Black Holes and their Spin

There are two basic characteristics that describe black holes: Mass and Spin. Mass determines the size of the event horizon, the gravitational mass, and many of the ways the black hole can gravitationally shred people, planets and just about anything else. Spin is related to the magnetic field (which can also shred people because of the magnetic properties of water), and it exerts many relativistic effects on its surrounding, such as frame dragging. Black hole spin also allows the black hole’s associated accretion disk to extend closer in toward the event horizon, creating a (with future higher resolution telescopes) a directly imaginable effect.

In a trio of spin related press releases, scientists described how to measure spin, the consequences spin has on how black holes merge, and results on a test to check if our understanding is wrong.

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A Brown Dwarf, A Black Hole, and 4 Jets …

A Brown Dwarf, A Black Hole, and 4 Jets …

phot-24-07-preview.jpgOpening my press release email folder this morning, I found what could have been the beginning of a good joke if I were actually a skilled humorist. So a black hole and a brown dwarf both start to form. As the black hole consumes his parent star, he shots powerful gamma ray jets off to announce his arrival. At the same time, a little brown dwarf, with a not so little planet, works to spring out of its proto-stellar cloud, and it blows with its feeble little jets as hard as it can to announce its arrival. The black hole looks at the brown dwarf and laughs and says “[insert something witty and demeaning]”. The brown dwarf, not one to be discouraged, just smiles and states “[Something thoughtful and witty that puts the black hole in his place]”

Not being a good humorist, I will not try to fill in the blanks. I will simply work to explain how two such very different objects can announce their formation via the same physical process.

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For every force there is an equal and opposite force…

One of the common questions I get is (averaged across many versions) “Why don’t all stars become black holes – don’t they all have gravity? And why don’t they start as Black Holes – didn’t they start with all that mass that made them become black holes?

Balancing stars against gravitational collapse is actually a process that is much more simple than many people think. When a star forms, the pressure and density in the center causes nuclear reactions to occur. These reactions release energy, partially in the form of photons, and the photons exert a pressure on the outer layers of the star. The light pressure pushes outward with the same force that the gravity presses inward. As long as nuclear reactions are occurring in the star’s center, the star doesn’t collapse. When stars die, their nuclear reactions stop and without the pressure from the light they collapse. If a star is similar to the Sun, it becomes a white dwarf, and the force of the electrons repelling one another supports the star. If a star is more massive, the electrons and protons in the stars atoms get crushed together and become neutrons, and the star is supported by the neutrons pushing against each other. If the star is even more massive, there is nothing left to support the star against gravitational collapse and it becomes a black hole.

So, for every force, their is an opposing force, and in black holes, well, inside the BH, we have no idea what is happening, but whatever it is, it kicks in after the material has collapsed small enough that we can get close enough to the center of mass that bad things can happen. Spaghetification anyone?

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