ScienceThe Universe is out there, waiting for you to discover it.
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
The story of how we built our Milky Way is like building a giant structure out of LEGOs. Only, instead of the LEGOs remaining the same over time, they’re actively changing form as we assemble our structure. It would be like starting with all the pieces to put together 100 different X-Wing LEGO fighters, and winding up with a Star Destroyer when we were done.
Galaxies, you see, don’t just grow by attracting other galaxies and merging together to form larger ones. Galaxies also evolve, meaning they:
funnel matter in towards the center,
generate density waves along their spiral arms,
attract additional matter from outside the galaxy along cosmic filaments,
and change shape and orientation based on the other galaxies and matter that falls into them.
Multiwavelength composite of interacting galaxies NGC 4038/4039, the Antennae, showing their namesake tidal tails in radio (blues), past and recent starbirths in optical (whites and pinks), and a selection of current star-forming regions in mm/submm (oranges and yellows). Inset: ALMA’s first mm/submm test views, in Bands 3 (orange), 6 (amber), & 7 (yellow), showing detail surpassing all other views in these wavelengths.(NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); HST (NASA, ESA, AND B. WHITMORE (STSCI)); J. HIBBARD, (NRAO/AUI/NSF); NOAO/AURA/NSF
While the earliest proto-galaxies that eventually grew into the Milky Way may have formed just 200-250 million years after the Big Bang, cosmic evolution continued all throughout that time.
The first stage was forming the earliest stars and star clusters, which takes around 100 million years, and formed out of the pristine material (hydrogen and helium) left over from the Big Bang. These star clusters evolved quickly, resulting in a very rapid end-of-life for their stars. When those stars died, they polluted the interstellar medium with heavy elements that then gave rise to the second generation of stars. By time 200-to-300 million years had gone by, star clusters had merged together with one another, giving rise to the very first galaxies.
Galaxies that are currently undergoing gravitational interactions or mergers are almost always also forming new, bright, blue stars. Simple collapse is the way to form stars at first, but most of the star formation we see today results from a more violent process. The irregular or perturbed shapes of such galaxies are a key signature that this is what’s occurring, and the evidence for these mergers can go back as far as our telescopes can see at present.NASA, ESA, P. OESCH (UNIVERSITY OF GENEVA), AND M. MONTES (UNIVERSITY OF NEW SOUTH WALES)
The cosmic web then begins to take shape. As more time goes by, gravitation can reach greater and greater distances, causing larger-scale clumps of matter to fall in. When a clump that’s smaller than the early galaxy falls it, it gets tidally torn apart and funneled into the galaxy’s interior gently and slowly, where it can simply be absorbed over time.
These minor mergers are common, and anything up to about a third the mass of the total galaxy falls into this category. Any internal structures, such as spiral arms, star-forming regions, a bar, or a bulge should all remain intact. Meanwhile, the additional gas and dust provides new fuel for new generations of stars. Star formation usually intensifies during merger events, even minor ones. For the first 2 or 3 billion years, this process was common.
When major mergers of similarly-sized galaxies occur in the Universe, they form new stars out of the hydrogen and helium gas present within them. This can result in severely increased rates of star-formation, similar to what we observe inside the nearby galaxy Henize 2-10, located 30 million light years away. This galaxy will likely evolve, post-merger, into a giant elliptical.X-RAY (NASA/CXC/VIRGINIA/A.REINES ET AL); RADIO (NRAO/AUI/NSF); OPTICAL (NASA/STSCI)
But as time goes on and the Universe expands, mergers become, on average, less common but more major. Galaxies clump and cluster together into groups of many different sizes, but occasionally can form large galaxy clusters with hundreds or even thousands of times the mass of our own Local Group. These dense galaxy clusters are some of the most spectacular sights in the Universe, but they’re also relatively rare: the majority of mass and the majority of galaxies are found in small groups like our own, not in the massive clusters that we see so prevalently in our Universe. By the time 4 or 5 billion years had gone by, it became clear we’d never become part of a massive cluster.
It’s important that we keep these mergers small, though. If we experience a major one, where two similarly-sized galaxies collide, they can induce an enormous burst of star formation, which can use up all the available star-forming gas and “mix up” the matter in the galaxy.
