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Secrets of the Stars

Red dwarfs could reveal mysteries of the Milky Way


An edge-on view of a spiral galaxy shows stars divided into strata, with the younger stars located mostly in the middle layers and older stars in the outer layers. Photo below by Vernon Doucette

Can the smallest stars in the night sky shed light on the structure, dynamics, and evolution of the entire Milky Way galaxy? Andrew West, a College of Arts & Sciences assistant professor of astronomy, is staking his career on it, and the data he has collected and analyzed so far suggest that he is onto something.

Occupying between one-thousandth and one-tenth the volume of the sun, red dwarfs are the smallest stars in the Milky Way. They are also the coolest and least luminous. But what they lack in size, temperature, and brightness, they make up for in number—they account for 70 percent of all stars in the galaxy.

Dubbing red dwarfs the “VWs of the Milky Way,” West says that “these stars are very dim, but fuel-efficient, and on average could burn for trillions of years. Because they’re so huge in number and last almost forever, large samples of them can allow you to probe the shape, structure, and evolution of our own galaxy like no other stars.”

Drawing on the Sloan Digital Sky Survey and other deep, all-sky surveys that show millions of red dwarfs extending across the night sky, West and his collaborators have built a sample set of about 50 million of the small stars. Likening the Milky Way galaxy to a Frisbee, he has devised a way to determine the likely age of each star based on its position with respect to the center line of the Frisbee, that part of the galaxy where all stars are born.

“As stars age, they interact with other stars and cold molecular gas, and their velocity increases,” West (right) says. “These aging stars gradually move away from the central plane of the galaxy. At any one moment, the youngest stars tend to orbit closer to the plane; the further you go from the plane, the older the stars are.”

By deducing the relative ages of the red dwarfs and obtaining light spectra that reveal information about their physical composition, temperature, and magnetic fields, West is tracing how the Milky Way has evolved over time. To achieve that objective, he is leading a research team that includes BU graduate and undergraduate students, Dan Clemens, a CAS astronomy professor, and astronomers from Cornell, MIT, and the University of Washington.

Noting that red dwarfs are the most common stars around which planets are likely to be found, West is homing in on how old a star needs to be for its magnetic field (and resulting large stellar flares) to dissipate sufficiently to sustain orbiting planets, and for its atmosphere to contain heavy elements indicative of the presence of carbon, oxygen, and other elements that support life.  

“While we already understand the basic sequence of the chemical and magnetic evolution of stars in the galaxy, it’s the timescale for that evolution that’s not well known,” says West. “The more we learn about this timescale, the more we will come to understand about the abundance of stars that can support habitable planets in the galaxy.”

Mark Dwortzan can be reached at dwortzan@bu.edu.

This story originally appeared in the 2010 issue of Research magazine.

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