Galaxy Collisions for Dummies

And How They’re Proving The Existence of Dark Matter


I geek out on space. I’m not an astronomer, but hope I can share my enthusiasm for this subject without any serious errors.

Most recently, I had my mind blown learning about galaxy collisions and dark matter, by watching two Hubble Hangouts with NASA / dark matter specialist astronomers and astrophysicists, and researching a bit further on some of the subjects. Here’s what I learned:

What is a Galaxy Collision?

Galaxies like our Milky Way have so much gravity they get sucked into each other. We have seen photos of these for years, but understanding how they work is another story.

Click this photo for 10 photos of unique looking galaxy collisions.

Because the collisions happen over such a long time period of time, we obviously cannot see them colliding in real time, but by looking at many galaxy collisions from around the universe, in different phases of crashing, scientists have now learned not only what happens when they collide but also, what happens to dark matter when it collides with regular matter and more of itself.

What is Dark Matter and How Do Know It Exists?

If you don’t know, dark matter is an invisible substance that (along with dark energy) seems to make up about 85% of the matter in the universe. We cannot it observe directly with our senses or technology, but we know exists by seeing its gravitational effect. Meaning, something that we cannot see in any way has a tremendous pulling effect on things we can see.

This sample of galaxy collision photos shows regular matter in pink and dark matter in blue. A group of astrophysicists recently spent 5 years to create accurate mathematical models to understand the photos, analyzing the data from roughly 70 collisions to make sense of them.

Reporting in a Hubble Hangout “dark matter is even darker than we thought” they explain that dark matter does not interact with itself at all when it collides from one galaxy to another. It seems to simply “move through itself”.

What they saw is that the dark matter (identified by invisible mass) zooms ahead in a collision, while the gases get “left behind”.

This observation is helping prove that dark matter is a real thing. For a great description of how we know this is happening and more, read Five Reasons We Think Dark Matter Exists on Medium’s wonderful list, “It Starts With A Bang”.

The Bullet Cluster

The most popular collision among researchers right now is called the bullet cluster, which “blasted through another galaxy cluster at 814 miles per second” and “weighs about 200 trillion times as much as Earth”. Having two relatively equal sized galaxies hitting straight on, makes for relatively easy analysis of what actually goes on, and in particular, how the dark matter (shown in blue) moves through itself unobstructed, while the gases (shown in red) toss and slow each other down.

the infamous bullet cluster

Here is a 16 second animation which shows an artist’s representation of the bullet cluster collision:

“Hot gas, containing most of the normal matter in the cluster, is shown in red and dark matter is in blue. During the collision the hot gas in each cluster is slowed and distorted by a drag force, similar to air resistance. A bullet-shaped cloud of gas forms in one of the clusters. In contrast, the dark matter is not slowed by the impact, because it does not interact directly with itself or the gas except through gravity, and separates from the normal matter.” (Credit: NASA/CXC/M.Weiss)

Here is NASA’s description of what is going on in the image, how the dark matter was identified, and how it’s proving dark matter’s existence:

“A mere 3.4 billion light-years away, the bullet cluster’s individual galaxies are seen in the optical image data, but their total mass adds up to far less than the mass of the cluster’s two clouds of hot x-ray emitting gas shown in red. Representing even more mass than the optical galaxies and x-ray gas combined, the blue hues show the distribution of dark matter in the cluster. Otherwise invisible to telescopic views, the dark matter was mapped by observations of gravitational lensing of background galaxies. The clear separation of dark matter and gas clouds is considered direct evidence that dark matter exists.”


Insanely Slow AND Insanely Fast

For geek points, try this riddle on your roommate:
What moves at hundreds of miles per second, looks like it’s still, and takes hundreds of millions of years to finish?

Galaxy collisions take place over hundreds of millions of years — a time frame FAR longer than humans have existed on Earth (PBS’s FAQs on Evolution tells me “Scientists estimate that hominids diverged from the ape lineage 5–8 million years ago. Homo sapiens, the species to which we belong, has existed for about 100,000 years”).

In fact, a collision that started when dinosaurs existed (beginning roughly 230 million years ago) could still be going on today!

