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Starts With A Bang

The most spectacular view of the galaxies next door

Just 12 million light-years away, the galaxies Messier 81 and 82 offer a nearby preview of the Milky Way-Andromeda merger.
m81 group
This multiwavelength view of the two largest, brightest galaxies in the M81 group shows stars, plasmas, and neutral hydrogen gas. The gas bridge connecting these two galaxies infalls onto both members, triggering the formation of new stars. If each star were shrunk down to be a grain of sand, this group would be 36 million km away, but the two galaxies would be separated only by a little over 400,000 km: the Earth-Moon distance.
Credit: R. Gendler, R. Croman, R. Colombari; Acknowledgement: R. Jay GaBany; VLA Data: E. de Block (ASTRON)
Key Takeaways
  • In four-to-seven billion years, the Milky Way and Andromeda, the Local Group's two largest galaxies, will merge.
  • Although we won't be around to see it, there's a slightly smaller, more advanced, analogous merger happening just 12 million light-years away: in the M81 Group.
  • This deep, multiwavelength image of the M81 and M82 galaxies interacting showcases a tremendous amount of physics at play, and offers a preview of our own future fate.

Right in our cosmic backyard, a preview of the Milky Way’s future unfolds.

The galaxy Messier 81, also known as Bode’s Galaxy, is one of the brightest and closest galaxies to Earth not found in our Local Group. By connectng the lower-left corner of the Big Dipper’s cup to the upper-right corner and then traveling that same distance in the same direction, you can find M81 and the other major galaxies of its group all clustered together. (Credit: E. Siegel/Stellarium)

Just outside the Big Dipper’s “cup,” Bode’s Galaxy, Messier 81, lingers.

This optical image of Bode’s Galaxy, M81, comes courtesy of the Hubble Space Telescope. The spiral arms are littered with hot, young, blue stars, while large extent of the arms indicates a gravitational interaction with one or more nearby neighbors. A wider-field and multiwavelength view supports that. (Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA))

12 million light-years away, it’s a naked eye object for those with acute vision and exceptionally dark skies.

M81 group
The two largest, brightest galaxies in the M81 Group, M81 (right) and M82 (left), are shown in the same frame in these 2013 and 2014 photos. In 2014, M82 experienced a supernova, visible in the 2014 (blue) image just above the galactic center. (Credit: Simon in the Lakes)

The largest galaxy in the M81 group moves ever-so-slightly toward us.

Most galaxies that are located beyond our own Local Group are redshifted from our perspective: evidence that they’re moving away from us as the Universe expands. M81, unusually, is very slightly blueshifted, but will not move toward us in the long term, as the M81 Group, which it’s the largest member of, moves away from us overall. (Credit: Ken Crawford)

The grand spiral arms and large surface brightness indicate rapid, recent star-formation.

This infrared view of Messier 81 taken by NASA’s Spitzer Space Telescope shows the emitted starlight (in blue) primarily coming from the center, the emission of carbon-based molecules in the interstellar medium (in green), and heated dust also in the interstellar medium (in red). The yellow/orange color showcases a combination of green and red. (Credit: NASA/JPL-Caltech)

The culprit? Gravitational interactions with other galaxies in its group.

M81 group
The bright galaxies of the M81 Group can be seen here, with NGC 3077 to the lower right of M81, and other, smaller galaxies visible amidst the (foreground) stars and neutral gas permeating the surrounding space. (Credit: Jason Guenzel/Astrobin)

The nearby Cigar Galaxy, Messier 82, is Earth’s closest starburst galaxy.

Most galaxies contain only a few regions of star-formation: where gas is collapsing, new stars are forming, and ionized hydrogen is found in a bubble surrounding that region. In a starburst galaxy, pretty much the entire galaxy itself is a star-forming region, with M82, the Cigar Galaxy, being the closest one with those properties. (Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (National Science Foundation))

NGC 3077 also contains young stars, infalling gas, and a distorted morphology.

