1. Black Holes Chapter 14

2. 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?

3. 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?

4. 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!

5. 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

6. 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.

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

8. Black Hole

9. 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?

12. 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