Latest / Daniel and Kelly’s Extraordinary Universe / Why do some galaxies have bars?

Transcript

Why do some galaxies have bars? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, Daniel, remember a few weeks ago when you insulted
• 0:10
  Speaker 1: America's favorite chocolate bar hershees.
• 0:14
  Speaker 2: How could I forget? I got some strong emails about
• 0:17
  Speaker 2: that comment.
• 0:18
  Speaker 1: Oh yeah, did you get us barred from the airwaves?
• 0:21
  Speaker 2: No? But I think I might have to raise the
• 0:23
  Speaker 2: bar in terms of my comments.
• 0:25
  Speaker 1: You mean we're not already at the bottom.
• 0:27
  Speaker 2: We're scraping the bottom of the bar roll.
• 0:30
  Speaker 1: Yeah, it sounds like you need to spend less time
• 0:32
  Speaker 1: in bars drinking away your chocolate sorrows.
• 0:35
  Speaker 2: I'll have a chocolate TEENI please.
• 0:37
  Speaker 1: Sure give it a shot. Hi am Hori, my cartoonist
• 0:54
  Speaker 1: and the author of Oliver's Great Big Universe. Hi.
• 0:57
  Speaker 2: I'm Daniel. I'm a particle physicist and a professor at
• 1:00
  Speaker 2: U SEE Irvine, and I do have strong opinions about chocolate.
• 1:03
  Speaker 1: Do you have opinions about strong chocolate? Or do you
• 1:05
  Speaker 1: have strong opinions about chocolate?
• 1:06
  Speaker 2: Yes? And yes, it's.
• 1:10
  Speaker 1: Because you're really passionate about chocolate. It gives you a
• 1:13
  Speaker 1: lot of comfort.
• 1:14
  Speaker 2: I feel like, if you're gonna eat chocolate, then you
• 1:16
  Speaker 2: should eat chocolate that you like. But you know, people
• 1:18
  Speaker 2: prefer lots of different kinds of chocolate. I don't mean
• 1:20
  Speaker 2: to judge people who like Hershey's. You know, all the
• 1:23
  Speaker 2: power to them if that's what they want. It's everywhere.
• 1:25
  Speaker 1: I feel like you're maybe being a bit of a
• 1:26
  Speaker 1: chocolate snob.
• 1:27
  Speaker 2: I mean, are you being a snob to know what
• 1:29
  Speaker 2: you like and know what you don't like. I don't
• 1:31
  Speaker 2: want to insult anybody else's preferences to anybody who is
• 1:34
  Speaker 2: offended by me calling Hershey's chocolate garbage, I do apologize.
• 1:38
  Speaker 1: That sounds like a non apology, Daniel.
• 1:41
  Speaker 2: It did sound like an apology. I agree, But it
• 1:46
  Speaker 2: sounded like I was trying, didn't it.
• 1:47
  Speaker 1: I don't think that counts these days. I think you
• 1:49
  Speaker 1: could raise your bar in terms of apologies to the masses.
• 1:54
  Speaker 2: Well, I'll say this, I enjoy chocolate. I hope everybody
• 1:56
  Speaker 2: else out there enjoys chocolate, whatever flavor or variety they prefer. Yeah,
• 2:00
  Speaker 2: there you go.
• 2:00
  Speaker 1: But anyways, welcome to our podcast Daniel and Jorge Explain
• 2:03
  Speaker 1: the Universe, a production of iHeartRadio.
• 2:06
  Speaker 2: In which we whip up a delicious concoction of everything
• 2:08
  Speaker 2: that's out there in the universe, all the dark energy,
• 2:11
  Speaker 2: all the dark matter, all the dark ideas that describe
• 2:14
  Speaker 2: how the universe actually works, what it's made out of,
• 2:17
  Speaker 2: and how it came to be. We melt it all down,
• 2:20
  Speaker 2: form it into squares, and ship it to you over
• 2:23
  Speaker 2: the airwaves.
• 2:24
  Speaker 1: That's right. We try to lower the bar of science
• 2:26
  Speaker 1: and make it accessible to everyone out there. We try
• 2:29
  Speaker 1: to make it silky and smooth and delicious and easy
• 2:32
  Speaker 1: to go down.
• 2:33
  Speaker 2: Because wondering about how the universe works is something everybody does,
• 2:36
  Speaker 2: and everybody should have access to what we know and
• 2:39
  Speaker 2: what we don't know about the universe. Thinking like a
• 2:42
  Speaker 2: physicist is not something only professional physicists can do. It's
• 2:45
  Speaker 2: something everybody can do and should do about how the
• 2:48
  Speaker 2: world works. And we're here to walk you through that.
• 2:51
  Speaker 1: It's right because we all live in the universe. We're
• 2:53
  Speaker 1: all members of the universe, and we can all wonder
• 2:56
  Speaker 1: about how it all works, how it's all put together,
• 2:58
  Speaker 1: and why things are the way they are.
• 3:01
  Speaker 2: Do you have like a membership card for the universe?
• 3:03
  Speaker 2: I don't have one.
• 3:04
  Speaker 1: Oh oh uh, awkward?
• 3:06
  Speaker 2: Awkward.
• 3:08
  Speaker 1: Maybe you just got lost in the mail, Daniel. I'm
• 3:10
  Speaker 1: sure that's what happened. Sure you are invited.
• 3:15
  Speaker 2: Maybe it's part of the Hershey's Cabal. That's secretly organizing
• 3:18
  Speaker 2: the whole universe.
• 3:18
  Speaker 1: Oh you've been you've been blacklisted or dark chocolate listed.
• 3:21
  Speaker 2: Yeah, milk chocolated.
• 3:23
  Speaker 1: That's what you get for insulting hours that be accessible
• 3:28
  Speaker 1: chocolate to the masses. Yeah, but you can still come
• 3:30
  Speaker 1: to the universe.
• 3:31
  Speaker 2: I guess, all right, thank you, you give.
• 3:32
  Speaker 1: Be somebody's plus one. Just don't invite physicist, maybe because
• 3:38
  Speaker 1: there they'll prefer to be on the outside of the
• 3:40
  Speaker 1: universe looking in.
• 3:42
  Speaker 2: No. I love being the part of the universe that's
• 3:44
  Speaker 2: looking at itself, right. That's what makes the questions so important.
• 3:48
  Speaker 2: That we're not outside the universe studying it like some
• 3:51
  Speaker 2: weird objects. It's part of our context, our lives. It's
• 3:54
  Speaker 2: our existence that we're trying to understand. Physics is sometimes
• 3:58
  Speaker 2: sold is like bigger than human But to me, it's
• 4:00
  Speaker 2: the fundamental question of humanity, answering the deepest questions about
• 4:04
  Speaker 2: our own existence.
• 4:05
  Speaker 1: Is it sold as bigger as humanities? Where is that listed?
• 4:10
  Speaker 2: That's how it was sold to me. I guess, so
• 4:12
  Speaker 2: maybe I shouldn't generalize from any equals one. But one
• 4:15
  Speaker 2: reason I got into physics originally was that it seemed
• 4:18
  Speaker 2: like the questions were universal, They weren't limited to things
• 4:22
  Speaker 2: happening on Earth. You know, you study biology, and you
• 4:25
  Speaker 2: know it's fascinating and important, but it might be very
• 4:28
  Speaker 2: different on other planets.
• 4:30
  Speaker 1: Maybe that's why you're not invited, Daniel. You think you're
• 4:35
  Speaker 1: bigger than the rest of us.
• 4:36
  Speaker 2: I think some of the pelophysics is that it does
• 4:38
  Speaker 2: seem to be universal, though of course we don't know,
• 4:41
  Speaker 2: and that's a question we can only answer in the
• 4:43
  Speaker 2: future when we meet aliens.
• 4:45
  Speaker 1: I guess it does sort of tap into that sense
• 4:48
  Speaker 1: of the cosmos, the grand pictures that we all kind
• 4:51
  Speaker 1: of have inside of us. And it is pretty amazing
• 4:53
  Speaker 1: what we've been able to understand and study and come
• 4:56
  Speaker 1: to terms with in terms of where we are in
• 4:58
  Speaker 1: the universe, within our Solar System, within our galaxy, within
• 5:02
  Speaker 1: our supercluster of galaxies, within the observable universe. It's pretty
• 5:06
  Speaker 1: amazing that we can see so much structure out there
• 5:08
  Speaker 1: just by looking out to the night sky.
• 5:10
  Speaker 2: Yeah, it's amazing what we know about the universe, never
• 5:13
  Speaker 2: having left our tiny little neighborhood. Everything we know about
• 5:17
  Speaker 2: the structure of galaxies and superclusters and all that stuff
• 5:20
  Speaker 2: is based just on the few photons that happen to
• 5:22
  Speaker 2: hit Earth from those distant locations. It would be incredible
• 5:25
  Speaker 2: to get in a warp ship and like actually visit
• 5:27
  Speaker 2: some of these places and learn more about what's actually
• 5:30
  Speaker 2: going on there right now. But we can do a
• 5:32
  Speaker 2: pretty impressive job even from here. But of course, there
• 5:36
  Speaker 2: are still lots of unanswered questions about how solar systems form,
• 5:40
  Speaker 2: what planets there are in our Solar system, what planets
• 5:42
  Speaker 2: there have been in our Solar system, and larger questions
• 5:45
  Speaker 2: about galaxies, why they exist, how they form, and why
• 5:48
  Speaker 2: they take their weird shapes.
