Mars Meets Monstrous, Mutable Mira


Mars can help us find the variable star Mira in the constellation Cetus the sea monster. The two are currently just a fist and a half apart. Mira is a pulsating variable star that physically expands and contracts, brightening and fading over a period of 332 days. Bob King

The Red Planet squares off with a red giant this month. Mars is currently just 15° (a fist and a half) above and to the left of the one of the best-known variable stars, Mira the Wonderful. Mira gives cause for wonder because it occasionally glows as bright as the North Star and but then disappears from naked-eye view a few months later. 

Mira animation. Merikanto / Wikimedia Commons. CC BY-SA 4.0

Mira's light varies because it pulsates. and changes in size which affects its temperature and brightness. Picture blowing up a balloon, letting out some of the air and then blowing it back up again. When Mira contracts it heats up and brightens, and when it expands it cools and fades. 

You need a small telescope to see the star minimum, but right now it's close to maximum brightness for the current cycle and shines at 3rd magnitude. That's a magnitude fainter than the stars of Orion's Belt, but it's still easy to see especially with Mars as your guide.

Mira is 1.2 times as massive as the sun but much larger! Its diameter varies from about 287 million miles (332 times the size of the sun) to 348 million miles (402 times solar). Mira is huge because it's old, about 6 billion years, and changes happen inside geriatric suns that make them puff up.

To find Mira, connect Mars and the bright stars Hamal and Algenib with Mira to form a giant "Y" in the sky. The Y measures 4 fists on the long side and 2 fists on the short. Mars is moving to the west this month, so its position is shown on two dates. I included the magnitudes of three stars near Mira if you'd like to estimate the star's brightness yourself. Stellarium 

The sun fuses hydrogen into helium in its core, liberating the energy that makes it hot and bright. Mira burns both hydrogen and helium. Helium burning occurs in a shell around its core and produces carbon and oxygen as byproducts. Hydrogen burns in a shell closer to the surface and causes the star to expand to gigantic proportions. If you replaced the sun with Mira it would extend two-thirds of the way to Jupiter. Earth would not fare well in this scenario.

Mira is so big that the Hubble Space Telescope shows its distended, football-like shape. The star's appearance may be caused by the expansion-contraction cycle or dark starspots on its surface. NASA / ESA 

Variations in the rate at which heat from inside the star escapes to the outside leads to pulsations. Unlike the sun, Mira is out of balance. Radiation generated in the sun's core neatly pushes back against the pull of gravity, and its diameter holds steady. Mira gets overly hot inside and expands. Expansion causes its outer layers to cool and then contract again, initiating another cycle. 

Mira's instability makes it fun to observe. If you watch the star starting now, when it's brightest, you'll see it fade over fall and winter. In your mind's eye you picture is growing larger and larger and cooler and cooler. By the time Mars resumes its normal eastward motion and passes near the star again in January, Mira will probably require binoculars to see.

Most stars appear the same brightness year in and year out, but you can always count on Mira to spice things up.


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