What is the smallest galaxy known?

The Hunt for the Teeny-Tiniest Galaxy: A Cosmic David vs. Goliath

We’ve been on a galactic treasure hunt for ages, searching for the absolute smallest galaxy out there. It sounds simple, right? But trust me, figuring out what really counts as a galaxy and which one’s the runt of the litter is trickier than you’d think. It’s a quest that’s forcing us to rethink how galaxies are born and the sneaky role dark matter plays in it all.

So, What Is a Galaxy Anyway?

Okay, before we get lost in the cosmic weeds, let’s nail down what we’re even looking for. Generally speaking, a galaxy is a bunch of stars, gas, dust, and that mysterious dark matter, all held together by gravity. The key thing? Most scientists believe galaxies are swimming in a sea of dark matter. This dark matter “halo” is what separates a true galaxy from, say, a really impressive cluster of stars. Think of it like the difference between a city (galaxy) and a really big neighborhood (star cluster).

Remember Segue 2? It Was a Big Deal

For a while, Segue 2 was the poster child for tiny galaxies. Discovered back in ’09, this little guy is a dwarf spheroidal galaxy, meaning it’s kinda roundish and hangs out near our Milky Way, about 114,000 light-years away. Now, Segue 2 is dim. I mean, its total light output is only about 800 times brighter than our Sun. That’s like comparing a flashlight to a stadium floodlight! It only has around 1,000 stars. But here’s the kicker: scientists estimate it weighs in at around 550,000 times the mass of the Sun! That huge difference between its mass and light tells us there’s a ton of dark matter lurking inside. Plus, its stars are ancient, some of the first to ever shine, which makes Segue 2 a fascinating fossil from the early universe.

Hold the Phone! Meet Ursa Major III/UNIONS 1

But the universe loves to throw curveballs. Recently, a new contender has entered the ring: Ursa Major III/UNIONS 1. This object, spotted during a sky survey, is even closer to us, at about 32,600 light-years. So, what makes it so special? Well, it might just be the lightest galaxy we’ve ever seen. We’re talking about a stellar mass of only 16 suns, with roughly 60 stars. That’s like a cosmic speck of dust! It’s also super compact, with a radius of about 10 light-years. And, just like Segue 2, it seems to be drowning in dark matter.

Is It Really a Galaxy, Though? That’s the Million-Dollar Question

Now, before we crown Ursa Major III/UNIONS 1 the winner, there’s a bit of a debate brewing. Is it a true galaxy, or just a star cluster masquerading as one? Some astronomers think it might be a globular cluster that’s being ripped apart by the Milky Way’s gravity. To know for sure, we need more data to confirm whether it’s truly held together by a dark matter halo. It’s like trying to decide if that weird-looking dog at the shelter is a purebred or a lovable mutt – you need to look closer!

Don’t Forget Andromeda XXXV

And let’s not forget Andromeda XXXV, another potential record-breaker orbiting the Andromeda galaxy (our Milky Way’s big sister). It’s incredibly faint and tiny, so much so that it’s making us rethink our theories about how galaxies can even form on such small scales.

Why Does Any of This Matter?

You might be thinking, “Okay, so what if we find the smallest galaxy? What’s the big deal?” Well, studying these ultra-faint dwarf galaxies is like archaeology for the universe. They could be the building blocks that formed larger galaxies like our own. Plus, their dark matter content makes them perfect labs for understanding this mysterious stuff that makes up most of the universe. It’s like studying the tiniest gears in a clock to understand how the whole thing works.

As our telescopes get better and better, I bet we’ll find even tinier galaxies lurking out there. It’s not just about bragging rights; it’s about piecing together the puzzle of how the universe came to be. And who knows what other surprises are waiting for us in the dark? The cosmos, as always, keeps us on our toes.