No Irish

17 March 2013

It’s St. Patrick’s day, and that means many will celebrate their Irish heritage and wear the green. But stars never wear the green. You will never see a green star.

You would think that stars could be green. After all, the color of a star depends on its temperature. Small cool dwarf stars and large cool supergiants are red. Warmer stars such as our sun are yellowish. Very hot stars appear blue. So what about a star hotter than our sun, but not hot enough to be blue? Why doesn’t it look green?

A green star? SOHO-EIT
A green star?

The reason is due to the way our eyes perceive color. We can detect red, green and blue light, but our range of colors is very narrow compared to the full range of light that is possible. So if a star is very bright in the range we see as green, it is also very bright in the range we see as red and blue. As a result our eyes see all three colors about equally, which our minds interpret as white. So we see stars that are red, yellow, blue, and white. But no green. So you’ll never see an Irish star.

There is, however, a long history of Irish astronomers, for example:

William Hamilton - He made advances in optics and dynamics. Also did a bit of mathematics. Devised a method used in computational physics and many other areas, now known as the Hamiltonian.

Annie Maunder - First woman inducted into the Royal Astronomical Society. She studied sunspots, and with her husband Walter demonstrated that the “little ice age” of the 1600s correlated with a period of unusually low sunspot activity, now known as the Maunder minimum.

So what is this green star above? It’s a false color image of the sun as seen at ultraviolet wavelengths. On St. Patrick’s day, even the Sun can pretend to be Irish.