Friday, March 22, 2013

Left Side of the Aisle #100 - Part 13

And Another Thing #5: Astronomers measure spin of supermassive black hole 56 million light-years away

For our final exploration, we star trek out to some 56 million light years away.

I want you to imagine a ball more than two million miles across - that's more than eight times the distance from Earth to the Moon - a ball two million miles across that's spinning so fast that its surface is traveling at nearly the speed of light.

Such an object actually exists: the supermassive black hole at the center of the spiral galaxy NGC 1365. And yes, that is a picture of NGC 1365.

This is the first time anyone has accurately measured the spin of a supermassive black hole, which is the huge kind of black hole that sits at the centers of galaxies, including our own Milky Way.

A black hole is an object where so much mass is concentrated in such a small volume that the gravity is so great that nothing, no even light, can escape. In fact, for a supermassive black hole, this gravity is so strong that, as the black hole spins, it literally drags spacetime around with it. It's called frame dragging.

As material spirals in toward the black hole, it accelerates and the friction among particles that results generates so much energy that it produces a stream of x-rays. As a result of the distortion of spacetime, the material can get closer to the black hole before being lost - so by seeing where the x-rays are coming from, astronomers can measure the black hole’s spin.

Why do they go through all this? Well, for one thing, just two numbers define a black hole: its mass and its spin. If you know those two numbers, you can know everything there is to be known about that black hole. More importantly, a black hole’s spin gives clues to its past - and therefore by extension, in the case of a supermassive back hole, the evolution of the galaxy in which it sits.

In other words, astronomers measure the spin of a supermassive black hole 56 million light years away because it can help us to understand how galaxies form. And that is just cool.


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