A really cool star, and I discovered it!!

[Admin]

I'll do some Googling.

[ceptimus]

If there is hydrogen fusion going on, then there must at least be some helium present (the 'ash' from the fusion reaction). Does helium count as a metal in this stellar context?

[wollery]

If there is hydrogen fusion going on, then there must at least be some helium present (the 'ash' from the fusion reaction). Does helium count as a metal in this stellar context?

Yes. And yes. As I said earlier, anything that isn't Hydrogen is a metal, including Deuterium and Tritium (isotopes of Hydrogen).

There are no stars that have no metals at all. The Big Bang produced small amounts of Deuterium, Helium & Lithium as well as Hydrogen. A star which has initial constituents which are identical in proportion to those produced by the Big Bang would be a member of the semi-mythical Population III. An interesting property of Population III stars is that they would have to be very massive, since the very small amounts of metals makes it very difficult to get the fusion reactions going. This means that they are also very short lived and end their lives as supernovae, leaving no obvious remnant. This means that we will never observe one, but assuming that the current Big Bang model is nearly correct (which is pretty reasonable) they must have existed in order to produce the metals we see in Population II (low metallicity) and Population I (high metallicity) stars.

[bonavada]

The extreme subdwarfs were the very first stars to form in our Galaxy, and only the very low mass ones are still around, the rest having burned all their hydrogen. They're fairly rare.

sorry if these seem like a daft questions.....but you state that these types of star are fairly rare. i take this to imply that they are only created at the formation of our galaxy? have these stars been located in other galaxies? if they are dark and low mass i assume they are difficult to spot.

BV

[wollery]

sorry if these seem like a daft questions.....but you state that these types of star are fairly rare. i take this to imply that they are only created at the formation of our galaxy? have these stars been located in other galaxies? if they are dark and low mass i assume they are difficult to spot.

BV

Yes, no and yes.

When the first stars were created there was very little metallicity content in the Galaxy, so the stars were made of almost pure Hydrogen. These are the population III stars that I mentioned in my previous post. They are extremely large and short lived, having masses (IIRC) of 500 times the mass of the Sun, and lifetimes of just a few million years, which in astronomical terms is just the blink of an eye. Essentially they form rapidly by gravitational collapse from the primordial clouds, burn Hydrogen for about 1 million years and then explode, ejecting all of their material back into the Galaxy.

Next come the extreme subdwarfs, which have some metals, thanks to the population III stars. (Please note, that in the context of astronomy, the term "dwarf" doesn't mean they are small, it is to differentiate them from stars which have finished hydrogen burning and expand to become "giant" stars.) The largest of the extreme subdwarf stars very quickly burn themselves out (the larger a star is the faster it uses up its hydrogen and the shorter its existence) and explode, showering heavy elements back into the Galactic medium. As time goes by lower mass stars do the same, then even lower mass stars, and so on, putting more and more metals out into the Galaxy

Shortly after the extreme subdwarfs begin to end their lifetimes come the subdwarfs, with slightly higher metallicity than the extreme subdwarfs. They follow a similar pattern, and the Galaxy settles down for a while. Extreme subdwarfs and subdwarfs are population II stars.

During the formation of the population II stars the Galaxy is still a swirling ball, with clumps of gas moving in fairly random directions, and thus the stars formed at this time, are also moving in fairly random directions. But as these stars are created the remaining gas and dust starts to form a disk. Nobody is exactly sure why some galaxies form disks while others remain elliptical, but current theory involves gravitational interactions between galaxies. As the disk forms it sets off a new round of star formation, and these are the thick disk stars, the first of the population I stars, with higher metallicity than the population II stars because of all the metals the population II have been chucking back out into the Galaxy. As the disk thins out more and the spiral arms form another round of star formation occurs - creating the thin disk population I stars. It is this population of stars that the Sun belongs to.

Of course, the above is a slightly idealised model. There are no hard and fast dividing lines between the periods of star formation, it's an ongoing process, and consequently there's no hard and fast dividing line between the stellar populations. Instead there's a range of metallicities with a sparsely populated gap between the population II and population I stars.

In fact there are stars being formed at this very moment in clusters all around the Galaxy, some with lower metallicity than the Sun, but most with higher metallicity. It all depends on where they are and how much metals the local Galactic medium contains.

To the best of my knowledge no subdwarfs have been found in other galaxies, mainly because we can only spot very bright stars at those distances (mostly giant variable stars such as Cepheids and Mira variables), although I'm not sure about the Magellanic clouds (but I doubt it).

[vbloke]

You'll be pleased to know that I covered this very topic in one of my recent classes - I even managed to namecheck youm so all my students have heard of you now.

[wollery]

You'll be pleased to know that I covered this very topic in one of my recent classes - I even managed to namecheck youm so all my students have heard of you now.

I'm famous!!