How much does the Milky Way weigh?

Chuck Bednar for redOrbit.com – @BednarChuck

How much does the Milky Way weigh? It seems like a simple enough question, but it’s one that astronomers have struggled to answer for many years, as the galaxy contains such a vast amount of stars that it has been difficult to narrow down just how heavy they all are combined.

Now, writing in The Astrophysical Journal, a team of researchers led by Andreas Küpper from Columbia University’s Astronomy Department has discovered a new, more precise method of determining the vital statistics of our home galaxy. Using stars located outside of its giant disk, they were able to precisely determine the weight of the Milky Way.

The stars beyond the disk orbit around the galaxy in a stream-like structure. In their study, Küpper’s team demonstrated that these streams (which are created by dissolving globular clusters) not only tells us how heavy the Milky Way is, but can also determine the location of the sun within the galaxy.

Density wiggles in the Palomar 5 cluster were key

“Globular clusters are compact groups of thousands to several millions of stars that were born together when the universe was still very young,” the lead author explained. “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.”

Using data from the Sloan Digital Sky Survey, a project that produced a comprehensive catalog of stars over the course of a decade, they tested the technique on a stream produced by a globular cluster called Palomar 5. This cluster was discovered high above the galactic disk in 2001, and it was found to contain density wiggles that helped the researchers collect highly precise data.

They created millions of models of the stream in different realizations of the Milky Way using a Columbia University supercomputer, and compared these models to the actual wiggle patterns in the Palomar 5 cluster. By doing so, they were able to infer the Milky Way’s mass within a radius of 60,000 light years to be 210 billion times the mass of the Sun with just 20 percent uncertainty.

“An important advance in this work was using robust statistical tools – the same ones used to study changes in the genome and employed by Internet search engines to rank websites,” said Ana Bonaca, a co-author from Yale University. “This rigorous approach helped in achieving the high precision in weighing the Milky Way.”

“Such measurements have been tried before with different streams, but the results were always quite ambiguous,” added co-author Kathryn Johnston, chair of the Columbia Astronomy Department. Our new measurement breaks these ambiguities by exploiting the unique density pattern that Palomar 5 created as it orbited around the Milky Way for the past 11 billion years.”

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