[♪ INTRO].

The first stars in the universe formed out of the primordial gases of the Big Bang, a bunch of fresh hydrogen and helium that had never been in another star before. But here's the thing: We've never actually seen those stars.

And last week, after looking farther back in time than any search yet, the European Space Agency announced, well, they still haven't found any. Which sounds a little disappointing, but it's also kinda interesting. Because just the absence of these stars at a certain distance is a clue in and of itself.

And it gives us a better idea of what the early universe must have been like. Even though we have never seen these stars, known as Population III stars, we know that they must have existed at some point before the first galaxies, less than a billion years after the Big Bang. And we even know what to expect when we find them, because astronomers have done a ton of modeling.

For one, these stars would have been enormous, like 60 or even hundreds of times more massive than the Sun. And unlike most stars, they'd be made almost entirely of hydrogen and helium. Now, if astronomers could just find any of ‘em, these stars could reveal tons of information, including when the universe got its first light and how galaxies got enriched with metals.

But astronomers have looked for these stars before, without any luck. So, using data from the Hubble Space Telescope collected between 2012 and 2017, a team at the European Space Agency set out to look further back in time than anyone had ever done before. They used special techniques for analyzing images, along with extra data from the Spitzer Space Telescope and the Very Large Telescope in Chile, and they managed to observe galaxies and galaxy clusters between 500 million and one billion years after the Big Bang.

But they did not still find what they were looking for. In fact, what they did find were lots of low-mass galaxies with a decent amount of metal in them. And that told the scientists that Population III stars must have existed even earlier in order to forge hydrogen and helium into the metals they saw in those galaxies.

So even though they didn't find what they wanted, that result actually gives us lots of insight into the early universe! For instance, it suggests that the oldest stars formed much earlier than we thought. And that gives us a better idea of when the first galaxies formed, which gives us a more complete picture of the entire early universe.

And when the James Webb Space Telescope launches next year, astronomers are hoping it will give us the observational capabilities to finally find these elusive stars and further understand our infant universe. Last week, scientists also announced that they have another mysterious star population on their hands. The whole thing started last fall with the observation of an unusual source of X-rays in between our neighboring galaxies, the Magellanic Clouds.

But this was actually not the first time astronomers had noticed this source. Back in 1993, researchers saw big bursts of X-rays coming from an unknown object in the same spot, almost 200 thousand light-years away. The X-rays happened in two big bursts over about six months, and then the source went quiet.

Astronomers weren't able to gather a whole lot of data on it, so it remained pretty mysterious, until the source suddenly flared back up again in November 2019! This time, scientists were able to collect much more data than they could in the 1990s. Back then, they thought it was probably a type of binary star system made up of a high-mass star and a neutron star orbiting close together.

In these systems, the neutron star can siphon off material from its companion, triggering bursts of X-rays. But there wasn't enough data for scientists to really nail down a hypothesis. So that's why it was so exciting when the X-rays started up again 26 years later!

This time, astronomers used the Indian space telescope AstroSat to study the newly emitted X-rays and see if they could better understand the object. And this time, they found something new: broad-band pulsations. These are regular pulses in brightness over a wide range of X-ray wavelengths.

And that was a pretty strong clue that they were looking at a specific type of rotating neutron star called a pulsar, and not just any pulsar, either. The pulsations coming from this one looked a lot like the signals emitted by stars known as ultraluminous X-ray pulsars, or ULXPs. ULXPs are super bright pulsars that are known to light up suddenly with bursts of X-rays.

But beyond that, astronomers don't actually know much about them, because this object was only the eighth one ever found. So we don't know what actually makes them so bright or drives their fluctuations. They might all be binary systems like astronomers originally suspected the 1993 object was.

Or maybe they've got something else going on. So on one hand, we've half-solved the mystery of the 1993 object:. It's a ULXP, and there are other objects like it.

But now it's part of a bigger mystery:. What exactly are these objects, and what makes them light up the way they do? That is still an open question.

So from the earliest stars to the brightest pulsars, astronomers are still working to understand the nature of all of the kinds of stars that are out there and how they shaped the universe as we know it today. Thanks for watching this episode of SciShow Space News! And if you're into stellar mysteries, you might like our episode on a hypothetical type of star called a blitzar, and the cosmic mystery that it could solve.

You can watch that one right after this! [♪ OUTRO].