We think of galaxies as ancient. our galaxy, Milky Wayformed 13.6 billion years ago, and The James Webb Space Telescope has allowed us to look back at some of the first galaxies in the early universe. But are galaxies still being born today?
It’s a fun question to tackle because it allows us to delve into the messy, complicated, and beautiful process of formation of galaxies. Let’s take a look at the possibilities.
First answer: No
galaxies are fairly easy to identify. They are vast collections of stars, gas and dark matter. They are mostly distinct from each other; a typical galaxy is approximately 100,000 light years across, while the typical distance between galaxies is approximately 1 million light years.
Sometimes, galaxies merge or cluster within clusters, but with a few exceptions, we can mostly tell one galaxy from another. They are like towns in the countryside: The distance between the towns is greater than the towns themselves, so they are easy to distinguish and define. Sometimes, cities crash into each other, and sometimes, a sprawling city consumes its neighbors. But in general, a city is just a city.
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Determining the beginning of a galaxy, however, is another matter. Galaxies appeared in the early universe through a gradual process beginning in the first second of the century. Big Bang. At that time, small pockets of higher-than-average density appeared and grew steadily over the next hundreds of millions of years. At first, alone dark matter it could be shed, as ordinary matter was busy messing with itself. But once the pockets of dark matter grew large enough, they pulled the regular matter around.
With the accretion of regular matter, it compressed, broke up, and gave light to the former STARS. These protogalaxies went on to consume more gas, merge with their neighbors, and grow to become the fully formed galaxies we see today.
So, in many senses, no new galaxies appear today. The process of building them up—of seeding them as tiny differences in density or the initial accretion of dark matter—is over and done with, an act that happened in the ancient cosmos and never again. There are no more protogalaxies – no more clouds of gas just waiting for the opportunity to compact and form a new galaxy – in today’s universe.
When it comes to galaxies, what we see is what we get.
Second answer: yes
But this is only one way to determine the beginning of a galaxy. We can also look at another key step: the appearance of the first stars. Returning to the city analogy, there is a difference between when a city is first planned—its outlines defined with boundary markers and survey lines—and when the first people start moving.
If we focus only on the formation of stars, we see that this is an ongoing process that continues even in our present-day universe. In recent years, astronomers have built a detailed understanding of a measure called the stellar mass function. This is a basic demographic record that maps how many stars are shining in each galaxy—or, put another way, how much mass there is in the form of stars within each galaxy at different epochs in the universe.
Stars make up only a small percentage of a galaxy’s mass; the rest goes to dark matter and occasional clumps of gas. However, stars make a galaxy what it is, and they are much easier to observe than any other galactic constituent.
With new surveys examining galaxies across the universe, astronomers have recently discovered that the stellar mass function is increasing across the board. This means that there are more small galaxies, medium-sized galaxies, and large galaxies than there were billions of years ago.
Young, small galaxies do not result from the emergence of protogalaxies in dark matter seeds; they are clumps of already existing material that have just begun to form stars. Larger galaxies, on the other hand, are driven primarily by the ongoing merger of smaller galaxies.
It won’t last forever
So in at least one important way, new galaxies are continuing to appear on the cosmic stage as they light up with new rounds of star formation. They’ve always been there, hanging out for billions of years, but they’re just now making themselves visible. This process is viable because star formation is incredibly inefficient. Most of the gas inside a galaxy will never turn into stars, and it can go on for very long periods without using up much material—and it can take a galaxy a long time to start in the first place.
But unfortunately, the party won’t last forever. The problem is that not only is it the expanding universebut its expansion is accelerating—an effect known as dark energy. Although astronomers still don’t understand what drives dark energy, they can observe its effects on the rest of the universe: It’s spreading everything.
As the universe ages, it is increasingly difficult for material to clump together to form new galaxies and fuel continued star formation. In fact, the peak of star formation passed billions of years ago. As new galaxies continue to shine, the rate of emergence is slowing, with fewer and fewer new galaxies appearing each year.
We still have plenty of time – galaxies will continue to form stars for hundreds of billions of years to come – but we should enjoy the party while it lasts.