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mardi 13 août 2013

What Is A Quasar ?

I love it when scientists discover something unusual in nature. They have no idea what it is, and then over decades of research, evidence builds, and scientists grow to understand what’s going on.

My favorite example? Quasars.

Astronomers first knew they had a mystery on their hands in the 1960s when they turned the first radio telescopes to the sky.

They detected the radio waves streaming off the Sun, the Milky Way and a few stars, but they also turned up bizarre objects they couldn’t explain. These objects were small and incredibly bright.

They named them quasi-stellar-objects or “quasars”, and then began to argue about what might be causing them. The first was found to be moving away at more than a third the speed of light.

But was it really?

An artist’s conception of jets protruding from an AGN.

Maybe we were seeing the distortion of gravity from a black hole, or could it be the white hole end of a wormhole. And If it was that fast, then it was really, really far… 4 billion light years away. And it generating as much energy as an entire galaxy with a hundred billion stars.

What could do this?

Here’s where Astronomers got creative. Maybe quasars weren’t really that bright, and it was our understanding of the size and expansion of the Universe that was wrong. Or maybe we were seeing the results of a civilization, who had harnessed all stars in their galaxy into some kind of energy source.

Then in the 1980s, astronomers started to agree on the active galaxy theory as the source of quasars. That, in fact, several different kinds of objects: quasars, blazars and radio galaxies were all the same thing, just seen from different angles. And that some mechanism was causing galaxies to blast out jets of radiation from their cores.

But what was that mechanism?

This artist’s concept illustrates a quasar, or feeding black hole, similar to APM 08279+5255,
where astronomers discovered huge amounts of water vapor. Gas and dust likely form a torus
around the central black hole, with clouds of charged gas above and below.
Image credit: NASA/ESA

We now know that all galaxies have supermassive black holes at their centers; some billions of times the mass of the Sun. When material gets too close, it forms an accretion disk around the black hole. It heats up to millions of degrees, blasting out an enormous amount of radiation.

The magnetic environment around the black hole forms twin jets of material which flow out into space for millions of light-years. This is an AGN, an active galactic nucleus.

When the jets are perpendicular to our view, we see a radio galaxy. If they’re at an angle, we see a quasar. And when we’re staring right down the barrel of the jet, that’s a blazar. It’s the same object, seen from three different perspectives.

Supermassive black holes aren’t always feeding. If a black hole runs out of food, the jets run out of power and shut down. Right up until something else gets too close, and the whole system starts up again.

The Milky Way has a supermassive black hole at its center, and it’s all out of food. It doesn’t have an active galactic nucleus, and so, we don’t appear as a quasar to some distant galaxy.

We may have in the past, and may again in the future. In 10 billion years or so, when the Milky way collides with Andromeda, our supermassive black hole may roar to life as a quasar, consuming all this new material.

If you’d like more information on Quasars, check out NASA’s Discussion on Quasars, and here’s a link to NASA’s Ask an Astrophysicist Page about Quasars.

We’ve also recorded an entire episode of Astronomy Cast all about Quasars Listen here, Episode 98: Quasars.

Sources: UT-Knoxville, NASA, Wikipedia

by FRASER CAIN on AUGUST 12, 2013

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