Red stars

[mikemasey]

We know that if a star is red then it is old.
My question is, what makes it red and is it to due with nearly all the fuel it has being used up. If so what is the elimant that shows as RED and is this conected with "spectogravy"?
thanks in advance:thanx:

[DavidU]

This is a good read Mike.
http://en.wikipedia.org/wiki/Red_giant
Spectogravy is a new word for me !:)

[Robh]

Mike,

The colour of a star has more to do with its surface temperature. As an analogy, consider a bunsen burner. A hot flame is bluish whilst a cooler flame is yellowish or reddish. The spectral class of a star correlates pretty well with its colour or surface temperature.
The one dimensional Harvard spectral classes were OBAFGKM in that order, with O class stars being hot blue stars (temperature 30000 to 60000 degs K), M class stars being cooler red stars (temperature 2000 to 3500 degs K).
The Morgan-Keenan spectral classification further subdivides each class from 0 to 9. A G9 star is cooler than a G0 star, which is closer to an F9 star.
A red M class star is not really that red, more orange. Carbon stars are much redder and this is actually due to a high level of carbon in the star's outer shell. The majority of carbon stars are old cooling giants or supergiants.

Regards, Rob.

[astro_nutt]

Hi Mike.
A nice carbon star to find is next to Mimosa in Crux.
Cheers

[renormalised]

Hi Mike. Also to add to what Rob has written, a red star is not necessarily old, either. Small stars...those with less than 0.5 times the mass of our Sun, are cool stars. As Rob has mentioned, these stars glow with a "red" heat and so are orange-red in colour. These stars can be as new as newborn babies. Because these stars are so small, they stay reddish in colour for all of their lives, which can be up to quite a few trillion years, for the smallest of them. These types of stars are the ordinary red stars, that like the Sun, are burning hydrogen in their cores. If the star is a large star and is red, then it can be an old star. When they're like this, they call them red giants or red supergiants. At this stage, they are red because they have gotten so huge their surface has cooled down to glow red and they have stopped burning hydrogen in their cores. They have become old stars, but even so, they may not be older than the Sun. When astronomers say a star is old, it maybe older than the Sun, but it also means that the star is getting on in age so far as the stage of life it is in. Very big stars age quickly, because they burn up their hydrogen very quickly. So they become red giants or supergiants before the smaller stars, like the Sun. Small stars can take a very long time to reach that stage in life, so they can hang around for a lot longer than the big stars.

So, you can see, when they say a star is red, it can mean many things.

[renormalised]

Also, Mike, to do with spectroscopy, a star is mostly hydrogen and helium, but it will also have other elements in its make up as well. Depending on how hot the star is, some of these elements will appear in the spectra of the stars, others won't. There are two types of spectra a star can have, absorption and emission. The type you normally see in the astronomy books is an absorption spectrum, where you see a rainbow of colours with dark lines in the rainbow. Those dark lines are caused by the gases in the star absorbing some of the light that the star is shining. Each element absorbs the light at particular wavelengths (like UV, visible or infrared) and from this you can tell what elements are in the star, how hot the star is and quite a few other things about the star. Sometimes an element can have more than one dark absorption line present in the spectrum, because the element absorbs more than one particular wavelength of light. However, the line that is the strongest (easiest to see) is the wavelength which the element is absorbing the light the most in. As I said before, astronomers can use this to tell how hot a star is.

Hope that has helped:)

[Liz]

Interesting Carl, I also thought that red stars were old stars. Yesterday I was reading up on Antares, and was surprised to see that its a lot younger than the Sun. It was once a blue supergiant with 12 x the mass of our sun, and was burning its fuel at a much quicker rate. Hmmm.
The article said that the bigger the star, the harder they fall.

http://my.execpc.com/60/B3/culp/astronomy/Summer/scorpius.html

[renormalised]

That's right...Antares is only about 5-6 million years old, or thereabouts. if you want the lowdown on the star, here's a pretty accurate description...

Antares (http://en.wikipedia.org/wiki/Antares)

Given its mass, it would've started life as a O9-B1 Main Sequence star with a radius of about 5-8 times the Sun and a surface temp up around the 32000-35000K mark.

Bigger stars burn their core hydrogen a lot faster than smaller stars like the Sun. I could workout the exact rate of burning difference between the two, but just to give you an idea the rate of core burning for a high mass star such as Antares, in its Main Sequence lifetime, would be several thousand times greater than it is for the Sun.

[ausastronomer]

Here is a list of 24 of my favourite red stars visible from Southern latitudes.

Some eye candy for those that enjoy red stars.

Cheers,
John B

[mikemasey]

Wow thanks guys, maybe I shouldnt have asked eh?

[MikeyB]

I'm very glad you did, Mike - I know I've certainly learnt a lot from the answers. And thanks for that list, John - it's much appreciated.

[astroron]

Mike, the question you asked was a good one:thumbsup:
and the answers you received would have given a lot of people reading this thread some valuable information they didn't know:)
Don't be afraid to ask questions like that, that's is one of the ways we learn:thanx: