Andromeda Is Coming Right At Us! Or Is It?


You might think that by working at an observatory, one might become “immune” to excitement over new science projects. That may be the case for some, but I still get ridiculously excited over a cool new idea. Lucky for me, I get to hear about a lot of interesting stuff before it becomes widespread. I’ll admit, you might even call me an astronomical hipster.

This month, I was particularly intrigued by an article by Jeremy Darling in the newsletter of the National Radio Astronomy Observatory. The title of the article asked, “Will the Andromeda Galaxy Collide with the Milky Way?”

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I was under the impression that it was already known that these two spiral galaxies would collide sometime in the next few billion years making a larger galaxy that some like to call “Milkdromeda.” Apparently, the case isn’t so cut and dry.

The Andromeda Galaxy is a large spiral galaxy 2 million light years away from our own spiral galaxy, the Milky Way. It was thought that Andromeda was almost twice as big as our galaxy, but more recent evidence suggests that our home galaxy is just as big and beefy. That may be comforting for some, especially in light of the evidence that the two galaxies are on a collision course.

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Most of the galaxies that we observe appear to be moving away from us. This is determined by the redshift of the galaxy’s light. Though this isn’t the most physically correct explanation, you can kind of imagine that the wavelength of the light is stretched out as an object moves away.

The Andromeda Galaxy, however, appears to be moving towards us at a speed of about 70 miles per second. Astronomers are not sure, however, whether it will be a head-on collision or a glancing blow.

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The colliding spiral galaxies known as the Antennae, as seen by the Hubble Space Telescope.

We have seen the beautiful violence of galaxy collisions in several places. The Antennae, for example (above), are made of two spiral galaxies that began colliding a billion years ago. The collision itself takes several billion years, so we are essentially seeing a snapshot in time of that activity.

Though a galaxy collision sounds destructive, the stars are spread so far apart that it is extremely unlikely that any two stars will ever collide. Instead, the gas in the disks of spiral galaxies collide, powering star formation regions that give birth to a news generation of stars and, possibly, planets. Seriously, just ask Felicia Day.

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Animated gifs are apparently back in style. So, here’s one of a galaxy collisions simulation!

Astronomers have built computer models that show the stars in a complicated and beautiful dance during the collision of two galaxies, and you can even simulate your own galactic smash up.

So how will Andromeda and the Milky Way collide? That requires knowledge of the direction and speed of Andromeda’s motion in the sky, or in the transverse direction.

The galaxy itself takes up an area of sky as large as five full moons, but it doesn’t move very quickly across the sky, so its transverse velocity has not been measured.

Here, astronomers are getting ready to use the Very Long Baseline Array (VLBA) to measure the galaxy’s motion with incredible precision.

The VLBA holds a special place for me since it is the first research telescope that I ever used. Also, it’s technically a telescope as big as the planet! By combining the data from several identical radio telescopes across North America, astronomers can make very precise images and track the small motions of objects through this sky. In this case, they are going to use masers.

Masers are kind of like lasers but shine in radio (specifically, microwave) light. They are produced naturally by molecules in interstellar clouds and are often used to probe star formation regions. The masers have been detected with a sensitive search using the Green Bank Telescope, and a plan is in place to monitor these bright points of microwave light for three to ten years in order to measure the motion of the Andromeda Galaxy.

So, by measuring the motions of these masers, astronomers will be able to figure out the transverse velocity of the entire galaxy. That, with the speed of the galaxy coming towards us, will confirm whether “Milkdromeda” will be a head-on collision or glancing blow. And don’t you want to know the fate of our home galaxy?

Image: Top – Black and white image is a Spitzer infrared image of M31 with colored crosses overlaid to indicate the maser positions. Credits - Gordon et al. 2006, Sjouwerman et al. 2010, and Darling 2011; Middle Credit: NASA/ESA; Bottom Credit: Mihos and Hernquist, adapted by Rieke.

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