Home > Aurorae, Ephemeris Program, stars > 03/17/16 – Ephemeris – Why are there no green stars?

03/17/16 – Ephemeris – Why are there no green stars?

March 17, 2016

Ephemeris for St. Patrick’s Day, Thursday, March 17th.  The Sun will rise at 7:50.  It’ll be up for 12 hours and 1 minute, setting at 7:52.   The Moon, 2 days past first quarter, will set at 5:10 tomorrow morning.

Today we celebrate the patron saint of the Emerald Isle.  And green is the color of the day.  When we look to the skies we don’t see a lot of green.  Well, maybe in the Hubble Space Telescope’s false color photographs like the original Pillars of Creation, where green represents hydrogen, and in the northern lights.  The colors we see in stars are red or orange if they are cooler than the Sun, yellow if they are the same temperature as the Sun, and white or bluish if hotter than the Sun.  In the spectrum of light we can see, green is in the middle, between yellow and blue.  As a matter of fact the Sun radiates its energy most heavily in the green.  So if you got rid of those other colors the Sun itself would be green.

Times are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

Star colors

Star cluster showing star colors, probably enhanced. Source uncredited image from planetsforkids.org.

Black body radiation

“Black body” Radiation Curves by temperatures. The Sun’s surface temperature is around 5800 Kelvin. Note the peak radiation besides increasing in amplitude slides from red to blue with increasing temperature.

Aurora looking north at 10/24/2011 at 10:52 p.m.

Aurora looking north at 10/24/2011 at 10:52 p.m.  The first color visible in an aurora or northern lights is green.  More active aurorae give off other colors.  Credit:  Bob Moler.

Aurora overhead

Looking overhead in an active aurora, and the variety of colors, even green. Credit: Bob Moler.

Pillars of Creation

Pillars of Creation in false color by the Hubble Telescope. Sometimes the colors are given to specific elemental emissions, of shifted because the colors represent radiation that is invisible to the human eye. Credit: NASA/ESA/HST.

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