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View Full Version : which kind of star has the longest life expectancy



Plat
2003-Nov-01, 01:58 AM
??? and what kind of star is Vega

Crimson
2003-Nov-01, 02:17 AM
Red dwarfs live the longest. Vega is a white A-type main-sequence star.

Plat
2003-Nov-01, 03:28 AM
does vega have a longer life expectancy than the sun....what type is the sun

Vermonter
2003-Nov-01, 04:13 AM
Red dwarfs do indeed have the longest life expectancy. Our Sun is a G type star, which is an average star.

Crimson
2003-Nov-01, 04:32 AM
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.

Vermonter
2003-Nov-01, 04:55 AM
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).

eburacum45
2003-Nov-01, 10:53 AM
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.

Crimson
2003-Nov-01, 02:13 PM
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).

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.

chaotica
2003-Nov-01, 02:21 PM
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

Crimson
2003-Nov-01, 02:22 PM
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.

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.

dgruss23
2003-Nov-01, 03:19 PM
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.

Eroica
2003-Nov-01, 04:04 PM
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.

TriangleMan
2003-Nov-01, 04:58 PM
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).

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

Eroica
2003-Nov-01, 05:10 PM
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.

dgruss23
2003-Nov-01, 05:13 PM
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).

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

Yep, in 1999 Kirkpatrick et al (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999ApJ...519..802K&db_key=A ST&high=3e5c03c80a21724) 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." :D

TriangleMan
2003-Nov-01, 05:15 PM
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." :D

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

chaotica
2003-Nov-01, 05:18 PM
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/allen/spectral_classification.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)

dgruss23
2003-Nov-01, 05:23 PM
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." :D

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

Is science flexible or what? :lol:

TriangleMan
2003-Nov-01, 05:32 PM
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.

dgruss23
2003-Nov-01, 05:35 PM
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.

Crimson
2003-Nov-01, 06:28 PM
Some M dwarfs are probably brown dwarfs, too.

dgruss23
2003-Nov-01, 06:48 PM
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?

Crimson
2003-Nov-01, 07:08 PM
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.

eburacum45
2003-Nov-01, 08:25 PM
Crimson is right. The Sun is only average in terms of the range of possible sizes stars can have.
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...

chaotica
2003-Nov-01, 09:00 PM
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.

Kebsis
2003-Nov-01, 09:45 PM
You guys are great at coming up with analogies.

Plat
2003-Nov-01, 11:37 PM
thanks for the classification link very useful

TriangleMan
2003-Nov-03, 12:26 PM
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 <snip>
The Two Micron All Sky Survey is infared, which is the wavelength these stars peak energy emissions should be in.

Thanks for the name of the survey dgruss23. I went to the site for the Two Micron All Sky Survey (http://www.ipac.caltech.edu/2mass/) (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?

chaotica
2003-Nov-03, 04:03 PM
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/davy/2mass/science/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.

Eroica
2003-Nov-03, 04:44 PM
First, T-class "stars" are not stars, they're brown dwarfs....
Debatable. Are brown dwarfs stars? I say, they are. So they can't fuse ordinary hydrogen. Big deal!

chaotica
2003-Nov-03, 05:04 PM
Yes, it IS a big deal. A brown dwarf is NOT a star, precisely because it can't ignite hydrogen fusion as do protostars when they turn into "normal" stars. And if you don't believe me, look it up on any serious astronomy page or in a book. You might as well say that Jupiter is a star (many scientists are not sure where to draw the line between brown dwarfs and planets, but there is a clear distinction between stars and brown dwarfs)

Blondin
2003-Nov-03, 09:49 PM
I can't remember where I got this from but it seems appropriate. I'll delete it if anybody thinks it's too over the top.