• 5:50
  Speaker 1: That's right, because we're orbiting around our Sun, and the
• 5:53
  Speaker 1: Sun is orbiting in a galaxy, which is a huge
• 5:56
  Speaker 1: cluster of stars. There are one hundred billion stars in
• 6:00
  Speaker 1: the Milky Way galaxy. But how these galaxies get form
• 6:03
  Speaker 1: is kind of a still up in the air, right.
• 6:05
  Speaker 2: It is, And it's an important question because galaxies are
• 6:08
  Speaker 2: kind of the basic building block of the universe. They're
• 6:11
  Speaker 2: like the atoms of the universe from which you can
• 6:13
  Speaker 2: put together structures and superstructures and all sorts of other stuff.
• 6:17
  Speaker 2: It's incredible that like galaxies actually exist and they tend
• 6:20
  Speaker 2: to be a certain size so it's an important thing
• 6:23
  Speaker 2: to understand, like why are universe features galaxies? How they
• 6:26
  Speaker 2: come to be, how long they will continue to be
• 6:29
  Speaker 2: the basic building block of the universe?
• 6:31
  Speaker 1: Yeah, and why do they have some strange features? And
• 6:34
  Speaker 1: so today on the podcast we'll be asking the question
• 6:41
  Speaker 1: why do some galaxies have bars? Are these like drinking bars?
• 6:46
  Speaker 1: Like our galaxy has a bar, can go and get
• 6:49
  Speaker 1: a drink.
• 6:49
  Speaker 2: There's definitely bars in our galaxy, right, I've been to
• 6:51
  Speaker 2: some of them. So that's a question we can definitively
• 6:54
  Speaker 2: answer today on the podcast.
• 6:55
  Speaker 1: They seem to be closer around Earth at least all
• 6:57
  Speaker 1: the bars we know are on Earth. Is that kind
• 7:00
  Speaker 1: of a physics question or is a biology question?
• 7:05
  Speaker 2: Well? I think there's some history of drinking alcohol in space, right.
• 7:09
  Speaker 2: I think Zach and Kelly's new book has some stories
• 7:11
  Speaker 2: about astronauts who have smuggled things up to the ISS,
• 7:14
  Speaker 2: So perhaps there is an unofficial bar off Earth as well.
• 7:19
  Speaker 1: I wonder if anyone has tried a like brew beer
• 7:22
  Speaker 1: on the space station.
• 7:24
  Speaker 2: Space moonshine, either purpose or by accident. The Moon does
• 7:28
  Speaker 2: shine more brightly in space, so maybe it's easier to
• 7:30
  Speaker 2: make moonshine m there you go. Yeah, And so of
• 7:33
  Speaker 2: course we don't know if aliens enjoy relaxing in bars
• 7:36
  Speaker 2: or whether they like Hershey's chocolate in their chocolate tenies.
• 7:40
  Speaker 2: But today we're not talking about places to drink. We're
• 7:42
  Speaker 2: talking about the structures at the centers of galaxies.
• 7:46
  Speaker 1: I see. So when you say a galaxy bar, it's
• 7:48
  Speaker 1: not like a like a buffet bar or a beer bar.
• 7:53
  Speaker 1: It's more like a destructor. Is a structure in a
• 7:55
  Speaker 1: galaxy that looks like a bar or a structures multiple ones.
• 7:58
  Speaker 2: Yeah, there's a kind of galaxy out there called a
• 8:01
  Speaker 2: barred galaxy, but not every galaxy has this kind of
• 8:04
  Speaker 2: bar at the center, and so it's an open question
• 8:07
  Speaker 2: about why some galaxies have bars.
• 8:09
  Speaker 1: Well, as usually, we were wondering how many people out
• 8:11
  Speaker 1: there had thought about galaxy bars and why some galaxies
• 8:15
  Speaker 1: have them, why some don't? Are they over twenty one?
• 8:18
  Speaker 1: Can you get your favorite drinks in them? And so
• 8:20
  Speaker 1: Daniel went out there into the internet to find out
• 8:22
  Speaker 1: what people think about this question.
• 8:24
  Speaker 2: And whether you like salad bars or chocolate bars or
• 8:28
  Speaker 2: any other kind of bar, You are welcome to participate
• 8:30
  Speaker 2: in this segment of the podcast. Just write to meet
• 8:32
  Speaker 2: you questions at Danielandjorge dot com and I'll set you up.
• 8:36
  Speaker 1: So think about it for a second. Why do you
• 8:37
  Speaker 1: think some galaxies have bars? Here's what peop glad to say.
• 8:42
  Speaker 3: So I didn't know galaxies had bars in the center.
• 8:46
  Speaker 3: So I'm going to say maybe it has it because
• 8:51
  Speaker 3: gravity is aliding them in a special way, but can't
• 8:55
  Speaker 3: be made into sphere because of the black holes gravity.
• 9:00
  Speaker 4: I think planets don't share orbits because the center of
• 9:05
  Speaker 4: gravity would have to be absolutely stationary or it would
• 9:09
  Speaker 4: oscillate and shake itself out of sync. I think that
• 9:13
  Speaker 4: the bars in the center of galaxies are there to
• 9:18
  Speaker 4: give people somewhere to go. But like if women's clothes
• 9:22
  Speaker 4: shops have bars, they have somewhere for men to go
• 9:25
  Speaker 4: instead of trying to look interested.
• 9:28
  Speaker 3: I expect that galaxies have bars at their centers because
• 9:31
  Speaker 3: you're going to need a place to kill some time
• 9:33
  Speaker 3: and have have a beer if your interstellar flight is delayed.
• 9:37
  Speaker 3: I have no idea what does a bar in the
• 9:40
  Speaker 3: center of abilitum?
• 9:42
  Speaker 1: All right, some fun ideas here, some of them a
• 9:46
  Speaker 1: little bit inappropriate.
• 9:47
  Speaker 2: Perhaps I think this might be the highest fraction of
• 9:51
  Speaker 2: joke answers we've ever gotten, which means people really just
• 9:54
  Speaker 2: don't know anything about bars and galaxies.
• 10:00
  Speaker 1: Maybe they're just a barring and not knowing anything about them.
• 10:03
  Speaker 2: Or maybe they'd already spent too much time at a
• 10:05
  Speaker 2: bar when they got these questions and they were in
• 10:07
  Speaker 2: silly mood.
• 10:08
  Speaker 1: Oh there you go. Yeah, maybe you shouldn't send these
• 10:11
  Speaker 1: questions up during happy.
• 10:12
  Speaker 2: Hours Monday morning nine am. Is that what happy hour is?
• 10:17
  Speaker 1: Yeah? Depends on how much you led your job, I guess.
• 10:22
  Speaker 2: And where you are in the world. Right, we have
• 10:24
  Speaker 2: listeners all over the world.
• 10:26
  Speaker 1: Yeah, we are a global podcast. All right, Well, Daniel,
• 10:30
  Speaker 1: maybe step us through this. What exactly do you mean
• 10:32
  Speaker 1: by a galaxy bar?
• 10:35
  Speaker 2: So, galaxy bars are features of one particular kind of galaxy,
• 10:40
  Speaker 2: and galaxies, it turns out, can have all sorts of
• 10:42
  Speaker 2: different shapes in them. Galaxies, of course, are clusters of
• 10:45
  Speaker 2: stars and they can be very very small, from dwarf
• 10:48
  Speaker 2: galaxies that have thousands of stars to super mega galaxies
• 10:52
  Speaker 2: that have billions and billions of stars. But we tend
• 10:55
  Speaker 2: to notice some features in them. And you know, the
• 10:57
  Speaker 2: beginning of any science is basically just bought it, just
• 11:00
  Speaker 2: like look at it. Categorize it, sort it, see what
• 11:03
  Speaker 2: patterns emerge. And very early on when people were studying galaxies,
• 11:07
  Speaker 2: they noticed that they could have different structures, even things
• 11:09
  Speaker 2: we could see here from Earth with limited telescopes.
• 11:13
  Speaker 1: Right, And we've talked a little bit about before the
• 11:15
  Speaker 1: different shapes that galaxies can have, right, And they can
• 11:18
  Speaker 1: be sort of like a blob like a football, or
• 11:20
  Speaker 1: maybe like a spiral, or maybe just like a pancake.
• 11:23
  Speaker 2: Right, yeah, exactly. And there are regular galaxies that are
• 11:26
  Speaker 2: probably in the process of merging between two galaxies. There
• 11:30
  Speaker 2: is a galaxy out there that looks like a question mark,
• 11:32
  Speaker 2: for example. But the most common galaxy out there is
• 11:35
  Speaker 2: a spiral galaxy.
• 11:37
  Speaker 1: How common is it and is it like fifty?
