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How do you weigh the Milky Way anyway?

A team of astronomers has calculated the weight of the Milky Way to a high degree of precision by observing stars outside the galactic disc.

Nearby spiral galaxy NGC 6744 is similar to the Milky Way.ESO

It's not as though, to gauge the weight of a galaxy, you can just pop it on scales and have the figure shown on a display. In order to calculate the mass of a cosmic object, astronomers need to observe the effect it has on other objects around it.

In the case of stars and planets, this is achieved by using Newton's Law of Universal Gravitation, which states that two bodies will be attracted to each other with a force that is directly proportional to the product of their masses, and inversely proportional to the square distance between them. It's maffs innit.

When it comes to galaxies, it's a lot harder to calculate mass. The Milky Way galaxy for example, contains around 100 billion stars, which form a disc with a diameter between 100,000 and 200,000 light-years. This wide variable makes it difficult to measure.

The weight of the Milky Way is hotly contested and has only been calculated within a factor of four -- that is, its estimated maximum weight is four times its estimated minimum weight.

To clarify the weight of the Milky Way, an international team of researchers led by Columbia University's Andreas Küpper investigated streams of stars outside the galaxy's borders, publishing their findings in The Astrophysical Journal.

The star streams come from dissolving globular clusters, born when the universe was still in its infancy. These streams, the team demonstrated, can be used to determine both the weight of the galaxy and also the location of the sun within it.

"They orbit around the Milky Way and slowly disintegrate over the course of billions of years, leaving a unique trace behind," Küpper explained on his blog. "Such star streams stick out from the rest of the stars on the sky as they are dense and coherent, much like contrails from airplanes easily stick out from regular clouds."

Sloan Digital Sky Survey image of the Northern Hemisphere sky. The Palomar 5 stream is the densest discovered so far. Ana Bonaca, Marla Geha and Nitya Kallivayalil with data from the Sloan Digital Sky Survey

Using data from the Sloan Digital Sky Survey, which collated a decade's worth of scans of the northern sky for a comprehensive star catalogue, the team examined a stream created by globular cluster named Palomar 5, discovered in 2001. Co-author Eduardo Balbinot of the University of Surrey, England, found density wiggles -- inconsistency in the density of the stream.

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

The team then created several million models of the stream using Columbia University's Yeti supercomputer. By comparing observational data of the wiggle pattern with the models, the team was able to calculate the mass of the galaxy: only within a certain narrow size and weight could the galaxy have produced the Palomar 5 wiggles. Smaller or larger models would have produced different patterns.

The team calculated the mass of the Milky Way within a radius of 60,000 light-years to be 210 billion solar masses, with an uncertainty of only 20 percent.

"In the future, we aim at using more structures like the Palomar 5 stream to gain an even higher precision and to create the most realistic model of the Milky Way to date. From the improved precision we hope to learn about the formation and composition of our home galaxy, and to understand how the Milky Way compares with other galaxies in the Universe," Küpper said.

"So far, the results indicate that the Milky Way is a healthy patient -- neither too skinny nor too heavy for its size."