The ultramassive, merging dynamical galaxy cluster Abell 370, with gravitational mass (mostly dark matter) inferred in blue. Many elliptical galaxies are found inside massive clusters like this, as the result of major mergers that occurred billions of years ago. There are still a large number of spirals, too, as the total mass of this galaxy cluster may exceed a thousand times that of the Local Group.NASA, ESA, D. HARVEY (SWISS FEDERAL INSTITUTE OF TECHNOLOGY), R. MASSEY (DURHAM UNIVERSITY, UK), THE HUBBLE SM4 ERO TEAM AND ST-ECF
This typically results in the creation of a giant elliptical galaxy: one that forms stars all-at-once in tremendous numbers, and then never again. This is the end-stage of galaxy evolution for most galaxies, but it relies on multiple large galaxies smashing together. This realization helps explain why giant ellipticals are common inside massive galaxy clusters, but much rarer in groups or in isolation.
It takes a lot of mass, built up over time, to create a major merger. So long as a galaxy is massive enough (as in Milky-Way sized or comparable), there is available material to form new stars (gas). So long as galaxies have angular momentum and a preferred rotation axis (which they do in the absence of a major merger), and so long as they have enough time to settle down into a stable shape (which they all have, unless there’s been a recent major merger), we expect them to have a spiral shape.
The isolated galaxy MCG+01-02-015, all by its lonesome for over 100,000,000 light years in all directions, is presently thought to be the loneliest galaxy in the Universe. The features seen in this galaxy are consistent with it being a massive spiral that formed from a long series of minor mergers, but having been relatively quiet on that front for billions of years.ESA/HUBBLE & NASA AND N. GORIN (STSCI); ACKNOWLEDGEMENT: JUDY SCHMIDT
Our Milky Way likely grew from a series of proto-galaxies that settled down into a spiral shape, then gradually gobbled up many of the smaller galaxies present in the Local Group. We didn’t even gather the majority of them; that honor goes to our neighbor, Andromeda. Nor are we done: there are satellite galaxies merging with us today, and a few galaxies on our outskirts, like the two Magellanic Clouds, that will likely be devoured in the next few hundred million years or so.
The cosmic story that brought the Milky Way to be is one of survival of the largest. When it comes to dominating the galaxy, mass is the overwhelming factor.
As time went on, this flat, disk-like shape began to wind up. Our spiral arms became more pronounced and developed more turns in them. Spurs came off of the arms, and gravitational interactions led to us forming stars along the tail ends of our galaxy. Additional gas flowed into the outskirts, eventually getting funneled to the center.
As galaxies continue to evolve, they also develop features we might recognize. A central bulge forms in the densest region of matter. There are pathways that are more successful at driving matter into the core: a central bar develops and grows. The dynamics of gas and stars causes the galaxy to become an even thinner disk, and to spread out towards the edges, increasing in radius but decreasing in thickness.
And finally, as gravity does the inevitable, all the galaxies bound together will eventually merge. The Milky Way itself is destined, approximately 4 billion years from now, for a merger with Andromeda.
A series of stills showing the Milky Way-Andromeda merger, and how the sky will appear different from Earth as it happens. This merger will occur roughly 4 billion years in the future, with a huge burst of star formation leading to a red-and-dead, gas-free elliptical galaxy: Milkdromeda. A single, large elliptical is the eventual fate of the entire local group.NASA; Z. LEVAY AND R. VAN DER MAREL, STSCI; T. HALLAS; AND A. MELLINGER
The cosmic story that led to the Milky Way is one of constant evolution. We likely formed from hundreds or even thousands of smaller, early-stage galaxies that merged together. The spiral arms likely formed and were destroyed many times by interactions, only to re-form from the rotating, gas-rich nature of an evolving galaxy. Star formation occurred inside in waves, often triggered by minor mergers or gravitational interactions. And these waves of star-formation brought along increases in supernova rates and heavy metal enrichment. (Which sounds like everyone’s favorite after-school activity.)
These continuous changes are still occurring, and will come to a conclusion billions of years in the future, when all the galaxies of the Local Group have merged together. Every single galaxy has its own unique cosmic story, and the Milky Way is just one typical example. As grown up as we are, we’re still evolving.