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Ephemeris: 06/25/2026 – The color of stars

June 25, 2026 Leave a comment

This is Ephemeris for Thursday, June 25th. Today the Sun will be up for 15 hours and 34 minutes, setting at 9:32, and it will rise tomorrow at 5:58. The Moon, halfway from first quarter to full, will set at 3:03 tomorrow morning.

Looking out at the night sky casually, the first impression is that the stars all appear to be white. Closer inspection shows that some appear with tinges of red or orange or maybe yellow while other stars have a bluish cast to them. The color of stars is due to their surface temperature which physicists call black body radiation. A rainbow is a spectrum of colors from red through orange yellow green blue and violet. These colors represent the different wavelengths of light. The peak wavelength determines the star’s surface temperature. The Sun’s in the green in the middle of our visual field. Cooler stars have their peak in the red, while hotter stars have their peak output to the blue end of the spectrum.

The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.

Addendum

The curved lines are the black body emission curves of these three particular stars the blue one the yellow one and the red one. The peak of each moves farther into the blue as the surface temperature of the star increases.
The curved lines are the black body emission curves of these three particular stars the blue one the yellow one and the red one. The peak of each moves farther into the blue as the surface temperature of the star increases. The temperature is in Kelvins. Think Celsius. Multiply by 2 (1.9 if you’re picky) to get the approximate temperature in Fahrenheit.
The spectral class of the star is determined by its surface temperature which is shown by its color.
The spectral class of the star is determined by its surface temperature which is shown by its color. The alphabetical classes used to be in alphabetical order by the strength of the hydrogen absorption lines in their spectrum. This turned out to be not useful. So about to turn it the last century Annie Jump Cannon, a Harvard College Observatory human “computer”, and an expert in determining spectral classes of stars, had them rearranged in this particular order by color. She even made-up a mnemonic to remember the order. It was “Oh Be A Fine Girl, Kiss Me”. Of course nowadays you could substitute guy for girl.
Annie Jump Cannon (1863-1941)
Annie Jump Cannon (1863-1941)

Categories: Concepts, Ephemeris Program, Stars Tags: ,