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C O U R S E 
From the Big Bang to Dark Energy
Hitoshi Murayama, University of Tokyo
C O U R S E   L E C T U R E 
The Milky Way and Its Black Hole
Notes taken on March 8, 2016 by Edward Tanguay
as you look far away in the universe, you are looking into the past
International Space Station
only very recent that humans have developed technology to have a station outside the earth
floating above the Earth at an altitude of 375 kilometers
the diameter of the Earth is 12,000 kilometers
suggests that moving out away from the Earth is not a very easy thing to do
travel to the moon
384,400 kilometers
nickname for SELENE (Selenological and Engineering Explorer)
launched on September 14, 2007
went to the other side of the moon
took pictures of Earth rise
because distances are so large in space, we use time to describe how far away objects are
light takes 1.3 seconds to reach the Moon
Moon is 1.3 light-seconds away
the Apollo astronauts put a mirror on the Moon
we can shoot a laser to it and it returns 2.6 seconds later
1.5 10^8 kilometers away
8.3 light-minutes away
an unmanned spacecraft developed by the Japan Aerospace Exploration Agency (JAXA) which returned a sample of material from a small near-Earth asteroid named 25143 Itokawa to Earth for further analysis
20 light-minutes away from the Earth (more than two times the distance to the Sun)
the command sent to the probe to deploy the Minerva lander arrived to late, since it took 20 minutes to get there
there is a 40-minute lag for making decisions of how the probe should interact with its environment
it had to decide as much as it could on its own
four light-hours away
human-made object that is the farthest from Earth
16-light-hours away
we are still communicating with it
closest star
Proxima Centauri
4.2 light-years
Voyager will have to travel for over 10,000 years to get to Proxima Centauri
Milky Way galaxy
we are in a galaxy which is a disc and when we look at the Milky Way we see the disc from the inside
we are 28,000 light-years away from the center of the galaxy
it's only 1500 light-years thick
our solar system is revolving around the center of the our galaxy
at a speed of 220 km/s
something is pulling us to the center of the galaxy
we know this to be a black hole
with infrared light we can see through the dust and debris in space to see the motion of stars at the center of the galaxy
something is pulling a star, and after it passes through, it speeds on quickly
it takes 20 years for a star to orbit around this very heavy object
by studying the motion of these stars, you can determine how massive the object is
the answer is staggering: the object in the middle of the Milky Way galaxy is four million times heavier than our Sun
nothing can be that heavy except for a black hole
black hole at center of the Milky Way
four million times our solar mass
other galaxies have heavier black holes
it can consume objects including gas around it
even the speed of light is not enough to escape the force of gravity from the black hole
just before gas enters the black hole, it flashes in light