QUESTION:

How does a black hole work?

ANSWER:

            A black hole is what is left over when a massive star has run out of fuel and collapsed. Why does that happen?

            There are two main, competing processes that shape stars. The fusion reactions are similar to tiny hydrogen bombs going off and tend to make the star bigger. At the same time, gravity tends to crunch all solar material and make the star smaller. These two forces are balanced throughout a star’s life, which typically lasts for billions of years.

             The size of a star is determined by this balance between gravity, making it smaller, and explosive forces, making it bigger, and this balance shifts only at the end of a star’s life, when the ultimate fate of any star is determined by its mass.

What happens to a star the size of our Sun? When nearly all the hydrogen is converted to helium, gravity will dominate and the star will collapse, ignite the nuclear ashes of helium, and fuse them into carbon. The star will then expand to the size of the orbit of Mars, at which point it is a Red Giant. After a few million years, the helium will be all burned out, the Red Giant will collapse, and the sun will become a cool cinder, called a Black Dwarf. Not a glorious end for our beautiful life-giving Sun!

            When all of the star’s hydrogen is used up, the fusion reactions stop, and the star’s gravity takes over, pulling material inward and compressing the core. This compression generates heat, which eventually leads to a supernova explosion, which blasts material and radiation out into space. This debris material goes into eventually making new stars. Stars are born, live out their lifetimes, and ultimately die.

            The story is quite different for a star three or four times the mass of the sun. Once nuclear fusion is done, the collapse doesn’t stop. The star not only caves in on itself, but the atoms that make up the star collapse so there are no empty spaces. What is left is a core that is highly compressed, very massive, and very dense. Gravitation is so strong near this core that light can’t even escape. The particles within the core have collapsed and crushed themselves out of visible existence. The star disappears from view and is now a black hole.

            If we can’t see a black hole, how do we know they exist? Though they’re not visible, we can detect or hypothesize about the presence of one by studying surrounding objects. Astronomers can see material swirling around or being pulled off a nearby visible star. The mass of a black hole can be estimated by observing the motion of nearby visible stars.

The core, or nucleus, of Galaxy NGC 4261, for example, is about the same size as our solar system, but it weighs 1.2 billion times as much as our sun. Such a huge mass for such a small disk indicates the presence of a black hole. The core of this galaxy contains a black hole with huge spiral disks feeding dust and material into it.

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