• 11:40
  Speaker 2: It's somewhere around two thirds of all the galaxies that
• 11:43
  Speaker 2: we have studied, our spiral galaxies. And there's a reason
• 11:46
  Speaker 2: for that. And we went into all of that in
• 11:48
  Speaker 2: our recent podcast about the various shapes that galaxies can have.
• 11:52
  Speaker 2: But basically, galaxies form from huge blobs of gas which
• 11:56
  Speaker 2: spin and collapse and then form a disc. And that
• 11:59
  Speaker 2: dense disc gas tends to form those stars, and the
• 12:02
  Speaker 2: spinning of the galaxy turns that disc into a spiral.
• 12:06
  Speaker 1: That's kind of like the default shape of a galaxy, right,
• 12:09
  Speaker 1: But then you can get other shapes, as we talked about,
• 12:11
  Speaker 1: when they merge with each other, other galaxies or they
• 12:13
  Speaker 1: crash into other galaxies.
• 12:15
  Speaker 2: Right exactly. So a spiral galaxy is like the basic
• 12:18
  Speaker 2: building block of the universe in that sense, it's the
• 12:20
  Speaker 2: most important one to understand because the other ones, ellipticals
• 12:23
  Speaker 2: and irregulars are all made out of spiral galaxies. And
• 12:27
  Speaker 2: also our galaxy is a spiral galaxy. So if we're
• 12:29
  Speaker 2: going to understand like the context of our lives, let's
• 12:32
  Speaker 2: begin at.
• 12:32
  Speaker 1: Home, right, And I guess maybe just to paint the
• 12:35
  Speaker 1: picture for folks, by a spiral, it sort of looks
• 12:38
  Speaker 1: like a like a toilet flushing, right, Like, It's not
• 12:42
  Speaker 1: like one like they're tracing out a spiral with a
• 12:44
  Speaker 1: pen and a paper. It's more like a swirl that's
• 12:48
  Speaker 1: kind of converging in the middle.
• 12:49
  Speaker 2: Yeah, And the spiral galaxy usually has several arms. They
• 12:52
  Speaker 2: can have two, sometimes they have three. There are galaxies
• 12:54
  Speaker 2: out there with four arms. Each one is like a
• 12:57
  Speaker 2: spiral that usually goes like one time around. The galaxy
• 13:00
  Speaker 2: starts from the center and then comes out, So it's
• 13:02
  Speaker 2: sort of more like a pinwheel.
• 13:04
  Speaker 1: Right, So that gives you the image of the spiral galaxy.
• 13:06
  Speaker 1: And so that's the basic shape of most galaxies out there.
• 13:09
  Speaker 1: But you say, you're saying some of them have a
• 13:11
  Speaker 1: bar through them or on them.
• 13:13
  Speaker 2: At the center. Some galaxies don't have bars. The spiral
• 13:15
  Speaker 2: arms just start at the very center. But a good
• 13:18
  Speaker 2: fraction galaxy is the center of them. Isn't part of
• 13:21
  Speaker 2: the spiral arm. It's a separate thing. It's like a bar.
• 13:24
  Speaker 2: Like the Milky Way galaxy has this big blob of
• 13:27
  Speaker 2: stars and gas and dust at the center of it,
• 13:29
  Speaker 2: which forms this big bar and the spiral arm that
• 13:32
  Speaker 2: comes out of each end of the bar. So the
• 13:35
  Speaker 2: spirals don't go to the very center of the galaxy.
• 13:37
  Speaker 2: They start at the edges of this bar.
• 13:39
  Speaker 1: WHOA, wait, wait, wait too, you're saying our Milky Way
• 13:42
  Speaker 1: galaxy is a bar.
• 13:44
  Speaker 2: Yes, we live in a bar galaxy.
• 13:46
  Speaker 1: Whoa is it a milk chocolate bar? Since we're the
• 13:49
  Speaker 1: Milky Way.
• 13:50
  Speaker 2: Now I'm afraid to answer because I don't want to
• 13:52
  Speaker 2: anger the Hershey universe cabal that's in charge of everything.
• 13:55
  Speaker 1: Yeah, what if the Milky Way is a hercheese product.
• 14:00
  Speaker 2: This is why it's so important for science to be
• 14:02
  Speaker 2: free of this kind of oppression. People should feel free
• 14:04
  Speaker 2: to speak their minds on chocolate, right.
• 14:06
  Speaker 1: Well, people shouldn't disparage other people if theyread chocolate. I
• 14:10
  Speaker 1: think that is maybe the most important part of your yeah,
• 14:14
  Speaker 1: se point take. But our Milky Way is a bar?
• 14:16
  Speaker 1: Are you saying so? Like our spirals, the arms of
• 14:20
  Speaker 1: our galaxy don't spiral into a single blob in the middle.
• 14:25
  Speaker 1: It's they spiral into a bar.
• 14:27
  Speaker 2: That's right. Our galaxy is two major arms, and then
• 14:30
  Speaker 2: there's some spurs that come off of them, and each
• 14:33
  Speaker 2: arm comes from a different side of the bar. So
• 14:36
  Speaker 2: the two major spirals that make the Milky Way galaxy,
• 14:39
  Speaker 2: they don't actually meet at the center. They come to
• 14:41
  Speaker 2: different sides of this bar that's at the core of
• 14:44
  Speaker 2: the galaxy.
• 14:45
  Speaker 1: Interesting, the picture I'm getting is sort of like I
• 14:47
  Speaker 1: don't know if you've seen those like Hawaiian fire dancers
• 14:50
  Speaker 1: where they have a stick and at each end of
• 14:52
  Speaker 1: the stick it's on fire and so then they spin it.
• 14:56
  Speaker 1: That's kind of sort of what's the picture that we
• 14:58
  Speaker 1: get of the Milky Way galaxy? Isn't it like there's
• 15:01
  Speaker 1: a bar and from the each end of the bar
• 15:03
  Speaker 1: you get these swirls that spiral out.
• 15:06
  Speaker 2: Well, I am not fancy enough to take vacations to Hawaii.
• 15:08
  Speaker 2: Tend to spend my money on chocolate instead. But it
• 15:11
  Speaker 2: sounds accurate.
• 15:12
  Speaker 1: You know that you can get chocolate in Hawaii. In fact,
• 15:14
  Speaker 1: it's kind of known for they're kind of known for
• 15:18
  Speaker 1: having good chocolate.
• 15:19
  Speaker 2: Macademian that chocolate here, I come, there you go. But yeah,
• 15:23
  Speaker 2: that sounds accurate. Wow. Or if you imagine like a
• 15:25
  Speaker 2: baton twirler at a football game and ribbons at the
• 15:28
  Speaker 2: end of the baton. If he or she spins the baton,
• 15:31
  Speaker 2: then the ribbons spiral around it. But the ribbons, of course,
• 15:34
  Speaker 2: don't meet at the very center. They come from the
• 15:36
  Speaker 2: edges of the bed.
• 15:38
  Speaker 1: Now, this is an interesting idea just because to me
• 15:41
  Speaker 1: and I wonder if to a lot of our listeners
• 15:43
  Speaker 1: this is kind of news. Right, Like most of the
• 15:46
  Speaker 1: time that you see a picture or a drawing of
• 15:48
  Speaker 1: the milk Away Galaxy, they don't it doesn't have the
• 15:50
  Speaker 1: bar in the middle, right like the famous isn't the
• 15:53
  Speaker 1: famous image of like you are here? Or it points
• 15:55
  Speaker 1: to a point in the Milk Away. Of a drawing
• 15:57
  Speaker 1: of the milk Away that one doesn't have a bar,
• 16:00
  Speaker 1: I don't think.
• 16:00
  Speaker 2: Yeah, that's a good point. I'm not sure if that
• 16:02
  Speaker 2: image is scientifically vetted. It also really doesn't have big
• 16:06
  Speaker 2: distinct arms the way you might expect, But a more
• 16:09
  Speaker 2: scientifically accurate picture of the Milky Way shows the specific
• 16:13
  Speaker 2: arms and the bar. The bar is not like a
• 16:15
  Speaker 2: very rigid rod. It's a bit of a blob also to.
• 16:19
  Speaker 1: Me now of stars, like a just an elongated cluster
• 16:22
  Speaker 1: of stars.
• 16:23
  Speaker 2: Yeah, it's stars, and it's gas and it's dust like
• 16:26
  Speaker 2: the rest of the galaxy. But you know, this is
• 16:28
  Speaker 2: us imposing order on a huge swirling mass of stuff.
• 16:32
  Speaker 2: So part of this, of course, is just the impression
• 16:34
  Speaker 2: we make, how our brains filter the full details of
• 16:38
  Speaker 2: the buzzing chaos of reality.
• 16:41
  Speaker 1: All right, Well, so then the Milkway has a bar.
• 16:43
  Speaker 1: Lots of galaxies maybe have bars in the middle of them,
• 16:47
  Speaker 1: and so the question is why do they have bars,
• 16:50
  Speaker 1: What's going on there in the middle of the galaxy?
