Small Magellanic Cloud

[Ray Moscow]

Here's a nice view of and commentary on one of our neighbouring galaxies, the SMC:

The terrible beauty of chaotic starbirth (http://blogs.discovermagazine.com/badastronomy/2010/01/05/the-terrible-beauty-of-chaotic-starbirth/)

All together, this image shows starbirth on a vast scale, thousands of light years across. And this may be a new phenomenon for the SMC: measurements of the elements in the stars there show that they have far fewer heavy elements (like oxygen, iron, and so on) than stars in the Milky Way, as little as 1/5th as abundant. Since these elements are created inside of stars over time, this indicates that stars in the SMC are on the whole younger than in the Milky Way.


Thanks to Phil Plait at Bad Astronomy (http://blogs.discovermagazine.com/badastronomy/)!

[DMB]

It is indeed a stunning picture. I don't know enough about the SMC to know the answer to this, so can anyone please explain the difference between young stars forming in a galaxy low in stardust (i.e. the remains of older stars) and a galaxy that for some reason has only very, very old stars, that came into existence long before much stardust was available.

[Ray Moscow]

I suppose there would be a big difference in the amount of heavy elements present, since these are formed in larger stars and released on their destruction.

IIRC, anything heavier (higher AW) than iron requires a supernova to produce it.

[Jet Black]

I suppose there would be a big difference in the amount of heavy elements present, since these are formed in larger stars and released on their destruction.

IIRC, anything heavier (higher AW) than iron requires a supernova to produce it.

in significant quantities yeah. Fusion of anything above iron is endothermic, and anything up to iron is exothermic, so a sustained nuclear reaction cannot be maintained with elements heavier than iron, since there would be no radiation pressure to maintain the balance against gravity, and so the core of the star collapses.

[DMB]

You didn't really answer my question, Ray. It is a given that in the absence of significant quantities of "stardust" (i.e. stuff from supernovas) there is going to be a shortage of stars containing heavier elements.

So in any case we are being told that in the SMC there is a shortage of stardust.

But how do you distinguish between the two cases where the galaxy is oldish but for some reason hasn't produced stardust or huge numbers of stars until now but where now there is a lot of new star formation without heavy elements and the galaxy is simply not making lots of new stars but has a lot of unusually old ones, formed before there would have been a lot of stardust around anyway.

The piece you linked to seemed to be in no doubt that it was the former situation, but how can they tell?

[Ray Moscow]

We have pretty good models of the life expectancy of stars of a given size and type, for example: Main Sequence (http://en.wikipedia.org/wiki/Main_sequence)

And so we (or rather, astronomers) can reliably estimate the age of a star that fits these patterns.

[DMB]

How easy it is to work out the size of a star that isn't even in our galaxy?

[Ray Moscow]

How easy it is to work out the size of a star that isn't even in our galaxy?

I don't know the details, but I imagine it requires extremely precise measurements that weren't possible until the latest telescopes came online.

Maybe an astronomer is in the house ....?

[Worldtraveller]

How easy it is to work out the size of a star that isn't even in our galaxy?

It's been a while since my astronomy classes, but we have a number of cosmological measuring sticks, if you will. Cepheid variables and other classes of specific star types can be used to get a good measure of of absolute magnitude. From that, and spectrum analysis, astronomers can calculate size and relative composition.

It obviously relies on our incomplete, but fairly reliable so far, theories and observations, but the universe seems fairly consistent in that way. :)

I'll try to answer this:
[C]an anyone please explain the difference between young stars forming in a galaxy low in stardust (i.e. the remains of older stars) and a galaxy that for some reason has only very, very old stars, that came into existence long before much stardust was available.
Again, going from memory....

A star forming in a 'light' gas cloud of almost exclusively hydrogen and heluim and a star forming in an older cloud with lots of dust from previous generations of stars will stil be fusing hydrogen as it's primary sequence. Let's assume they are about the same mass. They will both form and burn along the same main sequence. As far as I know, the only real difference will be in the absorption spectrums of the two stars. I think the nearby dust will either 'burn off' and not really affect the star itself, or it would be of such small percent compared to the hydrogen as to not really affect the star's life cycle.

The real difference from our perspective would, I think, only be in the spectrum of the star. I don't think even a nearby planet would be affected.

[B.H.]

I wonder if you were an inhabitant of the SMC and had a huge telescope you could make out our sun as an individual star?