics community is limited in its ability to describe what we see in our universe, in part because our information on astronomical objects is limit, and in part because the physics underlying astronomical objects is too difficult to solve with pencil and paper or to simulate with a computer. For this reason, the theoretical astrophysics community is somewhat estranged from the observational astronomy community. (continue)

General Relativity

Black Holes in Galaxies with Active Nuclei. The largest black hole candidates are found in the active nuclei of distant galaxies. These galaxies, which include quasars, can radiate more power from their nuclei than from all of the stars that they contain. This power is emitted over a broad electromagnetic spectrum, extending from optical frequencies to x-ray and gamma-ray energies. The power can change in less than a day, which inspires the association of the active nucleus with a massive black hole. Firm measurements of the mass of the black hole candidates at the centers of these galaxies give values of 10 to 100 million solar masses. (continue)

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