Celestia Forums

First, merry christmas!

Second, does anyone have any info on the distribution of masses of stars in our neighbourhood (within a few hundred lightyears)?

The only data I've got says that about 71% are M stars, 14% are K stars, 10% are G stars, 4% are F stars, and the rest are 0.9% are A stars, and the rest are O and B stars. However, I don't have a clue where I got that from now or if it's accurate, hence I'm looking for more up-to-date, confirmable info .

That said, I'd ideally prefer that the stars were broken down by mass rather than spectral class. i.e. how many stars are around 0.1 solar mass, how many are around 0.2 solar mass, 0.3, 0.4, etc. Does such data exist in that form?

Oh, Evil One,

Have you investigated the NStars database?

A mass distribution plot of stars known to be within 25 Parsecs is available at http://www.nstars.nau.edu/index.cfm?fuseaction=nstars.mass

Note, however, that the database is incomplete. Most of the nearby stars have yet to be discovered. See http://www.nstars.nau.edu/index.cfm?fuseaction=nstars.paper

Merry Christmas!

Hi Selden,

Thanks - that graph is something like what I was after, but as you say, it's incomplete. I suspect that's probably the best we can get too.

Still, it does seem to confirm that there are generally more stars in each mass range as the mass drops (i.e. there are more stars with 0.1 solar masses than 0.2 solar masses than 0.3 solar masses).

Do you know if my percentages were in the right ball park? ISTR it came from a picture showing all the stars as a big triangle, which was mostly red (M type), and then the other star types (orange for K, yellow for G, etc) were all crammed in at the top.

With so many objects in close proximity to the Sun, does anyone know of a formula to figure out what is the closest another star has ever passed to the Sun in the Sun's 5 billion year history? Maybe the answer would be in the form of "> 50% chance that another star has passed < 0.1 ly.

I found a formula once, but it only took into account the density of stars in the Sun's neighborhood, and their average velocities. But twice per galactic orbit the Sun passes through the much denser galactic plane. Also, the Sun is currently on the edge of a spiral arm. But often it should pass through the denser middle of an arm. Anyway, I can't find that formula anymore.

Many of the extra-solar star systems discovered to date have planets in highly eccentric orbits. Is is possible that they got this way because another star once passed through or near the first star's planetary region?

The planets in our solar system have very round orbits. Even Pluto's orbit seems round in comparison to some of the extra-solar discoveries. Obviously, another star has never passed through our planetary region, or the orbits of our planets would be seriously and permantly affected. Is our solar system just lucky to have successfully tip-toed through the galactic mine field for 5 billion years? Or is there so much space between the stars that such an encounter is unlikely? Or maybe the answer lies inbetween the two extremes. Maybe half of all star systems have suffered a close stellar encounter, and half have not.

Any guesses?

tony,

This won't answer your question, and it's probably no help at all really. But it does look really cool!

I recently found a java applet which allows you to create a fictional star (Rogue star) which passes through the solar system. Based on the parameters which you input (mass, perihelion etc) it calculates the gravitational effects of the star on the planets of the solar system graphically in real time.

http://janus.astro.umd.edu/orbits/rstar.html

Ok, so it's 2D, and not all that detailed, but how often do you get to see planetary pinball on this scale?!

Tony,

It's worse than you think!

The Sun's orbit around the galactic center isn't Keplerian. Since there's a lot of mass concentrated in the galactic plane, the sun is pulled back toward it rather strongly. The period of its vertical passage through the galactic plane seems to be about 70 million years, while the period of its orbit around the center is more like 240 million years. i.e. it passes through the galactic plane about 7 times during a "galactic year".

One discussion of this is at http://nedwww.ipac.caltech.edu/level5/Sept02/Bertin/Bertin13.html

Jack, thank you for the link. It's fun to play with stuff like that. You can also simulate it with GravitySimulator, a program I wrote, which is avaliable at GravitySimulator.com. This lets you do it in 3-D, so the star can pass from above or below. You can do further passages too that just knock the outer planets from their orbits, causing them to fall into the inner solar system for part of their orbits. I've set up some really cool looking simulations, where the intruding star actually steals Venus and Earth into orbit around itself. That certainly would have been the best "Top stories of 2003" if it happened last year.

Selden, I never knew that. I've read a lot about the theories of mass extinctions, and how they happen on periodic timescales. One of the theories why is because the Sun's passage through the galactic plane stirs up the Oort Cloud. But all I've ever read seemed to suggest that this only happens twice per orbit. Thanks for the info.

