Astronomy Cast » Nebulae http://www.astronomycast.com Take a facts-based journey through the universe. Tue, 07 Feb 2012 15:12:44 +0000 en hourly 1 http://wordpress.org/?v=3.3.1 Take a facts-based journey through the universe. Astronomy Cast no Take a facts-based journey through the universe. Astronomy Cast » Nebulae http://www.astronomycast.com/wp-content/plugins/powerpress/rss_default.jpg http://www.astronomycast.com/category/astronomy/nebulae/ Ep. 111: Nebulae http://www.astronomycast.com/2008/10/ep-111-nebulae/ http://www.astronomycast.com/2008/10/ep-111-nebulae/#comments Tue, 21 Oct 2008 17:44:48 +0000 admin http://www.astronomycast.com/?p=397 ]]> When you think about the best pictures in astronomy, almost every one is a nebula; the pillars of creation in the Eagle Nebula, or the complex Helix Nebula – or my personal favorite, the Ring Nebula. They’re beautiful, wispy clouds of gas and dust that signify both the birth and death of stars. Today we give tribute to nebulae.

  • Ep. 111: Nebulae
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    Birth!: Nebulae that create stars

    Death!  Nebulae that destroy stars

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    Transcript: Nebulae

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    Fraser Cain: It’s a very special day today that we’re recording.

    Dr. Pamela Gay: Oh, and what’s special day is that?

    Fraser: Phil Plait’s new book, ‘Death From the Skies’ which everyone must buy in hard cover if you can afford it and you like that kind of thing… but you should buy it.

    Phil is of course our good personal friend and we’re going to shill relentlessly on AstronomyCast for Phil to help him sell as many books as humanly possible. [Laughter]

    Pamela: I read a preview copy and I loved it. Anyone who reads my book reviews in Sky and Telescope knows I really don’t love everything.

    Fraser: When you think about the best pictures in Astronomy, almost every one is a Nebula. The Pillars of Creation in the Eagle Nebula or the complex Helix Nebula or my personal favorite the Ring Nebula.

    They’re beautiful wispy clouds of gas and dust that signify both the birth and death of Stars. Today we give tribute to the Nebulae. Okay Pamela should we start with birth or death?

    Pamela: Let’s start with birth. We always seem to end on death and it works well.

    Fraser: Alright let’s start with birth then. What are Nebulae?

    Pamela: Well, that’s neither birth nor death that’s sort of well, everything. Bottom line, Nebula is gas and dust. That’s all, it’s simple.

    Fraser: Is it? Alright so let’s go with the birth side. What are the constituents of a Nebula that will go on to create Stars?

    Pamela: The classic starting point is giant molecular clouds and then Bock Globules. Giant molecular clouds are just that. They are giant clouds of gas that in many cases has cooled off enough that in many cases you can start to get some really cool molecules.

    You can even get like formaldehyde in these things. They’re out there hanging out not generating any light. They’re just cold.

    When you try and look at them with optical telescopes – when you look at for instance the Kolchak Nebula or the Horsehead Nebula – you can’t really see any of the background Stars.

    You just see this blackness in the midst of quite often a bright cloud or just in the middle of nowhere suddenly there’s a patch of nothingness on the Sky. That’s because these things are just so big that as light tries to pass through them the light just all gets absorbed.

    Fraser: This is the situation where you’re out at night, looking up at the Stars and maybe there’s a single cloud or two in the Sky which is blocking out some of the Stars.

    You can’t see the cloud but you can see the region where there are no Stars and make out the shape of the cloud.

    Pamela: That’s exactly what happens when you go to extremely dark sites like out in west Texas or parts of Arizona.

    If you look out on a city night you’re going to see all the clouds as the bright points on the Sky.

    But in really dark locations, the darkest night is the one that has clouds everywhere.

    Fraser: Because you don’t see any reflection on the clouds from the ground which you see in the city.

    Pamela: Exactly.

    Fraser: Okay and I guess the temperature is the key, right? That cold temperature matters.

    Pamela: It’s the cold temperature that allows it to in some ways condense and that keeps it dark. If you heat up this gas it’s going to start giving off light in different colors.

    This is where we start to get to Emission Nebula and Reflection Nebula. You take a giant cloud of gas and you put Stars nearby. You can get two different situations. The simple situation is what we call Reflection Nebula. This is where you have some light passing through the gas.

    It’s not enough either in temperature or if the light is not perhaps intense enough you’re not going to get a lot of strong what we call Ionization Zones. Instead you’re just going to get scattering of the light. What happens is you have light rays from Star hit cloud.

    They start to pass through the cloud but as they hit different particles, as they hit different molecules, the light is going to scatter off of the dust just like your headlights scatter in fog off of all the water molecules in the Atmosphere. This scattering tends to scatter blue light more than it scatters red light.

    If you’re looking up in the Sky and you see a cloud of distant Nebular gas off in the distance and off at about the same distance you have off to the right a really bright Star, the light from that really bright Star is going to try and go through the cloud.

    But when it gets to the cloud, the blue light is going to get scattered towards you and you end up seeing this Reflection Nebula as blue. If you’re instead looking almost straight through the cloud of gas and dust at that Star, it’s the red light that would be able to make a straight line through the cloud of gas and dust.

    Then if you’re off on that other angle, you’re going to see the cloud as red. You can actually see the same cloud as either red or blue depending on where you are relative to the Star that’s illuminating the cloud.

    Fraser: Light is, you know the regular white light that we see coming from Stars is actually made up of the whole spectrum of light from red on the one side to blue on the other side and beyond.

    The different spectra get broken up in different ways depending on what they’re passing through. So red light will go straight through a Nebula and blue and green light will get kind of deflected away.

