Thursday, January 28, 2016

Winter Hexagon Star Color Comparison

The Winter Hexagon or Winter Circle is an asterism (not an official constellation) that takes up a huge portion of the southern sky in winter with some of the brightest stars (including the brightest Sirius). You really can't miss it! As the hexagon moves across the sky through the season, it becomes the Spring Arch setting in the west at dusk.

Did you know the human eye can detect the temperature of stars? Amazing! We know that cool stars radiate and emit energy in the red and infrared regions of the electromagnetic spectrum and so they appear red or orange. Likewise, hot stars appear white or blue because they radiate their energy in the blue and ultra-violet wavelengths.

Using the same technique from a previous color comparison, I wanated to look at the entire Winter Hexagon. I also used the same exposure settings to keep everything constant, so the difference in brightness is also relative to the star's perceived brightness to us (Sirius is clearly the brightest). This makes it a little harder to determine color, because some of the stars are dimmer, but it's clear at least that there is a variety of color out there in this popular region of the sky.

I arranged them in this composite photo to show their position in the hexagon. It looks like a jewel box of beautiful colored orbs or marbles. This was fun and easy to put together. I feel like trying it again with different exposures to hold the brightness constant and then trying to organize them from coolest to hottest. Do you think you could? Which one looks the most blue?

winter hexagon star colors
Composite out of focus stars from the Winter Hexagon in their relative positions, all taken at 300mm, f/5.6, 1 sec, ISO 200
winter circle star colors
Winter Circle star colors with labels

It looks like last time I used ISO 800 and 1.3 sec exposures, which would have blown out Sirius for comparison but made Rigel brighter blue so it's easier to see. I bet if I looked at the histogram instead and just focused on getting that centered for each star I could get a good color comparison while holding brightness constant (or at least not washing anything out to pure white).

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