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Messier 31

Persian astronomer Abd al-rahman al-Sufi’s The Book of Fixed Stars from the year 964 contains the first known report of Messier 31.

about Messier 31


2 million light-years

Apparent Magnitude




object type

Spiral Galaxy

This sweeping bird's-eye view of a portion of the Andromeda galaxy (M31) is the sharpest image ever taken of the galaxy.
NASA, ESA, J. Dalcanton, B.F. Williams and L.C. Johnson (University of Washington), the PHAT team and R. Gendler

Assembled from a total of 7,398 exposures taken over 411 individual pointings of the telescope, this image of our nearest major galactic neighbor, M31, is the largest Hubble mosaic to date. The 1.5 billion pixels in the mosaic reveal over 100 million stars and thousands of star clusters embedded in a section of the pancake-shaped disk of M31, also known as the Andromeda galaxy. Though the galaxy is over 2 million light-years away, Hubble is powerful enough to resolve individual stars in this 61,000-light-year-long stretch of the disk. It’s like photographing a beach and resolving individual grains of sand.

M31 mosaic, inset in ground-based image
Hubble’s M31 mosaic image, taken by the Panchromatic Hubble Andromeda Treasury (PHAT) program, is shown in context with a ground-based image of the entire galaxy. Despite the size of Hubble’s massive mosaic, it does not span even half of the galaxy.
NASA, ESA and Z. Levay (STScI/AURA); PHAT Mosaic: NASA, ESA, J. Dalcanton, B.F. Williams, L.C. Johnson (University of Washington), the PHAT team and R. Gendler; Ground-based Background Image of M31 (c) 2008 R. Gendler, used with permission
central core of M31
This image from Hubble spans 7,900 light-years and reveals M31’s crowded central region. The bright area to the right of the center is a grouping of stars nestled around the galaxy’s black hole. The blue dots sprinkled throughout the image are ultra-blue stars that have prematurely cast off their outer layers of material, exposing their extremely hot cores.
NASA, ESA and B. Williams and J. Dalcanton (University of Washington, Seattle)

Hubble traces densely packed stars extending from the innermost hub of the galaxy, seen at left. Moving out from this central galactic bulge, the panorama sweeps across lanes of stars and dust to the sparser outer disk. Cooler, yellowish stars dominate the center of the galaxy, toward the lower left. The blue, ring-like feature that wraps from the upper left to the lower right is a spiral arm with numerous clusters of young, blue stars and star-forming regions. The dark silhouettes trace out complex dust structures.

M31 is located in the constellation Andromeda and is best observed in November. Boasting an apparent magnitude of 3.1, the galaxy can be seen with the naked eye, even in areas with moderate light pollution. Because it is such an easily observed feature in the night sky, it is impossible to say who discovered the Andromeda galaxy. However, Persian astronomer Abd al-rahman al-Sufi’s The Book of Fixed Stars from the year 964 contains the first known report of the object.

For more information on Hubble’s observations of M31, see:

locator star chart for M31
This star chart for M31 represents the view from mid-northern latitudes for the given month and time.
Image courtesy of Stellarium

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