Tuesday, October 19, 2021

How Bright is Bright? Part II

Close up detail of Orion and Canis Major looking Southeast at 9 PM March 25. Submitted

Close up detail of Orion and Canis Major looking Southeast at 9 PM March 25. Submitted

Tonight mighty Orion has begun sinking towards the west while the Spring constellations are beginning to rise in the east. Betelgeuse is flagged as the brightest star in Orion in the 400 year old Bayer catalog, but it is a variable star and usually Rigel is brighter. Let’s look at Rigel then and ask how bright it really is.

Rigel is one of the brighter stars in the sky tonight – you can see this magnitude +0.18 bright blue giant star in Orion’s lower right corner. Not too far distant from Rigel is the even brighter star Sirius shining at magnitude -1.45; using the system discussed last time and a calculator we can figure out the Sirius is  4.48 times as bright as Rigel. (Remember the larger the magnitude the dimmer the star.) But is Sirius brighter than Rigel because it is intrinsically so, or is it brighter because it’s a lot closer? The magnitudes given are apparent magnitudes – the relative brightness as seen from Earth.

To answer which star is intrinsically brighter we must figure out what magnitude each star would be if they were both the same distance away from Earth. This magnitude value is called absolute magnitude and the standardized distance used is 32.6 light years. (To explain why this odd distance was chosen is the subject for another article someday.) When we go through the necessary calculations we find that the absolute magnitude of Sirius is  +1.42 and Rigel is -6.7; in other words if both stars were side by side Rigel would shine out 1,770 times brighter! If Rigel really was 32.6 light years from us it could be easily seen in broad daylight and would cast a well defined shadow at night.

Now that we know Rigel’s absolute magnitude it’s easy to calculate its brightness at any distance. For example, what would Rigel look like at the center of our Solar System? It would be a scorching, bright blue ball 35 degrees across, some 24,000 times brighter than the Sun! Needless to say the Earth would evaporate like a drop of water in a skillet. You might wonder how a star can keep putting out this kind of energy forever – the answer is that it can not. A star like Rigel will burn through it’s fuel in a few million years; our Sun will last a thousand times longer or more.

While on the subject of magnitudes, the faintest stars shown on the accompanying picture are about +6.5 which are visible at a site with a truly dark sky. On urban Marco Island you would count yourself lucky to see down to magnitude +4, about a quarter as many, because of local light pollution. See you next time!

Mr. Usher is President of the Everglades Astronomical Society which meets every second Tuesday at 7:00PM at the Norris Center, Cambier Park, Naples.


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