UNIT 3 INPUT 2: Notes on Black Holes (BH):
Process of Formation of BH:
The fuel of a giant star finish A giant star dies (3 times bigger than the Sun)  crushed into a single
point  black hole formed
Main structural features of BH:
Singularity: Giant star dies  mass squeezed into a single point singularity
Characteristics: space & time stop
Mass  no volume
Time  not pass
Event Horizon: Within a certain distance of singularity: gravitational pull  very strong  nothing (not
even light) could escape.
Characteristics: Not a physical boundary
The point-of-no-return nothing (even light) could get out
Size of black hole = Size of event horizon
The more mass of singularity = the larger event horizon
Out of event horizon  not pulled into BH
Travel faster than speed of light  get out of black hole (but nothing could travel
faster than speed of light)
All objects within event horizon (or black hole)  hit singularity (r=0)
Mass of a black hole= Mass of the star (Just compressed once it is black hole)
Not easy to detect:
Truly black; don’t shine; Light rays get too close  bend into BH and cannot get
out of BH. However, possible to detect by observing
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Emission of x-rays from dust particles that speed up and heat up
when pulled into black hole
Rotating stars  Stars at a safe distance from event horizon
circle (orbit) safely around BH
Gravity lenses
Key terms:
x-rays = dust particles pulled into black holes seed up and heat up and emit x-rays
Rotating stars= Stars rotate around black holes when they are at a certain distance from the event
horizon
Gravity lenses: When a big object passes between a star and the Earth, the object acts like a lens and
causes the star to brighten (used to detect BH)
Radial distance: How far away one is from the singularity
Escape velocity: the speed needed for a star to break free from another’s gravity
Spherical surface (SS): A BH from a large distance seen as unmoving SS.