Stars brightness

Hope I'm in the right place. While the sky is not permitting viewing, as usual, I'm spending time on Astrosuite spherical projection. I'm currently looking at two stars, Sirius and Rigel. It tells me that Sirius has a mag of -1.4 and a distance of 7 light years, while Rigel has a mag of .12 and a distance of 913 light years. Obviously the difference in distance isn't the reason for a comparatively small difference in magnitude - or is it? What is the cause of the difference. I'm assuming here that Rigel must be a huge star of sorts? If so what type is it and what type is Sirius?

Hope I'm in the right place. While the sky is not permitting viewing, as usual, I'm spending time on Astrosuite spherical projection. I'm currently looking at two stars, Sirius and Rigel. It tells me that Sirius has a mag of -1.4 and a distance of 7 light years, while Rigel has a mag of .12 and a distance of 913 light years. Obviously the difference in distance isn't the reason for a comparatively small difference in magnitude - or is it? What is the cause of the difference. I'm assuming here that Rigel must be a huge star of sorts? If so what type is it and what type is Sirius?

Differnt stars have different actual brightnesses (absolute magnitude), this is caused by a number of factors but the main ones are size and temperature. The bigger a star is the more area it has emitting light so the more light there is, the hotter a star is the brighter each square meter shines so the brighter it is. When seen from earth there is a third factor that determines the stars "apparent magnitude", how far away it is, the further away the dinner the star.

Hope that goes some way towards explaining the differences in star brightnesses.

Bart.

Absolute Magnitude: is how bright the stars actually are when seen from a uniform distance of 10 parsecs, or 32 light years.

Apparent Magnitude: is how bright the stars appear to be. This is what we see indicated by star atlases.

Therefore, even though there seems little difference in apparent magnitude between Sirius and Rigel, when their absolute magnitude is taken into consideration, you would now know that Rigel will far outshine Sirius hands down! Sirius is a White Dwarf, Rigel is a Blue Giant, hundreds of times bigger.

Luminosity: is the measure of how much energy a star emitts each second - depends on its size and temperature.

Changes in magnitude are 2.5 times the previous. For example, a 1st magnitude star is 2.5 times brighter than a magnitude 2 star. Therefore, a 6th magnitude star is 100 times fainter apparently than a magnitude 1 star.



Seanie.

Sirius is a White Dwarf

just to clear up . . . the companion to Sirius (Sirius is the White Dwarf and very faint. The primary is twice as massive as our Sun and has a surface temperature of over 9000°K.

a great resource to check out is;

www.astro.uiuc.edu/~kaler/sow/sowlist.html

John F.

Another site worth checking out is this:
zebu.uoregon.edu/~soper/Stars/hrdiagram.html
It explains the Hertzsprung-Russell Diagram - probably THE most important diagram in all of astronomy. It correlates star luminosity to radius, mass and temperature.
Clear skies,

Sirius is a White Dwarf

just to clear up . . . the companion to Sirius (Sirius is the White Dwarf and very faint. The primary is twice as massive as our Sun and has a surface temperature of over 9000°K.
.

Cheers John!

:oops: