THE SOUTHWORTH PLANETARIUM
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Founded January 1970
2021-2022: CXXVI
"When you become uncomfortable with uncertainty, infinite possibilities open up in your life."
-Eckhart Tolle
THE DAILY ASTRONOMER
Wednesday, May 18, 2022
The Mass of Sagittarius A*
How often do we astronomy people toss numbers around like so much healthy Romaine -as opposed to nutrient-bereft iceberg- lettuce in a bona-fide garden salad replete with greens but devoid of tasty. but high-caloric garnishments?* Examples: "The Sun is about 93 million miles away," or "the Milky Way Galaxy" is 100,000 light years across or "the supermassive black hole occupying the galactic nucleus is 4.3 million times more massive than the Sun." These numbers all sound quite impressive, which explains why we love to toss them about . However, one must wonder: how do we know these values? After all, our most distant space probe, Voyager 1, is only now pushing beyond the heliosphere surrounding our own tiny little solar system.
While we can't explain every single number in one article (or even in 100 articles), we will try to explain how astronomers determined the mass of Sagittarius A*, the SBH in the galactic nucleus. The determination of this mass involves observations of stars within the vicinity of Sagittarius A*. By measuring the speeds and estimating the sizes of these stars, one can estimate Sagittarius A*'s mass.
As we can see below in a composite image captured by the UCLA Galactic Group, the paths of many stars close to Sagittarius A* are plotted. By knowing the distance to the nucleus (approximately 10,000 parsecs) one can convert the angular displacement into kilometers. By marking the positions over a given time interval, one can calculate the speeds of these stars.
So, why does this speed matter? Simply because the mass of Sagittarius A* determines the velocities of the stars surrounding it. This relation holds for any orbiting bodies. If the parent body is highly massive, the orbiting objects will move very swiftly. If one were to remove material from that central object, the speeds of the bodies revolving around it would be reduced.
Mathematically, the calculations yield the combined masses of the central object and that which orbits it. However, in most instances, the smaller object's mass represents a negligible fraction of the parent body's mass. This is particularly true Sagittarius A*, which astronomers have determined is 4.3 million times more massive than the Sun.
This example, which we hope made sense, illustrates the governing principle of astronomy: that nearly all the information garnered about the Universe is gathered remotely: inference at a distance.
*Yes, I'm an attentive, as well as uxorious, husband.
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