• 16:52
  Speaker 1: And can you play galactic limbo with that bar? And
• 16:55
  Speaker 1: so let's get to these deep and profound questions. But
• 16:58
  Speaker 1: first let's take a quick all right, we're talking about
• 17:14
  Speaker 1: the Milky Way galaxy and the fact that it has
• 17:16
  Speaker 1: a bar of stars in the middle. The swirls of
• 17:19
  Speaker 1: the galaxy don't converge to a round blob in the middle.
• 17:22
  Speaker 1: They converse to kind of like a rod of stars. Right.
• 17:26
  Speaker 2: Yeah, it might be more accurate to call it like
• 17:27
  Speaker 2: a football because it's a little wider in the center
• 17:30
  Speaker 2: than it is at the edges. But yeah, there's sort
• 17:32
  Speaker 2: of a long.
• 17:33
  Speaker 1: But it's not a bar. Why don't we just call
• 17:37
  Speaker 1: them football galaxy?
• 17:38
  Speaker 2: Football galaxy? That sounds great. I am done defending the
• 17:44
  Speaker 2: naming schemes of astronomy.
• 17:47
  Speaker 1: Because it's indefensible, because you just can't do it.
• 17:50
  Speaker 2: Sorry, astronomers, you have no champion in me anymore.
• 17:53
  Speaker 1: Oh boy, you're throwing the astronomers under the bus.
• 17:56
  Speaker 2: No, I'm just throwing.
• 18:00
  Speaker 1: Naming peculiarities.
• 18:02
  Speaker 2: Oh, it certainly does. And nobody's ever asked me my
• 18:04
  Speaker 2: opinion when they make up these names.
• 18:07
  Speaker 1: Okay, So the Milky Way has a bar or football
• 18:09
  Speaker 1: kind of in the middle, which is weird, right, because
• 18:11
  Speaker 1: you would expect a galaxy to be sort of, you know,
• 18:14
  Speaker 1: round and symmetric, right.
• 18:16
  Speaker 2: You would kind of expect it. And that's exactly why
• 18:18
  Speaker 2: we are really interested in these questions like what structures
• 18:21
  Speaker 2: form in the universe. The answer to that question reveals
• 18:24
  Speaker 2: the forces at play. Like wire galaxies created in the
• 18:28
  Speaker 2: first place, Well, there's gravity, why don't they just collapse
• 18:31
  Speaker 2: into a central ball, while there's angular momentum that turns
• 18:34
  Speaker 2: them into disks? Why do the arms form at all?
• 18:37
  Speaker 1: Well, maybe a question we can ask here is like
• 18:40
  Speaker 1: what percentage of galaxies are barred galaxies? Like, is the
• 18:42
  Speaker 1: Milky Way super weird that it has a bar? Or
• 18:45
  Speaker 1: is it like most spiral galaxies have bars or some
• 18:48
  Speaker 1: of them or fifty to fifty.
• 18:49
  Speaker 2: That's a really interesting question, and the answer depends on
• 18:52
  Speaker 2: the mass of the galaxy in a super weird way
• 18:56
  Speaker 2: for galaxies about the mass of the Milky Way, like
• 18:58
  Speaker 2: billions and billions of solar masses. It's around two thirds
• 19:02
  Speaker 2: of spiral galaxies that have bars. The more massive the galaxy,
• 19:07
  Speaker 2: the smaller the fraction of the spirals have bars. And
• 19:10
  Speaker 2: also the less massive the galaxy, the smaller fraction have bars.
• 19:14
  Speaker 2: So there is this like sweet spot in the middle
• 19:16
  Speaker 2: where galaxies are more likely to have bars. And if
• 19:19
  Speaker 2: you move away from that lower mass or higher mass,
• 19:23
  Speaker 2: then the fraction that have bars decreases.
• 19:25
  Speaker 1: M interesting, So it depends on the size and mass
• 19:28
  Speaker 1: of the galaxy. But maybe let's take a back to
• 19:31
  Speaker 1: the basics, because it sounds like there's a lot of
• 19:33
  Speaker 1: physics lying on here, as you might expect from our podcasts,
• 19:37
  Speaker 1: But so what's going on at the center? Like how
• 19:41
  Speaker 1: can this bar still be there after all these billions
• 19:43
  Speaker 1: of years?
• 19:43
  Speaker 2: It all comes from asymmetries like little over densities. If
• 19:47
  Speaker 2: you start from a galaxy that was perfectly smooth, like
• 19:50
  Speaker 2: all the matter was evenly distributed, or like you had
• 19:52
  Speaker 2: a cluster of stars that were perfectly distributed in circles
• 19:55
  Speaker 2: on perfectly circular orbits, then it wouldn't form a bar.
• 20:00
  Speaker 2: You tweak it a little bit, like one little spot
• 20:02
  Speaker 2: has more density than another spot, then that spot's going
• 20:05
  Speaker 2: to have more gravity and it's going to attract more
• 20:07
  Speaker 2: stars to it, and so you get these density waves
• 20:10
  Speaker 2: forming inside the galaxy. So the same process that forms
• 20:14
  Speaker 2: the arms, theives density waves of the arms that we
• 20:17
  Speaker 2: describe as like traffic passing through a freeway, can also
• 20:21
  Speaker 2: form density structures in the center of the galaxy and
• 20:24
  Speaker 2: in this case, they form bars.
• 20:26
  Speaker 1: Well, yeah, I remember we spent a whole episode talking
• 20:28
  Speaker 1: about this idea of density waves in a galaxy. It's
• 20:32
  Speaker 1: not like the arm of a galaxy what you see
• 20:34
  Speaker 1: as a soirl like, it doesn't rotate around the galaxy.
• 20:37
  Speaker 1: It's more like a ripple through a whole bunch of stars.
• 20:41
  Speaker 2: Exactly. It's more like a ripple. It's like people doing
• 20:43
  Speaker 2: the wave in a football stadium. They're not moving sidewaves,
• 20:46
  Speaker 2: but the wave itself is moving, right, And so the
• 20:50
  Speaker 2: galaxy arms are waves in the density of stars, and
• 20:53
  Speaker 2: the bar is the same way. The bar is like
• 20:56
  Speaker 2: a density wave. So it's a structure that forms, but
• 20:59
  Speaker 2: it moves through the stars. It doesn't move at the
• 21:02
  Speaker 2: same speed the stars are moving around the galaxy. It
• 21:06
  Speaker 2: can move faster, it can move slower.
• 21:08
  Speaker 1: Wait, what do you mean the bar? Does a bar
• 21:09
  Speaker 1: rotate and bar devastates moving like, which direction is the
• 21:13
  Speaker 1: bar moving?
• 21:14
  Speaker 2: The bar rotates the same direction as the galaxy and
• 21:17
  Speaker 2: it definitely rotates.
• 21:18
  Speaker 1: Oh, kind of like the arms and a clock.
• 21:21
  Speaker 2: Kind of like the arms and a clock, and also
• 21:23
  Speaker 2: like the arms of the galaxy. Right, because the arms
• 21:26
  Speaker 2: are density waves, they move relative to the stars, So
• 21:30
  Speaker 2: one star might be in the density wave and then
• 21:32
  Speaker 2: they'll get passed by the density wave which moves on.
• 21:35
  Speaker 1: WHOA are there like videos online about this? That would
• 21:39
  Speaker 1: be cool to see.
• 21:39
  Speaker 2: You mean, like actual footage of bars rotating, not.
• 21:42
  Speaker 1: Like a simulation or something.
• 21:43
  Speaker 2: Yeah, I would love to see that, but the time
• 21:45
  Speaker 2: scales are ridiculous, Like the Milky Way takes two hundred
• 21:47
  Speaker 2: and fifty million years to rotate, So there's no chance
• 21:51
  Speaker 2: for like, watch a galaxy rotate unless you're gonna set
• 21:54
  Speaker 2: it your camera for tens of millions of years.
• 21:56
  Speaker 1: So wait, how do we know it's rotating.
• 21:58
  Speaker 2: Well, we build up a model of how galaxy works
• 22:00
  Speaker 2: that includes angular momentum, and you need those components that
• 22:03
  Speaker 2: rotation to explain what we see. If they weren't rotating,
• 22:07
  Speaker 2: there's no anglar momentum, and these things would collapse a
• 22:09
  Speaker 2: lot faster. A lot of galaxies and a lot of these
• 22:12
  Speaker 2: structures are supported by angular momentum. Like, where else would
• 22:15
  Speaker 2: the spiral arms come from if it wasn't spinning.
• 22:18
  Speaker 1: I don't know.
• 22:19
  Speaker 2: Yeah, So we do a lot of simulations to try
• 22:21
  Speaker 2: to explain the galaxies we see. We said, well, we
• 22:23
  Speaker 2: think we understand all the forces at play. We put
• 22:25
  Speaker 2: it in the computer, we press the button, it goes,
• 22:27
  Speaker 2: and we see what comes out of the simulation. Then
• 22:29
  Speaker 2: we compare that to what we see in the sky. Interesting,
• 22:32
  Speaker 2: that's what we do to try to explain what's happening
• 22:34
  Speaker 2: or try to gain some understanding of the forces at work.
