How do scientists weigh celestial objects?
Mike Sackheim
Evanston, Illinois
It's true that you can't simply place a planet or galaxy on a scale to measure its weight. Fortunately, astronomers have a few tricks up their sleeves.
The first trick is to understand that gravity and mass are inherently linked. It is important to note that weight, which measures the strength of the local gravitational attraction on an object, can change, while mass cannot. For example, if you You step on a scale on Earth and weigh 150 pounds. (68 kilograms), that same scale would read 379 pounds (172 kg) on Jupiter. Your personal mass is not what is changing, but your weight because more massive planets exert a greater gravitational pull on you than less massive ones.
So to find the mass of an object, astronomers can simply look at how long it takes nearby bodies to orbit that object. If they know the distance between the bodies, they can calculate the mass of the central body. In the case of binary stars, astronomers can observe the stars orbiting each other to determine their combined mass. If the stars are close and astronomers can see how closely each star orbits around their common center of mass, they can determine each star's individual mass. For galaxies it is a little different, but when examining How fast does a galaxy rotate?Researchers can similarly determine its mass.
There is another common trick that astronomers can use to estimate mass: luminosity. In most cases, a star or galaxy's luminosity (how brightly it shines) is approximately proportional to its mass. So as long as you know one, you can solve the other. For stars, scientists use computer simulations of how these objects evolve to better understand the relationship between a star's mass and its luminosity, as well as other parameters that can be observed, such as temperature and composition.
Caitlyn Buongiorno
Associate Editor
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