Thick Disc of Older Stars

The Science Teacher, April-May 2011 | Go to article overview

Thick Disc of Older Stars


An international team of astronomers has identified a thick stellar disc in the Andromeda galaxy, the nearest large spiral galaxy to the Milky Way.

The discovery of the thick disc, a major result from a five-year investigation, can help astronomers better understand the processes involved in the formation and evolution of large spiral galaxies, according to University of California--Los Angeles research astronomer Michael Rich and his colleagues from Europe and Australia.

Using the Keck Telescope in Hawaii, the astronomers analyzed the velocities of individual bright stars within the Andromeda galaxy, observed a group of stars tracing a thick disc--distinct from those comprising the galaxy's already-known thin disc--and assessed how these stars differ from thin-disc stars in height, width, and chemistry.

Approximately 70% of Andromeda's stars are contained in the galaxy's thin stellar disc. This disc structure contains the spiral arms traced by regions of active star formation and surrounds a central bulge of old stars at the core of the galaxy.

"From observations of our own Milky Way and other nearby spirals, we know that these galaxies typically possess two stellar discs, both a thin and a thick disc," says Michelle Collins, a doctoral candidate at the University of Cambridge's Institute of Astronomy, who led the study.

The thick disc consists of older stars whose orbits take them along a "thicker" path--one that extends both above and below the galaxy's thin disc. …

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