The first stars coalesced from the primordial hydrogen and helium synthesised in the first few minutes after the birth of our Universe just a few hundred million years after the Universe burst into existence.
All stars are born when a cool clump of hydrogen and helium gas condenses under its own gravity. As it does so the central regions of the clump heat up eventually to the millions of Kelvin required for nuclear fusion to occur. Once this critical point is reached the star fuses hydrogen to helium and begins to shine brightly. Stars grow by absorbing more material onto their surfaces from the surrounding nebula and hence they grow in mass. Eventually their increased energy output disperses the surrounding cocoon to the point where the star’s gravity is insufficient to attract anymore material and the star ceases growing.
In the current era of the universe this process is aided by heavier elements such as carbon and oxygen, these aid the cooling of the surrounding gas which allows for it to fall onto the star – if the gas was too hot it would have too much energy and escape off into space. The first generation of stars did not have any heavy elements to aid their formation as they had yet to be produced as such elements are only produced in the final stages of a massive star’s life. Thus they would have had a much shorter time to absorb matter from their surroundings before it escaped their clutches.
This may suggest that the first stars should be much smaller than those currently populating the Universe, though this is not the case. As the density of material in the early Universe would have been much higher they would have more material with their grasp to begin with and so all predictions produce stars that are much more massive than the current average – which is only a fraction of the mass of Sol.
It had previously been though that the first generation of stars would have been the most massive of all, with material equating to several hundred solar masses, similar to the most massive stars currently known – such as those within the Large Magelanic Cloud’s R136 open cluster. This new study casts doubt on this view on the first stellar newborns.
Simulations conducted at NASA’s Jet Propulsion Laboratory in California by a team of researchers lead by Takashi Hosokawa have indicated that such first generation stars would be ‘only’ a few tens of times a massive as Sol.
Whilst they are still significantly more massive than the vast majority of stars in today’s Universe this discovery removes a significant problem for Cosmology.
If the first generation of stars were as massive as previously predicted they would have produced a particular chemical signature of elements during their lives that would be detectably different from those of conventional mass stars – the higher mass equates to a higher core temperature – combine this with the unique blend of elements available to produce the first stars (~75 Hydrogen, ~25% Helium with tiny traces of Lithium, Deuterium and Beryllium) there should be a ‘bar code’ of the particular abundances of various heavy elements (as fusion reaction rates are easily altered by starting conditions) in the oldest of today’s stars that could only be attributed to the existence of such monstrous precursor stars. As of yet this has not been detected.
Whilst this proves to be a problem for the old model of 1st generation stars, the revised masses produced by this study removes it all together – with no such high mass stars the elemental fingerprint would have never have existed easily explaining the lack of detection – there is nothing to detect!
This fingerprint of elements would have been generated by the stars ending their lives as a yet undetected variety of Supernova. The new mass predictions would cause the 1st stars to detonate in a manner akin to their more recent brethren, allowing for the lack of unusual supernova detections to be explained satisfactorily as well.
As we have seen the masses of the 1st stars are now thought to be much lower than first expected but why would they be less massive than previously predicted?
The answer lies in the surroundings of the stars. The new simulations indicate that the region directly around a forming first generation star was heated to temperatures of up to 50,000 Kelvin or 8 ½ times the surface temperature of Sol. This extremely high temperature would have allowed the surrounding gas to disperse much more quickly than previously thought, thus the growing star has less time to absorb material leading to a lower final mass.
The simulation produced a star of just 43 solar masses, a far call from the 1000 solar mass superstars of early predictions (though I should add as predictions became more advanced the mass of the 1st generational stars has been falling continually and this is the latest study in a long line to reduce the value further).
You can read more here
This fantastic image has been created using X-ray data from the Chandra observatory (shown in shades of purple) with infra-red data from the Spitzer observatory (with varying wavelengths shown in red, green and blue).
It shows the star forming region NGC 281 focusing on the open star cluster IC 1590 affectionately called the Pacman Nebula by observers.
The nebula is located just 9200 light years from Earth it provides an important nearby laboratory for studies into high mass stars – those stars that are at least eight times the mass of the Sun.
As well as its proximity, its location nearly 1000 light years above the galactic plane from our view allows us to gaze at it almost uninterrupted and achieve precise measurements of what lies within.
Amateur astronomers with a good sized telescope and fortunate enough to have clear, dark skies.
After many delays the long awaited second part of our interview with Dr. Robert Simpson.
A transcript is being produced and will be uploaded shortly.
Before I go, If you have any questions for Hannah or myself you can get them too us via our emails or via our new formspring accounts: -
Hannah’s - http://www.formspring.me/Stellar190
Peter’s - http://www.formspring.me/Lightbulb500
Until next time
As promised (if slightly delayed), here is the ‘video’ version of our recent contribution to 365 Days of Astronomy.
Hannah and I would like to thank both Dr. Robert Simpson for his time, and 365 for the opportunity to get the Star Sailor podcast out to more people.
As indicated by the title a rather large section (actually the majority), of the podcast is yet unreleased due to time constraints on 365 podcasts. rest assured however that the remaining 12 minutes of the interview will be released soon, hopefully before the end of the year
Thanks for listening and reading.
365 Days of Astronomy - http://365daysofastronomy.org/
SDSS Image: – Credit SDSS
ESO Images: – Credit ESO
Edit: – Now with Transcript
[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.
[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!
Until next time
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