Andromeda and Milky Way galaxies are merging

Unless you specifically seek for it, the Andromeda galaxy, the nearest spiral galaxy to the Milky Way, is not visible in our night sky.

Milky Way and Andromeda merger has begun

However, in the absence of optical help, you can only faintly make it out as a fuzzy patch of light in the black skies. But someday in the far future, Andromeda will shine brightly in our sky, getting bigger and bigger as it approaches us. The eventual merger of our two galaxies has actually already started, despite the fact that they are still 2.5 million light-years apart.

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).

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.

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.

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.

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."

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.

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 Circum-galactic Medium of Andromeda*

Via Discover Magazine