As a reader of Cosmic Pursuits, you no doubt believe that all stars are wonderful. But one star, it turns out, is more wonderful than most. The star Mira, or Omicron Ceti, in the constellation Cetus, has been known since ancient times, but its nature began to emerge in the 16th century when a German pastor and amateur astronomer discovered the star’s brightness changed periodically, by a huge amount, every eleven months. And it was but the first discovered of many such stars.
Mira (“MY-rah”) is not a difficult object to observe when the time is right. At peak brightness, it shines at a respectable 2nd or 3rd magnitude, plenty bright enough for naked-eye observation. The star was near peak brightness, as it happens, in August 1596 when David Fabricius observed the star from his home in Germany, near what’s now the border with the Netherlands. When he observed it a few months later, it was gone. At first, Fabricius thought the star was a nova, a rare but remarkable type of star that suddenly brightens then fades from view for good. But when he saw the star brighten again in 1609, he knew he had discovered a new class of celestial object, the long-term variable stars.
For the record, Fabricius was a thoughtful but by no means expert astronomer. He first noticed Mira while looking for a comparison star to help estimate the brightness of what he thought was the planet Mercury. It turns out he was observing the planet Jupiter. But in 1610, Fabricius helped his son Johannes discover sunspots with a telescope, another major discovery for the time. Fabricius the elder is also famous for his unfortunate demise. The good pastor denounced a local goose thief from the pulpit in the town of Osteel. The thief, failing his Christian upbringing, smacked Fabricius over the head with a shovel and killed him.
Subsequent observations of this variable star through the 17th and 18th centuries revealed that it varied in brightness with a period of about 333 days, give or take. And it varied from about magnitude 2 or 3 down to about magnitude 10.0, a factor of about 1,500! That’s the largest variation in stellar brightness aside from the cataclysmic variables such as novae or supernovae, stars which exhibit variability because of immense (and often irreversible) explosions on their surfaces or in their cores.
The depth and period of variability of Fabricius’ star were not precisely regular like another famous variable star Algol, for example. But the periodic variability was clear enough, and the astronomer Johannes Hevelius was sufficiently impressed that he named the star Mira, Latin for “wonderful” or “astonishing”. Modern astronomers use the name Mira as well as its designation Omicron Ceti interchangeably.
What causes Mira’s variability? The short answer is that it’s going through a phase of evolution where the pull of gravity and the burning of hydrogen fuel in shells around the core fight against each other and cause regular pulsations, with the transparency of hydrogen ions in the outer atmosphere acting as an on-off valve to start each cycle anew. Gravity pulls the outer layers of the star inward for a few months. This warms the outer layers and increases the opacity of ionized hydrogen. The increased heat and radiation pressure builds up and causes the star to expand, the ions grow transparent and cool, and the outer layers fall back towards the star’s core. It’s a little like a weight on the end of a spring bobbing up and down in the Earth’s gravitational field: the spring pulls up, the Earth pulls down, again and again. The star is brightest as it nears its smallest radius and when it therefore grows hottest. The video below shows a simulation of a Mira-like variable star (Chi Cygni) as it does its thing.
Mira is the best known of an entire class of long-term variable stars called Mira variables. Other prominent Mira variables which brighten to naked eye visibility include Chi Cygni, R Hydrae, R Carinae, and R Leonis, among several others, all of which are visible at their peaks without optical aid. These stars are all deeply red in color and they vary over many months by several magnitudes. Mira variables are all red giant stars late in their life cycles, having long ago run out of hydrogen fuel in their cores. Their innards have compressed and heated to burn heavier elements such as helium, and in some cases they dredge up enriched material from their cores which helps add to their red color. Over the course of a few million years, a Mira variable will blow off its outer layer and become a beautiful planetary nebula such as the Ring Nebula or Dumbbell Nebula before its core settles down as a burned-out cinder called a white dwarf.
The evolution of stars into Mira-type variables has implications close to home: the Sun will also go through such a phase in about five billion years. So when we look at Mira, we glimpse the distant future of our home star as it begins its inevitable slide to oblivion. Indeed, Mira itself is just 20% more massive than our own Sun and may have strongly resembled our Sun in its earlier days. The star is about 300 light years away.
As if Mira is not wonderful enough by itself, it happens to have a companion star that has already evolved into a white dwarf. Mira itself, as it swells to immense dimensions, loses some mass to its companion (as seen in the image at the top of this page), and it also loses mass as it travels through interstellar space. In 2007, a space-based telescope discovered Mira has a comet-like tail caused by this mass loss. Here’s a spectacular image of what it looks like.
If you’re really keen, you can observe the variability of Mira for yourself and contribute useful data to professional astronomers who try to understand how such stars work. The star is located in the “neck” of the constellation Cetus, the Sea Monster, and it’s visible from approximately September through January, just over the southern horizon for northern observers and nearly overhead for southern-hemisphere observers.
If you can’t find Mira without optical aid, don’t worry. It means the star has dimmed. In fact the star spends most of its time below naked-eye visibility, and if its light curve and the ebb and flow of the seasons align, it’s possible for two or three years to pass without the star becoming bright enough to see without optics (this may explain why ancient stargazers did not notice its variability). Still, Mira is a stirring sight as it flares up to naked-eye visibility over the course of a couple of weeks.
To find out the current estimated brightness of Mira, and many other bright variable stars, and to estimate when it will next brighten to naked-eye visibility, use the light-curve generator app at the website of the American Association of Variable Star Observers (AAVSO). Enter the star’s name, the start and end date of interest (the ‘mm/dd/yyyy’ format is fine), and click on the ‘Plot Data’ button. The image above shows a sample of Mira’s light curve from 1/1/2011 to 10/18/2018 and it shows the star is beginning to brighten once again.
The star Mira, in its designation as Omicron Ceti, makes more than one appearance in science fiction. Star Trek fans will recall the episode “This Side of Paradise” in which Mr. Spock has a fling with a long-lost love interest played by Jill Ireland. The episode’s events take place on a planet called “Omicron Ceti III”, presumably the third planet in the Mira system. Given Mira’s huge variability, however, it’s unlikely there are any habitable planets in this star system. But it was an entertaining episode.
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