November 26th: Encore: Questions of a Stellar Nature

By on November 26, 2012 in

Date: November 26th, 2012

Title: Encore: Questions of a Stellar Nature

Podcaster: Peter Clark, Robert Simpson and Hannah Hutchins

Organization: The Witty Astronomers; the Star Sailor podcast –

This podcast orginally aired on November 29th, 2010

Description: Hannah Hutchins and Peter Clark interview astrophysicist Robert Simpson of the Zooniverse project on the latest advances in the knowledge of stars, from the formation of stars and stellar mass limits to star death.

Bios: Hannah Hutchins is a fifteen year old teenager in England who first discovered her passion for astronomy – especially astrophysics – when joining the citizen science project, Galaxy Zoo. She writes for the Galaxy Zoo blog (, the Witty Astronomers and the Young Astronomers (

Peter Clark is sixteen years old and lives in (light-polluted) Northern Ireland. He’s always had an interest in astronomy; especially the astrophysics side. He writes for the Witty Astronomers and the Young Astronomers.

Today’s Sponsor: “This episode of 365 days of astronomy was sponsored by Clear Skies Observing Guides, a Modern Day Celestial Handbook. ..Clear skies observing guides, or CSOG, is a new concept in visual amateur astronomy. The observing guides contain thousands of objects to observe through amateur telescopes, with matching tours for GOTO telescopes and matching AstroPlanner plan-files. CSOG allows you to target deep-sky objects and carbon stars you never observed before, night after night. Wishing astronomers around the world: Clear skies..! ”


[Peter] Hello and welcome to the 365 Days of Astronomy Star Sailor podcast ‘Questions of a Stellar Nature’ with Peter Clark, Hannah Hutchins and Doctor Robert Simpson of the University of Oxford. Hello and welcome!
[Robert] Hi, thanks!
[Peter] We’re just going to get stuck right in with the questions, so Hannah:
[Hannah] So what’s your favourite area in astrophysics and why?
[Robert] I ought to say that my favourite area of astrophysics is the one I studied in or schooled in if you like, so I ought to say it’s star formation but it is and it isn’t. I mean for most of us we fall into these things because we like them, and originally I was really into astrophysics because of things like Star Trek and I always thought time travel was cool (and I still do), and I always read papers about worm holes even though I shouldn’t. And so really star formation is the one I know the most about and it’s a very important and interesting area of astrophysics, but I also love cosmology for answering the really big questions, and I love the kind of really fringe stuff like time travel and worm holes because it really delves into areas that science fiction only dares touch. And of course I think like a lot of other people I love planets so solar system astronomy and astrophysics, and those beautiful pictures of the planets, the moons and understanding how we all formed and all these landscapes around us as well as the universe itself. There’s probably a lot I like about astrophysics so I should stop rambling and let you ask me the more important questions, but I like a lot about astrophysics so that would be the short answer.
[Peter] I am sure everyone listening would whole heartedly agree with you, so moving on to the main topic of the podcast and that’s stars:

Recently the ESO discovered what can only be described as a monster star located within the Tarantula Nebula. R136a1 has been measured to weigh in at a little under 300 solar masses with an estimated birth mass of 320 solar masses, which is more than twice the accepted mass limit of a star (150 solar masses). Anything more than 150 solar masses was expected to be unable to reach hydrostatic equilibrium and blow itself apart as it would be unable to balance its outward radiation pressure with its gravity, and so exceed its Eddington luminosity. Have we discovered something that’s a freak event or are we starting to get below the surface of how high mass stars work?
[Robert] Well we certainly haven’t really scratched the surface on massive star formation; this is a big problem area in lots of ways. The problem is that they’re not very common, and the reason we thought the largest stars would be about 150 solar masses was simply based on looking and going out there looking at big clusters of stars, and this was a paper back in the early part of the last decade basically empirically saying we can’t see stars that are bigger than 150 solar masses. And in fact we couldn’t see them at the time – it was measured to be 120. Up from there we thought well let’s say it’s 150 then. It’s not the most concrete number. The Eddington limit which you mentioned is to do with so for any given star it would have an Eddington limit, so the biggest stars themselves would have very large eddington limits, but the eddington limit (or eddington luminosity ) refers to the radiation that would be required to equal the gravitational energy of a star, so a very very big star would have a very very big eddington luminosity and even the most massive stars we’ve seen seem to be only approaching a 50 percent eddington luminosity if you like, meaning that the radiation pressure pouring out from those stars is only about half the gravitational energy, so there’s still a way to go in terms of their radiation, so perhaps they could get bigger.
This 300 solar mass star certainly does seem to be that big so what it does is it blows out the water, as you say, the 150 solar mass limit, but that limit being based on observation anyway so now we have a new upper limit. There are people that suggest that it was a binary system and we can’t resolve them as independent masses so therefore you’d have two 150 solar mass stars, but that seems unlikely based on measurements of the Doppler shift. If there really where two 150 solar mass stars orbiting each other we ought to be able to detect that in their spectra. But the measurement is pretty robust and certainly the way the technique for making that measurement stands up to the test of other measures of mass that we have, and so it really does look like the 150 solar limit is done and we may be moving on to 300 for now until of course we find the next one because theoretically people have put forward models that suggest we can have anything up to 400 – 450 solar masses.
[Peter] Wow!

