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Red Dwarfs

by Sastronaut
Tags: dwarfs
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Sastronaut
#1
Mar17-14, 09:34 AM
P: 66
Why are the nearest stars to Earth mainly red dwarfs? When inspecting a list of approx. the 12 brightest stars vs. the nearest stars I see clear differences in luminosity. Again, what explains why these lists are so different in terms of spectral type?! Does it have to do with the amount of interstellar gas present during star formation; in that their is less gas present closer to Earth than further away? I do know that red dwarfs are the dominate spectral type of star in our galaxy. Any thoughts on this would be great :) thanks pf!
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D H
#2
Mar17-14, 09:42 AM
Mentor
P: 15,170
Quote Quote by Sastronaut View Post
Why are the nearest stars to Earth mainly red dwarfs?
Because most of the stars in the galaxy, and presumably in the universe for that matter, are red dwarfs. They outnumber other stars 3:1.
Sastronaut
#3
Mar17-14, 09:49 AM
P: 66
DH I understand that completely, but what accounts for nearest stars being mainly dwarfs, the closest to Earth? Is it simply luck of the draw? Why isnt say an O5 III spectral type star among the list of the nearest stars?

D H
#4
Mar17-14, 10:22 AM
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P: 15,170
Red Dwarfs

By way of analogy, imagine a jar containing 10 million balls. Of those balls, about 7.65 million of them are red, 1.21 million are orange, 760,000 are yellow, 300,000 are a yellowish whitish yellow, 60,000 are white, 13,000 are a bluish white. In addition to those big numbers, three of the balls are a very pretty blue color. You get to draw thirty balls from the jar. What are the odds you picked one or more of those 3 rare blue balls?
Sastronaut
#5
Mar17-14, 10:32 AM
P: 66
very low! i think i see where you are going with this!
snorkack
#6
Mar17-14, 03:20 PM
P: 386
Another reason is that "red dwarf" is actually a very broadly defined term.
See this:
http://en.wikipedia.org/wiki/List_of_nearest_stars
Out of the 54 "stellar systems" within 5 pc of Sun, including the solar, 9 contain only brown dwarfs. This leaves 45 stellar systems, with 61 stars.
Of these 61 stars, just 15 are not red dwarfs (and of these, 4 are white dwarfs, leaving just 11 burning stars). So yes, 3:1... without white dwarfs 4:1.
But of these 46, the brightest is AX Microscopii, at +8,76 absolute magnitude. The dimmest is SCR1845-6357 which despite its magnitude +19,41 is recognized as red dwarf, not brown. Thus the brightest red dwarf is 10,65 magnitudes, or about 18 000 times, brighter than the dimmest one. And for comparison, the brightest star in the region, Sirius, is just magnitude +1,42, so about 7,34 magnitudes brighter than AX Microscopii - less than 1000 times as bright.
In terms of mass, AX Microscopii, at 0,60 solar masses, is about 8 times more massive than the dimmest red dwarf, yet only 3,4 times less massive than Sirius.
But while all stars so much lighter and dimmer than AX Microscopii are one spectral class M, the few brighter stars are divided in 4 different spectral classes: A, F, G, K.
Grouping the stars by first letter of spectral class:
A: 1 (Sirius A)
F: 1 (Procyon A)
G: 2 (Sun, Toliman A, tau Ceti)
K: 6
D: 4
M: 46
But see the first number of absolute magnitude:
1: 1 (Sirius A)
2: 1 (Procyon A)
3: -
4: 2 (Toliman A, Sun)
5: 2 (Toliman B, tau Ceti)
6: 2
7: 1 (61 Cygni A)
8: 3 (AX Microscopii, 8,76 is red dwarf, 61 Cygni B at 8,31 and Groombridgge 1618 at 8,16 are not)
9: 1
10: 8
11: 8 (1 white dwarf Pup, other 7 red)
12: 1 (white Procyon B)
13: 7 (1 white dwarf)
14: 6 (1 white dwarf)
15: 10
16: 5
17: 1 red dwarf and 1 brown dwarf
18: 1 red dwarf
19: 1 red dwarf and 1 brown dwarf

In the range 10...16, the red dwarfs are still rather numerous.
Drakkith
#7
Mar17-14, 10:04 PM
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Keep in mind that the divisions between stellar categories are mostly arbitrary. It's one smooth journey from microscopic dust particles to asteroids, planetary bodies, and all the way up to the largest stars. The larger and more massive the objects are, the less of them exist.
Sastronaut
#8
Mar20-14, 04:32 PM
P: 66
Thank you everyone I greatly appreciate all of your insight! Adding on to what already has been said; i believe the volume of sky that is being taken into account is also important. Also, stars on the upper left part of the HR-diagram live short lives relative to dwarfs (which can live for an upward of a trillion years) which is a broad term as snorkack stated. Since, for example our own sun is classified as a dwarf.


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