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Earth’s night sky as Milky Way and Andromeda merge

Composite image showing a very bright glowing oblique spiral several times larger than the crescent moon.

Andromeda galaxy actual size? Yes. This image truly depicts what the night sky would look like if the Andromeda galaxy – the galaxy next door – were brighter. Original background shot of the moon by Stephen Rahn. Andromeda galaxy image via NASA. Composite photo by Tom Buckley-Houston. Not convinced? Here’s a similar image from APOD.

The image above is making the rounds on social media this week. It’s true. The neighboring Andromeda galaxy occupies about the width of 6 moon-diameters on our sky’s dome. But, of course, the galaxy isn’t nearly this bright. You need a dark sky to see it, and, even then, it’s a barely visible fuzzy patch of light. In order to appear as bright as in the image above, the Andromeda galaxy would need to be closer. If it were close enough to look so bright, it would appear even bigger on our sky’s dome. And that’s going to happen someday! The Andromeda galaxy is currently racing toward our Milky Way at a speed of about 70 miles (110 km) per second. Ultimately, the two galaxies will merge. Between now and that eventual merger, any beings alive on Earth will see the Andromeda galaxy get bigger and bigger and BIGGER in our night sky.

The Andromeda galaxy is now about 2.5 million light-years away from us. The artist’s concepts below, released by NASA in 2012, show what will happen to Earth’s night sky as the Andromeda galaxy hurtles toward us.

Eight panels with night sky views ranging from today's through a chaos of stars to a smooth background glow.

View larger. | This series of illustrations shows the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy.
First row, left: Present day.
First row, right: In 2 billion years the disk of the approaching Andromeda galaxy is noticeably larger.
Second row, left: In 3.75 billion years Andromeda fills the field of view.
Second row, right: In 3.85 billion years the sky is ablaze with new star formation.
Third row, left: In 3.9 billion years, star formation continues.
Third row, right: In 4 billion years Andromeda is tidally stretched and the Milky Way becomes warped.
Fourth row, left: In 5.1 billion years the cores of the Milky Way and Andromeda appear as a pair of bright lobes.
Fourth row, right: In 7 billion years the merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky.
Image via NASA/ ESA/ Z. Levay and R. van der Marel, STScI/ T. Hallas/ A. Mellinger.

The descriptions above are based on painstaking Hubble Space Telescope measurements of the motion of the Andromeda galaxy, followed by computer modeling of the inevitable future collision between the two galaxies. A series of studies published in 2012 showed that – rather than glancing off each other, as merging galaxies sometimes do – our Milky Way galaxy and the Andromeda galaxy will in fact merge to form a single big elliptical, or football-shaped, galaxy.

The Milky Way and Andromeda galaxies won’t be the only ones involved in this merger. As shown in the video below, the other large galaxy in our Local Group of galaxies – M33, aka the Triangulum galaxy – will also play a role. In this video, you’ll recognize the Triangulum galaxy as the smaller object near the Andromeda and Milky Way galaxies. Although the Triangulum galaxy likely won’t join the merger, it may at some point strike our Milky Way while engaged in a great cosmic dance with the two larger galaxies.

Across the universe, galaxies are colliding with each other. Astronomers see galactic collisons – or their aftermaths – through their telescopes. In some ways, when a galactic merger takes place, the two galaxies are like ghosts; they simply pass through each other. That’s because stars inside galaxies are separated by such great distances. Thus the stars themselves typically don’t collide when galaxies merge.

That said, the stars in both the Andromeda galaxy and our Milky Way will be affected by the merger. The Andromeda galaxy contains about a trillion stars. The Milky Way has about 300 billion stars. Stars from both galaxies will be thrown into new orbits around the newly merged galactic center. For example, according to scientists involved in the 2012 studies:

It is likely the sun will be flung into a new region of our galaxy …

And yet, they said,

.. our Earth and solar system are in no danger of being destroyed.

How about life on Earth? Will earthly life survive the merger? Astronomers say that the luminosity, or intrinsic brightness, of our sun is due to increase steadily over the next 4 billion years. As the sun’s luminosity increases, the amount of solar radiation reaching the Earth will also increase. It’s possible that – by 4 billion years from now – the increase in the Earth’s surface temperature will have caused a runaway greenhouse effect, perhaps similar to that going on now on the planet next door, Venus, whose surface is hot enough to melt lead. No one expects to find life on Venus. Likewise, it seems likely life on Earth will not exist 4 billion years from now.

What’s more, our sun is evolving, too. It’s expected to become a red giant star eventually. The sun’s outer layers will swell into the space of the solar system so that Earth itself is swallowed by the sun’s outer layers. That’s expected to happen about 7.5 billion years from now.

Perhaps by that time, some earthly inhabitants will have become space-faring. Perhaps we’ll have left Earth, and even our solar system. Do you think so? Let us know in the comments below.

Read more: Hubble Shows Milky Way is Destined for Head-On Collision, via NASA in 2012

Andromeda galaxy and the Milky Way - both as we see them in our sky - shown close together, appearing to begin a merger.

View larger. | Artist’s concept of a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it’ll unfold over the next several billion years. In this image, representing Earth’s night sky in 3.75 billion years, Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull. Image via NASA/ ESA/ Z. Levay and R. van der Marel, STScI/ T. Hallas/ A. Mellinger.

Bottom line: Billions of years from now, our Milky Way galaxy and the Andromeda galaxy are expected to merge. This post contains photos and video illustrating the impending merger and showing how the Andromeda galaxy will appear in Earth’s night sky over the next 7 billion years.

The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements

The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass

The M31 Velocity Vector. III. Future Milky Way-M31-M33 Orbital Evolution, Merging, and Fate of the Sun


Deborah Byrd