    I get it, if you’re seeing a Star on the other side of the Nebula you see it red but if you’re seeing it kinda off to the side then you see the blue and the green. That’s really cool. So which one is which again? A Reflection Nebula is the blue/green one and …

    Pamela: And Emission Nebula I haven’t actually stopped to explain yet. An Emission Nebula is what you get when you crank that Star’s energy up or when you start to embed Stars directly into the cloud of gas.

    In this case you have so much light coming out and it’s at just the right temperatures that it heats the gas in the Nebula up. This act of heating the gas in the Nebula up causes some of the Electrons and various Atoms to get excited and jump to higher energy levels.

    Things that get excited don’t stay excited. Everything wants to just sit down and go to a lower energy level. The heat from the nearby Stars, heats up the gas, causes the Electrons to jump to higher energy levels as it absorbs the light.

    That light might be coming straight in from the Star and if you look through the cloud of gas and dust at that Star you’ll see the rainbow from that Star, the spectrum from that Star missing the colors that correspond to where the light has been absorbed by the Nebula.

    You get absorption lines when you look straight through the cloud of gas and dust at a background Star. The light that got absorbed that caused those dark lines when you’re looking straight through the cloud of gas and dust is going to get re-emitted. But it’s going to get re-emitted in every direction.

    So if instead of looking straight through the cloud of gas and dust of the background Star you’re off to the side. When you’re off to the side you’re going to see that light getting re-emitted as Emission lines and you’re not going to see the rainbow of light from the Star itself.

    This is kind of cool because you’re essentially taking light, removing it from one rainbow and spitting it out in all sorts of different directions. You’re redistributing the light, creating emission lines for one person and absorption lines for another.

    Fraser: The Star itself is heating up the gas in the Nebula and then the Nebula is giving off light of its own and that’s what we’re seeing. Right?

    Pamela: Exactly. This can lead to things like we see Oxygen lines in Planetary Nebulae because they’re still hot. Anytime you have hot gas you can get at these Emission lines.

    The general rule is Reflection Nebulae are going to be cooler when all they are is Reflection Nebulae because all they’re doing is scattering light.

    Emission Nebulae you have to actually heat the gas up, heat the dust up and this is where you start getting the Electrons bouncing up and down to different energy levels within the gas and cloud. It’s the gas within the Nebular cloud that’s emitting the light that it has absorbed from the Stars.

    Fraser: I have two questions there. My first one is how hot can that gas get?

    Pamela: Tens of thousands of degrees.

    Fraser: Like hot enough to start giving off x-rays, right?

    Pamela: Then we’re not so much dealing with Nebulae anymore.

    Fraser: Oh then we’re colliding Galaxies together.

    Pamela: Right so you can get gas and dust as hot as you want. Once you start getting it way hot, then you start getting x-ray emissions.

    This is what you get in centers of Galaxy Clusters. This is what you get when you have gas and dust falling into a Black Hole.

    Fraser: That’s kinda what I was imagining. I guess my second question or problem really is hot gas doesn’t form Stars. That’s the opposite of getting a Star, right?

    Pamela: It’s a balancing act.

    Fraser: So how does that balancing act kinda play out?

    Pamela: You have what’s called the Rayleigh Instance stability. When you get a certain amount of Mass crammed into a certain volume it’s going to collapse down. You can’t really stop this process. Within a gas cloud you’re constantly balancing the heat of the gas trying to expand that gas out.

    This is that old PERVNERT that we all learned when we were in Chemistry in High School. Pressure times Volume is equal to the number density times the constant times the temperature.

    Fraser: I never learned that.

    Pamela: You didn’t?

    Fraser: Well I mean I learned the formula but I didn’t learn the little acronym.

    Pamela: We called it PERVNERT – we’re strange in America I guess. So you had this balancing of you heat up the gas it wants to expand. But at the same time if you shock the gas if you have Stellar Winds that are pushing the gas so that it’s colliding together, as you get more gas into a smaller volume it’s going to gravitationally start to contract.

    So you have the gas pressure pushing outwards and the Gravity pulling the gas together and you could balance all of these different factors.

    Fraser: But you need to have that event, right?

    Pamela: You need something to cause the gas to start collapsing.

    Fraser: Right, so if you just let it sit there for millions and millions of years it won’t do anything until something else bumps it.

    Pamela: Yes, and there’s lots of different ways that you can trigger giant clouds of gas, the giant molecular clouds into collapsing. It could be as simple as they’re journeying around the Milky Way and on their orbit they encounter an area of higher density.

    The higher density regions are going to have a little bit more gravity. They’re going to accelerate the gas cloud into this higher density region. It’s potentially going to get condensed within this higher density region and start Star forming. This is actually where we get Star forming within the arms of Galaxies.

    We discussed this in an earlier episode. You can also get Super Nova going off. So all you need in some cases is just a shock coming along perhaps from a Super Nova and the shockwave from that can cause the gas to start to collapse. You can end up with two different Systems passing near each other and shocking one another.

    Lots of different things can start the gas collapsing down. Once you get a System that has active Star formation going on you’re going to get winds blasting, you’re going to get jets, and you’re going to get all sorts of high energy events and pressures and all these different things.

    Super Nova going off and this is going to move the gas around and heat the gas up but at the same time condense it into smaller volumes triggering collapse into Stars.

    Fraser: But aren’t you also getting the Stars’ Solar Winds blowing the gas away too?

    Pamela: Right, this is where you end up with the Stars emerging out of essentially cocoons of gas. There are some amazing images taken by the Hubble Space Telescope of both the Carinae Star forming region, the great Carinae Nebula and the Orion Star forming region where in both sets of images there are what we call proplets.

    These are essentially cocoons of Stars where you have the young Star that has ever so slowly collapsed which we can watch happening in clouds of different temperatures where when you look at the cooler clouds. You look specifically at how different Carbon molecules emit light. You can see pockets of heat where gases started to fragment and collapse.

