September Star Watch


Venus takes a tumble into the sunset this month, but that still leaves three bright planets to adorn the evening sky. Mars shines in the south after nightfall, with Saturn to the west and Jupiter even farther west. Jupiter also exits the evening sky soon, but in a few months it will join Venus in the morning sky.

Also in the west, the brilliant star Arcturus is dragging its kite-shaped constellation, Bootes, the herdsman, down toward the horizon. Arcturus is about 25 times the diameter of the sun and probably much older. But its main claim to fame is that it doesn’t circulate horizontally around in the disk of the Milky Way like the sun; instead, it is slicing its way down through the galactic disk. And a group of at least four dozen stars, called the Arcturus stream, is moving along with it.

The Summer Triangle of bright stars sails high above Saturn and Mars after nightfall. Lowest is Altair, in Aquila, the eagle. Brightest is Vega, in Lyra, the lyre of Orpheus. Note the parallelogram of stars below Vega; they outline the lyre and look beautiful through binoculars. East of Vega, Deneb completes the Triangle. It also marks the “top” of the Northern Cross, a feature of Cygnus, the swan.

The Summer Triangle abounds with small wonders. Look above Altair for thin Sagitta, the arrow. Then grab your binoculars and look immediately northwest of Sagitta’s feathers to see the dim but astonishingly realistic Coathanger hanging upside-down. And check out little Delphinus, the dolphin, just northeast of Altair.

Fall arrives with the autumn equinox at 8:54 p.m. on the 22nd. At that moment a space traveler would see Earth lighted from pole to pole. On the evening of the 24th, we’re treated to a lovely full harvest moon rising in twilight.

In astronomy news, on August 12 NASA launched the Parker Solar Probe spacecraft, which will fly closer to the sun than any spacecraft has gone before. Aboard are instruments designed by University of Minnesota space physicists to study electric and magnetic fields in the solar corona. Scientists believe these fields heat charged particles escaping from the sun’s surface to millions of degrees and accelerate them to form a constant torrent of particles traveling at up to a million miles per hour.

Known as the solar wind, this flow of particles bathes the solar system. Its interactions with Earth’s magnetic field often lead to spectacular auroras, but its strongest gales can leave damaged power grids in their wake. Data from the Parker Solar Probe is expected to settle a debate about just how the sun’s electric and magnetic fields generate the solar wind. That knowledge will make it easier to predict its fiercest outbursts and take precautions to protect power grids — as well as satellites and astronauts on future missions far from Earth.

Also, because our sun is the one star we can study up close, what we learn from it will tell us much about how stars everywhere in the universe live their lives.


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