• 22:37
  Speaker 1: Wow, so nobody's ever actually seen a galaxy swirling. We
• 22:40
  Speaker 1: just think it does. I mean, it's the most likely
• 22:44
  Speaker 1: or obvious explanation, but still it's a fact that we
• 22:48
  Speaker 1: haven't seen a galaxy swirl. Like, we've only seen a
• 22:51
  Speaker 1: photograph of it. It's like you've only seen a photograph
• 22:54
  Speaker 1: where Merry go round. Ever, like you've never seen a
• 22:57
  Speaker 1: mirror go around move.
• 23:00
  Speaker 2: Well, that's true when we've only even known about other
• 23:02
  Speaker 2: galaxies for like one hundred years. It was Hubble who
• 23:05
  Speaker 2: saw these little smudges in the sky and deduced their
• 23:09
  Speaker 2: distance from analyzing special stars in them, the sephids, whose brightness,
• 23:13
  Speaker 2: of course is famously connected to their pulsing frequency. And
• 23:17
  Speaker 2: he's the one that understood that these things were outside
• 23:19
  Speaker 2: our galaxy. So we only known that other galaxies exist
• 23:23
  Speaker 2: for like one hundred years and had good enough telescopes
• 23:26
  Speaker 2: to like resolve their structure for a few decades, which
• 23:29
  Speaker 2: is basically no time at all relative to their actual emotion.
• 23:33
  Speaker 2: So yeah, we're studying galaxies basically frozen in time.
• 23:36
  Speaker 1: Well that's interesting. Now you're saying that about two thirds
• 23:40
  Speaker 1: of galaxies have a bar in the middle of them,
• 23:43
  Speaker 1: and this bar is rotating and rippling through the center
• 23:47
  Speaker 1: of the galaxy. Like what's the mechanism for making these bars?
• 23:50
  Speaker 1: Like why not? Why a bar shape? Why not a
• 23:52
  Speaker 1: star shape or a banana shape.
• 23:56
  Speaker 2: I would love to see a banana shape at the
• 23:58
  Speaker 2: center of galaxy and probably some galaxy out there that
• 24:01
  Speaker 2: has that, because there's so many galaxies out there and
• 24:04
  Speaker 2: so many of them are doing weird things.
• 24:06
  Speaker 1: I guess if you think about a galaxy, is banana shape,
• 24:08
  Speaker 1: Like each arm is like a banana. So it's like
• 24:10
  Speaker 1: if you take if you take several bananas and arrange
• 24:13
  Speaker 1: them in a star shape, you would get a spiral galaxy.
• 24:16
  Speaker 2: Right, Yeah, that's true. That's a message from the universe
• 24:19
  Speaker 2: that bananas are important, and you.
• 24:21
  Speaker 1: Start with them in chocolate this they're galactic.
• 24:25
  Speaker 2: And that's why we sell frozen chocolate covered bananas here
• 24:28
  Speaker 2: in Orange County.
• 24:29
  Speaker 1: That is the purpose of the universe, isn't it? Is
• 24:31
  Speaker 1: it deep fried too? Why not fried chocolate covered bananas?
• 24:41
  Speaker 1: I think I've had that at a restaurant, actually.
• 24:43
  Speaker 2: But you asked a great question that wasn't about dessert,
• 24:46
  Speaker 2: which is why do some galaxies have bars? Why do
• 24:48
  Speaker 2: they form bar shapes and not other things? And the
• 24:51
  Speaker 2: answer is that we're not sure, Like we don't understand
• 24:54
  Speaker 2: why some galaxies form bars and some don't. We don't
• 24:58
  Speaker 2: totally understand the mechanism for the formation itself. Like we've
• 25:02
  Speaker 2: seen this happen in simulation sometimes and it comes out
• 25:05
  Speaker 2: of instabilities. You put a bunch of stars a little
• 25:07
  Speaker 2: bit closer to each other, they tend to form these
• 25:10
  Speaker 2: density waves. And what's happening here at the center is
• 25:13
  Speaker 2: that the stars are no longer totally in a circular orbit.
• 25:17
  Speaker 2: The bar like twists the orbits so that some stars are
• 25:19
  Speaker 2: moving more on radial orbits passing close through the center
• 25:23
  Speaker 2: of the galaxy.
• 25:24
  Speaker 1: What do you mean, radar like more oval shape orbits?
• 25:27
  Speaker 2: Yeah, more oval shape, So they're not equally distributed around
• 25:30
  Speaker 2: the center of the galaxy. They're like passing back and forth.
• 25:33
  Speaker 1: Oh interesting, I guess maybe it's sort of rare or
• 25:36
  Speaker 1: sort of a coincid is that you would have circular
• 25:38
  Speaker 1: orbit orbits, right Like, if I throw a rock at
• 25:40
  Speaker 1: the Sun right now, it would not necessarily go in
• 25:42
  Speaker 1: a round orbit. It might maybe more likely go in
• 25:45
  Speaker 1: an oval shape orbit.
• 25:46
  Speaker 2: Yeah, there's lots of possible solutions, and a circle is
• 25:49
  Speaker 2: like a special case of it. And also to maintain
• 25:51
  Speaker 2: a circular orbit requires resisting the tugs and the pulls
• 25:55
  Speaker 2: of everything else around you. Remember that you're not orbiting
• 25:58
  Speaker 2: by yourself. The Earth is being on by Jupiter, and
• 26:01
  Speaker 2: Jupiter is being pulled on by everything else. That's one
• 26:03
  Speaker 2: of the reasons why our orbits are eccentric. So the
• 26:06
  Speaker 2: more complicated dynamics you have, the more lack of symmetry,
• 26:09
  Speaker 2: the more these things are getting pulled by other stuff,
• 26:12
  Speaker 2: the more they're going to fall out of perfectly circular
• 26:14
  Speaker 2: orbits and clump together in other kinds of orbits.
• 26:18
  Speaker 1: So that you're saying, the stuff at the center of
• 26:20
  Speaker 1: our milk away galaxy is not necessarily going around in
• 26:23
  Speaker 1: a circle, it's maybe going in an oval shape around
• 26:26
  Speaker 1: the center. But then how does that explain the ripple
• 26:29
  Speaker 1: that is the bar or all of these things going
• 26:32
  Speaker 1: in an oval in sync.
• 26:34
  Speaker 2: Yes, the bar itself is like a denser region which
• 26:37
  Speaker 2: passes through the center of the galaxy, and it influences
• 26:41
  Speaker 2: other stuff. So gas near the center of the galaxy,
• 26:43
  Speaker 2: for example, moves at faster speeds than gas further out,
• 26:47
  Speaker 2: So the gas can catch up to the bar and
• 26:49
  Speaker 2: pass through it. But then the bar is stronger gravity
• 26:52
  Speaker 2: slows it down, so the material tends to lose some energy.
• 26:56
  Speaker 2: This actually ends up funneling a bunch more gas towards
• 26:58
  Speaker 2: the center of the galaxy and can create new stars
• 27:02
  Speaker 2: right there in the bar.
• 27:03
  Speaker 1: But then, I guess, like making the bar is so common,
• 27:06
  Speaker 1: you're seeing about two thirds of galaxies, spiral galaxies have
• 27:09
  Speaker 1: a bar in them. Why don't we see that effect
• 27:11
  Speaker 1: like in other places, like when I flushed the toilet,
• 27:15
  Speaker 1: or you know, in our Solar system. Even we don't
• 27:17
  Speaker 1: have a bar in our solar system. I guess we
• 27:19
  Speaker 1: don't have spiral arms in our Solar system either, Like
• 27:23
  Speaker 1: you know, things coalesced into the planets, although we do
• 27:25
  Speaker 1: have an asteroid built.
• 27:27
  Speaker 2: I think it comes down to the balance of all
• 27:29
  Speaker 2: the things at play. You know, we have gravity in
• 27:31
  Speaker 2: the Solar System, and of course in the galaxy. But
• 27:33
  Speaker 2: the galaxy is a lot more friction. You know, there's
• 27:36
  Speaker 2: gas and there's dust. This stuff is interacting much more
• 27:38
  Speaker 2: than things in the Solar system. So there's a lot
• 27:41
  Speaker 2: more transfer of energy between the stuff in the galaxy
• 27:44
  Speaker 2: than there is in the Solar system. Solar systems like
• 27:46
  Speaker 2: a cleaned up little galaxy. There's not nearly as much
• 27:48
  Speaker 2: stuff between the planets as there is between the stars.
• 27:51
  Speaker 2: In the galaxy. There's a lot more of this exchange
• 27:54
  Speaker 2: of energy and friction and stuff bumping into other stuff
• 27:57
  Speaker 2: in the galaxy, which can give you more interesting complex
• 28:00
  Speaker 2: basically turbulent structure. And we don't know how long these
• 28:04
  Speaker 2: bars last. There's some theories that say these bars will
• 28:07
  Speaker 2: last forever, and we see some galaxies that have had
• 28:10
  Speaker 2: their bars for like ten billion years. There are other
• 28:12
  Speaker 2: theories that the bars could be oscillatory, like maybe the
• 28:15
  Speaker 2: bar forms, but then the formation of the bar tends
• 28:18
  Speaker 2: to destroy the bar, and then another bar forms a
• 28:20
  Speaker 2: billion years later. So it could be that every galaxy
• 28:23
  Speaker 2: goes through phases where it has a bar and doesn't
• 28:25
  Speaker 2: have a bar. These are really open questions in the
• 28:28
  Speaker 2: study of galaxy formation.