    You follow this through to warmer, more fragmented clouds and you start to see the young Stars heating up, appearing in the infrared and they’re embedded in these cocoons of gas and dust. As they collapse more and heat up they start to blow off winds as they ignite with nuclear reactions in their cores.

    The winds as they are both eccreting matter onto them and blowing jets of materials off of their poles, these winds will clear out the areas around the Star. So, you go from cocooned in Star to Start that’s blown the region of Space around it completely free.

    Hopefully it has managed to maintain whatever Planetary Disc it might have had but we go through this entire system of everything collapsing down to then the Star basically putting the brakes on and blowing everything out from around it.

    Fraser: You get these amazing bubble structures in these Nebulas where these various Stars are starting to ignite and blow away all the Nebula material around them. It’s quite amazing; there are some great pictures from Hubble.

    Pamela: One of my favorite things to do is to look at these different systems across multiple different wavelengths. You can look at them in radio maps and you see in these dark cold molecular clouds occasionally these little pockets where Stars are starting to form.

    You look at them in the infrared and you can see through the clouds of gas and dust to objects behind them. You can also start to make out the cores of forming Stars within fragmenting clouds.

    As you crank up going from the longest wavelengths of the radio up toward optical, you see different features. You start to see different things because each of these different colors is able to penetrate the gas and dust in different ways and see different things.

    It’s essentially like probing with x-rays into the human body. Suddenly you’re able to see the bones. Well, as you probe into these clouds looking for red light, suddenly you’re able to see through the dust and see the Stars forming within.

    Fraser: Yeah, that’s just amazing. You see some of those pictures from Hubble and you just see these delicate structures and these clouds and some of the darker regions and you can see them not forming.

    You can see the Stars. It’s all just there. What kind of region did our Sun probably form out of? Is it something like the Orion Nebula?

    Pamela: That’s what we think. What’s really cool is this time of year (and we’re recording this in mid-October), it’s to the point that in the early evening you can go outside and look up and you can see Orion in the evening Sky. You can see Taurus in the evening Sky. You can see the Pleiades.

    Within Orion there’s the great Orion Star Forming region which is a fairly young Star forming area. It still has lots of really hot O type Stars and these are some of the shortest lived types of Stars. They only live for a few million years.

    So, we look at this and we see what a Star-forming region looks like when it is quite young. We then look over sideways up at the Pleiades – this is an older Star System that has cleared out most if not all of its gas and dust – you still see some young blue Stars but the stuff like whether you’re a Super Nova, it’s not there anymore.

    Then you look over toward Taurus, if you find binoculars you can see what is called the Heiades Cluster. It’s an open cluster of Stars that has fragmented to the point that you can only know its there because that area of Sky has more Stars in the regions around it.

    As we look around just this small area of Sky we can see the evolution of what the birthplace of our Sun probably looked like at different stages in its life. First you have Orion with the young hot Stars; the gas and objects still emerging from their cocoons.

    Then you have the Pleiades where the gas and dust has mostly been blown away and the Stars are starting to stand on their own fully formed. Next we have the Heiades System where it’s like the Stars are getting pushed out of their nest going off to live lives outside of the Cluster in which they formed.

    Fraser: So then the Sun and whatever Stars formed with it are way beyond what happened with the Heiades Cluster. We don’t even really know the Stars that formed with us anymore, right?

    Pamela: Right and that’s one of the fascinating things about looking out at the Galaxy around us. We know once upon a time we were part of a Star-forming Region. We were part of an Open Cluster.

    But as we’ve orbited around and around the Galaxy, this System has been stretched and fragmented to the point that the Stars that were furthest from the core haven’t made it around as many times.

    The Stars that were closest to the core have made it around a little bit more and everything has gotten stretched out to the point that we can no longer make out our original path.

    Fraser: That’s too bad. You can just imagine what it would have looked like back in the day being in the middle of a Star-forming Region.

    Although we kinda debunked what it would look like, you know [Laughter] what you would see with Nebula because you see these beautiful colors and pictures in the Hubble Space Telescope but you have to know that those come from long, long exposure times. If you actually were on a Planet in a Star in a Nebula, you wouldn’t see much.

    Pamela: And here the catch is that it’s spread out across your entire Sky. For us to look at the Orion Nebula, yeah we do see that there is Nebulosity in it when we look at it, but we’re seeing something where light has been confined into a small angle on the Sky.

    You take that light and spread it out and even when you bring it very, very close, it looks different. You’re still going to be able to know you’re in a Nebula and things like Super Nova going off and Massive X-ray Bursts from Giant Stars are sorta going to make your life a BIT scary.

    But it will be quite fabulous just to see all of these hot Stars in all these directions. I believe Lynette Cook has actually done some fairly good artwork on this, depicting it.

    Fraser: Alright, let’s switch gears then and go to the end of life then.

    Pamela: [Laughter] We always have to end on death.

    Fraser: We have to end with death. So where do these come from?

    Pamela: We have two basic forms of Dead Star Nebula and that’s the Planetary Nebula and Super Nova. What’s amazing is the diversity that both of these things can take.

    Especially with the Planetary Nebula, the idea behind a Planetary Nebula is fairly simple. Take a Star not too different from the Sun you can get a little bit bigger or a little bit smaller and you let it die. Eventually its core is going to stop undergoing Nuclear Reactions. When this happens, it’s going to undergo core collapse because there’s nothing left to push the core out.

    In the Millennia leading up to this, in fact the eons leading up to this, the outer Atmosphere of the Star is going to puff away as the Star basically resembles a Mira Variable. That puffed off Atmosphere that drifts away ends up forming a cloud of gas centered on the original Star that has now become a White Dwarf.

