A cosmic mystery, Part 2: Quasars.
TWO COSMIC MYSTERIES. I have personally encountered two cosmic mysteries early in my career – like about 40 years ago. They both involved massive, unprecedented sources of energy. The mysteries continued for decades and occasionally the two encounters kept popping up in my memory. Only in the last few years have both these mysteries been solved – and both explanations involve black holes. This is Part 2. Part 1 was posted about a month ago.
QUASARS. The first time I heard about this high-energy radiation was at the University of Arizona in 1981. I was doing research there on cosmic rays during a sabbatical from Oral Roberts University. A portly astronomer was giving a presentation on quasars, which were objects in the sky that he had mapped in visible light using the large Kitt Peak telescope in Arizona. I was amused by a red notch just above his belt where tummy pressure split the shirt and revealed red underwear. The red notch remained stable during the lecture as the astronomer walked us through his understanding of quasars.
The astronomer said he could measure the distance of these objects from earth using the redshift which was so large that these quasars must be near the boundary of the universe. He said the light had been traveling for 5-10 billion years to reach the earth! And there must have been phenomenal energy involved in making these quasars shine so brightly. But he had no idea what was the source of this immense energy.
Note: redshift is what explains the falling pitch (frequency) of a train’s horn as it travels away from a listener. The faster a train is moving, the more the pitch (frequency) drops, and the larger is the wavelength shift. For visible light from a star, if the star is moving away from earth this means a wavelength shift toward the red color, and this is what’s called redshift. A larger redshift means a star or galaxy is moving away from earth faster, and this means its located further away from earth.
SUPERMASSIVE BLACK HOLES are another kind of black hole. I talked about stellar black holes in Part 1, which form when massive stars die and collapse. They’re roughly 10 – 20 times the mass of our Sun. They’re scattered throughout the universe and there could be millions of these stellar black holes just in our own galaxy, the Milky Way.
Supermassive black holes are giants by comparison, measuring millions, even billions of times more massive than our Sun. Scientists can only guess how they form, but we do know they exist at the center of just about every large galaxy…including our own. Sagittarius A, the supermassive black hole at the center of our galaxy, the Milky Way, has a mass of roughly 4 million suns. But it has a very small diameter for such a heavy object — it’s diameter is about the distance between Earth and our Sun.
Because black holes are invisible, the only way for scientists to detect and study them is to observe their effect on nearby matter. If cosmic gas and dust fall in toward a black hole, they assume the shape of a disk called an accretion disk, a little like the rings of Saturn. This is followed by jets of radiation (charged particles and visible light and X-rays and Gamma-rays) that are blasted out of the supermassive black hole. And this radiation can be detected at earth if it’s pointed in the right direction.
These are periods of intense luminosity (visible light and X-rays, etc) that last for long periods of time – unlike the Gamma ray bursts in Part 1 which last only 2-30 seconds.
Gas and dust accretion is how black holes grow.
SO WHAT ACTUALLY POWERS THESE QUASARS?
Parts of the following are adapted from HERE. Quasars inhabit the center of active galaxies, and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way (our own galaxy) – and the Milky Way contains 200–400 billion stars!
Quasars are powered by accretion of material into supermassive black holes in the center of distant galaxies. Light and other radiation cannot escape from within the event horizon of a black hole, but the energy produced by a quasar is generated outside the black hole, by immense friction within the material nearest to the black hole as this material orbits and falls inward (see image below). The huge luminosity of quasars results from the accretion discs of supermassive black holes, which can convert up to one-third of their mass into energy.
All large galaxies have a black hole of this kind, but only a small fraction have sufficient matter in the right kind of orbit at their center to become active and power radiation in such a way as to be seen as quasars.
This also explains why quasars were more common in the early universe, as this energy production ends when the supermassive black hole sucks in and consumes all of the gas and dust near it.
THIS IS THE END OF PART 2.
The lecture I attended way back in 1981 was memorable because of the red notch underwear and the redshift of quasars. No-one could have guessed that further study would lead to supermassive black holes in the center of galaxies at the outer edge of the universe. And that these supermassive black holes are as massive as a billion suns (or more in a few cases).
I find this story of quasars and supermassive black holes mind-boggling. I hope you do too. To my mind the simplest explanation for such amazing science is that is has a cause that’s even more awesome – and the cause is God. I don’t find any conflict between science and God.
PS: I write blogs about three topics: Health and Hiking, and Inspiration and Hope, and Science and Energy.
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