This week, the "Wonderful Star" is expected to reach its peak brightness and should be readily visible to the naked eye.
This Wonderful Star is Mira, located in the constellation Cetus (the whale), and what makes it wonderful is its ability to vary in brightness by some 250-fold over a span of about 332 days, or nearly 11 months. Throughout this period, Mira varies in brightness from ninth magnitude or less (being visible only with a telescope) to third or fourth magnitude (visible without any optical aid). However, we cannot always tell what Mira will do; its peaks have ranged from as dim as seventh magnitude to first magnitude, the virtual equal of Aldebaran, the brightest star in the constellation Taurus.
Mira's peak brightness this year will occur around July 13, according to the American Association of Variable Star Observers (AAVSO). At that time, the star is predicted to be shining at around magnitude 3.4, although it appears that this upcoming maximum could very well surpass that forecast. Estimates placed Mira's brightness at magnitude 3.7 on June 26, so it seems quite possible that if the star continues to brighten at its current pace, Mira might reach close to second magnitude by mid-July, making it more than twice as bright as the original prediction indicated.
Mira appeared as bright as magnitude 2.1 just last year in August 2021, while the star was at magnitude 3.3 in October 2018, much closer to its "normal" maximum brightness. It certainly appears that this week, we'll see Mira again shining much brighter than usual.
Where to find Mira
Mira is located in the constellation Cetus, which is actually recognized as a fall constellation; in November, you can find it by merely stepping outside during the evening hours and casting a glance about halfway up in the south-southeast sky.
In July, however, you'll need to get up before the break of dawn — around 3:30 to 4 a.m. local time — and direct your attention low toward the southeast sky.
Cetus is a very large, sprawling constellation composed primarily of dim stars. To find Mira, it is best to use a brighter and more recognizable star pattern, namely the famous great square of Pegasus, which, at that early hour, will be very high in the south and nearly overhead.
If you draw an imaginary line diagonally from the square's upper-right star (Scheat) across to the lower left star (Algenib), and then continue for roughly three times that same distance, you'll be in the general vicinity of Mira. It's actually easier than it sounds, because there are so few stars in this region that even a faint one stands out. And if Mira reaches easy naked-eye visibility, you'll easily be able to find it.
While Mira's rise to maximum light tends to be rather rapid, its descent toward minimum is a bit more leisurely. Mira should remain in view without the need for any optical assistance until perhaps early October. Thereafter, you'll need a set of good binoculars to see it, and by the time the winter holidays come around, you'll need a good telescope and a star map to find it. By then, Mira will be about 15 times dimmer than a star that's at the threshold of naked-eye visibility.
Mira's strange behavior
Mira was the first variable star ever discovered, aside from a few novas, and today it is considered the prototype of all long-period variable stars.
Dutch astronomer David Fabricius assumed Mira was a nova or "new" star when he discovered it on Aug. 13, 1596. When it faded away by the following October, he did not look for its return. The star was again visible as a fourth-magnitude star when Johann Bayer mapped the constellations in 1603. Quite unaware of Mira's remarkable behavior, Bayer cataloged it in his star atlas as the Omicron star of Cetus.
In 1638, another Dutchman, Johannes Holwarda, discovered that this star repeatedly brightened and faded. To early skywatchers, this strange behavior seemed magical, if not almost miraculous — hence the star's moniker, Mira, which was first suggested by Polish astronomer Johannes Hevelius in 1662.
Exactly why the star brightened and faded posed a major conundrum for astronomers, and early theories ranged from creative to far-fetched. Ismael Boulliau (1605-1694) suggested Mira was a rotating globe that was uniformly dim save for one very bright spot. Pierre Louis Moreau de Maupertuis (1698-1759) suggested the star was in the form of a millstone seen at different angles at different times.
Meanwhile, Sir William Herschel (1738-1822), the discoverer of Uranus, observed Mira from 1777 to 1780 and came up with two hypotheses: Either the star was surrounded by Saturn-like rings that were sometimes seen face on and other times edge on, or the star had spots like the sun which caused its brightness to vary as it rotated.
Finally, in 1926, English astronomer, physicist and mathematician Sir Arthur Eddington demonstrated that Mira-type stars were pulsating red giants, somewhat similar to Cepheid variable stars but with much longer periods because of their more swollen physical size and lower surface gravity. Such enormous stars, he noted, can be characterized as thermodynamic heat engines.
A gelatinous stellar mass
Mira is located roughly 300 light-years away, and its mass is approximately twice that of the sun, but in a vastly different proportion. Its gaseous material is spread out, on average, as much as one-thousandth as thin as the air around us. If you weigh 150 pounds here on Earth, you would weigh only a few ounces at the surface of Mira!
Mira is also among the coolest known bright stars of the red giant class, with a temperature ranging from 3,000 to 4,000 degrees Fahrenheit (1,600 to 2,200 degrees Celsius). As with other long-period variables, Mira's deep red color at minimum pales to a lighter orange as the star brightens.
In addition, its diameter fluctuates from 400 to 500 times that of our home star. Its atmosphere is very loosely bound, forming huge, fuzzy envelopes; in fact, Mira and other stars similar to it have sometimes been called "jellyfish stars."
And indeed, a little of this so-called jellyfish's outer fringe is lost with each pulse. Try to envision a celestial body so enormous that its pulsations cast its outermost layers into space.
The odd couple
Mira may look like one star, but it is actually two stars that make a truly odd couple. Mira A is the star we see visually, a red giant that expands and contracts with regularity. Mira B is a much smaller white dwarf star that was first glimpsed in 1923 at the Lick Observatory in California and was resolved in images taken by the Hubble Space Telescope in 1997.
Here, we have a strange mingling of high- and low-energy radiation in widely different amounts. The two stars are currently separated by about 6.5 billion miles (10.5 billion kilometers), with Mira B taking about 400 years to orbit Mira A. And while Mira A is a cool supergiant star, Mira B is an intensely hot star measuring less than one-tenth the diameter of Mira A.
And as a final surprise, NASA's Galaxy Evolution Explorer satellite, which launched in 2003, discovered an exceptionally long, comet-like tail behind Mira. The tail measures roughly 13 light-years in length and was unveiled only when it was visible in ultraviolet light.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.
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Joe Rao is Space.com's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Joe is an 8-time Emmy-nominated meteorologist who served the Putnam Valley region of New York for over 21 years. You can find him on Twitter and YouTube tracking lunar and solar eclipses, meteor showers and more. To find out Joe's latest project, visit him on Twitter.