The constellation Cetus, part of which is outlined in this photo taken last night, is famous for the red giant variable star Mira (MY-ruh), which brightens to naked eye visibility for several months each year. You can use this photo and the maps below to guide you to it. Details: 24mm lens at f/2.8, 20-second exposure at ISO 1600. Photo: Bob King
Mira was true to its name last night. Around about moonrise I looked up in the southeastern sky in the direction of the constellation Cetus the Sea Monster and right before my eyes was a "new" star. Well, not exactly new. I’d seen it three weeks before through my telescope when it was still too faint for naked eye viewing. With all the cloudy nights between then and now, I lost track of Mira until last night’s pleasant surprise.
To find Mira, you can either use the little, finger-shaped Aries to drop down to Alpha Ceti or make a triangle connecting the Pleiades and bright Aldebaran in Taurus to Alpha. Two outstretched fists separate Aldebaran from Alpha Ceti. Mira is about four fingers to the right of Alpha. This map shows the sky around 9 o’clock local time as you face to the southeast. Maps created with Stellarium.
Mira means "the wonderful" in Latin, a name given it nearly four hundred years ago. The star was discovered in 1596 by David Fabricius (right), a German pastor and amateur astronomer. He assumed it was a nova or exploding star but saw it again some years later and realized he’d discovered a variable star instead. We’ve looked at variable stars several times over the past year in this blog. You might recall the Cepheid variable Delta Cephei in Cepheus or the eclipsing variable Algol in Perseus. Mira’s another type of variable star called a pulsating red giant. Since it was the first of its kind to be discovered and one of the brightest and easiest to observe, Mira has given its name to the entire class of "Mira variables". Over 6000 are known.
Mira variables are huge gasbags of stars with tiny, blazingly-hot cores. Mira contains nearly the same amount of material as our sun but spreads into a globe 700 times as large. Instabilities within Miras cause them to pulsate like a very slow beating heart. As the stars expand and contract, they get brighter and then dimmer. Unlike Cepheids, most Mira variables pulse with long periods ranging from around 100 to more than 600 days.
This Hubble Space Telescope photo reveals that Mira is shaped more like a football than a sphere. Astronomers speculate that Mira’s pulsations may distort its shape or that part of the star is covered with large starspots like our sun’s sunspots. Credit: NASA
Mira takes 331.6 days to rise from minimum light to maximum before fading back down to minimum again. Three weeks ago you needed binoculars to see it but last night to my astonishment, Mira was in plain view at magnitude 3.7. While that’s only about as bright as the brightest star in the Seven Sisters (Pleiades) star cluster it really jumped out considering there was "nothing there" last month. So yes, wonderful is exactly what I felt when I spotted Mira.
This map is a closeup version of the one above. The numbers next to the stars indicate brightnesses given to the tenth of a magnitude. The lower the number, the brighter the star. Last night I estimated Mira at magnitude 3.7. If you go out to see and take a moment to make an estimate, please e-mail me at firstname.lastname@example.org and I’ll include your result in a future blog.
You can find Mira too with the help of the Pleiades and Aldebaran. You can even watch it brighten and fade by comparing its light to several nearby stars. We’re coming up on maximum in mid to late November and no one’s sure just how bright Mira might become. Normally it reaches magnitude 3.5 but occasionally brightens to second magnitude — that’s as bright as the stars in the Big Dipper. By mid-winter Mira will fade and become invisible again to the naked eye.
Mira was once a star just like our sun but because of changes in what and how it burns matter inside its core, its atmosphere has expanded to vast proportions like a balloon blown to near bursting. Mira is so large that if put in the sun’s place it would reach all the way past Mars. A star that big with such a small core can barely hold onto its cool, red atmosphere. Over time, Mira’s red envelope will loosen and float away into space in a series of beautiful rings of gas called a planetary nebula. Its remnant core, now re-born as an Earth-sized star called a white dwarf, will continue to blaze on through the ages.
When you look at Mira tonight, you’ll not only see a remarkable star but gaze into the sun’s distant future.