Mnemonics for the
Harvard Spectral Classification Scheme

The modern stellar spectral classification scheme (also known as the Harvard Spectral Classification Scheme) was created by Annie Jump Cannon through her examination of spectra from 1918 to 1924. Originally, the scheme used capital letters running alphabetically, but was later reordered to reflect the surface temperatures of stars. In order of decreasing temperature, these types were O, B, A, F, G, K. and M. Three additional categories also in the scheme: R, N, and S types, were later realized to represent stars with peculiar heavy-metal abundances. Other types (Q for novae, W for Wolf-Rayet stars, etc) are not encountered frequently. According to astronomical myth, Henry Norris Russell suggested the following mnemonic to assist students in remembering the scheme:
Oh Be A Fine Girl, Kiss Me!
There have been many efforts since to improve on this mnemonic. The motivations are various: to include the R, N, and S and even W stellar types, to find a mnemonic for the vast number of astronomers who would really not want to be kissed by a girl, by the feeling that really we can't let Russell have the last word on this subject, or just as a fun homework assignment for students. Owen Gingerich (CfA) holds an annual contest in his "The Astronomical Perspective" course , and a summary of many winning submissions was published in his article "The Great Mnemonics Contest" in Phyllis Lugger, ed, ASTEROIDS TO QUASARS: A SYMPOSIUM HONOURING WILLIAM LILLER (Cambridge University Press, 1991). The Committee on the Status of Women in Astronomy also held a competition on the subject in 1995 and Russell's mnemonic (subject to the substitute of Guy or Gal for Girl) remained the most popular of the submissions. Our humble efforts, which draw on all these resources, plus many suggestions from others, are presented here.
The Traditional
1. Oh Be A Fine [Guy/Gal/Girl] Kiss Me (Right Now [Smack/Sweetheart]).
2. Oh Begone, A Friend's Gonna Kiss Me (Right Now Smack).
3. Only Boys Accepting Feminism Get Kissed Meaningfully.
Political
1. Official Bureaucrats At Federal Government Kill Many Researchers' National Support
2. Only Big Astronomy Federal Grants Keep Money. Research Needs Support!
The Joys of College
1. Oh Boy! Another Failing Grade Keeps Me Reconsidering Night School.
2. Oh Bother, Astronomers Frequently Give Killer Midterms.
3. Oh Bother, Another F's Gonna Kill Me.
4. Oh [Beautiful/Brutal/Big] And [Fine/Fearless/Ferocious] Gorilla, [Kill/Kidnap] My Roommate Next Saturday.
5. Old Boring Astronomers Find Great Kicks Mustily Regaling Napping Students.
6. Obese Balding Astronomer Found Guilty Killing Many Reluctant Nonscience Students.
Appeals to Physics and Astronomy
1. Observationalists Basically Are Fine Generous Kind Men (Really Not Sexist)
(credited to Wendy Haugen-Bauer, Wellesley College)
2. Oh Backward Astronomer, Forget Geocentricity;
Kepler's Motions Reveal Nature's Simplicity.
(Ohhhh... it rhymes!)
3. Organs Blaring and Fugues Galore,
Kepler's Music Reads Nature's Score.
(Oh... so does this one. Do I get impressed too easily?)
4. Out Beyond Andromeda, Fiery Gases Kindle Many Radiant New Stars.
5. Orbs, Bright And Fair, Generate Kinder Memories: Revolving Nighttime Skies.
6. Only Bright Astral Fires Going Kaput Make Real Neutron Stars.
For the old film buffs
1. Overseas Bulletin: A Flash! Godzilla Kills Mothra! (Rodan Named Successor).
Appeals to Mechanics
1. Oil Buffers A Fragile Gasket, Keeps Motors Running Nearly Smooth.
Alternative Cuisine
1. Oh, Bring A Fully Grown Kangaroo
My Recipe Needs Some!
[This 1969 prize winner claims to be a haiku, but poetic purists point out the match to the haiku form is imperfect]
2. Oven-Baked Ants, when Fried Gently, and Kept Moist, Retain Natural Succulence.
Ecology
1. When Obstreperous Beasts Approach, Fragrant Geraniums Knowingly May Receive Night's Stigmata.
[A rare mnemonic that includes W (Wolf-Rayet) stars]
2. Old Bottles And Filthy Garbage Kill Many Rare Natural Species.
3. Oregon Beavers Attack Famous Gardens, Killing Many, Rangers Now Shooting.
4. One Bug Ate Five Green Killer Moths.
Submissions which defy categorization...
1. Occupied Baker's Assistants Forget Giant Kitchen Mouse, Receive Nasty Scratches.
2. Only Bungling Astronomers Forget Generally Known Mnemonics!
3. Only Boring Astronomers Find Gratification Knowing Mnemonics!
4. Our Brother Andrew Found Green Killer Martians.

TriangleMan
2003-Nov-03, 09:55 PM
:lol: =D> :lol:

Spaceman Spiff
2003-Nov-03, 10:58 PM
from M stars to brown dwarfs, T dwarfs and planets, some of these links may be of some help...

The End of the Main Sequence (http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v482n1/35131/35131.pdf)

The Theory of Brown Dwarfs and Extrasolar Giant Planets (http://arxiv.org/PS_cache/astro-ph/pdf/0103/0103383.pdf)

Beyond the T Dwarfs.... (http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v596n1/58069/58069.web.pdf)

As noted just below, these are .pdf files and may be read with free-ware Adobe Acrobat Reader.

Vermonter
2003-Nov-04, 12:39 AM
Warning! Those are .pdf filess, so make sure you have Acrobat installed...

Wouldn't the definition between gas giant planet and a brown dwarf be dependant on mass and composition?

Also, I was not aware of the information regarding the Sun. I had assumed (I never read anything different) the Sun was average in our galaxy.

Eroica
2003-Nov-04, 05:38 PM
According to astronomical myth, Henry Norris Russell suggested the following mnemonic to assist students in remembering the scheme:
Oh Be A Fine Girl, Kiss Me!

I heard it was Edward Charles Pickering, director of the Harvard College Observatory (and Cannon's boss) who devised the mnemonic.

By the way, the spectral classes R and N have been replaced by C (for carbon stars). It was once thought that R and N stars were cooler than M stars, but it turns out that carbon in their upper atmospheres was absorbing some of the light from their photospheres, making them appear cooler. They're actually as hot as G, K and early M - which rather spoils those longer mnemonics.

chaotica
2003-Nov-04, 06:07 PM
By the way, the spectral classes R and N have been replaced by C (for carbon stars). It was once thought that R and N stars were cooler than M stars, but it turns out that carbon in their upper atmospheres was absorbing some of the light from their photospheres, making them appear cooler. They're actually as hot as G, K and early M - which rather spoils those longer mnemonics.
Not true. Classes R and N have not been replaced by C, astronomers just refer to R and N collectively as C. Except for the additional spectral features due to carbon compounds, R stars resemble K stars, N stars resemble M stars. The carbon compounds absorb blue light, so that the stars look redder.