Archive for radiation pressure

Off the Main Sequence…

Posted in Biographical, The Universe and Stuff with tags , , , , , , , on July 22, 2010 by telescoper

When I was at School, one of my English teachers enjoyed setting creative writing challenges for homework. One of the things he liked to do was to give us two apparently separate topics and get us to write a short story that managed to tie them together. Although I seldom got good marks I now realise that this is quite a useful skill to develop.  Sometimes, when I’ve been at a loss for something  to blog about, I’ve taken two items from the news and tried to link them somehow. That’s also how a lot of satire works – many of the best Private Eye skits involve putting two pieces of news together in a way that’s deliberately back to front. In fact many writers have commented along similar lines,  the most famous being E. M. Forster, whose advice to a young writer was “Only Connect”.

Yesterday the news was full of stories emanating from the discovery of a very massive star, in fact the most massive one ever found.  This news also got the Jonathan Amos treatment on the  BBC science website too. I think it’s quite an interesting discovery but it  didn’t generate much enthusiasm from Lord Rees who wrote in a Guardian article

I don’t view this discovery as a big breakthrough. It’s a bit bigger than other stars of this kind that we’ve seen and it’s nice that it involves British scientists and the world’s biggest telescope. It’s a step forward, but it is not more than an incremental advance in our knowledge.

What’s interesting about this star is that it may shed some light – actually, rather a lot of light, because it’s 10,000,000 times brighter than the Sun – on the properties of very big stars as well as possibly how they form.

There was even an item on local radio last night, which reported

The biggest star ever discovered was recently found by astronomers in Sheffield.

You’d think if it was that bright and so nearby somebody in Sheffield would have noticed it long before now…

A star this big – about 300 times the mass of the Sun – operates on the same basic mechanism as the Sun but the quantitative details are very different. Its surface temperature is about 40,000 Kelvin compared to the Sun’s, which is only about 6000K, so the radiation field it generates is very much more powerful. It’s also very much larger, probably about 50 times the Sun’s radius, so there’s more surface area to radiate. It’s a very big and very bright beastie.

The name of this star is R136a1 but given its new status as media star, it really needs a better one. In fact, there’s a suggestions page here. Let me see. Overweight and prominent in the media? No Eamonn Holmes gags please.

A star is basically just a ball of hot gas which exerts pressure forces that balance the force of gravity, which tries to make it collapse, in a form of hydrostatic equilibrium. With so much mass to hold up the pressure in the centre of the star has to be very large, and it therefore has to be very hot. The energy needed to keep it hot comes from nuclear reactions that mainly burn hydrogen to make helium (as in the Sun), but the rate of these processes is sensitively dependent on the temperature and density in the star’s core. The Sun is a relatively sedate pressure-cooker that will  simmer away for billions of years. A monster like the one just found guzzles fuel at such a rate that its lifetime will only be a few million years. Like megastars in other fields, this one will live fast and die young.

Nobody really knows how big the biggest star should be. Very big stars are produce such intense radiation that radiation pressure is more important than gas pressure in supporting the star against collapse, but if the star is too big (and therefore too hot) then the radiation field will blow the star apart. This is when the so-called Eddington Limit is reached.  Where the line is drawn isn’t all that clear. The new star  suggests that it is a bit higher up the mass scale than previously thought. I think it’s interesting.

I’ve written about this star partly to make a point about how wonderful astronomy is for teaching physics. To understand how a star works you need to take into account thermal physics, gravity, nuclear physics, radiative transport and whole load of other things besides. Putting all that physics together to produce a stellar model is a great way to illustrate the much-neglected synthetic (rather than analytic) side of (astro)physical theory education. Stars are good.

Cue cheesy link to another item.

The single biggest step towards the understanding of stellar structure and evolution was the Hertzsprung-Russel diagram, or HR diagram for short, which shows that there is a Main Sequence of stars (to which the Sun belongs). Main sequence stars have luminosities and temperatures that are related to each other because they are both determined by the star’s mass. That’s because they’re all described by the same basic physics – hydrostatitic equilibrium, nuclear burning, etc – but just come in different masses. They adjust their temperature and luminosity in order to find an equilibrium configuration.

Not all stars are main sequence stars, however. There are classes of stars with different things going on and these lie in other regions of the HR diagram.

With this in mind, the Astronomy Blog has constructed an amusing career-related version of the HR diagram which I’ve reproduced here:

Instead of plotting temperature against luminosity (or, to be precise, colour against magnitude) as in the standard version this one plots academic publications against google hits, which purport to be a measure of “fame”. A traditional academic will presumably acquire fame through their publications only, thus defining a main sequence, whereas some lie off that sequence because of media work, blogging, or (perhaps) involvement in a juicy sex scandal. I don’t think fame and notoriety are distinguished in this calculation.

I know quite a few colleagues have been quietly calculating where they lie on the above diagram, as indeed have I. Vanity, you see, is very contagious. I’m not named on the version shown, but I can tell you that I’m much more famous than Andy Lawrence, who is. So there.

Starchild

Posted in The Universe and Stuff with tags , , , on May 10, 2010 by telescoper

It’s been a busy day today,  so I’ve decided to be lazy and plunder the online stack of juicy Herschel images for a pretty picture to show. This one has done the rounds in the popular media recently, which is not surprising given how strange it looks.

Image Credits: ESA / PACS & SPIRE Consortium, Dr. Annie Zavagno, LAM, HOBYS Key Programme Consortia

This image shows a Galactic bubble (technically an HII emission region) called RCW 120 that contains an embryonic star that looks set to turn into one of the brightest stars in the Galaxy. It lies about 4300 light-years away. The star is not visible at these infrared avelengths but its radiation pressure pushes on the surrounding dust and gas. In the approximately 2.5 million years the star has existed, it has raised the density of matter in the bubble wall by so much that the material trapped there can now collapse to form new stars.

The bright knot to the right of the base of the bubble is an unexpectedly large, embryonic star, triggered into formation by the power of the central star. Herschel’s observations have shown that it already contains between 8-10 times the mass of our Sun. The star can only get bigger because it is surrounded by a cloud containing an additional 2000 solar masses.

Not all of that will fall onto the star, because even the largest stars in the Galaxy do not exceed 150 solar masses. But the question of what stops the matter falling onto the star is an astrophysical puzzle. According to theory, stars should stop forming at about 8 solar masses. At that mass they should become so hot that they shine powerfully at ultraviolet wavelengths exerting so much radiation pressure that it should push the surrounding matter away, much as the central star did to form this bubble in the first place. But this mass limit is must be exceeded sometimes, otherwise there would be no giant stars in the Galaxy. So astronomers would like to know how some stars can seem to defy physics and grow so large. Is this newly discovered stellar embryo destined to grow into a stellar monster? At the moment, nobody knows but further analysis of this Herschel image could give us invaluable clues.

It also reminds me a little bit of the Starchild from 2001: A Space Odyssey…