• 28:29
  Speaker 1: Mmmm, it's doing like pull ups or something on a bar,
• 28:34
  Speaker 1: going back and forth. But this is an interesting idea
• 28:37
  Speaker 1: that it maybe depends on time. Like you're saying, they
• 28:40
  Speaker 1: can come in and out. What do you see when
• 28:43
  Speaker 1: you look at the galaxies out there in the universe, Like,
• 28:45
  Speaker 1: don't can't we see them across different ages and times? Yeah?
• 28:50
  Speaker 1: Do we see that the older galaxies have more bars
• 28:53
  Speaker 1: or younger galaxies have more bars? What do we see?
• 28:55
  Speaker 2: Yeah, so we're not sure. We have a lot of
• 28:57
  Speaker 2: different clues that sort of point us in different directions. Like,
• 28:59
  Speaker 2: on one hand, we see some galaxies with really really
• 29:03
  Speaker 2: old bars. I was reading a paper about a galaxy
• 29:06
  Speaker 2: we're looking at ten billion years in the past, and
• 29:09
  Speaker 2: we can see the formation of a bar back then.
• 29:12
  Speaker 2: So that's just like some bars formed really early in
• 29:14
  Speaker 2: the universe and may have lasted a really long time.
• 29:18
  Speaker 2: Of course, you can't see the same galaxy over time.
• 29:21
  Speaker 1: Yeah, how do we know?
• 29:22
  Speaker 2: We don't know necessarily how long a bar lasts when
• 29:25
  Speaker 2: it's formed. But in this one particular galaxy, they could
• 29:27
  Speaker 2: see evidence of the bar over about ten billion years
• 29:31
  Speaker 2: because they could see how the bar had disrupted the
• 29:34
  Speaker 2: structure of the galaxy. So it's a little speculative, but
• 29:38
  Speaker 2: they think that that bar lasted for about ten billion
• 29:40
  Speaker 2: years in that galaxy because of how it like shepherded
• 29:43
  Speaker 2: the stars and influence the gas to create new stars.
• 29:47
  Speaker 2: So it's not everybody that believes that paper, but that
• 29:49
  Speaker 2: one paper argues that there are galaxies with really long
• 29:53
  Speaker 2: lived bars.
• 29:54
  Speaker 1: And then I guess, how do you know that they
• 29:56
  Speaker 1: can go away?
• 29:57
  Speaker 2: Yeah, another great question. That's again from simulation. We see
• 30:00
  Speaker 2: in some simulations that a bar can like drive gas
• 30:04
  Speaker 2: inwards towards the center of the galaxy. It's an over
• 30:06
  Speaker 2: density and so it pulls stuff in and that can
• 30:09
  Speaker 2: disrupt the motion of stars through the galactic core. So,
• 30:13
  Speaker 2: like the formation of the bar could create gravity which
• 30:16
  Speaker 2: pulls in more gas, which ends up disrupting the bar itself.
• 30:20
  Speaker 2: So we don't know. And this is the kind of
• 30:22
  Speaker 2: thing we study in simulations, and these simulations are never perfect, right,
• 30:25
  Speaker 2: You can't include everything in the galaxy in your simulation,
• 30:28
  Speaker 2: every tiny particle. It would take forever. So whenever they
• 30:31
  Speaker 2: do these simulations, they're always making approximations. Well, let's leave
• 30:35
  Speaker 2: out the dark matter, or let's ignore this effect. Or
• 30:37
  Speaker 2: something to make it tractable. And so you've got lots
• 30:39
  Speaker 2: of different simulations with sort of different answers, and people
• 30:42
  Speaker 2: are still exploring this, like right now, nobody really knows
• 30:45
  Speaker 2: the answers to these questions.
• 30:46
  Speaker 1: So when you simulate it, you add a little asymmetry
• 30:49
  Speaker 1: or like an offset to the distribution of the stars,
• 30:52
  Speaker 1: and then you get a bar. And then what you
• 30:54
  Speaker 1: see in the simulation is that sometimes the bar disappears
• 30:57
  Speaker 1: after a while exactly, and then maybe it comes back
• 31:00
  Speaker 1: later mm hmm.
• 31:01
  Speaker 2: And there's some other hints from the simulations. Some suggest
• 31:04
  Speaker 2: that it depends on the thickness of the disk. Like
• 31:07
  Speaker 2: our galaxies mostly flat, but it's not a totally thin disk.
• 31:11
  Speaker 2: It's not like it's a paper thin, right, There's a
• 31:14
  Speaker 2: width to the galaxy, and some galaxies have thinner disks
• 31:17
  Speaker 2: and some galaxies have thicker discs, And some of these
• 31:20
  Speaker 2: stimulations suggest that galaxies with thinner discs are more likely
• 31:24
  Speaker 2: to have bars than galaxies with thicker discs in them.
• 31:27
  Speaker 1: WHOA why would they be?
• 31:29
  Speaker 2: I think it has to do with basically the gravitational
• 31:31
  Speaker 2: environment in which a disruption forms, Like if there's a
• 31:35
  Speaker 2: lot of other gravity happening anyway, then the small over
• 31:38
  Speaker 2: density is less likely to cause a big pile up
• 31:41
  Speaker 2: in the galactic spin. Like, for example, you had a
• 31:44
  Speaker 2: huge amount of dark matter in your galaxy and then
• 31:46
  Speaker 2: the gravity of the galaxy is mostly dominated by that
• 31:49
  Speaker 2: dark matter. In the same way, if you have lots
• 31:51
  Speaker 2: of other stars like above and below where the over
• 31:53
  Speaker 2: density forms, then those stars are going to tend to
• 31:56
  Speaker 2: smooth stuff out because of their gravity. A thicker disc
• 31:59
  Speaker 2: might prevent a little over density from turning into a
• 32:02
  Speaker 2: big bar.
• 32:03
  Speaker 1: Interesting, it's like a two D effect, like it only
• 32:05
  Speaker 1: happens in thin sheets.
• 32:06
  Speaker 2: Yeah, it could be. Another study suggested that mergers can
• 32:11
  Speaker 2: destroy bars. Remember, spiral galaxies merge and form sometimes new
• 32:14
  Speaker 2: spiral galaxies, sometimes elliptical galaxies, And it could be that
• 32:19
  Speaker 2: bars are destroyed in these mergers, and then the new
• 32:21
  Speaker 2: galaxy doesn't necessarily have a bar. It depends on the
• 32:24
  Speaker 2: instability and the formation of the stars in that new galaxy. WHOA,
• 32:29
  Speaker 2: but there's another really interesting clue. And to answer a
• 32:31
  Speaker 2: question you asked a few minutes ago about whether bars
• 32:33
  Speaker 2: form more recently or not, we find that it also
• 32:36
  Speaker 2: depends on the mass of the galaxy. So a lot
• 32:39
  Speaker 2: of the bars tend to have formed more recently in
• 32:42
  Speaker 2: lower mass galaxies. So it might be that lower mass
• 32:45
  Speaker 2: galaxies take longer to form stars and then take longer
• 32:48
  Speaker 2: to form bars. So most of the bars that are
• 32:51
  Speaker 2: forming in the universe today we think are in the
• 32:53
  Speaker 2: lower mass galaxies. They might not just have had time
• 32:57
  Speaker 2: to make a bar until now.
• 32:59
  Speaker 1: So you need like time and mass and flatness to
• 33:02
  Speaker 1: make a bar.
• 33:03
  Speaker 2: And some sort of seed, right, some gravitational over density.
• 33:07
  Speaker 2: I think it's really fascinating that basically all the structure
• 33:09
  Speaker 2: in the universe comes from one place being heavier than another.
• 33:13
  Speaker 2: When the universe began, if it was totally smooth, nothing
• 33:16
  Speaker 2: would have happened because all the gravity would have been balanced.
• 33:19
  Speaker 2: Basically everything that happened in the universe is because of
• 33:22
  Speaker 2: gravitational imbalances.
• 33:24
  Speaker 1: M yeah, I definitely happen, and I definitely have a
• 33:27
  Speaker 1: mass imbalance in certain parts of my body. Gravity. Yeah,
• 33:32
  Speaker 1: it's all good. I'm blaeing the chocolate bars. So then,
• 33:37
  Speaker 1: but then, how does the mass of the galaxy affect
• 33:40
  Speaker 1: these bars? Like why would having more mass or less
• 33:43
  Speaker 1: mass affect the bar formation?
• 33:45
  Speaker 2: Because structure formation takes time, right, Gravity is very very weak,
• 33:49
  Speaker 2: and it takes time for gravity to overwhelm these other things.
