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Emily Lakdawalla • Jun 04, 2012

Artist's views of a night sky transformed by a galaxy merger

Last week, in the aftermath of the triumphal return of Dragon to Earth, there was a press briefing on quite a different topic altogether. It concerned three papers published in the Astrophysical Journal about a painstaking measurement of the proper motion of the Andromeda galaxy using Hubble data sets taken five or seven years apart. Andromeda, otherwise known as M31, is among our nearest neighbors, "only" 2.5 million light-years away. Its line-of-sight speed has been known for a long time (from Doppler measurements), but until now we didn't know whether its actual direction of motion is more toward us or more tangential to us.

The answer: Andromeda is coming at us, and will "collide" with the Milky Way in about four billion years. I put the word "collide" in quotes because while the collision will have a dramatic effect on the appearance of both galaxies, physical encounters between stars (and the planets they contain) will be really rare, because there is so much empty space between stars. Our solar system will most likely be unaffected by the collision, and will be having greater problems at that time due to our Sun's behavior; that's roughly the same time it's estimated that the Sun will expand into a red giant.

I find the introductions to the three papers to be very readable; you can download them from this link on author Roeland van der Marel's web page. But the reason I'm posting about it has to do with the illustrations included with the Space Telescope Science Institute press release on these articles. They show a progression of how a (very dark) night sky would appear to Earthlings as the collision progresses. I have always been fascinated by the kinds of changes that landscapes undergo, with the perspective of geologic time scales. This changing stellar landscape is equally fascinating. So here are eight illustrations of that changing sky. While I suggest you click to enlarge them to their full glamour, be warned that the links are to PNG-formatted images ranging in size from 11 to 16 MB. Alternatively, you can download all eight as a zip file here (107 MB!)

Also: I turned one of the images into an eCard for this week, which you can send from the link on our home page.

Nighttime Sky View of Future Galaxy Merger: Present Day
Nighttime Sky View of Future Galaxy Merger: Present Day This is a nighttime view of the current sky, with the bright belt of our Milky Way. The Andromeda galaxy lies 2.5 million light-years away and looks like a faint spindle, several times the diameter of the full Moon.Image: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
Nighttime Sky View of Future Galaxy Merger: 2 Billion Years
Nighttime Sky View of Future Galaxy Merger: 2 Billion Years The disk of the approaching Andromeda galaxy is noticeably larger.Image: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
Nighttime Sky View of Future Galaxy Merger: 3.75 Billion Years
Nighttime Sky View of Future Galaxy Merger: 3.75 Billion Years Andromeda fills the field of view. The Milky Way begins to show distortion due to tidal pull from Andromeda.Image: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger

The next two images are from the same future time; they're just different kinds of views.

Nighttime Sky View of Future Galaxy Merger: 3.85 Billion Years
Nighttime Sky View of Future Galaxy Merger: 3.85 Billion Years During the first close approach, the sky is ablaze with new star formation, which is evident in a plethora of emission nebulae and open young star clusters.Image: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
Nighttime Sky View of Future Galaxy Merger: 3.9 Billion Years
Nighttime Sky View of Future Galaxy Merger: 3.9 Billion Years During the first close approach, the sky is ablaze with new star formation, which is evident in a plethora of emission nebulae and open young star clusters.Image: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
Nighttime Sky View of Future Galaxy Merger: 4 Billion Years
Nighttime Sky View of Future Galaxy Merger: 4 Billion Years This is an artist's concept of what our night sky might look like in 4 billion years, after a collision between the Milky Way and Andromeda galaxies. Click the image for a full 8-image series.Image: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
Nighttime Sky View of Future Galaxy Merger: 5.1 Billion Years
Nighttime Sky View of Future Galaxy Merger: 5.1 Billion Years During the second close passage, the cores of the Milky Way and Andromeda appear as a pair of bright lobes. Star-forming nebulae are much less prominent because the interstellar gas and dust has been significantly decreased by previous bursts of star formation.Image: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger
Nighttime Sky View of Future Galaxy Merger: 7 Billion Years
Nighttime Sky View of Future Galaxy Merger: 7 Billion Years The merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky. Scoured of dust and gas, the newly merged elliptical galaxy no longer makes stars and no nebulae appear in the sky. The aging starry population is no longer concentrated along a plane, but instead fills an ellipsoidal volume.Image: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger

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