??? and what kind of star is Vega

Red dwarfs live the longest. Vega is a white A-type main-sequence star.

does vega have a longer life expectancy than the sun....what type is the sun

Red dwarfs do indeed have the longest life expectancy. Our Sun is a G type star, which is an average star.

The Sun is NOT an average star. It's more luminous than over 95 percent of the stars in the Galaxy.

The Sun has a longer life expectancy than Vega. Although the Sun was born with less fuel, it consumes its fuel more slowly. That's why it's less luminous than Vega.

Hate to argue with you, but the Sun *is* and average star. Look at the H-R diagram sometime. It's a main-sequence star, with a Luminosity rating of 1, an Absolute Magnitude rating of +5, and is a G2 spectral-type star. There are many more that are far brighter than the Sun is, like Rigel, Deneb, Betelguese, Antares, Acturus, Alderbaran, Mira, Vega, Regulus, Sirius A, Pollux, Altair, and Procyon A.

That's from my college textbook, Discovering the Universe, 6th Edition, by Comings and Kaufmann III (2003).

Red dwarfs are much more common than yellow dwarfs like our Sun. However yellow dwarfs can be said to hold an average, or median, position in the luminosity diagrams.


I thought at one time that brown dwarfs would last longer than reddwarfs but since they only fuse deuterium, which is a trace isotope, I think they probably will cease fusion long before the red dwarfs go out.

As a strategy, by the way, colonising red dwarfs would be sensible; if would be difficult to find a natural earth-type planet around such a small star, but if we can make artificial worlds of some kind the red dwarfs will still be shining tens of billions of years from now.

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The Sun is NOT an average star. It's not even close to being average. 80 percent of the stars in the Galaxy are red dwarfs. 5 percent are white dwarfs. 9 percent are orange dwarfs. ALL those stars are fainter than the Sun.

The bright stars you see at night--Rigel, Deneb, etc.--are NOT representative of the Galaxy. Most stars you see at night are far more luminous than the Sun. But most stars that actually exist are far less luminous.

The Sun is NOT an average star.

The problem with life around red dwars is that in order to have temperatures similar to those on earth, the planet would have to be much closer to the star. But that would probably cause the planet to be tidally locked like the moon is to the earth. So one side would always be exposed to the starlight while the other would always be dark and cold. Another problem is that many red dwarfs display flares many times as powerful as the Sun's.
However, I've found an article which explores the possibility of life on planets around red dwarfs in great detail: http://www.spacedaily.com/news/life-01c.html

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The first sentence is true, the second is not. "Median" means the point at which half of all stars are more luminous, half of all stars are less luminous.

In absolute magnitude, the Sun lies about midway between the most luminous star and the least. But that doesn't make it average. Over 95 percent of all stars are less luminous than the Sun.

Here's an analogy: Suppose a mean professor gave a really hard exam, on which scores range from 0 to 100. Almost everyone in the class gets scores around 10 or 20 or 30. But YOU score a 50. You're halfway between the best (0) and the worst (100). But you're NOT average--because you're scored higher than almost everyone else in the class.

The Sun is in the top 5 percent of all stars in the Galaxy. It is NOT average.

Crimson is right. The Sun is only average in terms of the range of possible sizes stars can have. When you look at the actual population numbers, the Sun is in the top 10%. The most populous type of star in the galaxy is the red dwarfs (unless the newer L-class turns out to be more populous - too early to say).

The naked eye stars are predominately larger than the Sun and this can give a false impression that the Sun is a small star (or "average"). Here's another analogy. Would you get a better estimate of the typical income of people living in the United States by finding the average income of the people on television or by taking an average of all the people in your community? Obviously the people we see on television tend to have higher paying jobs so this would skew the impression of what an average income is. Its similar to what happens if we consider the naked eye stars. On the other hand if you look at the stars within about 13 parsecs of the Sun, they are predominately red dwarfs.

As to the question of stellar lifespan. Think of the large stars like a lamborghini. Yeah they've got a large gas tank, but they burn it up quickly. The red dwarfs are like compact cars - small tank, but great mileage, so they can burn for many times the currently accepted age of the universe.

Of course the stars are not "burning" like gasoline. They use a variety of different nuclear fusion processes depending upon the mass of the star and the stage in its lifespan.

