https://www.forbes.com/sites/startswithabang/2018/12/26/what-was-it-like-when-the-milky-way-took-shape/#2414e2107375

What Was It Like When The Milky Way Took Shape?

The Sunflower Galaxy, Messier 63, tilted relative to our line-of-sight, with one half clearly appearing dustier than the other. This is an evolved spiral galaxy that hasn’t had a major merger recently, and is only somewhat more spiral-y (or flocculent) than our own.ESA/HUBBLE & NASA

The Milky Way galaxy may be just one of trillions in the observable Universe, but it’s uniquely special as our cosmic home. Composed of a few hundred billion stars, about a trillion solar masses worth of dark matter, a supermassive central black hole, and a plethora of gas and dust, we’re actually somewhat typical of modern galaxies. We’re neither among the biggest or the smallest galaxies, nor are we in an ultra-massive cluster or found in isolation.

What does make us special, though, is how evolved we are. Some galaxies grow up quickly, exhausting their fuel and becoming “red and dead” when they lose the ability to form new stars. Some galaxies undergo major mergers, transforming from spirals into ellipticals when that occurs. And others experience enormous tidal disruptions, leading to sweeping, distended spiral arms. Not the Milky Way, though. We grew up exactly like you’d expect. Here’s how we got there.

The Whirlpool Galaxy (M51) appears pink along its spiral arms due to a large amount of star formation that’s occurring. In this particular case, a nearby galaxy gravitationally interacting with the Whirlpool galaxy is triggering this star formation, but all spirals rich in gas exhibit some level of new star birth.NASA, ESA, S. BECKWITH (STSCI), AND THE HUBBLE HERITAGE TEAM STSCI / AURA)

At the present time, galaxies like the Milky Way are incredibly common. Here are some properties that they typically display:

  • hundreds of billions of stars,
  • concentrated into a pancake-like shape,
  • surrounded by globular clusters in a halo-like shape,
  • containing spiral arms that extend radially outward for tens of thousands of light years,
  • with a central bar-like feature emanating from a bulging region,
  • a tremendous amount of gas and dust concentrated in the galactic plane,
  • and young star-forming regions found where the gas and dust is densest.

Such a behemoth exerts a tremendous gravitational pull acting on everything else nearby. You can recognize a galaxy like this from afar, with the starlight streaming out of it being its characteristic giveaway. But it couldn’t have been this way forever. What we know as our Universe began with the Big Bang some 13.8 billion years ago, and galaxies couldn’t have always been this way. In fact, if we look back far enough, we can see the differences start to appear.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, more chaotic, and richer in gas in general than the galaxies we see today. For the first galaxies of all, this effect goes to the extreme. As far back as we’ve ever seen, galaxies obey these rules.NASA AND ESA

Compared to the Milky Way and other Milky Way-like galaxies that we see today, galaxies were:

  • younger, as evidenced by an increase in young stars,
  • bluer, since the bluest stars die the fastest,
  • smaller, because galaxies merge together and attract more matter over time,
  • and less spiral-like, because we are only see the brightest parts of the most active, distant, star-forming galaxies.

Our galaxy today, in other words, is the result of 13.8 billion years of cosmic evolution, where large numbers of small proto-galaxies merged together and attracted additional matter into them. We are what remains after countless other galaxies have been swallowed by our own.

Star formation, gas bridges, and irregularly shaped galaxies are just some of the features arising in Hickson Compact Group 31. Compact groups can often illustrate how galaxy mergers appear in a variety of stages and circumstances.NASA / STSCI / WIKISKY / HUBBLE AND WIKIMEDIA COMMONS USER FRIENDLYSTAR