PDA

View Full Version : What percentages of stars are Sun-like stars



Plat
2005-Apr-03, 03:46 PM
I have read that there are about 200 billion Sun-like stars in the Milky Way galaxy alone. What other types of stars are out there, the percentages of them?

stu
2005-Apr-03, 06:25 PM
I had to do this problem for an ASTR class in Fall 2003. Based upon the HD catalog of 272146 stars with complete spectral type information, there are:

O: 200
B: 19479
A: 72155
F: 51747
G: 50757
K: 71805
M: 5445
other: 558

Sun-like stars in this case would be considered G stars, which are 18.7% of the total, or loosely considered to be F-K stars, which are 64.0% of the total. I'd always heard that there are 200-300 billion stars in the Milky Way, so this would be 40-190 billion Sun-like stars in the Milky Way, depending upon how many stars you assume and depending upon what your definition of "Sun-like" is.

However, a big problem with basing spectral populations off of catalogs is that there is a faintness limit of magnitudes where it's much easier to see an O star far away than it is to see an M star. There are probably models to correct for this, but I think that every model gives a different answer.

eburacum45
2005-Apr-03, 06:48 PM
Every model does give a different answer, but they all seem to agree that class M dwarfs (red dwarfs) are by far the most common star in the sky. So of the 200-300 stars in the Galaxy, probably all but around 50 billion of them are small, dim red dwarfs. Perhaps only a third of those 50 billion are sunlike stars.

Kullat Nunu
2005-Apr-03, 07:36 PM
Atlas of the Universe (http://anzwers.org/free/universe/startype.html) gives a value of 3.5% for yellow main sequence stars. K stars comprise 8% of all stars and red dwarfs as much as 80%. One star per ten million is a class O star. They are the most luminous of all, but still only a few bright stars in the sky belong to the class O.

stu, the HD catalog is badly skewed towards more luminous stars.

Cougar
2005-Apr-03, 07:42 PM
...class M dwarfs (red dwarfs) are by far the most common star in the sky....
Makes sense, since these live for a relatively long time, hence there would be more of them around at any given time. Some stars are a lot more massive than the sun, but their lifetimes are a lot shorter.

It's important to note that the galaxy star population is not static. It's like an ecosystem. There are stars being born and dying all the time, just on a much longer timescale than earthly biological ecosystems.

stu
2005-Apr-03, 07:43 PM
stu, the HD catalog is badly skewed towards more luminous stars.

Yeah, well it was what we were told to use. The guy hasn't changed his homework assignments in many many years.

Kullat Nunu
2005-Apr-03, 07:47 PM
stu, the HD catalog is badly skewed towards more luminous stars.

Yeah, well it was what we were told to use. The guy hasn't changed his homework assignments in many many years.

Well, there is hardly any better spectral catalogue anywhere.

Romanus
2005-Apr-04, 03:06 AM
The skewed population of M-dwarfs isn't just because they live longest; from what I've read, nature seems to prefer to form low-mass stars to high-mass ones.

As per the question, the figure for the percentage G-stars I've seen quoted most often is about 4%, in line with the earlier posted link. If our Galaxy has 200-300 billion stars, that translates to 8-12 billion sunlike stars. This will probably include stars of all ages, so we could be looking at lots of old, metal-poor subdwarfs in that sample.

Plat
2005-Apr-04, 03:19 PM
Everyone always tells me different, one says 100 billion sun-like stars, another will tell me 50 billion, then one guy would go up as high as 200 billion, then 8 billion...

stu
2005-Apr-04, 03:32 PM
Everyone always tells me different, one says 100 billion sun-like stars, another will tell me 50 billion, then one guy would go up as high as 200 billion, then 8 billion...

I think the bottom line is that the number is still up for grabs. I think we all agree that currently, there are a few hundred billion stars within the Milky Way. Furthermore, we agree that O and B stars are well-represented in most catalogues, but that the fainter the star, the harder it is to see it, and so any catalogue you find will under-represent them.

Therefore, it is extremely difficult to get a "real" answer, and any answer that you get will be based on a model that makes one assumption or another that won't agree with the next person. That's why I tell people I'm not worried about job security as much as they might think once I get out of grad school ... which'll be once I get out of undergraduate school in a month.

Another question, at least from me, is what do you mean by "Sun-like?" Do you mean just G5 dwarfs, or all G stars, or do you extend your definition to other spectral types? When Kepler is launched, it's going to be looking for planets around "Sun-like" stars, but it's going to be looking at stars other than G. That's why I gave you a huge range in my first post.

TriangleMan
2005-Apr-04, 03:34 PM
A few months ago Romanus (http://www.badastronomy.com/phpBB/viewtopic.php?p=398847&highlight=type+g&#398847) quoted a source from the mid-90s that had 7.3% of stars in the galaxy as class-G.

George
2005-Apr-04, 03:46 PM
Just to add to the fun, what do you mean by "sun-like" stars?

Here is a site which explains variations in the term.... here (http://www.lowell.edu/users/jch/workshop/drs/drs-p1.html)


There are three classes of stars like the Sun that are defined by Cayrel de Strobel (1996): Solar-type (or Solarlike) Stars, Solar Analogous Stars, and Solar Twins

Odd, there was no mention of qualifying on the basis of matching intrinsic color. :)

Tensor
2005-Apr-04, 04:46 PM
Odd, there was no mention of qualifying on the basis of matching intrinsic color. :)

You mean like green or yellow? 8) :wink:

George
2005-Apr-04, 07:01 PM
Odd, there was no mention of qualifying on the basis of matching intrinsic color. :)

You mean like green or yellow? 8) :wink:
Yes. Just trying to do my part to make life simple. :)

[I doubt it will be yellow, contrary to the color version of Hertszprung-Russell. If we can't stand on their shoulders, then maybe we can at least stand on their toes. :wink: :) ]

Maddad
2005-Apr-04, 09:41 PM
Everyone always tells me different, one says 100 billion sun-like stars, another will tell me 50 billion, then one guy would go up as high as 200 billion, then 8 billion...Naturally. A sun-like star means something slightly different to each person, and there is a variance in our best estimates of the number of stars in the galaxy from 200 to 400 billion stars. If you include only G5 type stars as being sun-like, and you estimate the number of stars in the galaxy at 200 billion, then you will see a very low estimate. If you include all G type stars then you get a much larger estimate. If you say that F and K type stars are close enough to be called sun-like, or even maybe M type stars as well, then your estimate will most all the stars in the galaxy.