spaceSpace and Physics

How Do You Weigh The Milky Way?

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Caroline Reid

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324 How Do You Weigh The Milky Way?
The Milky Way by NASA

Does this globular cluster make my butt look big? It's a problem any weight-conscious galaxy would be concerned about. But how does a galaxy know if it's piling on the pounds? You can't find out a galaxy's weight by asking it to step on some galactic scales. It's even harder to ascertain when you're inside the galaxy you're trying to weigh: the Milky Way. (Do you Milky Weigh it?) Fortunately, scientists from Columbia University think they have a solution. 

Measurements have been made in the past on the weight of the Milky Way, so a weight prediction for the galaxy isn't new. The technique published in The Astrophysical Journal is though, and it's refreshingly creative.


Objects with more mass have a stronger gravitational force—the gravity on Earth is stronger than that of the moon, for example. It follows, then, that the mass of the Milky Way is able to affect the orbits of satellite stars outside of its disk. These stars have some special properties that cause them to emit stellar streams in their wake. They come from disintegrating globular clusters.

"Globular clusters are compact groups of thousands to several millions of stars that were born together when the universe was still very young," said Andreas Küpper. "They orbit around the Milky Way and slowly disintegrate over the course of billions of years, leaving a unique trace behind. Such star streams stick out from the rest of the stars in the sky as they are dense and coherent, much like contrails from airplanes easily stick out from regular clouds."

The team looked at the stellar stream of a star called Palomar 5. The stream contained wiggles in it that appeared periodically. 

Palomar 5 stream via Columbia University


"We found the wiggles to be very pronounced and regularly spaced along the stream," said Eduardo Balbinot. "Such variations cannot be random."

Using this rhythmic data, the team created a computer model of the Milky Way and Palomar 5 and adjusted different variables to see what happened. They found that the path of the star was the most similar to real life when the mass of the Milky Way was 210 billion times the mass of the sun. It also had to be contained within a radius of 60,000 light-years. 

The next stage for the researchers is to continue this research and map the Milky Way in more detail. This will hopefully allow scientists to make improved predictions about the Milky Way's formation. 

[Via The Astrophysical Journal, The University of British Columbia, Science Daily]


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