Andromeda and Milky Way galaxies are merging

Andromeda, composite image showing a very bright galaxy, next to a crescent moon.
View larger. | Andromeda galaxy actual size? Yes. This image truly depicts what the night sky would look like if the Andromeda galaxy – the galaxy next door – were brighter. Original background shot of the moon by Stephen Rahn. Andromeda galaxy image via NASA. Composite photo by Tom Buckley-Houston. The composite showed up on Reddit a few years ago. Not convinced? Here’s a similar image via APOD. As the Andromeda-Milky Way merger continues, the Andromeda galaxy will appear bigger – and bigger – in our sky.

Milky Way and Andromeda merger has begun

The Andromeda galaxy, the nearest spiral galaxy to our Milky Way, isn’t noticeable in our night sky, unless you look for it. Under dark skies, however, you can see it without optical aid, but only as a barely visible fuzzy patch of light. But one day, far in the future, Andromeda will be bright in our sky, growing larger and larger … as it gets closer and closer to us. And even though the two galaxies are still 2.5 million light-years apart, the eventual merger of our two galaxies has, in fact, already begun.

Galaxy near gibbous moon.
View larger. | Here’s another composite image showing the true size in our sky of the Andromeda galaxy. This one is from astrophotographers Adam Block and Tim Puckett. It was the Astronomy Picture of the Day for August 1, 2013.

The great extent of galactic halos

The Andromeda galaxy is currently racing toward our Milky Way at a speed of about 70 miles (113 km) per second. With this in mind, our merger will occur five billion years from now. But, in August 2020, the peer-reviewed Astrophysical Journal published new research revealing that the collision between our galaxies is already underway.

The news about the Andromeda galaxy came from Project AMIGA, which uses the Hubble Space Telescope to look at the deep-space surroundings of the Andromeda galaxy. AMIGA stands for Absorption Map of Ionized Gas in Andromeda. NASA called it:

… the most comprehensive study of a halo surrounding a galaxy.

The Andromeda galaxy, our Milky Way and other galaxies all sit enshrouded in a large envelope – called a galactic halo – which consists of gas, dust and stray stars. The halos of galaxies are faint, so faint, in fact, that detecting them is not an easy feat. These astronomers measured the size of the halo of the Andromeda galaxy by looking at how much it absorbed light from background quasars. They were surprised to find that the Andromeda galaxy’s halo stretches much, much farther beyond its visible boundaries.

Indeed, it extends as far as half the distance to our Milky Way (1.3 million light-years) and even farther in other directions (up to 2 million light-years).

Are the halos touching yet?

So, does this mean the halos of the Andromeda and Milky Way galaxies are touching?

It turns out that, from our vantage point inside the Milky Way, we cannot easily measure the characteristics of our galaxy’s halo. However, because the two galaxies are so similar in size and appearance, scientists assume that the halo of the Milky Way would also be similar.

In other words, it’s the faint halos of the galaxies that indeed appear to have started to touch one another. Thus, in a manner of speaking, the collision between our two galaxies has already started.

Visualizing Andromeda’s halo in our sky

Huge fuzzy purple sphere against star field with numerous pink dots in and around it.
Observing 43 background quasars, scientists used the Hubble Space Telescope to map out the halo of the closest spiral galaxy to our Milky Way, the Andromeda galaxy. The light from these very distant quasars (emission from very bright galaxies fueled by a central supermassive black hole) is absorbed as it travels through the halo. By studying the change in absorption depending on where in the halo you look, scientists not only see the large extent of the halo but also what it consists of. Illustration via NASA/ ESA/ E. Wheatley (Space Telescope Science Institute).
Earth's night sky above a ridgeline, with a huge purple halo around a galaxy.
This illustration depicts what the Andromeda galaxy’s gaseous halo might look like if it were visible to humans on Earth. At 3 times the size of the Big Dipper, the halo would easily be the biggest feature in the nighttime sky, according to NASA. Recent measurements of the halo show that the collision between the Milky Way and Andromeda galaxies has already begun. Image via NASA/ ESA/ J. DePasquale and E. Wheatley (STScI)/ Z. Levay.

So what will the Andromeda merger look like?

