Meet Mira, A Wonderful And Astonishing Star

Mira is a red giant star just 1.2 times as massive as the sun but 700 times larger! It’s big enough to show a shape in this photo taken by the Hubble Space Telescope. The odd, football-like shape may be caused by its expansion-contraction cycles or surface features. NASA/ESA

There’s a new star in the night sky and its name is Wonderful. Literally. It’s called Mira, Latin for “wonderful” or “astonishing.” Of course, it’s not new, but most people have never set eyes on it. It goes “missing” for much of the year because it’s too faint to see with the naked eye. But for about two months it returns to view for all to see.

Most stars we see with the naked eye produce a steady light and don’t vary. Like the sun their light is steady. Mira is different. It belongs to a class of stars called long period variables. Approximately every 333 days (about 11 months) the star brightens from magnitude 9 or 10, when it’s only visible in binoculars or a small telescope, until it shines as brightly as the stars in the Big Dipper. When brightest the star is at maximum. Then it fades back to obscurity, or minimum, and peaks again to start another cycle. The time from one minimum to the next is called its period.

There are a lot of ways to find Mira! You can shoot a line diagonally across the Great Square and extend it 5 ½ fists to the lower left. Or you can drop down from Triangulum to find the constellation Aries, then look a little more than two fists below Aries. You also triangulate to Menkar (a Dipper-bright star) and from there to Mira using the Pleiades star cluster (Seven Sisters) and Aldebaran. Stellarium

Mira reaches peak brightness roughly once a year, and that time is now. I’ve been eyeballing it the past month watching it brighten from barely visible to its current magnitude 3.1, a tad brighter than the faintest star in the bucket of the Big Dipper. In binoculars it looks red-orange. No surprise because Mira’s a red giant star hundreds of times larger than the sun. But here’s the amazing thing — the monster star is only 1.2 times as massive, meaning that it’s little more than a huge, distended gas bag resembling one of those giant bubble-wand bubbles that twists and stretches in the breeze.

If we could peer deep inside Mira through millions of miles of thin gas, we’d find a dense core of carbon and oxygen “ash” leftover from the fusion of helium. Mira-type stars are ancient. They’ve used up all the hydrogen fuel in their centers and have moved on to fusing helium into more complex elements. The core will eventually contract under the force of gravity and evolve into a hot, superdense star called a white dwarf.

I took this photo of Mira and friends last night (Oct. 7). Menkar shines at magnitude 2.5. Mira is currently about 3.1, just a little fainter. You can compare Mira to Menkar to track its changing brightness. Bob King

During the transition from red giant to white dwarf Mira is unstable and pulsates, expanding and contracting like the expansions and contractions that cause the beating of your heart. When smallest, about 660 times the sun’s diameter, Mira appears brightest. When it’s all puffed out at 800 solar diameters, it’s faintest. How amazing to realize that the simple winking of Mira reflects on a scale almost unimaginable.

Although the bright moon will soon interfere you can find Mira in the next few nights by returning to the Great Square of Pegasus we touched on earlier this week. Mira shines from the big, relatively faint constellation of Cetus the Sea Monster. It’s east of Pegasus so it rises later. Mira stands about 20° (two fists) high in the southeastern sky around 10 o’clock local time in early October, but it’s higher and easier to spot closer to 11.

This squiggly thing is called a light curve. It shows Mira’s changing brightness through six cycles of ~333 days. Mira is brightest at the top of each hump and faintest at the bottom. AAVSO with additions by the author

Start at the Great Square, swing left to Andromeda and then drop down to the little triangle, Triangulum. Keep going till you find the equally small constellation Aries the Ram, which looks like a bent index finger. Then look a little more than two fists below Aries for 2nd magnitude Menkar and Mira to its right. You can also shoot a line diagonally through the Great Square or use the Pleiades and Hyades star clusters (with Aldebaran) to triangulate your way to the star.

Mira is likely to brighten further. If we’re lucky it will equal or better its neighbor Menkar. Peak brightness varies year to year. Last year Mira topped out at just 3.5 magnitude. This year it’s already brighter than that. After maximum the star will slowly fade. If you find it now you can watch it disappear in the next few months but continue to follow it in binoculars for much longer.

Let Mira astonish you!

Want more astonishment? Join me at the Marshall W. Alworth Planetarium on the University of Minnesota-Duluth campus Weds. evening Oct. 9 where I’ll be signing my new book Urban Legends from Space starting at 6:30 p.m. Be sure to stick around for the free presentation and star show at 7 p.m. when we’ll talk UFOs, alien moon bases, whether or not you can see the Great Wall of China from space and much more!

2 Responses

  1. Alan Seamans

    After a month+ of cloudy nights or being otherwise occupied, I got the chance to check out Mira tonight. It was truly wonderful. Brighter than I’ve ever seen. Very easy to spot, even with the waning moon rising in the east. I’m not very skillful at estimating magnitudes but I felt it was brighter than mag. 3, almost rivaling Menkar. Thanks! Alan S.

    1. astrobob

      Hi Alan,
      I saw it too last night. Very bright! You’re right about the magnitude. I estimated 2.7. Thanks for writing.

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