[Robert] Yeah, we could have really big stars but of course the rarer they get the harder it is to spot them, certainly spot them nearby, which is where we’d need to see them to measure all this stuff, so it’s a tricky business and hopefully we’ll be finding them because the new technology we have and infrared data we now have lets us do a few things we couldn’t do before and one of those is looking at massive star formation in a great new way.
[Hannah] There’s a proto star that’s been discovered called IRAS134816124 that is still surrounded by the cloud that it formed from. And it’s been confirmed to weigh in at 20 solar masses, and this is particularly interesting as it goes a good way to showing that massive stars form via accretion rather than stellar mergers. Do you now think that we have discovered the secrets to massive star formation or could a spanner be thrown into the works?
[Robert] Yeah, this relates a lot to the last problem which is that these very massive stars are hard to come by because there aren’t as many of them. And infrared measurements that we have from the Very Large Telescope from ESO and a couple of other instruments that are out there at the moment and coming along soon, they all mean that we can look at these things in better detail than we could before and that’s because these things are often embedded within big dusty nebulae and molecular clouds and that makes them hard to see with conventional telescopes that we’ve had up till now.
This very very big one with the accretion disk as you say is notable because it’s the first time we’ve been able to say absolutely that this massive star, this very large star, I think it’s 20 solar masses, that it’s got an accretion disk around it. Now the problem was it was thought that with the lower mass stars it was easy to have accretion; material can fall onto the star through gravity and sort of swirl on and angular momentum gets delivered. But with the really big ones the radiation pressure, the energy pouring out through these new stars, ought to be powerful enough to stop things accreting onto the star. So we need to come up with ways to model that to show that it might be possible, and this observation in the near infrared it’s an Interferometric observation, meaning it’s using more than one dish and putting them together to create an even better picture, this shows that it’s definitely possible because it’s happening. And this is great for science when things like this happen because it’s always good to just totally challenge the conventional thinking which had been that perhaps large stars form by other stars coalescing together. The problem with that though anyway was I don’t think I was ever terribly comfortable with that idea, I mean if you look at the 300 solar mass one we mentioned earlier how many 100 solar mass stars that we see around could have formed to make that one? So it only works for stars that are medium sized, once you get really big you still need a way to create these things.
[Hannah] So have we now discovered the secrets of high mass star formation?
[Robert] Oh no absolutely not! I don’t think we’ve discovered the secrets of massive star formation at all, I think what’s great is to uncover a new piece of evidence that gives us a new line in enquiry so ok massive stars clearly can form through accretion; lets figure out how that works. And is that how they all form? Don’t know! But certainly that’s certainly how some of them form because we can see one. So yeah, spanner in the works? Definitely! More spanners in the works? Why not! That’s what science is all about right?
[Peter] Thank you for listening to the 365 Days of Astronomy Star Sailor podcast ‘Questions of a Stellar Nature’ with Peter Clark, Hannah Hutchins and Doctor Robert Simpson of Oxford University. Thank you!

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.

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  1. November 26th: Encore: Questions of a Stellar Nature « Northern Lights Expeditions - November 26, 2012

    […] [Robert] I ought to say that my favourite area of astrophysics is the one I studied in or Source: 365 Days of Astronomy Posted in Northern Lights […]