Look, I love me some majestic spiral galaxies, some weird peculiar galaxies, and some messy irregular galaxies. I even love puffy cotton-ball ellipticals. But dwarf galaxies need love too.

Dwarfs are smaller than galaxies like our huge spiral Milky Way, sometimes with only (“only”) a few billion stars, and sometimes even less. This makes them intrinsically small and faint, so they’re hard to see clearly if they’re not cosmically close to us. But they’re important: they greatly outnumber big galaxies, for one thing. For another, our theories on how galaxies formed when the Universe was young predict a certain number of them, and, stubbornly, the actual observed number seen doesn’t agree, which means there’s something we don’t fully grasp about the process. To understand galaxies, we need to study dwarfs.

There are roughly 80 dwarf galaxies at least in the Local Group, the small clutch of galaxies of which our Milky Way is one of the two largest members (the Andromeda Galaxy being the other). Most look like nothing more than fuzzy patches even through decent-sized telescopes, but when you use Hubble, individual stars are easily discerned.

UGC 4879 is just such a dwarf, though technically just beyond our Local Group at a distance of about 4 million light-years (it might be a member of the LG, just way out in the periphery). As you can see, it’s just…stars. No gas clouds, no obscuring dust lanes, just stars. In fact, you can see right through it to much more distant galaxies in the background!

It may be what’s called a relic, an ancient galaxy that formed when the Universe was young and has remained untouched ever since. It’s never gotten close enough to the Milky Way or Andromeda to be affected, so it’s a pristine example of what these galaxies were like billions of years ago. That makes it a great laboratory case to study!

Two closer dwarfs are Andromeda III and the Tucana Dwarf:

Andromeda III is a satellite of the Andromeda Galaxy, about 3 million light-years from us. A weird thing that’s not understood is that many such Andromeda satellites are aligned in the same plane, like the planets in our solar system. Why? That’s not usually seen around other galaxies, and current thinking is that they should orbit every which-way, like comets do around the Sun. It may be they all formed from the same immense gas cloud, which was near the much larger cloud that formed Andromeda, so they naturally all orbit in the same plane. Andromeda III, though, appears to be a few billion years younger than its host, so it’s not clear what’s going on here.

Tucana is about the same distance from us, 3 million light-years. It’s also quite isolated, like UGC 4879.

There’s also Leo A and the Pegasus Dwarf Spheroidal Galaxy (or Andromeda VI):

Leo A is about 2.6 million light-years from us, and Pegasus a wee bit farther (and also a satellite of Andromeda). Both are spheroidal, meaning very roughly spherical, as opposed to flat and disk-like, or irregular in shape.

These images were taken for different reasons, mostly to look at how stars have formed in these galaxies and how old they are. But another thing that can be done is to look at how the stars move over time! It’s not easy, because they’re so far away that the stars appear to move very slowly, but these images are so sharp that if they’re repeated years later many of the stars will have moved enough to measure. That gives all kinds of information, like how massive the galaxy is, whether it’s moving across the sky as a whole or not (its transverse velocity, which I wrote all about here), and how much dark matter might be present (which affects the gravity of the galaxy and so its total mass).

I don’t know if such observations will be taken — Hubble’s lifespan is finite, and it may not last long enough to be able to get enough images like these again to ascertain stellar motions — but in principle it can be done, even with other telescopes like JWST.

Such an idea was a dream a few decades ago, but here we are. A lot of astronomers’ dreams are now reality. Good thing we can dream even bigger! There’s a lot of stuff to see out there, much of which is small and faint, but still tells us big stories. 

Did you know I write a weekly column for Scientific American, called “The Universe”? Every Friday I cover some fun topic, from why the Moon looks bigger on the horizon to rogue planets in the Orion Nebula to questioning whether the Andromeda Galaxy is really going to collide with us in a few billion years or not. It’s kinda all over the place, but then, so is the Universe. 

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