Star Death, Page 51: [Previous Page] [Next Page] [Class Home Page] Textbook pages 345 - 365

Black Holes

Stars with masses greater than 10 M suns will likely form Black Holes when they go Supernova. This is NOT entirely accepted, as indisputable evidence in favor of Black Holes is difficult to find.

However, NEUTRON DEGENERACY, which holds up a Neutron Star, will fail for collapsed cores greater than several Msuns. What are THESE objects going to look like? More importantly: Does it matter?

First, we must learn the definition of: Escape Velocity

1.) The kinetic (speed) energy of an object of mass, m, with velocity, V is:

1/2 m V²


2.) The orbital energy of an object, m, orbiting a larger object, M, is given by (R is radius, G is the Gravitational Constant):

(GMm)/R


3.) In order for object, m, not to fall into object, M, it must have kinetic energy equal or greater than this orbital energy. If you set to the terms above EQUAL to each other, then solve for velocity you get:

V = (2GM/R)^1/2


This is the ESCAPE VELOCITY of the object, m. Or another way of putting it: object, any object, needs at least this velocity to escape the gravitational pull of object, M. Note: the mass of the object, m, is not related to the velocity! The speed is the same, whether you are sending a baseball or a rocket into orbit!


Examples of escape velocities:

• The escape speed from the Moon's surface: 2 km / second

• The escape speed from the Earth's surface: 11.2 km / second

• The escape speed from the Sun's surface: 620 km / second

• The escape speed from the surface of a neutron star: 0.4 c (speed of light)

• The escape speed from the surface of a black hole: The speed of light!