Sirius A nd B

Hi All,
This is a question that I've been pondering on for a while now, I would like to know is their an equation linking a stars life expectancy, spectral class and mass,
i.e how old is a star given its spectral class and mass and how long is it going to live.

The reason I'm asking this is because of Sirius A and B.
As Sirius B is now over 7 arc seconds and is entering the realm of visibility in amateur scopes I was wondering how Sirius B actually exists, Sirius A is a main sequence star 2.14 times the suns mass, so been an A1 class star is not very far into its life, yet has a shorter life expectancy then the sun would have.

There is a white dwarf in orbit, this dwarf must have had a mass less then 1.4 solar masses otherwise it would have turned into a neutron star (correct me here if I'm wrong), so it should have lived longer then Sirius A, yet Sirius B died off first, of course I'm presuming that the 2 star started life at approximately the same time in the same nebula.

Does anyone have any thoughts on this?

Perhaps Sirius B had more mass then sirius A to begin with but succeeded in getting rid of it all at the planetary nebula stage so it was able to fall under the 1.4 mass limit to form a white dwarf. Thats why it died frist.

Good luck imaging it

Sirius A & B may have formed in the same nebula but there are several possibilities to explain the “little and large” duo we see today. It is likely that the two stars developed separately in different parts of the nebula with Sirius B forming the larger of the two from the greater gas/dust density in its vicinity. This may explain why it is now a white dwarf while Sirius A (which may have formed in a less dense part of the nebula) has burned slower and longer. The two may then have captured each other at some stage. It is also entirely possible that I am completely mangling the H-R Diagram of stellar evolution… so treat this with caution!

At any rate, Burnham’s Celestial Handbook suggests that Sirius B may have been in the Red Giant stage as recently as 2000 years ago. See Vol I – pages 387 – 434 for a little bed-time reading!

Burnham’s is a useful reference and contains some fascinating descriptions, but it is a wee bit out of date at this stage, in terms of modern theory. If you want a good ‘readable’ book on astronomy and astrophysics (that won’t make your brain bleed trying to get around heavy concepts!) then I would recommend “Universe” by Kaufmann. It has an excellent section on stellar evolution amongst other things.

Phil

A red giant only 2000 years ago, I suppose that sirius A either swallowed or blew away the gas from the planetary nebula in that scenario.
As for imaging it, that would be quite a job !

Various books say that it is possible to view Sirius B with even a small scope, just that it requires very steady seeing. Another trick might be to put an occulting bar across the exit pupil of the eyepiece (a strip of black electrical tape will do... as long as it doesn't touch the lens!) You could then rotate the eyepiece until Sirius A is just covered or occulted by the tape. Sirius B should pop into view!

Phil.

Hi Dave,

There is a big smile on my face since this is something I have already looked into AND experimented with.

The primary, Sirius A, is so bright that it floods the immediate area on the imaging chip and which exceeds the 7" distance between the primary and secondary. For example, I am imaging with an image scale of 1.27"/pixel (AP160, ST2000XM) which means I have Sirius B at a mere five to six pixels from the primary. Now, add the blooming due to the great magnitude of Sirius A and you have a serious (impossible?) challenge.

An occulting bar was mentioned but I am not sure how it would be incorporated when imaging. I thought of trying to move Sirius just off the FOV of the imaging chip but no luck yet.

Also, add less than ideal seeing and the five to six pixels (in my case) become even less.