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For most of human history, we didn't know that galaxies were a thing. So over the past century, astronomers have been working to understand how galaxies come to be and how they evolve over time. And for a full decade, there was one telescope helping to light the way.

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[♪ INTRO] For most of humanity’s history, galaxies… weren’t a thing.

I mean, they existed, but before the 1920s, we didn’t know our Milky Way was just one of many throughout the universe. Or that they come in an array of shapes and sizes.

So over the past century, astronomers have been working to understand just how galaxies come to exist, how they grow, and how they die. And for a full decade, they used a space telescope called GALEX to light the way. Or, more accurately, capture the invisible light that lit the way.

You know what I mean. The invisible light we’re talking about is ultraviolet light, which is past blue and violet visible light on the electromagnetic spectrum, and has a shorter wavelength. And UV light is key if we want to understand how old a galaxy is and how it came to be.

The more massive a star is, the more ultraviolet and blue-adjacent visible light it emits compared to longer wavelengths, like red and infrared. So stars around 3 times the mass of our Sun look blue. And blue stars have shorter lifespans.

We’re talking hundreds of millions of years, compared to the least massive stars, which appear more red and can live for trillions of years. But baby stars of all sizes and colors also emit more UV light than their older counterparts. So, astronomers have turned to UV to understand the populations of stars in galaxies, and how many stars a galaxy is making at a given moment.

Basically, where it is in its life cycle. And taking a bunch of galaxies together, you can understand how they evolve over time. But our atmosphere blocks most wavelengths of UV light.

While that’s good for humans most of the time, it means astronomers have to get above our atmosphere to study how galaxies evolve. The most recent space-based telescope dedicated to this area of research is NASA’s Galaxy Evolution Explorer, better known as GALEX. Launched into Earth orbit back in April 2003, GALEX focused on capturing those UV wavelengths our air will not let through.

It could see back in time to about 3.7 billion years after the Big Bang, covering about 80% of the universe’s history. GALEX used a 50-centimeter-wide primary mirror to collect UV light. After that, a beam splitter sent two ranges of UV wavelengths into two different detectors.

That means GALEX could make two kinds of observations at the same time. The beam splitter also helped correct for distortions caused by GALEX’s field of view, which was super wide compared to most other telescopes. It could see about one and one quarter degrees of the sky while looking at a single target.

And that might not sound like much, but it’s three times the width of the Moon. And it’s over a hundred times wider than what the Hubble Space Telescope can see in ultraviolet. With GALEX’s view, astronomers could study the regions around galaxies in greater detail.

And they’ve made some pretty important discoveries over the years. Some of those include evidence supporting prior hypotheses. Galaxies that are making a lot of new stars look bluer, thanks to all that blue and UV light.

So astronomers use the terms “blue” and “red” galaxies to describe galaxies that are either making a lot of new stars, or are not. It so happens that a lot of blue galaxies are spirals, and a lot of red galaxies are ellipticals. This led to the hypothesis that galaxies start out as blue spiral galaxies, and evolve into red ellipticals.

As a galaxy ages, it runs out of fuel to make more stars. So not only are there fewer baby stars spewing out UV rays in any given millennium, all those short-lived blue stars eventually start dying out without getting replaced. Only the red ones are left shining.

But confirming that hypothesis would require finding so-called “teenage” galaxies in between these two types. And GALEX found those teenagers, providing evidence that astronomers were on the right track with this hypothesis. But there were also several times that GALEX disproved common perceptions about the lives of galaxies.

Take the galactic outer boonies. These are places where astronomers didn’t think there was enough material to condense into stars, but GALEX’s wider field of view revealed that stars totally could form out there. And in a similar sense, observations also disproved the idea that our universe was too old to make new, big galaxies any more.

But just a few billion light years from home, GALEX found about three dozen galaxies that were as young as 100 million years old. Compare that to the Milky Way, which is about 100 times older. Those galaxies are younger than some dinosaur fossils.

Usually, if astronomers want to study baby galaxies they have to look at ones in the furthest reaches of our universe. The further away a galaxy is, the younger we see it, because light can only travel so fast across the universe so fast. But the further away a galaxy is, it's also way harder to see it!

So these younger neighbors give astronomers a much more detailed look at galactic evolution. But maybe the coolest discovery that GALEX made, with some assistance from the Hubble Space Telescope, is that dead galaxies can reanimate themselves. Red galaxies that have run out of fuel to make new stars can gravitationally scoop up gas from the space in-between galaxies, the intergalactic medium, and roar back to life.

GALEX was able to identify 30 galaxies which to the human eye were past their star-forming years. But in the UV, they were super bright. Unfortunately, GALEX could not see enough detail in these images to figure out why.

So that’s where Hubble came in. And it found three quarters of these presumed dead galaxies had massive ultraviolet ring structures, indicating star formation. If the extra fuel came from something like merging with another galaxy, those rings could only form through an almost impossible dead-on collision.

So it’s more likely they’re slowly stealing matter from intergalactic space around them. GALEX was originally slated for a 29-month mission, but it kept collecting data until June 2013. That’s a whole decade of exploration, and its data is still being used by astronomers to explore how galaxies evolve.

So like the galaxies it studied, in a way, it’s still burning bright. Like galaxies, all telescopes eventually die. And like galaxies, sometimes they can flare back to life… at least, this one can, as this month’s SciShow Space Pin of the Month.

Starting now through the end of this month, you can preorder this pin to add to your collection. We will only accept orders through the end of this month, then we will start manufacturing and shipping. And next month, we’ll have a new, great pin for you – so keep your eyes peeled.

And thanks for watching! [♪ OUTRO]