Michigan residents who look up on clear nights this September will be treated to a great view of four of our solar system’s nine planets and the Summer Triangle—a large constellation made up of three bright stars, according to astronomer Richard G. Teske.
“During September, Michigan skywatchers will be able to see the Summer Triangle—a large figure composed of the three bright stars Vega, Deneb, and Altair,” he explains. The Summer Triangle announces the change of seasons and the coming of the brilliant constellations of winter. “Deneb and Vega are nearly overhead at mid-evening in Michigan, while Altair lies midway down the sky toward the south. To recognize Altair, look for two guard stars flanking it,” he adds.
Teske, professor of astronomy, explains that while the three stars making up the Triangle seem to be roughly the same brightness, they are actually at very different distances from Earth.
“Deneb is the most distant. It is 1,600 light years away,” Teske adds. “Look for it at the tail of Cygnus, the Swan, a mythical bird which flies southward along the Milky Way each autumn. The star’s name comes from an old Arabic word for ‘tail.’”
Deneb belongs to a category of stars that astronomers call “supergiants.” About 60 times the size of our sun, Deneb emits more than 60,000 times as much radiant energy, making it one of the brightest stars in Earth’s sky despite its remoteness.
Vega is about 27 light years from Earth. Chief star in the constellation of Lyra the Harp, it is the fifth brightest in Earth’s sky. “Our sun and solar system are drifting slowly through space among the other stars, heading roughly in Vega’s direction. But during the 400,000 years it will take us to get there, Vega will move to some other position.
“Astronomers have made Vega their `user-friendly’ star,” Teske says. “They have carefully measured Vega’s radiant output at many wavelengths and use it as an accurate standard of comparison when determining the brightness of other objects.”
Of the three Triangle stars, Altair is closest, only 16 light years away, and is one of the sun’s nearest neighbors in space. Forming the head of Aquila the Eagle, Altair’s major claim to fame is its very rapid rotation speed.
“The sun, for example, rotates once every 25.4 days, with the leisurely equatorial speed—leisurely for a star—of 4,500 miles an hour,” Teske says. “But Altair would win the Rotational Olympics hands down. At the equator its surface spins along at almost 600,000 miles an hour, and the star completes one rotation every six and one-half hours. As a result, it must bulge fatly at the equator like a squashed-down tennis ball.”
In early September the planets Mars and Jupiter can be seen close together low in the western sky just after sunset, Teske adds. They appeared to be closest on the evening of Sept. 6; after that Mars climbs higher past Jupiter until both are lost in the glare of the setting sun by mid-month. Brilliant Venus remains our morning “star” in the east before sunrise. By late November, Venus will be close to the rising sun and difficult to see. The ringed planet Saturn is visible throughout the night during September.
Teske notes that “fall begins in Michigan at 8:22 p.m. on Sept. 22 when the sun passes exactly above Earth’s equator on its annual southward journey through the constellations.”
