365DaysDate: February 12, 2009


Title: Eta Carinae and Supernova Imposters

Podcaster: Michael Koppelman

Organization: Slacker Astronomy

Description: Michael from Slacker Astronomy interviews Dr. Roberta Humphreys and Dr. Kris Davidson about one of the most interesting stars in the galaxy, Eta Carinae, a super massive star which became a “supernova imposter” in 1890 and may become an actual supernova in the astronomical near future.

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.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored in celebration of the 200th Birthday of Charles Darwin.  His observations about nature indelibly affected our understanding of how living things on Earth are related.


Michael: In our last episode on January 12, 2009 we talked to Roberta Humphreys and Dr. Kris Davidson about massive stars.  They are also experts on Eta Carinae, a massive star in the constellation Carina which is a likely supernova candidate sometime in the astronomical near future.

Our conversation last time concluded with a discussion on supernova impostors and we pick up the discussion there.  It sounds like we’re saying that with a massive star they end catastrophically in general, right?

Dr. Kris Davidson: Those are supernovae, yes.

Michael: Yes, these are supernovae.  Sometimes we get things that look like supernovae that are not and then we have the star Eta Carinae which has been enigmatic over the last century.  Tell us a little about that.

Dr. Davidson: It’s just the most famous because it happened only 7500 light years away from us.  We have no right to have such a rare object that close.  When astronomers first began to look at stars in the southern hemisphere about three hundred years ago, they put this object down on the list of stars – Eta Carinae, just a star in a southern constellation.

In the 1840s it blew up and for about ten years it was one of the brightest stars in the sky.  For a few months in 1843 it was the second brightest star in the sky.  But it’s almost a thousand times farther away than the brightest star, Sirius which means the amount of light coming out was really prodigious.

After a while it faded and astronomers began to forget about it. But now we see a nebula blowing out, expanding out around it which is clearly the remnant of the stuff shot out by that explosion, the ejecta I mean.  Those ejecta we can weigh now by various tricky methods.

It looks like it is about ten times the mass of the sun. I don’t mean the star. I mean the material that the star lost in just a few years back, 160 years ago.  The brightness of that thing got confused for awhile with real supernovae.

One famous astronomer classified it as an unusual type of supernova in the historical records.  But, supernovae don’t leave survivors and this one left survivors.

Michael: Do supernovae ever fail like they start and then stop or his this a whole different mechanism?

Dr. Davidson: It looks lately as though there are some massive stars that start to become supernovae and just what you said they fail.  It’s a dud. Then they become real supernovae in ten, a hundred or a thousand years later.

Some people suspect that might be what Eta Carinae is.  I doubt it myself but if it is true then it will become the real supernova sometime in the foreseeable future

Michael: Massive stars which I think many of our listeners know don’t last very long, compared to a lower mass star and I was just sort of curious, are there many massive stars?

You mentioned Eta Car as being sort of a very nearby one.  Are there other nearby massive stars?  Do we see a lot of massive stars in the galaxy or are their life spans so short that we’re lucky to catch them?

Dr. Davidson: They are very rare.  One of the reasons is that they don’t form very often.  It’s kind of like taking a census of people restricted to those more than 6 feet 8 inches tall and in the worst cases 7 feet 6 inches tall.  There are such people but you don’t see them every day just because they are so rare.

The next thing is that the lifetimes of all these very massive stars are around three million years compared to ten billion for the sun.  That makes them even rarer because they go through their lifetimes, blow up and disappear much, much quicker even than an ordinary star that makes an ordinary supernova.

I said earlier, we’re lucky to have a thing like Eta Carinae so close.  In the total list of supernova impostors there have been what, a few dozen candidates?  Just a few really good ones, in the lists of stars above 80 times the mass of the sun, it’s hard to keep up with but I think there are only six or twelve or so that we’re sure about.

Michael: Galactic, you mean?

Dr. Davidson: Mostly but some are in other nearby galaxies.

