The universe can be an amazing, terrifying,
and incredibly weird place. We’ve talked about planets so strange they
probably shouldn’t exist, but that’s just the tip of the iceberg in a universe with
hundreds of billions of galaxies. Over the years, astronomers have discovered
a lot of galaxies that they never would have expected to find — huge collections of stars
that formed in ways we still can’t explain, or because of a rare coincidence. About 600 million light years away, for example,
is a place so odd that for decades astronomers weren’t even sure whether it was a galaxy. Today we call it Hoag’s Object after its
discoverer, Art Hoag, who discovered it in 1950. It’s a striking example of a rare type of
galaxy known as a ring galaxy. The middle of a ring galaxy is pretty normal-looking. But it’s surrounded by a huge loop of stars,
dust, and gas. And Hoag’s Object is especially remarkable
because of the contrast between the galaxy and the ring. The inner clump shines brightly with a ton
of older, redder stars, while the outer ring is mainly made up of younger, bluer ones. And the area between the center and the loop
seems almost totally empty! Astronomers have a few ideas about how ring
galaxies might form, but Hoag’s Object doesn’t line up neatly with any of them. Some ring galaxies are thought to form when
a smaller galaxy strikes a larger one and the resulting shockwave pushes material out
to form the ring. But if that happened to Hoag’s Object, where’s
the second galaxy? You’d expect it to be somewhere nearby. Ring galaxies might also form when one galaxy
passes close enough to another for its gravity to strip away some of the stars. But that would be a messy process and probably
wouldn’t result in such a neat, confined ring. So even after decades of studying it, astronomers
really don’t know how Hoag’s Object could have formed. Now let’s travel back more than ten and
a half billion years, to a galaxy named BX442. It’s the earliest known grand design spiral
galaxy, a category of galaxies identified by their long, graceful spiral arms. These days about 10% of galaxies have this
sweeping appearance, but the farther back you go, the less often they appear. That’s probably because in the past, galaxies
were closer together, which led to a lot more spiral-destroying collisions. Older grand design spiral galaxies are so
rare that in a sample of more than 300 ancient galaxies examined in one study, BX442 was
the only one to display any sort of spiral structure. And astronomers think they might know why. There’s a nearby dwarf galaxy that might
have passed close to BX442, and this smaller galaxy’s gravity could have helped with
the formation of spiral arms. In that case, BX442’s beauty might have
lasted only a hundred million years. So this galaxy only existed because that dwarf
galaxy happened to be passing by. And we’re only seeing it because we happened
to catch it at the right time. Another strange galaxy is even older — it
existed just 700 million years after the Big Bang. That’s only a couple hundred million years
after the earliest known galaxies. But A1689-zD1 looks downright old. That’s because it’s full of something
that’s thought to have been rare in the early Universe: dust. Interstellar dust contains elements heavier
than hydrogen and helium, and those heavier elements weren’t around right after the
Big Bang. Instead, they were produced by supernovas,
the explosive deaths of giant stars. When a star explodes, it turns lighter elements
into heavier ones and spreads them through space. Over time, this builds up large clouds of
interstellar dust. To create the amount of dust seen in this
galaxy, there must have been a lot of star formation and star death going on. And the same stuff that makes up dust also
forms the building blocks for planets, which means this galaxy might have had more developed
star systems, too. So it was really mature for such a young galaxy. Like Hoag’s Object and BX442, this ancient
galaxy is one of the most surprising galaxies astronomers have ever discovered. And we’ve only seen a tiny fraction of all
the galaxies out there. So who knows what else we’ll find? Thanks for watching this episode of SciShow
Space, and thanks especially to our patrons on Patreon who help make this show possible. If you want to help us keep making episodes
like this, just go to patreon.com/scishow to learn more. And don’t forget to go to youtube.com/scishowspace
and subscribe!
