THE SOUTHWORTH PLANETARIUM

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70 Falmouth Street   Portland, Maine 04103

43.6667° N                   70.2667° W 

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

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. 

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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