The Andromeda galaxy (M31), as imaged from a ground-based telescope. Image credit: Adam Evans, under a c.c.-by-s.a. 2.0 license.

The stars of Andromeda, inside and out

How the bulge, disk, and halo stars of Andromeda reveal lessons we cannot see about our own galaxy.


“He who would search for pearls must dive below.” -John Dryden

The Milky Way’s plane obscures our view of most stars in our own galaxy, but an even grander spiral — Andromeda — lies 2.5 million light years away.

A Mosaic of the 117 million resolved stars — plus many more unresolved ones — in the disk of the Andromeda galaxy. Image credit: NASA, ESA, J. Dalcanton, B.F. Williams, L.C. Johnson (University of Washington), the PHAT team, and R. Gendler.

Even at this modest distance, incredible telescope and camera technology is needed to resolve individual stars in a galaxy beyond our own.

The Hubble Space Telescope recently completed the Panchromatic Hubble Andromeda Treasury, mapping a third of Andromeda’s disk and resolving over 117 million individual stars.

Closeup of a large region of the Andromeda galaxy’s disk, containing hundreds of open star clusters (identifiable as bright blue sparkles). Image credit: NASA, ESA, J. Dalcanton, B.F. Williams, L.C. Johnson (University of Washington), the PHAT team, and R. Gendler.

The most metal-rich stars are found near the central bulge, with the newest, bluest stars found in the open clusters.

Six of the most spectacular star clusters in Andromeda. The brilliant red star in the fifth image is actually a foreground star in the Milky Way. Over a thousand new clusters were found in this survey. Image Credit: NASA, ESA, and Z. Levay (STScI); Science Credit: NASA, ESA, J. Dalcanton, B.F. Williams, L.C. Johnson (University of Washington), and the PHAT team.

Far outside of the center, in the outer disk and the faint galactic halo, a different set of populations thrive.

Image credit: NASA, ESA and T.M. Brown (STScI), of the stars in Andromeda’s outer disc.

The outer disc of Andromeda (above) shows a wide variety of stars, including many Sun-like ones and older variables.

Image credit: NASA, ESA and T.M. Brown (STScI), of the stars in Andromeda’s giant stellar stream. The Milky Way’s foreground stars are clearly identified by their diffraction spikes.

The stars from the giant stellar stream are also densely packed, obscuring the Universe beyond.

Image credit: NASA, ESA and T.M. Brown (STScI), of the sparser, older stars in Andromeda’s halo, along with background galaxies.

While the diffuse halo’s low-density regions contain many of the oldest, least evolved stars.

Image credit: NASA, ESA and T.M. Brown (STScI), of another view of the sparser, older stars in Andromeda’s halo, along with background galaxies.

They’re lower in heavy elements than any stars found in the disk,

Image credit: NASA, ESA and T.M. Brown (STScI), of background galaxies seen through the halo of Andromeda.

with galaxies up to billions of light years away visible through the gaps in the halo stars.


Mostly Mute Monday tells the story of a single astronomical phenomenon or object in visuals, images and video in no more than 200 words.

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