01:750:109 Chapter Notes - Chapter 10.2: Observable Universe, Strong Gravity, White Dwarf
Black Holes
●Sometimes the gravity of a stellar corpse becomes so strong that nothing can prevent it from
collapsing under its own weight
●The stellar corpse collapses without end, crushing itself out of existence and forming perhaps
the most bizarre type of object in the universe: a black hole
What are black holes?
Escape Velocity
●Conditions required for gravity to keep something from escaping
●The velocity a rocket needs to escape the Earth's orbit is the escape velocity
bc the velocity it
needs in order to escape does not depend on the spacecraft's mass
○The escape velocity from Earth's surface is about 40,000 km/hr, or 11km/s
■This is the minimum speed required to escape Earth's gravity for any object that
starts near Earth's surface
○Ex: imagine if we could make Earth smaller while keeping its mass the same. Making
Earth more compact would raise the escape velocity bc the strength of gravity at Earth's
surface depends on the inverse square of its radius
○If we could make Earth the size of a golf ball, its escape velocity would reach the speed
of light
●Light emitted by such an object ^^ would behave like a rock thrown upward, eventually slowing
to a stop and falling back down
●Einstein's theory of relativity shows that extremely compact, massive objects have the basic
idea: it is indeed possible for an object's gravity to be so strong that not even light can escape
○Aka "black holes"
○"black" bc no light can escape
○"hole" bc its like a hole in the observable universe in the sense that if you enter a black
hole, you leave the region of the universe that we can observe with a telescope and can
never return
Black Hole Formation
●Forms when a massive object is somehow squeezed down to a much smaller size
●We have seen that extreme squeezing in a massive star with its iron core reaching the white
dwarf limit 1.4MSun causes electron degeneracy power to no longer resist gravity and the core
collapses to form a neutron star, causing a supernova explosion
○Calculations show that the mass of a neutron star has a limit that lies somewhere btwn
2 and 4 solar masses
■Above this mass, neutron degeneracy pressure cannot hold off the crush of
gravity in a collapsing stellar core
Document Summary
Sometimes the gravity of a stellar corpse becomes so strong that nothing can prevent it from collapsing under its own weight. The stellar corpse collapses without end, crushing itself out of existence and forming perhaps the most bizarre type of object in the universe: a black hole. Conditions required for gravity to keep something from escaping. The velocity a rocket needs to escape the earth"s orbit is the escape velocity bc the velocity it needs in order to escape does not depend on the spacecraft"s mass. The escape velocity from earth"s surface is about 40,000 km/hr, or 11km/s. This is the minimum speed required to escape earth"s gravity for any object that starts near earth"s surface. Ex: imagine if we could make earth smaller while keeping its mass the same. Earth more compact would raise the escape velocity bc the strength of gravity at earth"s surface depends on the inverse square of its radius.
