A century ago, the American astronomer Edwin Hubble first established that this so-called ‘spiral nebula’ was approximately 2.5 million light years away from our Milky Way Galaxy. Now, the NASA/ESA Hubble Space Telescope has accomplished the most comprehensive survey of the Andromeda galaxy. It took more than 10 years to collect data for this colorful portrait, which captures the glow of 200 million stars, and was created from more than 600 snapshots.
This the largest photomosaic ever assembled from Hubble observations. It is a panoramic view of the neighboring Andromeda galaxy, located 2.5 million light-years away. Image credit: NASA / ESA / B. Williams, University of Washington.
The Andromeda galaxy (Messier 31), which is located 2.5 million light-years away, is the Milky Way’s nearest large galactic neighbor.
Hubble’s sharp imaging capabilities can resolve more than 200 million stars in the galaxy, detecting only stars brighter than our Sun. They look like grains of sand across the beach. But that’s just the tip of the iceberg.
Andromeda’s total population is estimated to be one trillion stars, with many less massive stars falling below Hubble’s sensitivity limit.
“Photographing Andromeda was a herculean task because the galaxy is a much bigger target on the sky than the galaxies Hubble routinely observes, which are often billions of light-years away,” said University of Washington astronomer Zhuo Chen and colleagues.
“The full mosaic was carried out under two Hubble observing programs. In total it required over 1,000 Hubble orbits, spanning more than a decade.”
“This panorama started with the Panchromatic Hubble Andromeda Treasury (PHAT) program about a decade ago.”
The Andromeda galaxy is seen almost edge-on, tilted by 77 degrees relative to Earth’s view; 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 is 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 the Andromeda galaxy; (e) dark dust lanes across myriad stars. Image credit: NASA / ESA / B. Williams, University of Washington.
“Images were obtained at near-ultraviolet, visible, and near-infrared wavelengths using Hubble’s Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3) to photograph the northern half of Andromeda.”
“This program was followed up by the Panchromatic Hubble Andromeda Southern Treasury (PHAST), which added images of approximately 100 million stars in the southern half of Andromeda.”
“This region is structurally unique and more sensitive to the galaxy’s merger history than the northern disk mapped by the PHAT survey.”
“The combined programs collectively cover the entire disk of Andromeda, which 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 fields of view.”
The results are described in a paper in the Astrophysical Journal.
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Zhuo Chen et al. 2025. PHAST. The Panchromatic Hubble Andromeda Southern Treasury. I. Ultraviolet and Optical Photometry of over 90 Million Stars in M31. ApJ 979, 35; doi: 10.3847/1538-4357/ad7e2b