NASA released the images below in 2012. They are artist’s concepts of what someone on Earth might see as the Andromeda galaxy hurtles toward us.

The depictions below are based on painstaking Hubble Space Telescope measurements of the motion of the Andromeda galaxy, with computer modeling of the inevitable collision between the two galaxies. Also, a series of studies published in 2012 showed that – rather than glancing off each other, as merging galaxies sometimes do – our Milky Way galaxy and the Andromeda galaxy will in fact merge to form a single big elliptical, or football-shaped, galaxy.

Roeland van der Marel, an astronomer at the Space Telescope Science Institute, told Discover Magazine in February 2022:

Whether it’s fully a head-on collision or more of a glancing blow doesn’t really affect the end result.

And that is a new, giant elliptical galaxy.

Eight panels with night sky views ranging from today's through a chaos of stars to a smooth background glow.
View larger. | The merger between our Milky Way and neighboring Andromeda.
1st row, left: Present day.
1st row, right: In 2 billion years, the disk of the approaching Andromeda galaxy is noticeably larger.
2nd row, left: In 3.75 billion years, Andromeda fills the field of view.
2nd row, right: In 3.85 billion years, the sky is ablaze with new star formation.
3rd row, left: In 3.9 billion years, star formation continues.
3rd row, right: In 4 billion years, Andromeda is tidally stretched and the Milky Way becomes warped.
4th row, left: In 5.1 billion years, the cores of the Milky Way and Andromeda appear as a pair of bright lobes.
4th row, right: In 7 billion years, the merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky.
Image via NASA/ ESA/ Z. Levay and R. van der Marel, STScI/ T. Hallas/ A. Mellinger.

Another video of the Andromeda merger

The Milky Way and Andromeda galaxies, however, won’t be the only ones involved in this merger. As shown in the video below, the other large galaxy in our Local Group of galaxies, that is, M33, aka the Triangulum galaxy, will also play a role.

In the video below, you’ll recognize the Triangulum galaxy as the smaller object near the Andromeda and Milky Way galaxies. Although the Triangulum galaxy likely won’t join the merger, it may, nevertheless, at some point strike our Milky Way while engaged in a great cosmic dance with the two larger galaxies.

What happens to stars and planets when galaxies merge?

Across the universe, galaxies are colliding with each other. Astronomers observe galactic collisions – or their aftermaths – with the aid of powerful telescopes. In some ways, when a galactic merger takes place, the two galaxies are like ghosts; they simply pass through each other. That’s because stars inside galaxies are separated by such great distances. Thus the stars themselves typically don’t collide when galaxies merge.

That said, the stars in both the Andromeda galaxy and our Milky Way will be affected by the merger. The Andromeda galaxy contains about a trillion stars. Meanwhile, the Milky Way has about 300 billion stars. Stars from both galaxies will be thrown into new orbits around the newly merged galactic center. For example, according to scientists involved in the 2012 studies:

It is likely the sun will be flung into a new region of our galaxy …

And yet, they said,

… our Earth and solar system are in no danger of being destroyed.

Will humanity see the Andromeda merger?

So, how about life on Earth? Will earthly life survive the merger? Well, the sun will eventually become a red giant in about 7.5 billion years, when it will increase in size and consume the Earth. But even before then, the luminosity, or intrinsic brightness, of the sun will increase. This will happen, ultimately, in a timeline of about four billion years.

As solar radiation reaching the Earth increases, Earth’s surface temperature will increase. We may undergo a runaway greenhouse effect, similar to that going on now on the planet next door, Venus. So there’s a good change that earthly life won’t be around when the merger concludes.

But by that time, maybe some earthly inhabitants will have become space-faring. Perhaps we’ll have left Earth, and even our solar system. We may still get the view of Andromeda crashing into the Milky Way, just from a slightly different perspective.

Read more: Hubble Shows Milky Way is Destined for Head-On Collision

Bottom line: The Milky Way and Andromeda merger has already begun. The two spiral galaxies will form one giant elliptical galaxy in 5 billion years.

Source: Project AMIGA: The Circumgalactic Medium of Andromeda*

Via Discover Magazine

March 2, 2022

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