The dusty, irregular galaxy NGC 3077 is actively forming new stars, has a very blue center, and has a hydrogen gas bridge connecting it to M81. One of 34 galaxies in the M81 Group, it’s an example of gravitational interactions triggering new star-formation. (Credit: ESA/Hubble and NASA)

Radio light reveals neutral hydrogen bridges connecting these M81 Group galaxies.

The M81 triplet, consisting of M81 (right-center), M82 (top) and NGC 3077 (left) are all connected by a vast bridge of neutral hydrogen. Gas infall, star formation, and gravitational tidal effects are all related. (Credit: Blok et al. 2018, ApJ)

This galactic triplet is analogous to our Local Group’s Andromeda-Milky Way-Triangulum collection.

This three-dimensional view of the Local Group showcases the three largest galaxies and their relative positions in space: Andromeda (M31), the Milky Way, and Triangulum (M33). Over the next few billion years, these galaxies will interact and merge, but will be billions of years behind the timeline of M81, M82, and NGC 3077. (Credit: Antonio Ciccolella/Wikimedia Commons)

The M81 Group contains ~1 trillion solar masses, only slightly below the Local Group’s mass.

This zoomed-in view of the stars, gas, dust, and hydrogen surrounding Messier 81 showcases a tremendous population of young, newly formed stars, but represents only a fraction of the M81 Group’s mass. Although this galaxy is only about 70% the diameter of the Milky Way, the mass of the group adds up to over ~1 trillion solar masses. (Credit: R. Gendler, R. Croman, R. Colombari; Acknowledgement: R. Jay GaBany; VLA Data: E. de Block (ASTRON))

Copious star-formation and abundant supernovae are hallmarks of these galaxies.

This close-up view of Messier 82, the Cigar Galaxy, shows not only stars and gas, but also the superheated galactic winds and the distended shape induced by its interactions with its larger, more massive neighbor: M81. In only a few billion years, this galaxy will be no more, as it’s destined to be consumed by its larger neighbor. (Credit: R. Gendler, R. Croman, R. Colombari; Acknowledgement: R. Jay GaBany; VLA Data: E. de Block (ASTRON))

Gravitational distortion, tidal forces, and gas infall trigger these phenomena.

M81 Group
This map shows the velocity field of the neutral hydrogen gas as mapped out by radio data for the M81 Group. Note how the gas bridges connect the major galaxies of the M81 Group, and that the center of M81, where blue-and-red meet, has gas both moving toward us (blue) and away from us (red) from our perspective: evidence of outflows. (Credit: Blok et al. 2018, ApJ)

Modern views reveal a somewhat downsized preview of our upcoming merger with Andromeda.

A series of stills showing the Milky Way-Andromeda merger, and how the sky will appear different from Earth as it happens. When these two galaxies merge, their supermassive black holes are fully expected to merge together as well. At present, the Milky Way and Andromeda move toward one another at a relative speed of ~109 km/s. (Credit: NASA; Z. Levay and R. van der Marel, STScI; T. Hallas; A. Mellinger)

Overall, the Universe’s expansion drives the M81 Group away.

Our local supercluster, Laniakea, contains the Milky Way, our Local Group, the Virgo cluster, and many smaller groups and clusters on the outskirts, including the M81 Group. However, each group and cluster is bound only to itself, and will be driven apart from the others due to dark energy and our expanding Universe. After 100 billion years, even the nearest galaxy beyond our own Local Group will be approximately a billion light-years away, making it many thousands, and potentially millions of times fainter than the nearest galaxies appear today. (Credit: Andrew Z. Colvin/Wikimedia Commons)

Owing to dark energy, it will eventually recede from view: the final galactic group to become unreachable.

M81 Group
The humble M81 Group will likely be the last group of galaxies to leave our view, as it’s very nearby and only very slowly receding from us in the expanding Universe. These humble galaxies, and the eventual single galaxy they merge into, will likely be the last galaxy beyond ‘Milkdromeda’ that an observer in our future location will be capable of spotting in the very, very far future. (Credit: Skatebiker/Wikimedia Commons)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.


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