Andromeda: Facts, Similarities, and a Mysterious Core

Andromeda Facts

Our Milky Way galaxy has a Big Sister, the Andromeda Galaxy. Andromeda lies 2.5 million light years away from us. Which is 2.36×1019 kilometers away. That is 2 with nineteen zeros behind it! The Andromeda galaxy is the most distant object you can see with the naked eye, given good observing conditions. 

Andromeda of course lies in the Andromeda constellation, which is next to the Cassiopeia constellation. 

Similarities to the Milky Way

andromeda Similarities to the Milky Way

Like the Milky Way, Andromeda is a spiral galaxy. The reason we can think of it as an older sister is that there are many similarities between the two, but Andromeda is much larger. This galaxy contains over a trillion stars, whereas our Milky Way contains 200-400 billion.

Andromeda is also more than twice as wide as our galaxy measuring 220,000 light years in diameter, and our Milky Way only being 100,000 light years across. 

Both the Milky Way and Andromeda were formed around the same time near the beginning of the Universe, 13.5 billion years ago. These two galaxies have grown throughout their lifetimes by consuming other galaxies they have collided with.

We are actually on track to collide with the Andromeda galaxy in approximately 4 billion years! While the initial collision with Andromeda will take place in 4 billion years, computer simulations predict that it will take another two billion years for our galaxies to completely merge.

Once merged, we will no longer be a spiral galaxy, but morphed into an elliptical galaxy. Below is an artist’s depiction of what our sky will look like in 3.75 billion years, just before our Milky Way collides with Andromeda.

What’s In the Center of Andromeda
Illustration Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger NEWS RELEASE: 2012-20 >

What’s In the Center of Andromeda?

Another similarity between the Milky Way and Andromeda is that they both have a central bulge of stars and a supermassive black hole at their cores. Supermassive black holes are just that, black holes that are extremely massive, weighing anywhere between millions to billions of times the mass of our Sun.

Astronomers do not know why these black holes are so massive, but they believe it could be linked to their presence at the center of their host galaxy. It is assumed that in the center of every massive galaxy there resides a supermassive black hole.

The supermassive black hole at the center of our Milky Way was captured in the world’s first picture of a black hole in 2019. 

andromeda center
Image Credit: The Event Horizon Telescope

While both the Andromeda galaxy and our Milky Way have a central bulge of stars around their black holes, in 1993 astronomers were surprised to see two blobs of light surrounding Andromeda’s core. 

One blob appears blue and the other more red. By 2012, astronomers had understood that the blue blob is a cluster of young hot stars surrounding Andromeda’s central supermassive black hole.

The Milky Way’s central supermassive black hole is also surrounded by a young blue cluster of stars, so this may be a common feature in many galaxies. The red blob seen at Andromeda’s core is actually a lopsided elliptical disk of redder stars surrounding the blue cluster and the supermassive black hole.

We do not see this red stellar disk in our milky way so it has led astronomers to investigate what happened in Andromeda’s past that could have created this stellar disk and what made it lopsided?

What’s Different About Andromeda’s Core?

In 2021, astronomers may have found an answer to this question. It involves Andromeda’s supermassive black hole and a cosmic collision. The supermassive black hole at the center of Andromeda is about 100 million times the mass of our Sun, and about 25 times more massive than the Milky Way’s central supermassive black hole.

Andromeda’s black hole was not likely born this massive but grew over its lifetime by consuming the occasional object, even other black holes. 

This could have occurred when galaxies collided with Andromeda. As a smaller galaxy collides into Andromeda, the objects in that galaxy will begin to flow around the center of mass in Andromeda, its original central black hole.

Occasionally a black hole from the colliding galaxy may fall into the black hole at the center of Andromeda. These black holes begin to orbit with each other and eventually merge in a powerful event. 

This massive collision event produces a burst of gravitational waves. If one of these black holes is much smaller than the other, this burst may not radiate evenly in all directions but can be more focused one way or the other. This lopsided blast can give the newly formed merged blackhole a kick in one direction. 

What’s Different About Andromeda’s Core

This can push the black hole to the edges of the galaxy’s nucleus or even out of the galaxy entirely if sufficiently powerful. If the black hole stays in the nucleus of its galaxy, it will oscillate back and forth, eventually slowing down by interacting with stars, and reaches the center of the galaxy once again. 

This is what may have happened in Andromeda’s core. The assumed timeline of the events is proposed to have played out as follows:

Over a billion years ago a galaxy collides and begins to merge with Andromeda. 

The black hole from the colliding galaxy drops closer to Andromeda’s larger black hole and they begin to orbit each other.

As this happens, material from the galaxies fall towards the black holes and begin to orbit a few light years away. This material begins to condense and form stars making a ring around the black holes. 

Finally, when the black holes merge there is a lopsided kick as described previously and the resulting black hole shoots off at high speeds, potentially 150-200 km per second. 

As this happens the orbiting stars get a kick as well and their circular orbits get disturbed creating a new elliptical orbit. With this new orbit, they speed up as they get closer to the black hole, then slow down when they are further away.

This is why the disk we see in Andromeda’s center looks lopsided with one brighter side: there is a pile up of stars as they slow down when they are further from the black hole. 

The blob of blue stars we see are younger so they would have come along later through a few different possible scenarios. 

This however is only one possible series of events that could have resulted in what we see today at the center of Andromeda. While this new work is a great first step in understanding what is going on, it is just a start. 

As technology improves, we will be able to dig more deeply into the cores of nearby galaxies, giving us more insight into how galaxies like andromeda and even our own Milky Way have evolved. 


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Cassie Hatcher

Cassie is a lifelong learner with a passion for communicating high level science in a conversational matter. She holds a B.S. and M.S. in physics and has written two astronomy theses, one of which is published. She earned an internship at NASA’s Goddard Space Flight Center in 2016 and got the chance to see the James Webb Space Telescope while it was being built.
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