Tony,

Here's a "popular" article describing how crater impacts seem to happen more often every 35 million years -- consistant with the 70 MYear period: http://www.giss.nasa.gov/research/intro/stothers_03/

An abstract of the original articles (published in Nature in 1984 and 1985) are here: http://pubs.giss.nasa.gov/abstracts/1985/Stothers.html
and here:http://pubs.giss.nasa.gov/abstracts/1984/RampinoStothers1.html

I found a few more recent publications, but they all had similar numbers.

We're in the "much denser galactic plane" at the moment. The Sun is only 50 light years north of the galactic plane, and, according to Allen's Astrophysical Quantities, stellar density is constant for 50 parsecs (~160 light years) either side of the plane.
The spiral arms don't help much, either - although they're very much richer in gas and dust, the overall density of stars within the arms is only about 10% greater than in the gaps between the arms. They just look as if they're full of stars because they're regions of rapid star-birth - so they contain the highly visible OB associations, which are so short-lived they never orbit out of the arm they were born in; whereas long-lived stars are orbiting in and out of spiral arms all the time. But from a distance all you see are the brightest stars, which therefore mark out the spiral arms.

A formula you may have seen is:

N = pi*D^2*v*rho

where N is the encounter rate, D is the distance of the encounter, v is the Sun's velocity relative to the local stars, and rho is the local stellar density.

Local star density is around 0.08/pc^3; the Sun's peculiar velocity relative to the surrounding stars is ~20km/s (~20pc/Myr, since the number of kilometres in a parsec is roughly the same as the number of seconds in a million years). So the above equation reduces to:

N = (5*D^2)/Myr

So once in a million years a star will pass 0.45 parsecs from the Sun; once in a billion years within 0.014 parsecs; and once in 5 billion years within .006 parsecs (~1300AU).
That's not to say that closer encounters don't occur, but it does seem that in this neck of the woods they'll be too rare to explain the frequency of close-in elliptical orbits we're seeing in other solar systems.

Grant

Grant,

Do you have a reference for the sun being in the plane? I've seen quite a few comments that the sun is quite a ways away from it. (Although, of course, lots of people tend to repeat obsolete information without realizing it's outdated.)

Added seconds later:
http://www.aas.org/publications/baas/v26n4/aas185/abs/S2206.html (dated 1995) says 8-16 parsecs. which isn't much.

Thanks anyhow.

My reference is from The Guide to the Galaxy, Henbest & Couper, 1994 - a summary of the relevant section appears here. Frank Bash's original calculations in the 1980s (Henbest and Couper's source) were (I think) based on the dynamics of the situation - the Sun's velocity relative to the local average, coupled with the estimated mass distribution of the galaxy. It's good to see his calculations falling within the error box of an independent test.

Grant

Sorry to resurrect this, but I thought an update might be in order. I found a star list at http://www.chara.gsu.edu/RECONS/TOP100.htm of the 100 nearest stars (which goes out to 6.86 parsecs from Sol), and according to that the spectral type distributions break down as follows:

M: 73.6% (103 stars)
K: 13.6% (19 stars)
G: 5% (7 stars, including Sol)
F: 0.7% (1 star - Procyon (F IV-V))
A: 1.4% (2 stars (one V, one IV-V))

White Dwarf: 8
Brown Dwarf: 5
Planet: 3 (+ solar system)

So the M and K V stars roughly match the proportions I posted at the top of the thread, but the G, F, and A stars are somewhat underrepresented compared to that. I suspect that's more down to "small statistics" though (it'd be odd if there are really more A stars than F stars, for example).

Annoyingly enough, they don't seem to have made the full 10pc survey publicly available .

Well, there is the catalog

V/101 Nearest stars until 10pc (Zakhozhaj, 1979-1996)

http://vizier.u-strasbg.fr/viz-bin/Cat?V/101

It includes 356 stars and claims to be 70% complete, whatever that means.

selden wrote:Well, there is the catalog

V/101 Nearest stars until 10pc (Zakhozhaj, 1979-1996)

http://vizier.u-strasbg.fr/viz-bin/Cat?V/101

It includes 356 stars and claims to be 70% complete, whatever that means.

Hrm. A bit old, I think... but thanks anyway. Besides, I just converted those closest 100 stars by hand to galactic coordinates and distances, so I've had enough of doing that for now