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The last days of massive stars
Outer layers expand as helium
core contracts
• Helium fuses to form carbon,
carbon fuses with helium to
make oxygen, and heavier
and heavier nuclei get built
until iron (Fe) is made
• Iron does not fuse because it
takes energy to fuse Fe to
heavier elements (star
doesn’t have enough energy
to do so)
• Whole things
stops…equilibrium breaks
down and something bad is
about to happen
Supernovae
“Crab Nebula” remnant of supernova noted
by Chinese, Japanese, and Korean astrologers
in 1054
Now that there is no pressure pushes
out….gravity wins the long-lasting battle
• In less than 1 second, the entire star
collapses in on itself, hits the iron core
and bounces off to release an EXTREME
amount of energy
• As bright as an entire galaxy of 10 billion
stars
• Only high-massive stars go supernovae
(must be at least 3x the mass of Sun)
• Can emit as much energy in this
explosion as the Sun does in its entire
lifetime
• All of the material is explodes out up to
10% the speed of light (30,000 km/s)
• No supernova has been witnessed since
1604 (Kepler)
• On average, this event occurs 3 times
every century in the Milky Way
Black Holes
Black holes are insanely dense….
• Think of a star ten times more massive
than the Sun squeezed into a sphere
approx. the size of New York City!
After some massive stars (greater than 10
suns) explode as supernovae, they will retain
a mass of 2 to 3 solar masses in their cores
• Nothing in the universe is strong enough
to hold up the remaining mass against the
force of gravity, so it collapses into a black
hole
• Matter that falls into a black hole
disappears from contact with the rest of
the Universe….not even light can escape
the gravitational pull of a black hole
• The “escape velocity” exceeds the speed
of light
• Black holes are a consequence of
Einstein’s theory of gravity which is called
General Relativity
Escape Velocity
Think about throwing a tennis ball up
• The faster the ball is traveling when
it leaves your hand, the higher the
ball will go before turning back
• If you throw the ball hard enough it
will never return, the gravitational
attraction will not be able to pull it
back down (e.g. a rocket into space
is moving this fast)
• The velocity that the ball must have
to escape is known as the escape
velocity (you must be traveling
approx. 25,000 mi/hr to escape
Earth’s gravitational attraction)
• The more massive or denser a
planet (or star), the larger the
escape velocity
• Black holes are so massive that the
escape velocity is greater than the
speed of light
Recent History of Black Holes
1915, just after Einstein published his theory of gravity,
Karl Schwartzchild showed that black holes could
theoretically exist
• Until the 1907’s, most physicists believed that black
holes wouldn’t happen in nature
• In 1977, Stephen Hawking & Roger Penrose proved
that black holes are a generic feature in Einstein’s
theory of gravity, and cannot be avoided in some
collapsing objects
• The expression “black hole” was coined by theoretical
physicist John Wheeler in a public lecture in 1967
Black Holes
Scientists can’t directly observe black
holes with telescopes that detect x-rays,
light, or other forms of electromagnetic
radiation
• We can infer the presence of black
holes by detecting their effect on
other matter nearby
• If a black holes passes through a
cloud of interstellar matter, it will
draw matter inward in a process
known as accretion
• A similar process can occur if a
normal star passes close to a black
hole
• As the attracted matter accelerates
and heats up, the black hole “eats
too much at one time” and emits xrays that radiate into space and
powerful gamma-ray bursts
Event Horizon
1916 K. Schwartzchild showed that if an
object was smaller than a characteristic
radius “the Schwartzchild radius” then
even light moving straight out would not
escape
• This imaginary radius is known as the
“event horizon”
• If matter (or light) falls within this
radius, it is doomed to be pulled in
Frequently Asked Questions About
Black Holes
How is time changed in a black hole?
•
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Einstein proved that time is relative (my time may not be the same as your time depending
on our different velocities)
Faster you travel, the slower time “ticks” for you when observed by someone who is not
going as fast as you are (you would observe time as “ticking” at its normal rate)
If you were just outside the event horizon, time would be slowed time dramatically for you
while your friends back on Earth would go through time as normal
If you stayed around this event horizon long enough and returned back to Earth some
number of years later, you would find that your friends became old (maybe even dead) while
you had barely aged at all
How big can a black hole get?
•
There is no limit to how large a black hole can be. However, the largest black holes we think
are in existence are at the centers of many galaxies, and have masses equivalent to about a
billion suns. Their radii would be a considerable fraction of the radius of our solar system
(150,000 light years)