Monday, October 26, 2009
Watch for variable star, meteor shower
John Goss
John Goss is chairman of the Mid-East Region of the Astronomical League and a former president of the Roanoke Valley Astronomical Society.
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Astronomers are known for their astonishingly accurate celestial predictions. The next few weeks offer two recurring sky events whose predicted details are often way off the mark.
While a few stars vary in brightness, no star ever decreases its brightness enough to disappear from view. No star except one, that is.
In 1596, David Fabricius, an amateur astronomer, observed what he thought was a new star. Surprisingly, after a few months, it completely faded away. Other observers saw it intermittently over the next 40 years, not realizing its periodic nature until 1638.
That star, named Omicron Ceti, or Mira the Wonderful, is the brightest member of a class of stars known as long period variables. On average, it takes 332 days for Mira to cycle from its minimum brightness through its maximum then back to its minimum.
When Mira reaches its dimmest luminosity, such as it did in June, it is invisible to the unaided eye. Nearly four months later, it brightens sufficiently so that it becomes barely visible. Mira continues to slowly increase for another six to eight weeks, as it is doing now, to finally become the brightest star in its part of the sky.
Mira's maximum luminosity varies unpredictably. Normally, it attains a brightness level not quite matching that of the stars of the Big Dipper, making it easily seen but certainly not eye-catching. Rarely though, Mira has topped the brightness of Fomalhaut, which lies low in the south this time of year.
Mira is predicted to reach maximum a few days before Thanksgiving and to remain at that brightness for a couple of weeks. After that, it slowly fades, much as David Fabricius saw it do in 1596, finally disappearing from view in February.
The physical characteristics of Mira are astounding. First of all, it is huge. If it replaced our sun, its bloated bulk would easily swallow Mercury, Venus, Earth and Mars and, at times, the giant star would extend nearly to the orbit of Jupiter. Second, Mira's intrinsic luminosity level ranges from less than that of our sun to more than 1,500 times that of our sun. All this is because of the depletion of the star's hydrogen and helium fuel that powers its nuclear furnace.
The other upcoming unpredictable sky event is the Leonid meteor shower. Ten years ago, many people witnessed a "once in a lifetime" Leonid meteor show with more than 500 bright meteors per hour raining down on Southwest Virginia.
In the wee hours on Nov. 17, there is a possibility that skywatchers can enjoy a short display of more than 200 meteors per hour. But because meteors originate from the very tenuous and very uneven debris stream shed from a comet passing long ago, there exists much uncertainty predicting the rate of Leonid meteors, or whether the strong meteor shower will materialize at all.
The Leonids are most likely to be seen just before morning twilight, about 5 a.m. By then, the shower's radiant will be high in the east between the bright star Regulus in Leo and Mars in nearby Cancer.
This week holds one more sight for early morning viewers, this one being very predictable. Beginning tomorrow and continuing through the first week of November, bright Mars creeps in front of the distant background stars of the Beehive star cluster, also known as M44. A pair of binoculars and a dark observing location are needed to best appreciate this empyreal encounter.
When will Mira reach its maximum brightness and how bright will it grow? How many Leonid meteors will fall on Nov. 17?
For the answers, step outside and observe these events for yourself.
