The star Omicron Ceti lies 420 light-years away in the constellation Cetus the Whale, an unremarkable patch of the night sky along the celestial equator, well to the west of hard-to-miss Orion. Omicron Ceti is visible to the unaided eye – except when it isn’t, which is most of the time.
This remarkable star varies in brightness. Its changes happen on a regular schedule of about 11 months. But how bright it becomes isn’t as predictable. The star’s unusual fluctuations were known as far back as at least the late 16th century. In 1662, German-Polish astronomer Johannes Hevelius named it “Mira” (meaning “wonderful” or “astonishing” in Latin).
Throughout the centuries, Mira has been as bright as 2nd magnitude (approaching the noticeability of the stars of the Big Dipper), but it usually peaks at about magnitude 3.5, putting it below the brightness of other stars in Cetus. At its dimmest, Mira falls to 10th magnitude, below the visibility limit of modest binoculars. That’s an overall factor of more than 1,500 times. Astonishing, indeed!
Mira varies because it’s past its prime as a “normal” star. It has exhausted most of its hydrogen and puffed up to become a red giant. The last gasps of its stellar furnace make the star pulsate and throw off its outer layers. Eventually, most of it will be gone, leaving behind a shell of gas called a planetary nebula that will surround the stellar cinder called a white dwarf.
As modern astronomers study the star, Mira continues to amaze. In 2007, observations by a satellite viewing in ultraviolet light discovered that Mira has a luminous tail of gas more than a dozen light-years long. This is the material that Mira has shed, leaving it behind as it speeds through the galaxy at some 80 miles per second (130 km per second) – very speedy for a star! The invisible tail spans about 2 degrees in the sky, about four times the diameter of the full Moon.