Andromeda-Milky Way collision: what happens when the galaxies collide?

Will our Milky Way galaxy really collide with the Andromeda Galaxy? When will it happen, and how does our Galaxy compare to the Andromeda Galaxy?

A simulation showing Earth’s night sky in 3.75 billion years as Andromeda collides with the Milky Way. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

Our galaxy is on a collision course. In roughly 4.5 billion years’ time the Milky Way will smash into the rapidly approaching Andromeda Galaxy, and astronomers are still attempting to predict what it will be like when the two galaxies collide.

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That a collision between our galaxy and the Andromeda Galaxy is inevitable has been known for a little while. It’s been clear from measurements of the Doppler shift of its spectral lines that Andromeda was approaching, but such measurements can only tell you about the motion directly towards or away from the observer.

Working out if we were in for a direct hit or a near miss therefore required careful study of the motion of Andromeda’s satellite galaxies, and they reveal Andromeda’s trajectory makes collision inevitable.

A pair of colliding galaxies nicknamed 'The Mice', captured by the Hubble Space Telescope. When galaxies merge, stars are incredibly unlikely to collide, due to the vast distances between them. Credit: NASA, Holland Ford (JHU), the ACS Science Team and ESA
A pair of colliding galaxies nicknamed ‘The Mice’, captured by the Hubble Space Telescope. When galaxies merge, stars are incredibly unlikely to collide, due to the vast distances between them. Credit: NASA, Holland Ford (JHU), the ACS Science Team and ESA

Plugging this information on the relative velocities, estimates of the masses of the galaxies and details of their structure into their model, the authors of an interesting paper on this galactic collision have watched the event unfold.

Though the details depend to some extent on galactic structure and in particular on assumptions we make about the haloes of dark matter that contain the visible discs of the Milky Way and Andromeda, the general picture is clear.

After a spectacular series of close passes lasting billions of years – and which will distort the structure of both galaxies – a final merger of the Andromeda Galaxy and the Milky Way galaxy will occur about 10 billion years from now.

What will happen when Andromeda and the Milky Way collide?

The result of the collision between Andromeda and the Milky Way will be a new, larger galaxy, but rather than being a spiral like its forebears, this new system ends up as a giant elliptical.

The authors of the paper in question have named this new galaxy ‘Milkdromeda’. There’s something about this name that really bugs me. Surely we need something grander sounding? If you can think of a better name, let us know by emailing contactus@skyatnightmagazine.com.

For the first time, the simulation includes details of what happens to the supermassive black holes that lurk at both galaxy centres. The pair will end up forming a binary at the heart of the new, larger galaxy.

Left: elliptical galaxy NGC 4621. Right: Spiral galaxy NGC 1015. Credit: ESA/Hubble & NASA, P. Cote / ESA/Hubble & NASA, A. Riess (STScl/JHU)
Left: elliptical galaxy NGC 4621. Right: Spiral galaxy NGC 1015. Credit: ESA/Hubble & NASA, P. Cote / ESA/Hubble & NASA, A. Riess (STScl/JHU)

This binary isn’t stable, though. Interaction with their surroundings means that the pair will spiral inwards, emitting gravitational waves as they do so.

In only about 17 million years, the two will be close enough that the loss of energy through these gravitational waves drives a final inward spiral and merger.

The gravitational wave signal of an event like this can’t be detected by LIGO (the Laser Interferometer Gravitational-Wave Observatory), which is sensitive only to the merging of lower mass black holes and neutron stars.

An artist's impression of two binary black holes in orbit around each other. Credit: The SXS (Simulating eXtreme Spacetimes)
An artist’s impression of two binary black holes in orbit around each other. Credit: The SXS (Simulating eXtreme Spacetimes)

In the future, the simulations indicate that ESA’s LISA (Laser Interferometer Space Antenna) mission should observe such mergers out to an approximate redshift of two – so in a decade or so we may be able to see how predictions of the fate of the Milky Way’s black hole match up to similar events elsewhere.

The paper ends on a melancholic note: the authors’ estimate of 10 billion years for the merger to reach its end is longer than in previous studies, but means that the Sun will not live long enough to find a home in the new galaxy.

We’ll have to enjoy the results of simulations instead.

Andromeda vs the Milky Way

Milky Way

The Milky Way galaxy sets over Fajada Mesa at Chaco Canyon, New Mexico. Credit: Eric Lowenbach / Getty Images
The Milky Way galaxy sets over Fajada Mesa at Chaco Canyon, New Mexico. Credit: Eric Lowenbach / Getty Images
  • Size 100,000 lightyears across
  • Number of stars 100 billion stars
  • Type of galaxy Spiral galaxy
  • Age 13.6 billion years

Andromeda Galaxy

The Andromeda Galaxy Nishant Revur, Washington, USA, 19 June 2020. Equipment: ZWO ASI 294MC Pro colour camera, William Optics RedCat 51 apo refractor, Celestron Advanced VX⁠ mount
The Andromeda Galaxy, by Nishant Revur, Washington, USA, 19 June 2020.
  • Size 220,000 lightyears across
  • Number of stars 1 trillion stars
  • Type of galaxy Spiral galaxy
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  • Age 10 billion years

Chris Lintott was reading Future merger of the Milky Way with Andromeda galaxy and the fate of their supermassive black holes by Riccardo Schiavi et al. Read it online at arxiv.org.

Chris Lintott is a Professor of Astrophysics at the University of Oxford and a co-host of The Sky at Night. This article originally appeared in the April 2021 issue of BBC Sky at Night Magazine.