Sirius B (APOD 6 Oct 2000)
Posted: Mon Oct 16, 2006 9:47 pm
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??
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??
).