Date:  January 12, 2009

Title:  Massive Stars:  with Dr. Roberta Humphreys and Dr. Kris Davidson

Podcaster:  Slacker Astronomy

Description:   Michael from Slacker Astronomy interviews Dr. Roberta Humphreys and Dr. Kris Davidson from the University of Minnesota about the most massive stars in the universe.

Links:  Slacker Astronomy

Bio:   Slacker Astronomy is a light-hearted podcast about the astronomical road-less-traveled hosted by Michael Koppelman and Doug Welch, with frequent contributions by Aaron Price and Travis Searle.

Sponsor:  No one.  Please consider sponsoring an episode of 365 Days of Astronomy by clicking on the “Donate” button on the lower left.

Transcript:

Michael Koppelman: Hello, I’m at the University of Minnesota. This is Michael from Slacker Astronomy and I’m here with Dr. Roberta Humphreys and Dr. Kris Davidson, professional astronomers here at the University of Minnesota and experts on massive stars.

We’re going to talk a little bit about massive stars today. To the uninitiated, the star they’re most familiar with is the sun obviously. What is a massive star and how could you relate it to what we know about our own sun?

Dr. Kris Davidson: It depends on whether you mean massive or very massive. Massive stars behave quite differently from the sun. Their nuclear reactions are different. They produce a lot more radiation from their sizes, they’re ultra-violet, and a lot of things like that. But when you get up to the top of the scale, a hundred times the mass of the sun and that is about the top, then radiation pressure begins to dominate instead of ordinary gas pressure.

We’re all familiar with gas pressure, but radiation pressure so far as I know there isn’t anywhere on Earth where it’s strong enough to feel. In the sun the pressure of radiation is something like 140 thousandth of the total pressure holding the sun up. In these stars, they’re basically the most massive stars, are basically held up by the pressure of the photons of light. That makes a big dynamical difference because it turns out that a star supported totally, absolutely by radiation pressure would not be stable.

It would either collapse or come apart and discombobulate just because of some dynamical theorems. So the behavior is very different from the sun. These things are not very stable. They don’t hold themselves together with much reliability. They show all kinds of effects that theorists haven’t predicted.

Dr. Roberta Humphreys: They’re also very different from stars like the sun or what we sometimes call low-mass stars in terms of their evolution and interior structure. A star like the sun is going to eventually become what we call a red giant. When it does that it forms a white dwarf at the center. That is an object in the state of electron degeneracy.

Eventually a star like the sun will shed its outer envelope as a red giant or sometimes as a second-stage red giant, becoming what we sometimes call planetary nebula, eventually just ending its life as a white dwarf. Now, the mass range of stars that do that or the upper limit of the mass range of stars that do that is a little bit uncertain but most of the time people put it at three to five solar masses, somewhere in that range.

Stars with initial masses above about nine or ten have a very different interior structure and a different evolution. They do not form electron-degenerate cores. In other words, they don’t become white dwarfs at the center. They will evolve into a red super giant stage where they will initiate successive stages of nuclear burning. From hydrogen to helium it goes helium to carbon and on up the table of elements to producing a core of iron.

After that, as many people probably already know, the star is going to go super nova. But what we’ve been learning the last 20 years or so, ever since super nova 1987a, is that we don’t know exactly what part of the famous HR diagram that star is going to explode in. It may explode as a red super giant or like 1987a explode as a blue super giant.

Now that’s probably the state for any star with initial mass above about ten solar masses. But as Kris said, when you get upwards of 60 to 80 solar masses the story gets much different and also much more complicated.

We’re really not entirely sure how those stars end their lives, ultimately as a super nova no doubt. But maybe they might have some very violent eruptions even before they become super novae.

Michael: So, it sounds like maybe three to ten solar masses you might call a massive star and then if we get up to a hundred solar masses it’s kind of really the upper limit of the very massive stars?

Dr. Davidson: Right, there aren’t any clear definitions here because a few people talk about super massive stars which mean 200-400. Some other people say that the first stars ever formed had a really big proportion or percentage of these extremely massive objects compared with what we see today.