    Now you’d think that you’d consistently get nothing more than a big sphere of gas or maybe if there’s a Planetary System you’re actually going to get a figure eight of gas because the Planetary Disc ends up essentially acting like a corset forcing the gas to go off to the two poles of the Star.

    But what we find when we look out there is Planetary Nebula come in a variety of amazing shapes, everything from boxes to nested circles to every possible geometry. It’s like someone took one of the Spirograph from when we were children and played with it on the Sky using different gases that emit different colors of light.

    The Hubble catalog of these objects is truly spectacular and they’re some of my favorite objects to look at through telescopes with my eyes just because they do have so much diversity of shape and color to them.

    Fraser: That’s why I was mentioning the Ring Nebula. That’s the one that I always look for. If I get my hands on a telescope and I’m done showing people the Planets, I try to show them the Ring Nebula because you can see it. It’s a little ring.

    Pamela: Yeah and if you’re playing with something like an Obsession twenty-inch dob or something you can actually start to make colors out on some of these, especially if you’re using Oxygen filters. A lot of these things have Oxygen gas in them that’s giving off emission lines that is just this brilliant green color.

    Fraser: What creates those amazing shapes?

    Pamela: We’re still [Laughter] sorting that out. It’s one of the mysteries; it’s still being determined. That’s just the way the Universe works.

    Fraser: Right, but there has to be a guess.

    Pamela: You have to look at what are the different processes that cause the gas to get emitted. So you have a Pulsating Star that gives off perhaps breaths – one breath at a time – over millions of years perhaps and with these occasional outflows of gas – these occasional exhalations – you end up with a dense region.

    Now you have that object, the Star that’s giving off this gas moving through Space that’s going to affect the geometry. You have Planetary Discs, you have Magnetic Fields and you start adding all the different Physics and you can affect how gas moves and you start getting crazy shapes.

    Fraser: Right you might have like a Binary Companion that’s plowing through the Nebula. So now why can we see them? What allows us to see a Planetary Nebula?

    Pamela: It’s the hot White Dwarf in the center and the fact that the gas is still cooling off in some cases. But mostly it’s the hot White Dwarf in the center.

    Fraser: Right so the White Dwarf is still illuminating the shells of expelled material.

    Pamela: And over time it’s all going to fade away as that Star fades away and as the gas continues to expand away.

    Fraser: Oh, so that’s why we don’t see these Planetary Nebulae around all the White Dwarf Stars. We can see only the ones that recently sloughed their outer layers.

    Pamela: Right and what’s cool is especially with some of the Super Nova remnants, you can actually measure the expansion of this gas. The Crab Nebula is one of the most famous examples because it’s so pretty and it’s so big on the Sky that people have been taking photos of it pretty much since photographic technology was invented.

    We can go back through the archives and look at these pictures of the Crab Nebula and see it on image, a Star was not yet blocked by gas and dust but over in another image, it is gone. And in another image, the one next to the Star is gone.

    We can actually put together essentially movies of the expansion of the Crab Nebula using these old photographic images and comparing them with modern images.

    Fraser: Now you had another way that Nebulae are formed, right through Super Nova?

    Pamela: And that’s where the Crab Nebula came from. In this case rather than having the Nebula form over long periods of time, you sort of have one giant burst.

    Fraser: Splat!

    Pamela: Yeah, no more Star – Giant Nebula. Well the Nebula actually takes time and this is like we can watch this happening. Super Nova 1987 A is another example. A Super Nova that went off in the nearby Magellanic clouds. This is a Star that went off – and our good friend Phil Plait actually studied it – and we saw it as bright Star in the Sky that didn’t used to be there.

    We could go back and actually identify what was the Star before it exploded. We watched it go through the brightening; the fading and now we’re watching the Nebula expand away like a figure-eight. What does that mean, a figure-eight?

    Well it probably means there is some sort of a Disc constricting it on the inside. We have these two rings that we’re seeing and it’s not actually rings it’s two spheres but we only see the edges of the spheres. They’re not quite lined up with each other so that means that the pole of the explosion wasn’t pointed straight at us.

    We can start to learn lots about the geometry of the System that did the exploding many light years away just by looking at how the Nebula expands away.

    Fraser: It’s the same kind of situation where no two Super Nova Nebulas look the same so you’ve got all of the dynamics. Did it turn into a Black Hole? Did it turn into a Neutron Star? Were the poles facing towards us or away from us? How fast was it spinning? What was it made of?

    That just changes the way it looks. I mean the Crab Nebula and Super Nova 1987 A look totally different.

    Pamela: And they are at totally different stages in their life cycle. Super Nova 1987 A is what 21 years old now? So, I have students that aren’t that old.

    Fraser: Right and the Crab Nebula was in the year like 1,054, right?

    Pamela: Yes so we have lots of time that. Nebula has been expanding and it’s been starting to actually interact with the Interstellar Media. The gas and dust that isn’t quite Nebula-like, but is still out there affecting things out between the Stars.

    What’s wonderful about models of Super Nova explosions is you can actually see they don’t just blow off their own Atmosphere but the shock waves of this Super Nova compress the gas in the Interstellar Media.

    They create bubbles around themselves. They can actually blow bubbles out the disc of the Milky Way. So Super Nova explosions can do amazing things to the distribution of gas and dust that are visible things.

    Fraser: And there’s another Nebula that I’ve reported on a couple of times and that’s the V838 Monocerotis… I don’t seem to be pronouncing that right.

    Pamela: It’s one of those impossible to pronounce Constellations so I usually just move on with life and call it V838 Mon. This is one of those really weird objects. It’s Nebulosity doesn’t come from it giving off gas and dust the way a Planetary Nebula or a Super Nova does but rather this really strange object that we’re still trying to figure out went through these two flash periods. It flared back in 2002 and the light that it gave off moved away.

