Hubble finds Milky Way's central bulge to be an active stellar region

Bulge stars' motions seem to vary based on their chemical compositions.
By Laurel Kornfeld | Jan 18, 2018
Once thought to be a quiet region inhabited largely by old stars, the Milky Way's central bulge has now been found to be an area pulsing with stellar activity.

The new findings come from a nine-year Hubble Space Telescope study of 10,000 Sun-like stars in the galaxy's central bulge.

A research team led by Will Clarkson of the University of Michigan at Dearborn studied data captured by Hubble's Advanced Camera for Surveys and Wide Field Camera 3 in two separate projects--the Wide Field Camera 3 Galactic Bulge Treasury Program and the Sagittarius Window Eclipsing Extrasolar Planet Search.

Their study marks the first time anyone has successfully measured the motions of stars in the galactic bulge, approximately 26,000 light years away.

Stars in this crowded region are of various ages and travel at many different speeds. Those composed largely of hydrogen and helium appear to move chaotically and slowly while their counterparts containing heavier elements move in a more orderly fashion and orbit the galactic center more quickly.

The researchers determined stars' chemical compositions by studying their colors.

"By analyzing nine years worth of data in the archives and improving our analysis techniques, we have made a clear, robust detection of the differences in motion for chemically deficient and chemically enriched Sun-like stars," Clarkson stated.

"We hope to continue our analysis, which will allow us to make a three-dimensional chart of the rich chemical and dynamical complexity of the populations in the bulge."

Hubble is the only telescope with a high enough resolution to simultaneously measure the motion of the numerous stars in this region.

Accurate measurement of stellar motion also requires observations over a long period of time.

Earlier studies of the galactic bulge had focused solely on red giant stars, which are stars nearing the ends of their lives.

"Hubble gave us a narrow, pencil-beam view of the galaxy's core, but we are seeing thousands more stars than those spotted in earlier studies," emphasized Annalisa Calamida of the Space Telescope Science Institute in Baltimore and member of the Hubble study team.

Understanding stellar motions in the bulge will help scientists better piece together the evolution of the Milky Way. Currently, there are several competing theories regarding the time the bulge region formed.

"Some say the bulge formed when the galaxy first formed about 13 billion years ago," Calamida explained. "In this case, all bulge stars should be old and share a similar motion. But others think the bulge formed later in the galaxy's lifetime, slowly evolving after the first generations of stars were born. In this scenario, some of the stars in the bulge may be younger, with their chemical composition enriched in heavier elements expelled from the death of previous generations of stars, and they should show a different motion compared to the older stars. The stars in our study are showing characteristics of both models."





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