Celestia Forums

Any photometrists around here ?

Here is a question :

is there any effect (other than reddening) that the color indices U-B, B-V and V-I (Johnson-Cousins system) are sensitive to ? (i.e. metallicity, age, etc)

And another :

what is the inferior mass for a supergiant star ?

Since increasing metallicity introduces more absorption lines into the spectrum, it affects the colour indices. And since young stars tend to have higher metallicity (because of the existing metals generated by previous generations of stars), they will also potentially have differences in their colour indices for the same spectral class - you can imagine, for instance, that a very old red dwarf would have a different spectrum from a young red dwarf. I'm away from my references sources at present, so I can't give you hard data; sorry.

Grant

OK, here we go. The whole group of subdwarfs represents metal-poor dwarf stars. Their relatively low metal concentration shifts their colour indices towards blue (relative to a dwarf of the same mass and average metallicity), because less blue light is soaked up my absorption in the metals' spectral lines. (This more transparent atmosphere also means that they lose more energy, and so burn smaller and hotter than their main-sequence counterparts.)

Grant

This arises some thoughts I had a few weeks ago:

Does there exist some spectral absorption diagrams for certain stars out there?
I'd like to compile them into an info-plugin addon, so that they can be viewed when watching a star from closeup.

maxim.

Giant stars have at least 8 solar masses (Chandrasekhar limit).

I don't know for Supergiants...

Kendrix wrote:Giant stars have at least 8 solar masses (Chandrasekhar limit).

No they don't. Giant stars can currently enter the RGB phase with as little as about 0.9ish solar masses at the very lowest (that drops as they shed mass during that stage) . They aren't necessarily massive, they're just evolved.

Think of it this way... in 5 billion years or so, the sun will turn into a Red Giant. It doesn't suddenly magically gain mass in the process - it'll still be a 1 solar mass star, it'll just be filling up a much bigger volume.

That's not the usual usage of the phrase "Chandrasekhar limit", either. It usually refers to the maximum mass of a white dwarf star - around 1.44 solar masses. Above that mass, electron degeneracy can no longer support the pressure of the collapsed star, and so collapse necessarily continues to a neutron star (or black hole).

Grant