Looking Up: A River Runs Through It

Jan 13, 2020

Rigel, the Witch Head Nebula, and gas and dust that surrounds them lie about 800 light-years away.
Credit Mario Cogo (Galax Lux) / nasa.gov

This week's Looking Up has a certain flow to it as Hal talks about the river constellation.

There are lots of constellations up in the Colorado night sky all year round, with 88 total constellations all told. Today I want to tell you about one you may never have heard of, but is low in the Colorado sky right now, the river constellation, formally known as Eridanus.

Eridanus means, like I said, the river. It starts up near the celestial equator and runs around and below the far more famous constellation of Orion. Interestingly, Eridanus is actually one of the original 44 constellations mapped in ancient times by Ptolemy. It was associated with the Nile river, and contains one of the brightest stars in the sky, Achernar, but if you are much north of Texas, you won’t see that blazing blue star.

But here in Colorado you can see several interesting stars as well as IC 2118, better known as the Witch Head Nebula. This nebula is a very faint, defuse cloud of gas about 900 ly from Earth. It seems to glow with a blue light, but since the gas itself can’t really glow, that light must be coming from somewhere else. And in this case, the super bright Orion star Rigel is the source. Rigel is a blue star, but the nebula’s blue color doesn’t come from Rigel, but rather from the actual gases in the nebula reflecting more blue light than any other color, kind of in the same way we end up with a blue sky on Earth. So take a look at the Witch Head and get blued away.

If you’d like to take a closer look at Eridanus, or any of the other wonderful and amazing things in the sky, please visit csastro.org for a link to information on our monthly meetings and our free public star parties.