    As the light moved away from the Star, it progressively lit up different shells of gas and dust. It turns out that this Star is apparently embedded within a fairly thick region of the Interstellar Media. So you’ve been able to watch through a series of Hubble Space Telescope images as progressively more and more gas and dust was eliminated as the light had time to travel further and further from the Star.

    We call this light echoes – it’s basically where light starts to head off towards the right, hits a particle and reflects off of it and echoes back toward us the way sound, when you yell in a large empty room will travel away from you and then echo off of the walls.

    People will hear both the first holler of you yelling straight toward them and then will hear a second sound of the sound reflecting off the walls and then traveling toward them.

    Fraser: Right and if you see the pictures – the Hubble folks have been taking so many pictures of this they could now do animations of the change of this Nebula over time. It’s just amazing, it really looks like that.

    You can see a sphere of light moving away from the central Star and illuminating different parts of this ball of gas around the Star. I think it’s one of the most amazing images and videos in Astronomy that I’ve ever seen.

    Pamela: The Hubble Heritage Project has done a great job so we’re going to put a link in to what they’ve done.

    Fraser: Alright Pamela, I think we’ve covered our Nebulae. So, we’ll talk next week.

    Pamela: Sounds good and I’d like to put final plug in and it’s not for Phil’s book. We’re nominated for the People’s Choice in Podcasting Awards. It would be so cool if we could bring home a People’s Podcast Award because you guys supported us.

    ]]>     http://www.astronomycast.com/2008/10/ep-111-nebulae/feed/ 4 When you think about the best pictures in astronomy, almost every one is a nebula; the pillars of creation in the Eagle Nebula, or the complex Helix Nebula - or my personal favorite, the Ring Nebula. They're beautiful, When you think about the best pictures in astronomy, almost every one is a nebula; the pillars of creation in the Eagle Nebula, or the complex Helix Nebula - or my personal favorite, the Ring Nebula. They're beautiful, wispy clouds of gas and dust that... Astronomy Cast no     Ep. 75: Stellar Populations http://www.astronomycast.com/2008/02/ep-75-stellar-populations/ http://www.astronomycast.com/2008/02/ep-75-stellar-populations/#comments Mon, 11 Feb 2008 20:46:00 +0000 admin http://www.astronomycast.com/astronomy/ep-75-stellar-populations/ Episode 75: Stellar Populations (14.4MB)
    ]]>     After the big bang, all we had was hydrogen, a little bit of helium, and a few other trace elements. Today, we’ve a whole periodic table of elements to enjoy, from oxygen we breathe to the aluminium cans we drink from to the uranium that powers some people’s homes. How did we get from plain old hydrogen to our current diversity? It came from stars, in fact successive generations of stars.

  • Episode 75: Stellar Populations (14.4MB)
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    Fraser: After the big bang, all we had was hydrogen, a little bit of helium, and a few other trace elements. Today, we’ve a whole periodic table of elements to enjoy, from oxygen we breathe to the aluminium cans we drink from to the uranium that powers some people’s homes. How did we get from plain old hydrogen to our current diversity? It came from stars, in fact successive generations of stars.
     
    Let’s start back at the beginning. Pamela, can you give us what the conditions were like right after the big bang?

    Pamela: Right after the big bang, I’m going to skip forward and skip to right after the cosmic microwave background formed. Go back, listen to our shows on the big bang and right after the cosmic microwave background formed, we have a universe that’s gas. It’s gassy. It needs something to come along and clear up the gas, sort of like a person who ate too much.
     
    In this gas, it’s pretty much all constant everywhere, but there’s slight inhomogeneities, slight differences from place to place, where some places have a little bit more matter (and by matter I mean both luminous matter and dark matter).

    Fraser: But what is the gas?

    Pamela: It’s hydrogen.

    Fraser: It’s just hydrogen, a little bit of helium, trace elements.

    Pamela: Yeah, and trace amounts of lithium and beryllium.

    Fraser: Right, and the hydrogen is what was formed right at the big bang, the helium and others were formed when the whole universe was acting like a big, giant star and in a compressed state.

    Pamela: Yeah.

    Fraser: Yeah, and we covered that in the big bang episode. I don’t want to over do it, but bring everyone up to speed.

    Pamela: This universe of gas, these slight differences from place to place in the density caused it to start fragmenting. If you look at different images of nebulae, you’ll see that in some of them you can actually see the chunkiness in them. You can see the fragmentation and the process of stars forming. You can see this some in the Orion Nebula and the Eagle Nebula. The entire universe was basically one of these giant star forming regions.
     
    Because the universe was only hydrogen, some helium, and trace amounts of lithium and beryllium, the types of stars we have today wouldn’t have been able to form. We actually require metals to get small stars. This is because the metals help the energy escape from the star and cool them down. Without that ability to cool off, the stars get really big, really hot, really powerful, and live very short lives.

    Fraser: But what if you don’t have enough gas? Like, right in the beginning the whole universe was a lot more compressed than it is today. I can see how there must have been gigantic quantities of gas that came together in various places, but if you had a small amount of gas, wouldn’t that come together as a small amount of star?

    Pamela: This is one of those things that there’s a lot of debate going on about in the astronomy community. I have to admit that as a community, we aren’t of a single mind. Observationally, if there had been any small stars forming, if any stars that were even 80% the size of the Sun, they’d still be hanging around. We’d be able to see them somewhere. Perhaps not in large numbers, but we probably would’ve found one by now that had this primordial ratio of abundances: had no iron, no carbon. We haven’t found any stars like that.

    Fraser: Oohh, okay. Right, because a star that’s smaller than the Sun is going to burn its fuel more slowly and last a lot longer. It might not even be done its main sequence phase and we would see it. Because the universe is only 13.5 billion years old, we’d look at these stars and go, “that is a 13.5 billion year old star, it must have formed right from those primordial elements�?.