• 33:52
  Speaker 2: It takes time for gravity to pull stars together, it
• 33:55
  Speaker 2: takes time for those stars to then influence each other.
• 33:58
  Speaker 2: So all these things take time, and lower mass galaxies
• 34:01
  Speaker 2: have less gravity, so it just takes them longer to
• 34:03
  Speaker 2: sort of come together and form these things. And structure
• 34:06
  Speaker 2: is sort of hierarchical, right, it's bottom up. You build
• 34:10
  Speaker 2: the stars, the stars build a galaxy, You build a
• 34:13
  Speaker 2: structure on that, and then you build structures on those structures.
• 34:16
  Speaker 2: The barred spiral galaxy is sort of like the pinnacle
• 34:18
  Speaker 2: of universal structure. It's like the most complex organized thing
• 34:22
  Speaker 2: the universe has made so far.
• 34:25
  Speaker 1: Well, it definitely sounds like we've raised the bar here
• 34:27
  Speaker 1: in terms of galactic mysteries. It seems like this is
• 34:29
  Speaker 1: still a question for many scientists, and in fact, it
• 34:32
  Speaker 1: might even have interesting consequences for our conception of dark
• 34:36
  Speaker 1: matter and how they influence the formation of these galaxies.
• 34:40
  Speaker 1: So let's dig into those dark matters. But first, let's
• 34:43
  Speaker 1: take another quick break. Right, we're talking about galactic bars now, Daniels,
• 35:00
  Speaker 1: this is where single stars go to meet other single stars.
• 35:05
  Speaker 2: This is where molecular clouds hang out hoping to one
• 35:08
  Speaker 2: day make stars.
• 35:09
  Speaker 1: They're like celebrity bars. You see a lot of stars
• 35:11
  Speaker 1: in them.
• 35:13
  Speaker 2: This is where stars are born. Baby.
• 35:15
  Speaker 1: All right, Well, we talked about how we're not quite
• 35:18
  Speaker 1: one hundred percent sure why some galaxies are bars. It
• 35:21
  Speaker 1: seems like it's just a matter of the physics, like
• 35:24
  Speaker 1: the weirdness of how stars and the gravity interact and
• 35:27
  Speaker 1: form ripples in a cluster of stars. But there is
• 35:30
  Speaker 1: sort of another component to it, which is dark matter. Right.
• 35:33
  Speaker 1: Dark matter plays a big role in how galaxies form,
• 35:37
  Speaker 1: how fast they form, what shapes they have, and so
• 35:40
  Speaker 1: what do these bars tell us about dark matter?
• 35:42
  Speaker 2: Yeah, you're exactly right. And remember that the stars themselves
• 35:46
  Speaker 2: are just like tracers that tell you where most of
• 35:49
  Speaker 2: the mass is in the universe. So most of the dynamics,
• 35:52
  Speaker 2: the gravitational forces, are hidden from us. We live in
• 35:56
  Speaker 2: a dark universe where we only see a tiny, little
• 35:58
  Speaker 2: fraction of what's going on out there. So by studying
• 36:01
  Speaker 2: the motion of the gas and the dust and the stars,
• 36:03
  Speaker 2: we could try to get a glimpse for what's going
• 36:06
  Speaker 2: on sort of behind the curtain. What the fuller picture
• 36:09
  Speaker 2: of the universe is, and you're absolutely right that dark
• 36:12
  Speaker 2: matter controls the formation of the galaxies. We wouldn't have
• 36:14
  Speaker 2: galaxies this early in the universe if it wasn't for
• 36:17
  Speaker 2: dark matter. Eighty percent of the matter in the universe
• 36:20
  Speaker 2: is dark matter, but more like ninety percent of the
• 36:23
  Speaker 2: matter in galaxies is dark matter. So galaxies are basically
• 36:27
  Speaker 2: mostly dark matter. And because dark matter feels gravity, it
• 36:31
  Speaker 2: has a huge influence on gravitational turbulence, like the formation
• 36:35
  Speaker 2: of bars.
• 36:36
  Speaker 1: Absolutely so, all this time that we've been talking about
• 36:39
  Speaker 1: the mass of a galaxy, I thought we were talking
• 36:41
  Speaker 1: about the stars, But are we also talking about the
• 36:44
  Speaker 1: mass due to dark matter.
• 36:45
  Speaker 2: Yeah, we're talking about the total mass, not just the
• 36:47
  Speaker 2: visible mass of the galaxy.
• 36:50
  Speaker 1: Or are they pretty correlated, like the more stars you
• 36:52
  Speaker 1: see in a galaxy, the more dark matter it has.
• 36:56
  Speaker 2: Yeah, great question. We did a podcast recently about the
• 36:58
  Speaker 2: variation of dark man matter in galaxies. Most galaxies have
• 37:02
  Speaker 2: around ninety percent of their mass is dark matter, but
• 37:06
  Speaker 2: there is a variation. There's some that are like ninety
• 37:08
  Speaker 2: nine percent dark matter and some that have almost none
• 37:11
  Speaker 2: or maybe zero dark matter. It tells a really fun
• 37:14
  Speaker 2: story about the history of galaxies and how they form
• 37:16
  Speaker 2: and merge and might even strip each other of dark
• 37:19
  Speaker 2: matter on the whole. There's a reasonable correlation, and like
• 37:22
  Speaker 2: ninety percent of a galaxy's mass is invisible ninety percent
• 37:26
  Speaker 2: ninety percent.
• 37:27
  Speaker 1: Yeah, but isn't like the proportion of the universe something
• 37:31
  Speaker 1: like five to one dark matter to normal matter.
• 37:33
  Speaker 2: Yeah, so galaxies tend to have more dark matter than
• 37:36
  Speaker 2: most of the universe.
• 37:38
  Speaker 1: Why is that? Is it just because there's a lot
• 37:39
  Speaker 1: of like stars and does kind of in between galaxies
• 37:42
  Speaker 1: out there.
• 37:43
  Speaker 2: It's because dark matter is really responsible for the formation
• 37:45
  Speaker 2: of galaxies, and once a galaxy forms, it's hard for
• 37:49
  Speaker 2: it to lose its dark matter, but it's possible for
• 37:51
  Speaker 2: it to lose its gas and its dust and even
• 37:53
  Speaker 2: its stars. There's all sorts of processes that tend to
• 37:56
  Speaker 2: blow out gas and dust from a galaxy, you know,
• 38:00
  Speaker 2: supernova or radiation from the central black hole. All these
• 38:03
  Speaker 2: things can push normal matter out of a galaxy, but
• 38:06
  Speaker 2: they don't affect its dark matter. So the dark matter
• 38:09
  Speaker 2: is sort of there in the background, forming the gravitational
• 38:11
  Speaker 2: structure on which the rest of the universe lives, and
• 38:14
  Speaker 2: it's much less influenced by everything else that happens to
• 38:17
  Speaker 2: the normal matter.
• 38:19
  Speaker 1: Okay, so if our galaxy is ninety percent dark matter,
• 38:22
  Speaker 1: I would think that was like the most powerful thing.
• 38:25
  Speaker 1: That term is the shape with a galaxy. So does
• 38:27
  Speaker 1: that mean that the dark matter in our galaxy is
• 38:31
  Speaker 1: also in a spiral shape and maybe also has a
• 38:33
  Speaker 1: bar of dark matter in the middle.
• 38:35
  Speaker 2: Yeah, great question. We don't know the answer to that
• 38:38
  Speaker 2: because we don't know the distribution of dark matter in
• 38:40
  Speaker 2: the galaxy with a lot of precision. We know roughly
• 38:43
  Speaker 2: that it's denser in the core and then it's spread
• 38:46
  Speaker 2: out like a big halo. But we also know something
• 38:49
  Speaker 2: about the physics of dark matter, like we know that
• 38:51
  Speaker 2: dark matter doesn't feel friction with other dark matter the
• 38:54
  Speaker 2: way normal matter does. Like two clouds of gas that
• 38:57
  Speaker 2: pass through each other will exchange energy them will tend
• 39:00
  Speaker 2: to fall towards the center for example. These kinds of
• 39:03
  Speaker 2: energy exchanges just don't happen for dark matter because dark
• 39:05
  Speaker 2: matter passes right through itself. It only feels gravity, and
• 39:09
  Speaker 2: gravity is not strong enough for those kinds of exchanges.
• 39:12
  Speaker 2: So the complex structures you see forming in normal matter
• 39:15
  Speaker 2: don't always happen in dark matter. That's why we think
• 39:18
  Speaker 2: dark matter is sort of a bigger, fluffier halo than
• 39:21
  Speaker 2: the more compact visible matter in the galaxy.
• 39:24
  Speaker 1: The shape of the regular stars is mostly due to gravity, right,
• 39:27
  Speaker 1: and dark matter feels gravity with itself, doesn't it.
