The “Police Star”

This time of year is a good time to get out an observe the brightest star in the night sky, Sirius (mag -1.46). Sirius is one of only five stars that has a negative apparent magnitude (Apparent magnitude is a measure of how bright an object appears to us on Earth: The smaller the number, the brighter the star. Compare this with Absolute Magnitude which is a calculation of how bright the star would be if it were at the standard distance of 10 parsecs). Three other stars with a negative apparent magnitude are: alpha Carina (Canopus), alpha Centauri (Rigil Kentaurus), and alpha Bootis (Arcturus). Since I have several kids reading this blog now, I will leave the identity of the fifth star with a negative apparent magnitude as an exercise for them.

Sirius is about twice as bright as the next brightest star, Canopus.

Siruis is actually two stars, Sirius A, the scorchingly bright bluish-white one we see, and a nearly hidden white dwarf companion, Sirius B. There is some discussion of whether there is a Sirius C. If there is, its estimated mass is small, about 0.05 Msun, which places it very near the border between ‘really small star’ and ‘really big planet’.

Sirius A is so bright because it is close (only 8.7 light years away) and about twice as heavy (2.2 solar masses) as the Sun, which makes it burn brighter than the Sun. Sirius A is a spectral type A star, and it is expected to stay on the main sequence for only about 1 billion years (as opposed to 10 billion years for the Sun). It is about 300 million years old, and so we can expect it to start post-main-sequence evolution in another 700 million years.

I call it the “Police Star” because when it rises in the evening, it is very common to see it flash red and blue. This only happens with very bright stars, and only when they are low on the horizon. This effect is called ‘scintillation’.

Scintillation happens because stars are so far away that they are effectively a point sources. The Earth’s atmosphere is constantly in motion due to buffeting of warm and cool air, which have different optical densities. The very narrow path of light from a star is easily deflected by this air turbulence. Usually this deflection causes the stars to ‘twinkle’ (vary in brightness). The light from the star is being deflected so that in one moment you see it, and in another moment most of the light is deflected away from your eye.

If you have ever split a beam of light in a prism, you’ll recall that light refracts differently depending on its frequency (color). This is what makes the sky blue, the sunset red, and what breaks sunlight in to a rainbow. So, in the case of very bright stars, the light from the star is broken up just like sunlight through a prism. As its path to you bends and twists, you are sometimes in the path of the red part of the light, and sometimes in the path of the blue or some other color ( Green is also a very common color to see). In fact, if our eyes were sensitive enough, we could see this effect in more stars… it’s just the bright ones have enough light energy to show this effect for our eyesight.

A list of the brightest stars in the sky can be found on the SEDS website.


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