Sirius B (APOD 6 Oct 2000)

[Orca]

Sirius, the brightest star in the sky, is in fact a binary star system. There are two stars...Sirius A, a main sequence white-blue star, and Sirius B, a small companion thought to be a white dwarf star.

I never thought much about it...but something occurred to me of late. Why would a white dwarf be orbiting a relatively heavy main-sequence star? How could they have formed together?

The problem is that only low to medium-mass stars become white dwarfs. Those stars live for many billions of years. The sun for example, which is thought to have white dwarf-dom in it’s future, is about 5 billion years old and only "middle-aged."

For a star to be whitish-blue, it ought to be pretty heavy, which suggests that it would have a shorter life than a mid-mass star. This is because the heavier a star is, the more fuel it has, and the faster it fuses that fuel.

White dwarfs are formed by mid-mass stars, which have much longer life spans. If these two stars formed together, how could the low-mass star have already become a white dwarf and the high-mass star still be in main sequence?!?

Again: we have a relatively hot massive star orbiting what must be an extremely old white dwarf.

What else could be going on? Could the two stars have formed separately and through gravitation "captured" each other? Unless they formed from the same molecular cloud, it seems unlikely, because most stars are light years apart.

Could Sirius B have been much more massive, but blown the majority of its material off in the supernova explosion...so it didn't have enough mass to condense into a neutron star...and thus became a lighter white dwarf instead? If so, where are the nebulous remains of such an explosion?

If Sirius A and B formed together, for B to have collapsed already (with relatively heavy Sirius A still in main sequence) it would have had to be extremely massive to have gotten through it's main sequence and eventual collapse so much faster than A. Yet that much mass suggests at the very least a neutron star; more likely still a black hole.

Am I missing anything??

[astro_uk]

Hi Orca

I don't think it is a problem, Sirius B supposedly had a mass around 5Msun whereas Sirius A has a mass of 2.1 Msun, its estimated that Sirius B became a white dwarf 120 Myr ago. (im getting this from Wiki ).

http://en.wikipedia.org/wiki/Sirius

So everything seems fine, they both formed together, Siruis B would have evolved very rapidly, with a main sequence lifetime of less than 100Myr, whereas the smaller Sirius A has a main sequence liftime of about 1Gyr. (Main sequence lifetime approx= const x 1 / M^3 where mass is in solar units and the constant is the lifespan of the sun, about 10Gyr.) So Sirius A still has about 800Myr left.

The number of 5Msun for Sirius B is a bit big to just form a white dwarf, but if the star could eject a lot of its mass before the end of nuclear burning it is possible to avoid becoming a neutron star. It is however very close to being one, it has a mass now of about 1Msun, and 1.4Msun is usually the limit for neutron star formation.

[Orca]

Astro, it just doesn't seem like it adds up!

The Chandrasekhar limit is only 1.4 solar masses. While it is true that many stars that have much more mass are able to lose enough of it, usually you can see some trace of it, like a planetary nebula or remnants of a supernova.

Shouldn't the Sirius system be enshrouded in a large nebula...or at least have some remnants extending outward, such as the Veil Nubula?

Edit: maybe this thread should get tossed over into the newly opened Cafe...I hadn't noticed it was open.

[astro_uk]

I don't think so, in 120 Million years is a very long time, more than enough time for the gas thrown out to be swept away or incorporated into a new generation of stars. The stars will have made about half an orbit of the Milky Way in that time, during the passing through the disc things ususually get stired up and stripped. The Veil Nebula is only 5000-10000 years old, it will not be around forever.

[Qev]

Also, don't stars in binary pairs that go into the red giant phase tend to lose a lot of mass due to gravitational interactions with their partner? This could also help to explain why Sirius B ended up a white dwarf instead of a neutron star...

[lewishb]

the ancients insist that Sirius was a red star...... it may be past its red giant stage,,,, if so what would that mean?