Skyward for February 2021.
By
David H. Levy.
Orion in Winter.
Orion and vicinity as shown from Casdtor House,
one of the smaller buildings at Jarnac Observatory. A bright Geminids
meteor is in the picture
As twilight deepens these evenings, Orion is just clearing the
eastern horizon. Robert Frost wrote
eloquently in his famous poem “The Star Splitter”
“You know Orion always comes
up sideways,
Throwing a leg up over our
fence of mountains.”
Whenever
I see Orion rising, which is almost every night from fall to midwinter, I am
reminded of how poets like Robert Frost saw the mighty hunter as it entered the
sky to take command of winter. Even if
you have difficulty finding some constellations, the three stars in a row that
form Orion’s belt are a giveaway. And if
you have a telescope, as Frost did, the view is even better. Just below the belt lies a fainter set of
three stars. Surrounding the middle one
is a gigantic cloud of hydrogen gas which is the Great Nebula in Orion. It is one of the richest star forming
regions in our whole galaxy.
During
that first winter I enjoyed watching lots of the fainter stars within the
nebula change their brightness over time scales of days, hours, or in one case,
minutes. According to Janet Mattei, the
late director of the American Association of Variable Star Observers, these
variable stars can “flicker” as they go through their carefree cycles of
stellar youth.
Near the top of Orion, marking his left shoulder, is a much
older, grandfather star. Named
Betelgeuse, this star is at the other end of the stellar life cycle. An old, very large and massive sun,
Betelgeuse varies lazily from being almost as bright as Rigel, the star marking
Orion’s lower right knee, to not much brighter than Bellatrix, the star marking
Orion’s right shoulder.
Last winter Betelgeuse faded
more than usual, and throughout 2020 it was setting off alarms that it was
about to explode as a supernova. Probably not now, though it will likely happen
within the next hundred thousand years or so.
In the spring Betelgeuse began to brighten again, but when I saw it
rising above the eastern horizon in late August, it had faded once more. Around
that same time, the Hubble Space Telescope, observing in ultraviolet light,
provided data that suggested that the unusual dimming was caused by an ejection
of some very high temperature gas from within the star into space.
When Betelgeuse is finally done being the
star we love, its core will collapse almost instantaneously, within a few
seconds. Betelgeuse will increase exponentially in brightness. It will shine as brightly as the first
quarter Moon and will be easily visible in daylight for three months or more.
It will be brighter than Tycho’s great exploding star of 1572, and brighter
even than the brilliant supernova of 1006. As large as it is, Betelgeuse is
probably not massive enough that its core will shrink to a black hole. Instead, it will probably form a new neutron
star, small, dark, very dense, and cold.
Stars
are people too. They age just as we do.
They enjoy the carefree times of youth, go through a long middle age
like our Sun, and then get strange again as they grow old. Please go out and enjoy Orion rising over the
eastern horizon these evenings. It is
time to settle back and enjoy this magnificent king of the winter sky. As you look, imagine how young stars like
those in the nebula, and old ones like Betelgeuse, tell their beautiful story
of the life cycle of distant suns.
