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Object type: Pulsating variable star
Distance: 300 light years
Size: 300 to 400 times the sun’s diameter

In 1617, David Fabricius lay on a church floor at the age of 53, by a man wielding a shovel, who Fabricius had just accused of stealing a goose. It was a sorry end for the person who was first to spot one of the most important objects in the history of astronomy – a flashing star called Mira.

Not only did Mira go on to shatter cherished notions of a constant universe – to the dismay of Catholic theologians – it has recently gained another distinction: it is the first and only star with a comet-like tail that we know of. And this eternal hellraiser refuses to go quietly. The latest news is that Mira has been spotted enveloped by a mysterious spiral structure.

To fully understand the effect Mira had when it was first discovered, rewind to 20 years before Fabricius’s death. The Dutch minister had been using a star to chart the position of what he thought was the planet Mercury (but later turned out to be Jupiter), so he noticed when it dimmed out of sight over a few months. Ten years later, the faithful amateur astronomer was rewarded by the star’s unexpected return.

The star was then largely forgotten until, long after Fabricius’s death, Polish astronomer discovered in the mid-1600s that it continued to appear and disappear. He gave it the name Mira, meaning “wonderful” or “astonishing” in Latin.

Cultural earthquake

Word of Hevelius’s observations reached the French astronomer , who carefully charted Mira’s brightness variations and found they were highly regular, with a period of 333 days.

What ensued was a cultural earthquake. These variations the idea that the realm of the stars was eternal and unchanging – the received wisdom of the time, underwritten by Aristotle, says historian of the University of Florida in Gainesville. The Catholic church itself, which only recently had banned Galileo’s heretical theories, endorsed the Aristotelian view.

And unlike two supernovae that appeared and shone for about a year each in and , Mira returned again and again, almost tauntingly. “Mira kept showing up and saying, ‘Hey, here I am, you’ve got to explain me,'” says Hatch. “It’s the worst nightmare for the Aristotelians.”

What’s more, several other stars were discovered during the 1600s to be variable. “The variable stars are not as dramatic, but they’re more important in some ways than the two supernovae,” Hatch says.

Dark side of a star?

In addition to adding to mounting evidence for a mutable celestial realm, the regularity of the variable stars’ changes suggested that the universe could be understood and predicted in a rational way.

Bouillaud suggested that Mira had one side much brighter than the other. As it rotated, the bright side would come in and out of view, explaining why it seemed to regularly dim and disappear, then reappear and brighten again.

It wasn’t until the early 20th century that the real reason for Mira’s changes was discovered. It is a giant star whose atmosphere is – making it bigger and brighter at some times and smaller and dimmer at others. The pulsations happen because radiation gets periodically trapped in the star’s atmosphere, exerting an outward force that causes the atmosphere to bloat.

Spiral mystery

Variable stars have become immensely important in astronomy. One class, called , is useful for measuring distances and was crucial in how truly enormous the universe is.

Mira itself is still surprising us. In 2007, observations by the revealed an extraordinary comet-like tail behind Mira – the first ever spotted behind a star. As Mira travels through the galaxy, the gas it is shedding slams into ambient gas, causing a glow at ultraviolet wavelengths that we see as a tail.

More recent observations by the have revealed another curious detail – what appears to be a spiral structure in the that contains Mira itself and a companion star. The spiral may result from the way the companion disturbs the gas shed by Mira as it moves around in its orbit.

If only Fabricius could have known. The star that puzzled the poor man turns out to have changed the world.

Journal reference:

Read previous Astrophile columns: Star exploded? Just another day in Arp 220, Giant star comes with ancient tree rings, Frying pan forms map of dead star’s past, The most surreal sunset in the universe, Saturn-lookalike galaxy has a murky past, The impossibly modern star, The diamond as big as a planet.