    Pamela: We’re not finding those stars.

    Fraser: Ever? Not a single star has been found like that?

    Pamela: None!

    Fraser: Really! Huh.

    Pamela: So, when we do the computer models (well not we – when other people who are far better programmers than I am go off and do the models) they’re finding the stars are coming out naturally when you balance all of the thermodynamics, when you balance all of the energy generation and gravity. They’re coming out huge, they’re coming out hundreds of times the size of the Sun.

    Fraser: I don’t want to keep going into this – obviously the evidence is there. Aren’t there clouds of hydrogen still out there in the universe that haven’t collapsed yet?

    Pamela: Yes, and those clouds that haven’t collapsed yet aren’t pure. They’re not virgin clouds.

    Fraser: They’ve been seeded from other generations of stars.

    Pamela: So not all the fuel got used up. Think of it this way: when you’re making bread dough on the counter, there’s still flour on the counter when you’re done. A lot of It goes into making the bread, but you still have leftover bits. In our galaxy, that first generation of stars didn’t suck all the gas into star formation, in fact we think that the population II stars, the next generation of stars actually started to form before that first generation of stars had finished lighting up all the gas and clearing it out and making it transparent.

    Fraser: Okay, well now you’re just skipping to the second chapter. Let’s go back to the first chapter here.

    Pamela: Okay.

    Fraser: You’ve got these gigantic clouds of gas coming together to make monster stars.

    Pamela: Monster, monster stars. There’s actually a few papers, though I’m not sure I believe them, that say that dark matter might’ve played a role and these first generation stars might’ve helped coalesce these giant stars, might’ve helped seed the early nuclear fusion that was very different from what we get in the Sun. it was a lot hotter because you didn’t have any metals to help cool these stars.

    Fraser: So let’s talk about one of the lifecycles of one of these stars. So the vast cloud of gas with potentially hundreds of times the mass of the Sun comes together to form a star. What kind of star are we looking at here?

    Pamela: We’re looking at a supergiant. Not too different in some conceptual ideas from the giant stars that we periodically see in star forming regions today. What’s different here is in real star forming regions in the modern universe, you might get two or three huge, ginormous, frighteningly scary-large stars. In the early universe, all the stars were like that.

    Fraser: Right, so one of these stars would be as big as… is there a theoretical limit? Is there a limit to how big a star can be, and why?

    Pamela: There is. The reason that there’s a theoretical limit is because you have to look at the balancing of light pressure and gravitational collapse. Once a star gets too big, it starts generating so much light, heat and pressure in the centre of the star that it starts flinging off the outer layers of its atmosphere faster than that stuff can gravitationally collapse in.

    Fraser: That’s able to actually escape, right?

    Pamela: Yeah, so the gas that’s trying to become part of the star gets blown away – radiated away.

    Fraser: Right, and that mass being blown away might bump into other clouds nearby and cause those to collapse. It’s almost like you could get a whole pile of the largest possible stars everywhere you looked.

    Pamela: Exactly.

    Fraser: That must have just been insane to see.

    Pamela: This lit up our universe. That’s the cool thing about it. Up until that moment, all this gas had been like a fog over the entire universe, where even if you were there with a tiny mag-lite, you couldn’t see anything because all this gas is opaque and there’s nothing creating light.
     
    When these stars turned on, not only did they create light, but the light they created ionized all the gas and made it transparent. It’s like some of those new windows they have that you flip a switch and it changes the qualities of the material making the window so it goes from being completely transparent to completely opaque.

    Fraser: Okay, so these stars were not only ionizing the gas and making it clear, they also had powerful ultraviolet radiation and stellar winds that were blasting huge cavities in this gas and sort of pushing it all away, clearing out space around the stars.

    Pamela: The light itself was changing the properties, the transparency of the gas that wasn’t getting pushed out. Stuff’s getting blown and moved around. These stars are lighting everything up, and the light they’re generating is making the universe transparent all at the same time. It was a tremendously dramatic period in the universe’s evolution.

    Fraser: Have we actually seen this? Or is this just theoretical?

    Pamela: We can see the light of this first generation of stars, but we can’t see the individual stars, which is a bit frustrating. There’s actually a background glow that’s in the infrared that we attribute to the first generation of stars. When we look at certain gravitationally lensed galaxies that are so far back that we couldn’t see them if it wasn’t for the fact that they’re gravitationally lensed, we can start to get hints of this first generation, this first round of star formation taking place.

    Fraser: One thing – you mentioned they’re in infrared. Why are they in infrared? They must be really hot stars pumping out ultraviolet radiation. Why would they be in infrared?

    Pamela: This is the crazy thing. Because of the universe’s expansion, things like the cosmic microwave background weren’t emitted in the microwave – it just arrived in the microwave. Light from most distant galaxies we see in the red, but it didn’t start off there. In a lot of cases it started off in the ultraviolet. The same is true of extremely hot stars. They were extreme ultraviolet emitters, but as their light has travelled through the expanding universe, the wavelengths have gotten expanded, the velocities have Doppler-shifted everything. By the time the light gets to us it’s in the infrared.

    Fraser: These stars must not have lasted long.

    Pamela: Only a few million years. It was a quick birth, a quick death and because of them the next generation of stars that were born were kind of neat. One of the weird things about this first generation of stars is they didn’t create all the elements equally. For instance, they didn’t do a lot of carbon or oxygen creation. What was left behind by this first generation of stars was a lot of iron.

    Fraser: So in the final stages, as they died as supernovae, they went through the same process, fused heavier and heavier elements, hit iron… is that right?

    Pamela: They hit iron and that was the signature they left behind. When we look at the next generation of stars that formed them, we see stars that have extremely low carbon and oxygen abundances compared to their iron abundances. All of their metals are extremely, extremely low.

