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RalofTyr
2007-Feb-24, 12:41 AM
Why does Procyon and Sirius have white dwarfs orbiting them? To my knowledge, through stellar evolution, white dwarfs come from G, K and M stars, which have a lifespan of at least 10 billion years. So, why do Procyon and Sirius, stars maybe only a billion years old, have a white dwarf orbiting them?

Van Rijn
2007-Feb-24, 01:20 AM
Due to mass loss during the red giant phase, stars perhaps eight times the mass of the sun while on the main sequence could become white dwarfs. Sirius B was probably about five times the mass of the sun. So white dwarfs could evolve from stars with a spectral class of F, A, or B.

Ken G
2007-Feb-24, 02:19 AM
Yeah, a star with a mass 5 times the Sun can probably make a white dwarf in a billion years or so, and it seems like a billion years is about the shortest time you can make one. Also, one always has to look out for mass transfer in the case of a close binary.

Romanus
2007-Feb-24, 06:57 AM
That's a pretty good question. As the others have said, the WD companions were once the more massive stars in the binaries, but lost most of it in the transition.

BigDon
2007-Feb-24, 07:35 AM
Aren't white dwarves technically naked stellar cores?

Ken G
2007-Feb-24, 09:31 AM
Aren't white dwarves technically naked stellar cores?

Sort of. They are quite a lot smaller than the core of our Sun right now, but they are what the core of our Sun will become in time.

RalofTyr
2007-Feb-24, 04:59 PM
If the white dwarf on both Sirius and Procyon were more massive than the A stars, wouldn't the A stars have orbited the more massive star, then when it became a red giant, shedding mass, the orbits were reversed? What's the odds any worlds remained in a stable orbit after that?

Kaptain K
2007-Feb-24, 05:20 PM
If the white dwarf on both Sirius and Procyon were more massive than the A stars, wouldn't the A stars have orbited the more massive star, then when it became a red giant, shedding mass, the orbits were reversed?
Both stars orbit the barycenter of the system, whatever the relative masses.

What's the odds any worlds remained in a stable orbit after that?
Huh? Why shouldn't they?

RalofTyr
2007-Feb-25, 12:09 AM
Then the barycenter would shift, disrupting the orbits of a few worlds, if they existed.

Secondly, wouldn't some of the shedded mass of the heavier star be accreeted to existing or perhaps forming new worlds in the Procyon/Sirius system?

Kaptain K
2007-Feb-25, 01:46 AM
Then the barycenter would shift, disrupting the orbits of a few worlds, if they existed.
First: The barycenter is not a physical object. It is merely the point around which two (or more) objects revolve. The shift, in and of itself, does not affect the planets of the two stars.
However, the barycenter shift is due to the "B" component losing mass. As the star loses mass, its grip on its planets is reduced and their orbits expand. Some of its outer planets could be lost to the other star or even be flung out of the system completely.


Secondly, wouldn't some of the shedded mass of the heavier star be accreeted to existing or perhaps forming new worlds in the Procyon/Sirius system?
Probably not. The stellar wind is very hot and thin. Any amount captured by the planets would be negligible.

Van Rijn
2007-Feb-26, 03:18 AM
Secondly, wouldn't some of the shedded mass of the heavier star be accreeted to existing or perhaps forming new worlds in the Procyon/Sirius system?

It probably wouldn't form worlds. However, Sirius B might have increased the metallicity of Sirius A.

Van Rijn
2007-Feb-26, 03:22 AM
Yeah, a star with a mass 5 times the Sun can probably make a white dwarf in a billion years or so, and it seems like a billion years is about the shortest time you can make one. Also, one always has to look out for mass transfer in the case of a close binary.

Much less than that. A star five times the mass of the sun has much greater luminosity and evolves much quicker than a star with twice the sun's mass. The Sirius system is thought to be about 250 million years old, and Sirius B went off the main sequence 120 million years ago or more.

For instance, see here:

http://www.astronomy.com/asy/default.aspx?c=a&id=3391

Ken G
2007-Feb-26, 03:02 PM
Good point, somehow I took 5 to the 2.5 power (the usual approximation for finding the lifetime of a star) and didn't get 50 or so, which is the factor one would expect the main sequence lifetime of Sirius B to be less than the Sun's value of about 10 billion years. So 10/50 ~ 200 million years, not a billion. My bad.

RalofTyr
2007-Feb-26, 11:38 PM
Too bad Sirius and Procyon's ** stars were probably further away and did not provide any light for the dinosaurs to read bed time stories.

It's odd that Procyon and Sirius's B stars were both red giants at the same time.


As the star loses mass, its grip on its planets is reduced and their orbits expand. Some of its outer planets could be lost to the other star or even be flung out of the system completely.

Most planets, like Jupiter's moons, would have to orbit close to their parent stars. Any worlds orbiting the B stars would have been gobbled up by the red giant. Any outer worlds, most likely Oort objects, could remain stable.


I say there is an invisible elf in my backyard. How do you prove that I am wrong?


With a napalm flamethrower. If he's in your backyard, napalm's sticky and I'm sure he'd catch on fire, and scream and run.