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Starts With A Bang

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

The other great spiral galaxy in the local group demonstrates why we’re #2, even in our own backyard.

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.

The Andromeda Galaxy resides in our local group, and is perhaps twice as large in diameter as our Milky Way, estimated to contain anywhere from 2.5–5 times as many stars. (ADAM EVANS / FLICKR)

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.

As showcased in this Hubble image of Andromeda, background galaxies can be seen through the halo of Andromeda. Thousands of such galaxies have been discovered, many of which are in various stages of mergers and acts of galactic cannibalism. (NASA, ESA AND T.M. BROWN (STSCI))

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))

Mostly Mute Monday tells the cosmic story of an astronomical object or phenomenon in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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