    Fraser: We’ve got this second generation of stars that formed out of the rubble leftover from that first generation. Once again, it’s kind of astonishing when you think about it. The gas cooled down to the point that they started to gather into stars and after a couple millions of years, that whole time period was over. You had the next generation – the population II stars.

    Pamela: That next generation of stars started forming before the first generation of stars was finished living. So you did have overlap between the generations.

    Fraser: Right, so that first generation is called population III, right?

    Pamela: We do generally refer to it as population III, though there are some sloppy individuals (and I have to admit I’m one of them occasionally) that refers to the most metal-poor stars that we find, the ones that have truly aberrant metallicities, that had to have their parents be that first generation stars, also sometimes get lumped in with population III.

    Fraser: Right. That first generation of stars, some of them are still alive, while others had detonated and that’s where the ones that had already detonated… their material started to form this next generation of stars while the first generation was still around.

    Pamela: Yeah.

    Fraser: Right. The overlap.

    Pamela: We have this fascinating mixture. We’ve actually found two stars that we think belong to this very next generation of stars. They’ve terribly boring names: HE0107-5240 and HE1327-2326. These stars have iron abundances that are 200-300 times smaller than the iron amount found in the Sun, which is pretty spectacular.

    Fraser: Would this second generation of stars have the same problem as that first generation? They’d still be fairly hydrogen-rich, so they would probably want to be fairly large – is that right?

    Pamela: They’re going to have different temperatures as a function of size. They’re always going to run a little bit hot. Take a mass the size of the Sun, and you’re going to get a star a little bit hotter, a little bit bluer than the Sun. you’re still able to start getting the smaller stars though.

    Fraser: Shouldn’t you have the same problem? Shouldn’t we see these stars everywhere, if smaller stars were possible? Since we’ve only found two, they probably didn’t happen so much – or didn’t last. They blew up too.

    Pamela: This is one of the things we just don’t know. We do refer to this as the missing G-dwarf star. We’re missing stars the size of the Sun that should be out there and aren’t. We’re not sure why there aren’t as many as we would expect, and part of this is knowing where they’d be located.
     
    We are still sorting out the problem of how galaxies form. It could be that these stars are simply so far out in the halo that they very rarely get close enough that we can see these little tiny things well enough to get spectra to look for their extremely low mass.

    Fraser: Okay, give me a bit of a contrast. What would a population II star look like compared to a population I star?

    Pamela: Well, at first glance you can’t tell that big a difference. They do have slightly different colours, as a function of their mass. But then you have to build and measure their mass. So if you’re simply looking at two stars on the sky and all you have is a picture, you can’t tell the difference.
     
    Where you have to start looking for the difference is we take what are called spectra that spread the light out into a very detailed rainbow. This allows us to look for gaps in the rainbow that are caused by absorption of certain atoms of certain colours of light. Different atoms have specific fingerprints on what colours of light they absorb and emit.
     
    We can determine what chemicals make up a star based on their intricate fingerprints. This is a very precise science, but it requires huge telescopes and a lot of time on those huge telescopes. If you’re trying to pore through thousands of stars, that can be a lifetime’s worth of work sometimes. So we’re working on poring through the stars in the outer parts of the Milky Way, looking to see if we can find these extremely metal-poor stars.
     
    We do look for hints in the colour: there are different fingerprints where if you use a filter when you look in just the red light, it might have a bunch of lines in the red light that caused the red light to be a little bit lower than the blue light when you also compare the green light. So you start using multiple filters and sorting things out and you can start to make guesses.

    Fraser: Can you look at a galaxy as a whole and then say that whole galaxy seems fairly metal-poor?

    Pamela: Not so much. There’s so many things involved in the light from galaxies that you can’t just get the metallicity of a galaxy.

    Fraser: Right.

    Pamela: You have light coming from nebulae, individual stars, random high-energy events, all of these different sources make it hard to get a big picture until you start to take detailed spectra. Then we’re just basically looking at fingerprints and saying that’s the fingerprint of carbon or iron and looking to see how they’re layered on top of each other, which one’s stronger, which one’s weaker.

    Fraser: Okay. So we’ve got this second population of stars. Was that one quick round, or would stars stay in that population II through successive generations?

    Pamela: There were successive generations of population II stars. It’s actually kind of fuzzy to define what makes a population II star and what makes a population I star.

    Fraser: That’s just all astronomy, right?
     
    [laughter]
     
    What’s a planet?

    Pamela: So now we start getting into much fuzzier definitions. With population III, at its core, it’s the first generation of stars. Population II stars, these are made out of leftover materials, recycled materials. You start to see the abundances changing. The amount of carbon related to iron starts changing.
     
    We also look at where they’re located. Part of the definition of a population II star is they’re in the halo of the Milky Way. they’re in globular clusters. They’re in places other than where our Sun is. So you also get a kinematical definition, but how do you apply that when you’re looking at a small galaxy that’s spherical with all the stars mixed together?
     
    So we do have chemical definitions still. In general, the breaking point is when you start to get objects that are two or three times less metal-rich than the Sun. Mileage may vary on that definition. You start to get into some fuzziness as you get more and more metal-rich – is it population II or population I? Different people will give you different answers.

    Fraser: Right, okay. Let’s say these things have died, some exploded as supernova, some are still going, but the ones that exploded as supernova spread their elements out. When do we hit population I then?

    Pamela: We have different ways of making the definition. You start getting population I when you get the disk of the galaxy starting to form. The stars in the disk of the Milky Way – these are what we call population I stars. The stars that are like our Sun in some ways.
     
    You also start to get population I when you start getting metallicities – when you start getting carbon, oxygen, iron contents that are within a factor of two or three of the Sun.
     
    These are the stars that can start forming planets. That’s not part of the official definition, but that’s what we’re finding. When you look at population I stars, they can have planets. We aren’t finding any planets around population II stars.