This diagram is drawn with the time-spans to scale. Humans have been around for 100 to 200,000 years. That’s less than one eight-hundredth the amount of time that dinosaurs were on Earth. (source)

While they are insanely high speed crashes, galaxies are mind-bogglingly large, so it still takes hundreds of millions of years for the crash to happen.


Why Galaxy Collisions are a Non-Contact Sport

We use the word crash because that’s what it looks like, but in fact, when these galaxies “collide” the stars and planets do not touch. That is correct; they collide and destroy each other, creating a hotbed of star formation at a rate of a hundred or thousand times faster than stars are formed normally, because of increased heat that causes nuclear fission… and yet, there is basically no physical contact.

How can this be?

The inside of galaxies, like atoms, is mostly empty space. Yet, the space between galaxies is relatively small. When you compare the size of two stars and the distance between them, it is in the scale of a billion times away from each other. Yet when you compare galaxy sizes and the distance between them it is in scale of 10 times away from each other. This is why star and planet collisions essentially never happen, yet many galaxy collisions do.

Now, it’s not that NO contact happens; Gas and dust collides, forming massive molecular clouds, which get compressed, extremely hot, and form stars.

Get a Sense of Scale — How Big Are Galaxies and How Far Apart Are They?

Let’s let Morgan Freedman and IMAX help us get an experience of the vastness of this universe…

Props to the original Powers of Ten video (1977)

Here, every ten seconds we view the starting point from ten times farther out until our own galaxy is visible only as a speck of light among many others.


How Do We Get Such Clear Images of Galaxies and Their Collisions?

A stunningly detailed photo of The Antennae Galaxies in Collision.

The Hubble telescope is not on Earth, but orbits just outside Earth’s atmosphere, which distorts and blocks the light that reaches our planet. While we have more powerful telescopic cameras here on earth, they just cannot compete with the clarity achieved in a photograph taken from space.

Also, just like we can take a long exposure photo here on Earth and get more light in one shot, Hubble and other telescopes takes very long exposures (multiple days even) to see this incredible detail. (So much so, that they've even found thousands of galaxies in what looks like a dot of nothingness! Fly into the furthest photograph ever taken)

Astronomers then overlay photographs from Earth with photographs from space in order to create the images that seem to detailed to be real.

Backstage Time-Lapse: Watch Team Hubble Make Galaxy Photos

These aren't single photos either. In this 2 min fast forward Photoshop video, Hubble shows how they stitched together 1400 screen captures taken one every 10 seconds over the course of 3 weeks to create a clear color image of a galaxy.

Hubble on the meaning of colors in their photos: “We often use color as a tool, whether it is to enhance an object’s detail or to visualize what ordinarily could never be seen by the human eye.” Read more

The Earth May Miss It’s Own Galaxy Collision

Even our galaxy, the Milky Way, is on a crash course, moving at many light years per second to collide with the Andromeda galaxy next door. Luckily, NASA doesn’t think this will happen for another 4 billion years.

How fast is the milky way moving towards andromeda?
About 250,000 miles per hour (fast enough to travel from here to the moon in one hour).

So, the next time you feel your problems are very big, just remember our galaxy is going to collide with our neighbor galaxy in 4 billion years. But not to worry, we will never touch.

Meanwhile, our sun is expected to die at around the same time and in doing so, turn into a red giant star so large that it will engulf the earth. While the most common estimate for its death is around 5 billion years, some postulate just 2.8 billion.

So, which will happen first?

Will we get eaten up by our sun, or ripped away from the sun and whipped out of orbit in a no-contact collision with Andromeda?


Astronomy for Stress Relief

Did this article give you some perspective? Even my meditation guru says astronomy is a good way to get over worries =)

“This universe has been there for millions of years and it will continue to be there for millions of years. Our life is so short. We will be here for a few years and then we will be gone. As long as we are alive, let’s do some good work…
If at all you have to worry, worry about the big things, like me! I am so worried. You also worry big, for the world. Even for a fraction of a second if these bigger issues worry you, you would be able to come up instead of sinking with small little things.”
— Sri Sri Ravi Shankar