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Messier 31

M31, also well-known as the Andromeda Galaxy, is the nearest major galaxy to our own, the Milky Way.

Distance

2.5 million light-years

Apparent Magnitude

3.1

constellation

Andromeda

object type

Spiral Galaxy

The Andromeda galaxy, a spiral galaxy, spreads across the width. It is tilted nearly edge-on to our line of sight so that it appears as an extreme oval on its side. The borders of the galaxy are jagged because the image is a mosaic of smaller, square images. The outer edges are blue, while the inner two-thirds are yellowish with a bright, central core. Dark, dusty filamentary clouds wrap around the outer half of the galaxy’s disk. At 10 o'clock, a smaller dwarf elliptical galaxy forms a fuzzy, yellow blob. Hubble's sharp vision distinguishes about 200 million stars within the image. The background of space is black. There are what appears to be steps toward the bottom, mainly toward the middle, which indicates where no data were taken.
In January of 2025, NASA's Hubble Space Telescope released this panoramic view of the neighboring Andromeda galaxy. It took over 10 years to make this vast and colorful portrait of the galaxy, requiring over 600 Hubble overlapping snapshots that were challenging to stitch together. The galaxy is so close to us, that in angular size it is six times the apparent diameter of the full Moon, and can be seen with the unaided eye. For Hubble's pinpoint view, that's a lot of celestial real estate to cover. This stunning, colorful mosaic captures the glow of 200 million stars. That's still a fraction of Andromeda's population. And the stars are spread across about 2.5 billion pixels. The detailed look at the resolved stars will help astronomers piece together the galaxy's past history that includes mergers with smaller satellite galaxies.
NASA, ESA, Benjamin F. Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern); Image Processing: Joseph DePasquale (STScI)
Download this image (10,552 x 2,468)(9 MB)
Download this image (42,208 x 9,870)(203 MB)

Our neighbor, the Andromeda galaxy is a proxy for spiral galaxies in the universe at large. Astronomers would know much less about the structure and evolution of our own spiral, the Milky Way, if they didn't have Andromeda to study. That's because we are embedded inside the Milky Way. Our inside view is like trying to understand the layout of New York City by standing in the middle of Central Park.

Because of its proximity, photographing the disk of Andromeda is a herculean task. The galaxy is a much larger target on the sky than the galaxies Hubble routinely observes, which are often billions of light-years away. The full mosaic above was created with observations from two Hubble observing programs. In total, it required over 1,000 Hubble orbits, spanning more than a decade.

The panorama above started with the Panchromatic Hubble Andromeda Treasury (PHAT) program (see image below) about a decade ago. Images were obtained at near-ultraviolet, visible, and near-infrared wavelengths using the Advanced Camera for Surveys and the Wide Field Camera 3 aboard Hubble to photograph the northern half of Andromeda.

This sweeping bird's-eye view of a portion of the Andromeda galaxy (M31) is the sharpest image ever taken of the galaxy.
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 2.5 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.
NASA, ESA, J. Dalcanton, B.F. Williams and L.C. Johnson (University of Washington), the PHAT team and R. Gendler

Hubble traces densely packed stars extending from the innermost hub, seen on the left side of this image, of Messier 31 (M31), the Andromeda Galaxy. 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 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
Photo mosaic of Andromeda galaxy and five regions of interest. A spiral galaxy spreads across the width. It’s tilted nearly edge-on to our line of sight, appearing as an extreme oval on its side. Its borders are jagged because the image is a mosaic of smaller, square images. The outer edges are blue, while the inner two-thirds are yellowish with a bright, central core. Dark, dusty clouds wrap around the outer half of the galaxy’s disk. At 10 o'clock, a smaller dwarf elliptical galaxy forms a fuzzy, yellow blob. There are about 200 million stars within the image. The background of space is black. There are what appears to be steps toward the bottom, mainly toward the middle, which indicates where no data were taken. Interesting regions: (a) Clusters of bright blue stars embedded within the galaxy; background galaxies seen much farther away; (b) NGC 206, a concentration of bright blue stars; (c) A young cluster of blue newborn stars; (d) The satellite galaxy M32; (e) Dark dust lanes across myriad yellow stars.
This is the largest photomosaic ever made by the Hubble Space Telescope. The target is the vast Andromeda galaxy that is only 2.5 million light-years from Earth, making it the nearest galaxy to our own Milky Way. Andromeda is seen almost edge-on, tilted by 77 degrees relative to Earth’s view. The galaxy is so large that the mosaic is assembled from approximately 600 separate overlapping fields of view taken over 10 years of Hubble observing—a challenge to stitch together over such a large area. The mosaic image is made up of at least 2.5 billion pixels. Hubble resolves an estimated 200 million stars that are hotter than our Sun, but still a fraction of the galaxy’s total estimated stellar population. Interesting regions include: (a) Clusters of bright blue stars embedded within the galaxy, background galaxies seen much farther away, and photo-bombing by a couple bright foreground stars that are actually inside our Milky Way; (b) NGC 206 the most conspicuous star cloud in Andromeda; (c) A young cluster of blue newborn stars; (d) The satellite galaxy M32, that may be the residual core of a galaxy that once collided with Andromeda; (e) Dark dust lanes across myriad stars.
NASA, ESA, Benjamin F. Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern); Image Processing: Joseph DePasquale (STScI)
Download this image (2,000 x 1,125)(1.5 MB)
Download this image (7,680 x 4,320)(16 MB)

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

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