Michael: When you say massive people tend to think big and I think with stars we really mean more of weight than size, but how big would a ten solar mass or a hundred solar mass star be compared to the sun in terms of its physical size?

Dr. Davidson: It depends when you catch it in its evolution. When it starts out it is about 50 times I suppose. I think I’m talking about a 100 solar mass star right now. I think maybe 50 times as big as the sun when it is young and then it grows.

But it doesn’t grow very fast and there is some kind of instability that no one understands that prevents the most massive stars from ever becoming gargantuan red super giants. If these things became red super giants they’d be as big as the orbit of Saturn or Uranus, but they can’t manage that and we don’t understand why not.

Dr. Humphreys: Well the largest stars that are in terms of size, are indeed the red super giants and the largest ones we know of actually do reach sizes approaching that of the size of Saturn’s orbit. But, those are red super giants.

What Kris is referring to is some of the very most massive stars actually stay blue the whole time. Therefore even Stars like Eta Carinae with an initial mass maybe of around 150 solar masses might only be about half an au to one au in size but that’s because it’s hot.

Dr. Davidson: You can read in books about how a star in a textbook way is supposed to start out its life fairly blue and evolve by growing. As it grows the light comes out at longer wavelengths and it gets redder and redder and redder and gets bigger and bigger and bigger and that’s the red super giant gig.

But the kind I was just talking about as I emphasized we do not understand why but at a certain stage before it becomes red, these most massive stars begin to lose their mass in big outbursts. They are unstable, they just can’t do it and so they reach a kind of a limit and bounce off that limit in size and color.

Michael: Is that the Humphreys-Davidson limit? [Laughter]

Dr. Davidson: Afraid so.

Dr. Humphreys: It’s the observational of the limit. [Laughter]

Michael: I wanted to ask you about that because it’s rare that people get things named after them in astronomy and you guys have this limit named after you. Can you again explain what that is the limit of?

Dr. Humphreys: The way I always explain is it is basically an observational upper limit, an empirical upper limit. It’s basically above that limit or to the right of this line in the HR diagram we find no more stars. I think of it as a fundamental upper limit to the mass or initial mass of the star that can become a red super giant.

Basically it tells us above a certain mass and that mass kind of depends on whose models or theory you’re using, but it’s somewhere around 40-50 solar masses a star does not become a red super giant. That’s how I think of this empirical upper limit.

So what happens with the stars above the 40-50 solar masses? Well, I think this is why we got strongly identified with this empirical limit is because when we first noticed it, we suggested the reason they don’t become red super giants is because they undergo a major instability in which they lose a large amount of mass and that these high mass loss episodes were sporadic.

It is not continuous mass loss but sporadic episodes of high mass loss that prevented their further evolution to what we call the red or cooler temperatures. In other words they’d all become red super giants. This high mass loss event, or more than one, keeps them relatively hot.

Dr. Davidson: We use the word eruptions a lot. Just like a geyser or volcano and it means pretty much what it sounds like. Instead of gushing out of the surface, you just get a blast of material coming off the star but the star survives.

Dr. Humphreys: A lot of these types of objects, stars undergoing some kind of eruption are now being found, particularly in the super novae surveys. A lot of those are in progress basically covering the entire sky. So a few every year of these so-called super novae, objects that get a super nova designation, turn out on closer investigation not to have been a true super nova. Either a bit under luminous or people get a spectrum and they watch the spectrum and they say, “Hey, this isn’t behaving like a true super nova.”

Sometimes they’re called Eta Car analogs or Eta Carinae-like variables because they remind people of Eta Carinae. If Eta Car had its great eruption today, not here in our galaxy but let’s say in a fairly nearby galaxy and we witnessed it, what do you think we’d call it? We’d call it a super nova. And then a few decades later we discover the star was still there and it would become what we now call a super nova impostor.

Dr. Davidson: By the way, they’re more mysterious than the super novae. We know what makes a super nova explode. There’s more than one kind of explosion and we know what they are. These impostor events though are simply not understood. There’s no good theory for them.

Michael: We’re at the automated plate scanner lab here at the University of Minnesota talking with Kris Davidson and Roberta Humphreys. Thanks a lot guys.

This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Cindy Leonard.

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