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.
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Birth!: Nebulae that create stars
Death! Nebulae that destroy stars
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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.
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 theInstance 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– 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
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 hottype 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.
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 theI 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. 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 differentof 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.