The tail end of 2019 also brings an opportunity to observe and follow one of the most fascinating variable stars out there: Mira. Located in the constellation Cetus (the whale), Mira will be well-positioned in the sky above Quebec in late fall and throughout winter.
The variability of Omicron Ceti was officially discovered by Dutch astronomer David Fabricius in 1596, although this star had been observed by Hipparchus of Nicaeae in the 2nd century BCE and spotted by many other observers for centuries. At the time, Fabricius assumed the star was a nova that had burned out after shining for a few weeks, but much to his surprise, he saw it again in 1609. However, it was only in 1642 that Johannes Hevelius named the star Mira, meaning “the wonderful” in Latin.
Astronomers soon realized that Mira would reappear at regular intervals, varying in brightness over a 332-day cycle. At its maximum, Mira is easily visible to the unaided eye, but fades at its minimum until it can only be seen with a telescope.
During this particular cycle, Mira is expected to reach peak brightness around November 15, and will remain a naked-eye star until the end of January 2020. Note, however, that its luminosity varies from cycle to cycle, making it impossible to accurately predict just how bright or dim the star will appear.
What causes Mira’s variability?
Mira is a red giant star, just like the Sun will become near the end of its life. After having exhausted the supply of hydrogen in its core, the star has entered the next stage of its evolution. The core is now surrounded by an outer shell of unfused hydrogen, resulting in an extremely large star with very high luminosity and a relatively cool surface temperature.
Mira is what we call a pulsating variable, because its diameter varies periodically. When the star’s diameter is at its minimum, the heat forces the outer layers to expand outward into space, where they cool and emit more light. Eventually, the heat is no longer able to keep the star’s outer layers puffed out. The process then reverses course and the layers contract and heat up again.
When Mira is at its minimum, it is 664 times bigger than the Sun and its surface temperature is 3,200 degrees Kelvin (that of the Sun is 5,750 K). At its maximum, Mira is 804 times bigger than the Sun and its temperature drops to 2,900 K. It mainly emits infrared light, but in visible light, the star is about 1,500 times brighter at maximum than at minimum—which explains how it transitions from totally invisible to apparent.
So make time over the next few weeks to observe the wonderful Mira while it is still visible to the naked eye.
It’s a planet party!
In December, dazzling Venus lights up our early evenings and glows brilliantly above the southwestern horizon at twilight.
At the start of the month, Jupiter and Saturn accompany Venus in the sky. Then around 5 p.m. on the evening of December 1, the Moon joins our planetary trio. Venus will hover 7 degrees above the horizon, with Jupiter to its lower right and Saturn to its upper left. As for the lunar crescent, it will be positioned a little higher and to the left of Saturn.
Jupiter inches closer to the Sun with each passing day and becomes harder to make out. By mid-month, the giant planet all but disappears in the solar glare. Saturn follows the same path as Jupiter, but will remain visible until the end of the year.
Before night sets in on December 10 and 11, be sure to check out Venus and Saturn as they glide past each other, less than 2 degrees apart. On December 27 and 28, the Moon will join the two planets at twilight. On the 27th, the thin lunar crescent will lie to the upper left of Saturn, low on the southwestern horizon, 30 minutes after sunset. The next day, the Moon will hang just 3 degrees below Venus.
Mars is visible at the crack of dawn about 15 degrees above the southeastern horizon. On December 22, the crescent Moon will shine above the Red Planet, and the next morning, the thin lunar crescent will move to the lower left of Mars.
Mercury will be visible in our morning sky for the first half of December, approximately 45 minutes before sunrise. Unfortunately, it will sit low on the southeastern horizon, a mere 10 degrees high at the beginning of the month. Over the next few days, the planet draws ever closer to the Sun and the horizon, making it increasingly difficult to spot. By mid-December, it will be lost in the light of dawn.
And finally, the solstice will occur on December 21, at 11:19 p.m. EST, marking the official start of winter in the Northern Hemisphere.
We wish you Happy Holidays and a feast of astronomical observations!