Andromeda Is Big, Massive, And Full Of The Stars Our Milky Way Is Missing

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The Milky Way is our cosmic home, containing hundreds of billions of stars across 100,000 light-years.

The largest galaxy in the Local Group, Andromeda, appears small and insignificant next to the Milky Way, but that's because of its distance: some 2.5 million light years away. It is moving towards our Sun, at the present moment, at around 300 km/s, slated for a merger with Andromeda in 4-7 billion years. SCIENCETV ON YOUTUBE / SCREENSHOT

But 2.5 million light years away, our big sister, Andromeda, outclasses us in every way.

Our Local Group of galaxies is dominated by Andromeda and the Milky Way, but there's no denying that Andromeda is the biggest, most massive, and contains the most stars. In the past, our cosmic neck-of-the-woods contained even more. ANDREW Z. COLVIN

It's double our diameter, with around a trillion stars.

This 1887 picture of the Great Nebula in Andromeda was the first to show the spiral armed structure of the nearest large galaxy to the Milky Way. The fact that it appears so thoroughly white is because this was simply taken in unfiltered light, rather than looking in red, green, and blue, and then adding those colors together. All of the features identifiable from this image are unchanged in the 131 years since it was composed. ISAAC ROBERTS

It's the Local Group's biggest, most massive and luminous galaxy.

When we look at the stars within Andromeda, with space telescopes like Hubble, the biggest differences emerge.

The stars visible in the Andromeda galaxy, in a dust-rich region and a dust-poor region. Hubble's observations, along with the observations of space telescopes operating at different wavelengths, have allowed us to learn even more about Andromeda than we know, in many ways, about our own Milky Way. ILLUSTRATION CREDIT: NASA, ESA, AND Z. LEVAY (STSCI/AURA); SCIENCE CREDIT: NASA, ESA, J. DALCANTON, B.F. WILLIAMS, AND L.C. JOHNSON (UNIVERSITY OF WASHINGTON), AND THE PHAT TEAM

Over 117 million stars in the disk were measured by PHAT: the Panchromatic Hubble Andromeda Treasury.

A Mosaic of the 117 million resolved stars -- plus many more unresolved ones -- in the disk of the Andromeda galaxy. Only a portion of the central bulge was imaged, but the metallicity of stars in that region is unparalleled anywhere else in the Local Group. NASA, ESA, J. DALCANTON, B.F. WILLIAMS, L.C. JOHNSON (UNIVERSITY OF WASHINGTON), THE PHAT TEAM, AND R. GENDLER

The stars near the central bulge are far richer in heavy elements than our Sun.

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. These star clusters represent some of the newest stars found by the Panchromatic Hubble Andromeda Treasury. 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

New, blue stars shine in a slew of open clusters.

In the Andromeda galaxy, as in the Milky Way, the oldest individual stars are found in the galaxy's halo. However, there are stellar streams and a population of stars in the halo that can be traced back to later times and more recent star-formation events. NASA, ESA AND T.M. BROWN (STSCI)

The low-density, outer halo contains stars just as ancient as the Milky Way's oldest: 13+ billion years of age.

This image from Hubble showcases the stars in Andromeda’s giant stellar stream. About a third of the stars are young: about 6-8 billion years of age, compared to the more typical 11-13 billion years of age for the other stars in Andromeda's halo and almost all the stars in the Milky Way's halo. The Milky Way's foreground stars are clearly identified by their diffraction spikes. NASA, ESA AND T.M. BROWN (STSCI)

Andromeda has stellar streams populating that halo, with a third of those stars just 6-8 billion years old.

We can successfully reconstruct a progenitor galaxy that gave rise to the combination of a satellite of Andromeda, halo stars, stellar streams, and a population of stars ~2 billion years old in both M32 and Andromeda. The stars found in the halo and stellar streams of M31 point to another such merger even earlier: 6-8 billion years ago. AMANDA SMITH, INSTITUTE OF ASTRONOMY, UNIVERSITY OF CAMBRIDGE

This means a major act of galactic cannibalism recently occurred.

This ultraviolet composite image of the Andromeda galaxy, taken by the GALEX spacecraft, showcases the youngest, bluest stars of all, which trace out the spiral arms and the galactic bulge. Note the extended nature of the arms, which indicates that new waves of star formation may be triggered by mild tidal disruptions. NASA / JPL-CALTECH / GALEX

Ultraviolet images showcase the newest stars, tracing out spiral arms and peaking in the center.

The ESA's Herschel Space Observatory found large populations of infrared dust and gas, but interestingly found them at different temperatures, which indicates where new stars are more likely to form going forward into the future. ESA / HERSCHEL

Infrared imaging pinpoints the galactic fuel that will birth future generations of stars.

Thousands of background galaxies, seen through Andromeda's halo, showcase our chaotic, evolving Universe.

The sparsely populated regions in Andromeda are a prime location to view the nearby and distant Universes together. Along with the 6-8 billion year old stars, the halo showcases 11-13 billion year old stars, along with scores of background galaxies in this image alone. NASA, ESA AND T.M. BROWN (STSCI)