Download Black Hole at Galactic Center

Mystery at the Galactic Core
Here is a simulation plot* of 21 stellar motions around the core of our Milky Way Galaxy. Notice that in the 16
years we have been following these stars, S0-2 has managed to finish one complete orbit around the mysterious,
non-luminous object at the galactic core. S0-2 is the only star to complete a galactic orbit in the time you have
been alive! (Remember, it takes the sun about 230 million years to complete an orbit around the galactic center.)
A) What angle θ does the semi-major axis of
star S0-2 (which is rave ) subtend? Convert
this angle to degrees. Note that 0.1” means
0.1 seconds of arc. Use the scale
Center of our Milky way Galaxy
B) Now calculate the size of the semi-major axis, rave, in meters. Draw the relevant triangle! Label the
distance to the Galactic center d, where d = 26,000 LY
* http://www.astro.ucla.edu/~ghezgroup/gc/pictures/orbitsMovie.shtml Simulation runs from 1995.5-2010.3
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C) Find the mass M of the object being orbited, first in kg, then in solar masses. (Use the 3 Step Plan, beginning
with a sketch of the equivalent, circular orbit with radius rave.)
D) The Keck/UCLA group did more careful measurements, concluding that T = 15.78 years and rave = 980
AU’s. Do a better mass calculation on the object at the core, first in kg, then in solar masses. Just plug your
converted numbers for T and rave into your algebraic solution for M from part C).
(You should have gotten about 4 million solar masses! Did you?)
E) One star (S0-16) is known to pass within 90 AU’s of the galactic center! Watch it wiz by on the simulation.
This means that all the mass you calculated above must reside in a non-luminous object roughly smaller than
the size of our solar system. Humm, black and super high density…What could it be?
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