Cetus the Whale

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Winter nights in Oregon are typically known for showing us the remarkable staying power of clouds. Nonetheless, there are often some pretty clear nights to take advantage off. One constellation worth searching out is Cetus, the Whale - in the South sky during an Oregon winter. You can find it by starting with the Square of Pegasus, which is located high in the South sky. By tracing a line connecting its easternmost stars, then continuing downwards, you'll run across Cetus.

Cetus and Neighboring Constellations

Mythological lore portrays Cetus as Neptune's sea monster that menaced the princess Andromeda. If you recall the mythology of Perseus, Andromeda, and Cassiopeia, you might remember that Perseus turned this sea monster into stone by use of Medusa's head. Maybe it was this failure that led to Cetus being "demoted" to the more benign figure of a whale. Neptune was a pretty peevish character, after all.

So what does this constellation have to offer astronomers? As the name suggests, Cetus is large; spanning more than fifty degrees across the South sky. For most of the year the fairly bright stars Diphda & Menkar distinguish it. Both are fine examples of orange giant stars. Their cooler temperature in relation to the Sun produces orange hues that make for a pleasing contrast to stars in the white-blue end of the spectrum. They're also quite larger than the Sun, which explains why they appear bright in spite of their distance from us - 60 light-years in Diphda's case and 150 light-years for Menkar.

But there is another sight in Cetus that predominates in astronomer's eyes. It's a star called Mira, which helps demonstrate how a star is a tug-of-war between gravity and the nuclear energy it creates. This conflict has led to a large class of what are called variable stars - stars that change brightness. Subdivisions abound within the variable category. A long-period variable (LPV) star like Mira is essentially a very large, old star in the later stages of its life. With this age comes an exhausting of the hydrogen fuel for the nuclear fusion inside of the star. This forces the star to "burn" other gases, such as helium. But this induces instability in an LPV star's output and size. Its energy causes the star to bloat outwards considerably. But it doesn't have nearly enough mass and energy to sustain this size. Gravity takes over, then, compressing the star to a much smaller diameter until the star's nuclear reaction compels another bloating. This rhythm just repeats, usually on a yearly period, to the point that it seems that the star is pulsating regularly. With this pulsation comes dramatic changes in the light output for the star - a typical LPV will brighten by as much as 250X throughout its cycle. Some can even brighten by a factor of 4,000X.

In Mira's case, it was discovered as a variable star back in 1596, well before the invention of the telescope. Its propensity for changing brightness led to the name of Mira - Latin for "wonderful". At its brightest, Mira will appear almost as bright as Menkar & Diphda. When it's dimmer, you'll need a good star chart and an abundance of patience to locate it. It'll take about a year to go through a full cycle, so there's plenty of time to track it.

Astronomers who track Mira's brightness over several years quickly notice another LPV quirk. They are very cool stars. Many in fact, have a surface temperature in the range of only 2,000 - 2,500 Celsius. That's about a third of the Sun's temperature. This translates into the deepest shades of red that you'll find in the skies. Many of us, in fact, got hooked into looking at the huge pulsating stars because of this wonderful color that they give to the skies.