Age of visible stars?

[Yora]

I was just reading some philosophically ponderings about the ancient age of rock. Stars are also ancient, but unlike stars, rock is something you can touch.

Wich had me wondering: Almost all the stars that can be seen without a telescope are larger than the sun. Under perfect conditions with very good eyes, there are just about 8000 stars that can be seen, and I think only 4 of them are smaller than the sun. To be visible from Earth, a star either has to be extremely close or very massive. And massive stars have shorter lifetimes than smaller stars, and that by order of magnitude.

I believe the Earth is almost as old as the sun, and the sun is also halfway through its total lifespan. So probably half the stars with a size similar to the sun are older, and the other half younger. And some stars that are somewhat bigger than the sun that are now approaching the end of their lives could also be older than the Sun and the Earth. But the bigger they get, the shorter they live. And the smaller they are, the lower the chance they are visible.

Does that mean that most of the stars we see are younger than the Earth? Is rock generally speaking older than the stars?

[Gnoman]

Current scientific consensus is that most of the Solar system formed roughly around the same time as the Sun did, from the same matter. This was around 4800 megayears ago. The youngest star I can find in the list of brightest visible stars is Betelgeuse (8-8.5), and the oldest is Arcturis at 7100. This suggests that you may be right, with the caveat that a great many stars do not have an estimated age.

[Lvl 2 Expert]

The earth's crust is a bit younger than the Earth, closer to 2 billion years, for the continental/dry parts. But that's still older than quite a few large stars.

[Yora]

I found one calculator that gave an output for stars of 1.5 solar masses to have a lifetime of I think 3.7 billion years. And none of the stars on the wikipedia list of brightest visible stars with both mass and age estimates comtradict that.
Since rock gets frequently recyled and reformed through lava erruptions, the oldest rocks on Earth are about 4 billion years old.

This means any stars larger than 1.5 solar masses can't be older than ancient rocks. Of the 93 brightest stars listed by wikipedia, I found 10 stars that can't be ruled out to be older than 4 billion years:
Alpha Centauri A and B, Arcturus, Aldebaran, Gacrux, Hamal, Gamma Leonis A, Theta Centauri, Epsilon Scorpii, Alpha Phoenices. Of which 7 are within 65 lightyears, and only one more than 90 lightyears away.

Assuming that the same ratio persists once we go down the list of brightest stars (which I have doubts being the case), that would still mean that only 10% of visible stars could be older than really old rock.
Lots of rock is nowhere near 4 billion years old, but since most visible stars are short lived giants that live for one millions of years, I think one might be able to say many rocks are older than the average visible star.

[GrayDeath]

Didnt think about it that way before, but yeah, pretty obviously you are correct.

Hmmnnn....

[factotum]

This is certainly true of *visible* stars. Of course, the vast majority of stars are not visible and are rather small and dim--I think more than three quarters are classified as red dwarfs, and very few of those are naked-eye visible from Earth.

[Chronos]

No red dwarfs (assuming by that you mean M-class main sequence) are naked-eye visible from Earth at all. The brightest is Lacaille 8760, at an apparent magnitude of 6.67 (compared to a visibility cutoff of 6-- Lower numbers are brighter).

Of stars brighter than red dwarfs but dimmer than the Sun, you're looking at the K class and most of the G class (the Sun is a G2, putting it towards the bright end of G). There, you're looking at four or maybe five K stars (61 Cygni B is right on the edge, at 6.03), plus two G stars (tau Ceti, a G8, and alpha Centauri A, a fellow G2 that's just a hair brighter).

Actually, there are probably a few more: I'm working from a table of stars within 5 pc, and the Gs and some of the Ks (but none of the Ms) should be visible from somewhat further away than that. There might reasonably be about ten times that. But that's still a tiny, tiny proportion of all of the naked-eye visible stars, and almost none of the stars bright enough for anyone to notice or care about.

[hamishspence]

Almost all the stars that can be seen without a telescope are larger than the sun. Under perfect conditions with very good eyes, there are just about 8000 stars that can be seen, and I think only 4 of them are smaller than the sun.

From an earlier thread.

A bit more than that.

Ones listed with an apparent magnitude of less than 6.00 (the limit for suburban skies) but an absolute magnitude of more than the Sun's 4.83 (sometimes this required a little googling as well as using the Wikipedia list:

https://en.wikipedia.org/wiki/List_o...t_bright_stars

Alpha Centauri B (technically you're only seeing the pair, but B would be visible if it was on its own)
Epsilon Eridani
61 Cygni A (61 Cygni B would be at, or slightly below, the unaided eye visibility limit if seen on its own)
Epsilon Indi
Tau Ceti
40 Eridani A
70 Ophiuchi A
Sigma Draconis
Gliese 570
36 Ophiuchi star system (A and B would each be just about visible if on their own)
82 G. Eridani
HR 8832
Xi Boötis A (Xi Boötis B would be below visibility if seen on its own)
Gliese 105 A (others are much too faint to contribute to the system's visible brightness)
HD 4628
107 Piscium
Mu Cassiopeiae A (other is too faint to contribute)
p Eridani A & B (each is close to the unaided eye visibility limit - the combined system will be a little brighter)
Chi Draconis B (A is more luminous than the Sun, B is less luminous but still above magnitude 6)
41 G. Arae A (B is too faint to contribute)
Xi Ursae Majoris B (A is more luminous than the Sun, B is less luminous but still above magnitude 6)
HD_192310
Kappa 1 Ceti
HD 102365/HR 4523/66 G. Centauri A (B is too faint to contribute)
61 Ursae Majoris
HR 4458 A (B is too faint to contribute)
12 Ophiuchi
HR 511
Alpha Mensae
Zeta Herculis B (A is more luminous than the Sun, B is less luminous but still above magnitude 6)
54 Piscium
11 Leonis Minoris A (B is too faint to contribute)
HR 5553 A (B might be contributing a little to this - the whole system is at magnitude 6)
Zeta Reticuli 1
Zeta Reticuli 2 (the Zeta Reticuli system is a wide binary, separable with the eye: 2 is exactly as bright as the Sun, but very slightly less massive)
85 Pegasi A (B is dim enough not to contribute very much)
55 Cancri A (B is too faint to contribute)
HD 69830 (285 G. Puppis)
HD 147513 (62 G. Scorpii) - this one is a bit tricky - absolute magnitude is listed as 4.82 rather than below 4.83 - but luminosity is listed as 0.98x sun's - so I'm going with that.
HD 172051 (86 G. Sagittarii)
58 Eridani
HD 166 (V439 Andromedae - Gliese 5)
Pi 1 Ursae Majoris
Nu 2 Lupi
Psi Serpentis A (B is far too faint to contribute)
HD 38858


So, that's 46 systems with one or more stars in them with a lower luminosity than the Sun, but still above magnitude 6 - counting Zeta Reticuli as two since they are far enough apart that both are visible.


If you restrict it to apparent magnitude 5 however, the number plummets:


Alpha Centauri B
Epsilon Eridani
Epsilon Indi
Tau Ceti
40 Eridani A
70 Ophiuchi A
Sigma Draconis
Xi Boötis A
Xi Ursae Majoris B
Kappa 1 Ceti
HD 102365 A

Only 11 there.

No red dwarfs (assuming by that you mean M-class main sequence) are naked-eye visible from Earth at all. The brightest is Lacaille 8760, at an apparent magnitude of 6.67 (compared to a visibility cutoff of 6-- Lower numbers are brighter).

To be fair, exceptionally dark skies and good viewing conditions will push the visibility cutoff magnitude to the point where it may scrape in - so that particular star has been spotted with the naked eye under such conditions.