    Fraser: Hmm. Why not?

    Pamela: Well, it’s these metals, these carbon atoms and all these atoms that are heavier than helium. They start to chunk up and form planets. You need these extra elements, these heavier objects that hang out, outside of the solar nebula’s inner glowing, fusing part that becomes the proto-star, to start forming planets. Without the heavier objects, you don’t generally get hydrogen-pure versions of Jupiter. You need to have other things in it. Planets are formed in that third magical generation.

    Fraser: Right. That’s where we are, right? The Sun is of that group.

    Pamela: The Sun is one of the most metal-rich stars out there. We go from stars in the population II in these two most metal-poor objects we know of, with 200-300 thousand times less metal than the Sun to those in the population II that are 10 times less metal-rich than the Sun. The most metal-rich things we know of are only three and a half times more metal-rich than the Sun. So you go from 300 thousand times less to three and a half more, and not more than that.

    Fraser: I wonder what impact having heavy metals in the solar nebula had as an influence on the planets and life. I wonder where – we might not even be able to get terrestrial planets.

    Pamela: This is what we’re trying to figure out. This is something we have to answer observationally, and where future programs like Darwin and the Terrestrial Planet Finder start to become so important. We can look out and say, “that star has this amount of iron/carbon/oxygen and it has a rocky planet. This star that has a little bit less has no planets at all.�?

    Fraser: Right. We can’t see rocky planets apart from going around pulsars.

    Pamela: Right.

    Fraser: So it’s really hard to do. But we’re close – there’s Darwin, potentially the Terrestrial Planet Finder and I know there’s a whole series of ground-based techniques that people are starting to develop: some super-large telescopes that we talked about, telescopes in Antarctica.
     
    We should, within the next decade, start finding these terrestrial-sized planets, and I guess we can map them together. We might say that if a star has exactly this much metal or more, it will have terrestrial planets, and if not then it won’t. then we could just stop looking for planets around certain kinds of stars.

    Pamela: This is one of the most interesting but also one of the hardest things to deal with. We live right in the centre of the metal-rich area of the galaxy. It’s easiest for us to look for planets around nearby stars. All the nearby stars unfortunately are just like us.
     
    So when we start asking the question, “how different can a star be and still form planets?�? now we have to start looking at things that are uncomfortably far away. It’s going to take more technology. There are some people who have already done some pretty good work ruling out certain places.
     
    The Hubble Space Telescope did a long campaign looking at the globular cluster M15 (I think). In looking at it, they’re taking round after round after round of images. If there had been any planets transiting stars passing in front and causing the light to get a little bit fainter, they would’ve seen it. There was no evidence in their work that showed transits. They found some variable stars, lots of binary stars, but they didn’t find transiting planets.

    Fraser: So they’re finding to put some limits on this.

    Pamela: Yeah. Scott Gaudy’s also done a lot of work on this: looking at different clusters of stars and seeing what he could rule out with his team of observers. Every time these people have looked at large populations that have lower metallicities, no planets have been found.

    Fraser: Is there a limit to how much metal a star could have and still be a star?

    Pamela: This is another question. We’re still trying to figure out how to answer. We don’t have any to observe.
     
    As you add more and more metals, it allows the star to radiate away its energy much more efficiently. It cools the star down and makes it harder for it to build up temperatures in the centre. It could be that there’s a limit eventually, but in the interim you can just build the star a little bit bigger and the temperature will get hotter as you end up with more mass.
     
    It’s this balancing line between effectively having smaller stars with some (but not a lot of) metals and are able to effectively cool but still be hot enough to burn. Then you start cooling too efficiently, so you need to build the star up a little bit more.

    Fraser: I want to go back to the beginning for a second. With the James Webb Telescope, which is going to be a much more powerful version of Hubble, but specifically designed to look at the infrared… it should be able to look back at some of those most distant stars and galaxies. Is it going to be able to sense the population III stars?

    Pamela: Only in terms of their aggregate properties. They’re so far away that you’ll never be able to make out individual stars. Not with James Webb.

    Fraser: Right, they’re going to be 13.5 billion light years away.

    Pamela: Right. We struggle to see individual stars in our local universe. It will be able to look and see this little tiny blob of light is a galaxy that’s light-output can only be described by these giant stars that we can’t see individually. We’re going to get there, we just aren’t going to be able to get all the way there. That’s always frustrating, but you can figure out the shape of an elephant if you have enough blind men observing it and they talk to one another effectively. Astronomers are pretty good at talking to one another.

    Fraser: Right. Good. I think that’s it. Now, hopefully, everyone can really understand the successive generations that stars went through, from the big bang to today. When we talk about metallicity, metal-poor, metal absorption, and some of the requirements for being able to form planets, hopefully everyone will have a much better understanding of that.
     
    I know it comes up a lot in a lot of the articles I do on Universe Today. It’s one of those things I think you kind of take as a given that either you’re not even going to talk about it or you assume that people know it. I think it’s good to go into that level of detail. Thanks Pamela.

    This transcript is not an exact match to the audio file. It has been edited for clarity.

    ]]>     http://www.astronomycast.com/2008/02/ep-75-stellar-populations/feed/ 2 After the big bang, all we had was hydrogen, a little bit of helium, and a few other trace elements. Today, we’ve a whole periodic table of elements to enjoy, from oxygen we breathe to the aluminium cans we drink from to the uranium that powers some ... After the big bang, all we had was hydrogen, a little bit of helium, and a few other trace elements. Today, we’ve a whole periodic table of elements to enjoy, from oxygen we breathe to the aluminium cans we drink from to the uranium that powers some people’s homes. How did we get from plain old hydrogen to our current diversity? It came from stars, in fact successive generations of stars. Episode 75: Stellar Populations (14.4MB) Astronomy Cast no