I believe the formula for calculating the main sequence lifetime (t, in years) of a star of initial mass M kilograms is:

• t = (5.46e85)/(M^2.5)

Vega, with a mass 2.6 to 3.1 times that of the Sun, should remain on the main sequence for around 600-900 million years. Red dwarfs have main sequence lives that are measured in trillions of years.

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L-class? You mean there is another class after OBAFGKM?
(Hmmm, do I have that order right?)

Brown dwarfs are classified as spectral types L and T.

L stars have a surface temperature around 1000-2100 Kelvins.
T stars have a surface temperature around 800-1000 Kelvins.

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Yep, in 1999 Kirkpatrick et al defined the L-class. I haven't followed it too closely since that paper, but they're smaller and cooler than M-class stars. If I remember right they said that not all the L-class stars were true stars fusing hydrogen. Some of the L-class objects are probably more akin to brown dwarfs. In fact that is the other new class. Brown dwarfs are given the class "T".

So now the sequence has an "L" on the end which means the traditional way of remembering the spectral sequence: "Oh Be A Fine Girl/Guy, Kiss Me" can be changed to "Oh Be A Fine Girl/Guy, Kiss My Lips."

Well, if you add 'T' I'd use "Oh Be A Fine Girl, Kiss Me Like That"

TriangleMan, you might be surprised to learn that there is also a T-class. L-class stars are very, very cool stars and T-class means brown dwarfs - failed stars, so to speak. And there are yet more (special) classes than OBAFGKMLT, WC and WN for Wolf-Rayet stars, for example. More about spectral classification: http://lheawww.gsfc.nasa.gov/users/a...ification.html
(Here, T is used in another context and L is not mentioned at all - probably because it's quite a new addition to the classification scheme)

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Is science flexible or what?

Thanks Eroica, dgruss23 and chaotica for the info and links. One question: how did we locate stars that we would consider L or T class? Their surface temperatures seem so low that they wouldn't be very luminous, especially once you get below 1000K since that wouldn't even melt many metals.

The Two Micron All Sky Survey is infared, which is the wavelength these stars peak energy emissions should be in.

Some M dwarfs are probably brown dwarfs, too.

Interesting, I've never heard that before. Do you happen to have a link or reference on that? I suppose it would have to be really young brown dwarfs that are still relatively hot from their formation or a deuterium fusion stage?

Spectral types are based on spectral characteristics, not directly on mass. The dividing point between main-sequence stars and brown dwarfs is around 0.08 solar masses. Where does that mass fall in spectral type? Probably in the late M's. Thus, late M dwarfs, plus all L and T stars, would be brown dwarfs.

Trouble is, we have very few direct mass measurements of stars, especially M stars.

Sorry, that was what I was trying to say in my typical hamfisted way;

but what about the lifespan of brown dwarfs?

If they don't fuse hydrogen won't they cool down quite a bit sooner than fully fledged red dwarfs?
Just curious...
such information might be vital in about a trillion years time...

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Red dwarfs can fuse hydrogen for trillions of years while, as you said, brown dwarfs never fuse "normal" hydrogen. However, they are thought to gain heat from fusing deuterium for some time after their birth and contracting slowly. But as soon as the deuterium is depleted, they slowly fade to blackness. If you want to define "lifespan" by how long a stellar body maintains fusion then a brown dwarf lives perhaps 10 million years. Anyway, brown dwarfs certainly start to cool down earlier than red dwarfs.

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You guys are great at coming up with analogies.

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thanks for the classification link very useful

Thanks for the name of the survey dgruss23. I went to the site for the Two Micron All Sky Survey (known as '2MASS') and they note that nominally they can detect infrared sources at up to 15.8 magnitude. I did a rough calculation and found that a 800K star half the size of the Sun would have an absolute magnitude of 14.9 so while it can detect the hotter L-Class stars I don't know if 2MASS would be sensitive enough to locate T-class stars. Is the existance of T-class stars still theoretical?

First, T-class "stars" are not stars, they're brown dwarfs, that means failed stars, which don't have enough mass to ignite normal hydrogen fusion.
Second, brown dwarfs are not just theoretical objects, there are even pictures of them, here's one by 2MASS (yes, it is sensitive enough): http://spider.ipac.caltech.edu/staff...tdwarf_all.jpg

And there are pictures of brown dwarfs by other observatories like Keck and Gemini, too. L-class objects are also often brown dwarfs.

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Debatable. Are brown dwarfs stars? I say, they are. So they can't fuse ordinary hydrogen. Big deal!

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