THE SOUTHWORTH PLANETARIUM
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Altitude: 10 feet below sea level
Founded January 1970
Julian date: 2458774.16
2019-2020: XXXIII
"“It's not all bad. Heightened self-consciousness, apartness,
an inability to join in, physical shame and self-loathing—they are not all
bad. Those devils have been my angels. Without them I would never have
disappeared into language, literature, the mind, laughter and all the mad
intensities that made and unmade me.” -Stephen Fry
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Happy 25th Birthday, Nick!
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THE DAILY ASTRONOMER
Thursday, October 17, 2019
Weighing the Black Hole
[image: gcradio_arc_vla.jpg]
The center of the *Milky Way Galaxy*, as seen in radio waves. IMAGE: NRAO
Today, when we dipped into Pandora's Jar, the vessel in which we store
astronomy questions written on pieces of parchment, we withdrew one of our
favorite types of questions: the "how do we know" question. This
subscriber's query pertains to the supermassive black hole located within
Sagittarius A*, a powerful radio source in the galaxy's center. The
question reads
*"I heard that the supermassive black hole in the galaxy's center is 4.6
million times more massive than the Sun. I also know that we're 23,000
light years away from it, so how can we possibly know how much that black
hole weighs?" -Peter D, Woolwich*
We can begin answering this question by first playing pretend in our own
solar system. Imagine that we magically add a significant amount of
matter to the Sun while maintaining Earth's orbital distance. What would
happen? Would Earth fall into the Sun? Not exactly. It would merely
revolve around the Sun more quickly. We know that a planet's orbital
period relates directly to its mean distance. The masses of both bodies
also affect this period directly. If the Sun were more massive, Earth would
revolve more rapidly. After astronomers determined Earth's mean distance
from the Sun, they were able to determine the masses of both Earth and the
Sun through the relationship between mass and orbital period. As
Earth's mass is negligible compared to the Sun's, this measurement
effectively yielded the Sun's mass.
[image:
1920px-Artist_impression_of_a_supermassive_black_hole_at_the_centre_of_a_galaxy.jpg]
The *black hole in the galactic center* is about 4.6 million times more
massive than the Sun. Astronomers were able to make such a determination
based on measurements of the velocities of stars within this region.
The image above is an artistic depiction. Image: Wikipedia
By measuring the velocities of stars within the vicinity of this black
hole, astronomers were able to measure the black hole's mass. The cubic
central parsec around Sagittarius A* contains approximately 10 million
stars. These stars are moving far more rapidly than the stars within
outer parts of the galaxy. Examinations of specific stars in the galactic
center region yielded the mass of about 4.6 million solar masses (4.6
million times more massive than the Sun.)
This black hole is estimated to be about the size of Earth's orbit around
the Sun (though this value is difficult to measure). Such matter
compression into such a small volume indicates the presence of a black
hole, as opposed to a region of unusually high stellar density.
In astronomy, if it is not observed, it is inferred. Astronomers can't
measure a black hole's mass directly. They can, however, measure the
motions of stars around the galactic center and from these measurements
yield a value of the black hole's mass.
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