How do we know what parameters to use when simulating the collision of the Milky Way and Andromeda? (Advanced)

I am 10 years old and I am in 5th grade. I learned about astronomy from a Discovery program. I want to know about the Andromeda and Milky Way galaxy collision. What instruments do you use to collect the values that you put into your computer simulations? What I mean is, did the Hubble telescope collect the values? What instruments were used? How do you know that your values are right?

Edit by Michael Lam on August 2, 2016: Here is a simple answer, with a more thorough answer to the question below. We use lots of different instruments and telescopes to understand how Andromeda is moving relative to the Milky Way. The Hubble telescope has been used to help us understand the motions, as well as many other telescopes observing many different kinds of light, not just the kind of light that our eyes can see. By understanding how light changes as objects move towards or away from each other on the Earth, we can extend that understanding to how objects like stars in Andromeda are moving with respect to us, and the fact that lots of different measurements agree means that we have good scientific belief in our results.

You ask a great question! There are many observational parameters that factor into the way in which we think that the Milky Way and the Andromeda galaxy will merge. Just so that we are clear on what the merger will look like, check out this simulation video by NASA.

The major ingredients in the simulations are :

  1. The relative velocities of Andromeda and the Milky Way
  2. The direction of rotation, and orientation of the Milky Way relative to Andromeda
  3. The masses and composition of the Milky Way and Andromeda
  4. The distance between Andromeda and the Milky Way

All of these properties of the Milky Way - Andromeda system are observable:

1. It turns out that in astronomy, it is very easy to measure velocities relative to the Earth: that's because light changes energy depending on whether the object that emits the light is moving towards, or away from you. The phenomenon responsible for this energy change is called the Doppler shift. When we look at distant galaxies, we often look at the light emitted by atoms and molecules therein. Since we know at what the energy of the light should be from experiments on Earth, most of the change in energy that we see can be attributed to a the relative velocity ,between us and that object. We are also able to build detectors of light that are sensitive enough to determine these velocities quite well. We can do this for the Andromeda galaxy, and since we know (roughly) the Earth's motion in the Milky Way, we can infer the relative velocity between the two. Although the Hubble Space Telescope can be used to infer velocities, radio telescopes can also do a good job from Earth. The catch is that we are able to infer the velocity of Andromeda only along the line of sight: we therefore have to infer a 3-dimensional orbit of Andromeda relative to us with only 1 dimension's worth of information. This is perhaps the most uncertain aspect of the simulations.

2. Because we can measure velocities so well and because the Andromeda galaxy is nearby, we can point our telescopes at each side of the galaxy and determine which side is coming towards us and which side is moving away. This, together with observations of dust lanes in the disk of M31, tells us its orientation and direction of rotation relative to the Milky Way.

3. We can infer the masses of the Milky Way and Andromeda by using, again, the observed velocities in different parts of the disk. It turns out that the velocity with which matter orbits the centre of a galaxy is related to the amount of mass interior to the part of the galaxy we are looking at. This technique is fairly straightforward for Andromeda, but rather tricky for the Milky Way (since we are inside it!); in the latter, we use the orbital velocity of small satellites of the Milky Way to estimate its mass.

The composition of the two galaxies is obtained by looking at them at different wavelengths, and hence with different telescopes; since different parts of the galaxy emit and absorb energy at different wavelengths, we gain insight into things like the shape and size of the galaxy disk, the size of the bulge, and the extent of the gas in the disk by observing both the Milky Way and Andromeda with different instruments. The Hubble Space Telescope is used in this context, especially to examine sites of star formation in Andromeda.

4. In order to determine a distance to Andromeda, we need to observe something for which we know the intrinsic brightness (or luminosity); the apparent brightness or the object then tells you how far away it is. The most popular method used to determine the distance to Andromeda is to observe a type of variable star called a Cepheid Variable, for which we think we know the absolute brightness. This method, however, gives you a distance accurate to only a few percent.

The prediction of the merger of the Milky Way and the Andromeda galaxy is a good example of the importance of using both observations (like the ones above) and theory (like the simulations you saw on TV) to do astronomy: in most cases, neither one is possible without the other.

This page was last updated on August 2, 2016

About the Author

Kristine Spekkens

Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

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