Black Holes Chapter 14

Review What is the life cycle of a low mass star (<8 solar masses when on the main sequence)? What is the life cycle of a high mass star (>8 solar masses when on the main sequence)? After a supernova, what are the two fates of the core of the star? What determines whether the core will be a neutron star or a black hole?

Topics Black hole what is it? why is it so hard to detect? how do we detect it? how big can it be? how small is it?

How it begins A type II supernova is the explosion of a giant star. The core collapses. If M<3 solar masses, it will be a neutron star. If M>3 solar masses, even a neutron gas cannot withstand the large gravitational force. The core continues to collapse; its volume decreases to zero. The density becomes infinite. Note: the mass is still finite!

Characteristics At the center of the black hole, the gravitational force is infinite. Light inside a certain radius cannot escape the gravitational force. Schwartzchild radius Imaginary surface at this radius is the event horizon Light inside this event horizon would orbit the black hole. 3 solar mass black hole, R=9 km

Is it a cosmic vacuum cleaner? No, as long as you’re outside the Schwartzchild radius. For example, if the Sun were a black hole of the same mass, the Earth’s orbit would be NO different.

What does a black hole look like? Look very closely!

Black Hole

So how do we detect it? The same way you detect Aunt Edna - by the effect she has on others. If black hole is part of a binary system with giant, it accretes matter from the giant. as the matter accelerates, it radiates x-rays accretion disk it could be a neutron star, so we must measure mass; how do we do that? Bends light that passes nearby gravitational lensing Nearby objects orbit with very high orbital velocities and small periods centers of galaxies are likely to be very massive black holes so how do we know how fast something is moving?

What makes supermassive black holes? Typically at centers of galaxies blackholes gobble up stars and even each other M~ millions of solar masses see press release