• 39:30
  Speaker 2: It's mostly due to gravity, but also crucially relies on
• 39:34
  Speaker 2: losing energy. Like if stuff is just swirling around gravitationally,
• 39:37
  Speaker 2: it won't collapse nearly as easily if it's not sticky,
• 39:40
  Speaker 2: if it just passes right through itself. When things are
• 39:43
  Speaker 2: orbiting a black hole, for example, they could orbit there forever,
• 39:46
  Speaker 2: but they're much more likely to fall in if they
• 39:48
  Speaker 2: exchange some energy, so one of them gives up energy
• 39:51
  Speaker 2: to another one rather than passing right through.
• 39:53
  Speaker 1: You mean, like if the stuff swirling around crashes with
• 39:56
  Speaker 1: itself or like bumps into itself and it's going to
• 39:59
  Speaker 1: lose some energy.
• 40:00
  Speaker 2: Yeah, So things are just much more stable if there's
• 40:03
  Speaker 2: no interactions other than gravity. The other interactions, they tend
• 40:07
  Speaker 2: to exchange energy and then stuff can fall into the
• 40:09
  Speaker 2: center to make stars or to make galaxies, or to
• 40:12
  Speaker 2: make arms, this kind of stuff. But the truth is
• 40:15
  Speaker 2: we don't really know what structure there is to the
• 40:17
  Speaker 2: dark matter. We don't have a lot of great probes
• 40:19
  Speaker 2: for exactly where the dark matter is throughout the galaxy.
• 40:23
  Speaker 1: It could be a dark chocolate bar in the middle.
• 40:25
  Speaker 1: It could be just like a moose spread out throughout
• 40:28
  Speaker 1: the whole galaxy.
• 40:30
  Speaker 2: It could be We're getting better and better techniques to
• 40:32
  Speaker 2: map where the dark matter is in the galaxy, but
• 40:35
  Speaker 2: we don't have a very accurate picture. But you know,
• 40:38
  Speaker 2: simple models of the physics of dark matter tell us
• 40:41
  Speaker 2: how it should be distributed, though we don't know if
• 40:43
  Speaker 2: those models are accurate, of course.
• 40:45
  Speaker 1: But then in all these simulations you're talking about where
• 40:48
  Speaker 1: we are finding out all these things about the bars,
• 40:51
  Speaker 1: do they include dark matter in those simulations? And kauds
• 40:54
  Speaker 1: of assumptions do you have to make there?
• 40:55
  Speaker 2: They do include dark matter, and they treat dark matter
• 40:58
  Speaker 2: as like a collisionless fluid, like they're in the backdrop
• 41:01
  Speaker 2: of providing more gravity. And the dark matter does definitely
• 41:05
  Speaker 2: influence the formation of bars in these simulations. The more
• 41:08
  Speaker 2: dark matter you have, the less often you get a bar,
• 41:12
  Speaker 2: even when you have some sort of like gravitational instability
• 41:15
  Speaker 2: or over density in the visible matter.
• 41:17
  Speaker 1: So does dark matter help the bars or prevent the bars?
• 41:21
  Speaker 2: It prevents the bars from forming. So the more dark
• 41:24
  Speaker 2: matter you have, the more it tends to smooth out
• 41:26
  Speaker 2: the gravity of the disc, just like having a thicker
• 41:29
  Speaker 2: disc right, as you were saying earlier, sort of like
• 41:31
  Speaker 2: a two D effect. The more three d R galaxy becomes,
• 41:34
  Speaker 2: the less susceptible it is to these effects. In the
• 41:36
  Speaker 2: same way dark matter basically reduces the effect of gravity
• 41:40
  Speaker 2: of the visible matter provides this like background gravity pulling
• 41:44
  Speaker 2: on everything.
• 41:45
  Speaker 1: I see, like it evens out the strength of gravity
• 41:48
  Speaker 1: in the galaxy. Right, because you think dark matter is
• 41:51
  Speaker 1: distributed spread all over the place, the more significant that is,
• 41:54
  Speaker 1: the more generally speaking, gravity is in that galaxy exactly.
• 41:59
  Speaker 1: And so if you have no dark matter, do you
• 42:01
  Speaker 1: always get a bar?
• 42:02
  Speaker 2: Nothing happens every time in simulation, but the proportion definitely
• 42:05
  Speaker 2: goes up if you have no dark matter. But we
• 42:07
  Speaker 2: don't expect that to happen in the universe very often,
• 42:10
  Speaker 2: as we talked about recently on the podcast. We don't
• 42:12
  Speaker 2: think galaxies can form without dark matter, though they might
• 42:15
  Speaker 2: later be stripped of their dark matter in some collisions,
• 42:19
  Speaker 2: But then collisions are also going to destroy a bar.
• 42:22
  Speaker 1: So is the bar kind of a sign of how
• 42:25
  Speaker 1: much dark matter there is in a galaxy, then.
• 42:27
  Speaker 2: Yes, exactly. And people have used bars to try to
• 42:30
  Speaker 2: measure the dark matter fraction in galaxies. Like if you
• 42:33
  Speaker 2: look up at the night sky and you measure the
• 42:35
  Speaker 2: fraction of galaxies that have bars in them, that tells
• 42:38
  Speaker 2: you something about how much dark matter there can be
• 42:40
  Speaker 2: in those galaxies, because if there's a lot of dark
• 42:43
  Speaker 2: matter in all those galaxies, they wouldn't have bars, And
• 42:46
  Speaker 2: if there was less dark matter in those galaxies, more
• 42:48
  Speaker 2: of them would have bars. So we can actually use
• 42:50
  Speaker 2: the bars as a way to like indirectly estimate the
• 42:53
  Speaker 2: dark matter fraction of the universe in a way that's
• 42:55
  Speaker 2: separate from the galactic rotation speed or any of the
• 42:58
  Speaker 2: other probes.
• 43:00
  Speaker 1: Like the bar sets the bar for dark matter.
• 43:05
  Speaker 2: Yeah, it raises the bar on our understanding.
• 43:07
  Speaker 1: Our precision to measure dark matter.
• 43:10
  Speaker 2: Yeah, this is not a very precise measurement because we
• 43:13
  Speaker 2: don't have that many galaxies where we can measure this
• 43:15
  Speaker 2: kind of thing. But it's consistent with everything else we
• 43:18
  Speaker 2: know about dark matter. So the fraction of galaxies with
• 43:21
  Speaker 2: bars is consistent with our understanding that eighty percent of
• 43:24
  Speaker 2: the matter in the universe is dark matter, and something
• 43:26
  Speaker 2: like ninety percent of the matter in galaxies is dark matter.
• 43:31
  Speaker 1: And then what percentage is Hershey's chocolate versus snob chocolate.
• 43:36
  Speaker 2: I see, now you're disparaging people's choice chocolate. It goes
• 43:40
  Speaker 2: both ways. I'm all right, I can take it. If
• 43:47
  Speaker 2: I can dish it out, I can take it. That's fair.
• 43:49
  Speaker 2: I'm sure that Hershey's dominates the chocolate market, but I'm
• 43:52
  Speaker 2: glad that there are now a variety of chocolate offerings
• 43:55
  Speaker 2: for people to enjoy.
• 43:56
  Speaker 1: All right. Well, another interesting feature of our galaxy and
• 43:59
  Speaker 1: of our u universe the way physics works to create
• 44:01
  Speaker 1: these weird characteristics of the things we see out there,
• 44:04
  Speaker 1: which then kind of reveal what things are made of
• 44:07
  Speaker 1: and how things work in the grand cosmic scale things.
• 44:11
  Speaker 2: Absolutely, and it's just part of the game we're playing,
• 44:13
  Speaker 2: looking out at the night sky and out deep into
• 44:15
  Speaker 2: the universe to try to get a handle on what's
• 44:18
  Speaker 2: going on out there, what structures form, why do they form,
• 44:21
  Speaker 2: and what do they reveal about the flash of titanic
• 44:24
  Speaker 2: forces that's happening underneath it all.
• 44:26
  Speaker 1: It sounds like these bars are giving us a good
• 44:28
  Speaker 1: handle on galaxy formation and dark matter content of the Universe.
• 44:34
  Speaker 2: I'm going to go hit the lunch bar after this.
• 44:37
  Speaker 1: Yeah, just don't hit the dessert bar too hard. It
• 44:42
  Speaker 1: depends on the kind of chocolate ban Yeah, that's right.
• 44:44
  Speaker 1: The brand apparently.
• 44:48
  Speaker 2: Love all you and love all your chocolate choices. Keep
• 44:51
  Speaker 2: chomping on bars. That sounds really credible, Daniel. I'm working
• 44:54
  Speaker 2: on it, man, I'm working on it all right.
• 44:56
  Speaker 1: Well, we hope you enjoyed that. Thanks for joining us.
• 44:58
  Speaker 1: See you next time.
• 45:04
  Speaker 2: For more science and curiosity, come find us on social media,
• 45:07
  Speaker 2: where we answer questions and post videos. We're on Twitter, Discorg, Insta,
• 45:12
  Speaker 2: and now TikTok. Thanks for listening, and remember that Daniel
• 45:15
  Speaker 2: and Jorge Explain the Universe is a production of iHeartRadio.
• 45:19
  Speaker 2: For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts,
• 45:24
  Speaker 2: or wherever you listen to your favorite shows.