Dr. Roberta Humphreys: A lot of people focus on these extremely massive stars.  Stars like Eta Car which probably started out about 150 solar masses and maybe even more.  But a number of other very massive stars, not extreme but some say stars, 30 or 40 or 50 solar masses also have periods of high mass loss.

Eruptions, they’re not as dramatic perhaps as Eta Car but as a result of these episodes of high mass loss, they also produce extensive and complex nebulosity around them.  What we’re seeing is this phenomenon of high mass loss episodes is now being among stars almost across the H-R diagram – for massive stars that is.

Even other evolved massive stars – by that I mean post main sequence, post hydrogen burning massive stars seem to have these episodes as well.  It’s interesting now to kind of speculate how frequently they have them. That’s something we still don’t know and if there might be some other restrictions on which stars will do it and which ones don’t.

Dr. Davidson: Or do they even do it more than once?  We don’t know.

Dr. Humphreys: I’d like to mention that some of these stars and Eta Car itself is a naked eye object.  It’s a fifth magnitude star.  Another one that is well observed by amateur astronomers and quite famous is VY Canus Majorus. 6:38

It is no longer a naked eye star.  It was about 150 years ago but it is a very famous large red super giant.  It is particularly interesting to amateurs because it has a very visible red reflection around it.  It’s about ten arc seconds across so you can even see it with a small telescope.

Michael: Cool, I think I’ll look that one up.

Dr. Davidson: In fact in that connection, trends and fads affect professional astronomy as well.  I know some pretty famous objects in astronomy like the crab nebulae and Eta Carina that have sometimes gone for years without professionals remembering to monitor them.

The big difference with Eta Carina is that when you’re not watching it often does something unpleasant.  It completely changed its state back between 1940 and 1950 and we have practically no observations then because astronomers, professionals had lost interest.

More recently it has been getting brighter lately, faster than it was doing before and a critical part of that story is a period in the 1990s and recently has actually been covered by amateur astronomers, or non-professional astronomers with considerably more frequency than the professionals have done.

It’s just that the professionals have so many other things on their plates and often they concentrate on one object for a few years and then forget about it and go to something completely different.

The records are spotty for many famous objects.  The crab nebulae may not be there tonight for all I know [Laughter] because we don’t get reports on this once a month.

Michael: Maybe we’ll end with the end of Eta Car because it is one of the more likely supernova candidates in terms of our galaxy and nearby and I believe it would be visible during the day, right?

I’ll post a picture of Eta Car if you haven’t seen it.  It’s a pretty famous object.  It looks sort of like it is kind of pointed at us and I know people often ask if that thing were to go supernova or do a gamma ray burst or something is there any danger here on Earth?

Dr. Davidson: Gamma ray bursts tend to go along the axes of very massive supernovae.  Eta Carina is about the only star we know where we know which direction the axis points. As you’ll see when you look at a picture of the homunculus you’ll see it right there.  The homunculus is that nebulae that shot out.

We are not in the path but I did some geometry calculations a few years ago. Its polar axis points right down a galactic spiral arm.  So when it becomes supernova, if it makes a gamma ray burst, that will be bad news for literally thousands of planets along that beam because it’s a crowded part of the galaxy.

What’s more, it won’t be a normal supernova.  They saw the brightest supernova on record a couple years ago it has a name: Supernova 2006GY.  It wasn’t just brighter than the runner up.  It was something like fifty times brighter.  It was a lot brighter, it was a monster.

The only interpretation that people have had is that it was a very massive star like Eta Carinae.  So if it became a supernova, yes you’d see it by broad daylight easily.  You’d have to go south of Texas to see it.

Michael: That concludes our interview with Dr. Roberta Humphreys and Dr. Kris Davidson from the University of Minnesota.  I want to thank them for taking the time to talk to me.  For more information on Eta Carinae, check out  The 365 Days of Astronomy continues.

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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365 Days of Astronomy
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