Stars over Sisters 8/31/2021

By late summer and early fall, a few of the “water” constellations begin to materialize in the southeast at nightfall. First to arrive on the scene are Capricornus the Sea Goat, Delphinus the Dolphin, and Aquarius the Water Carrier. The smallest of the three is Delphinus, our featured constellation for September.

Although the size of the celestial dolphin is quite modest, four relatively bright stars form an easily recognizable shape resembling a flattened diamond, which outlines the main body of the aquatic mammal. A fifth star, Epsilon Delphini, marks the tail. Look for Delphinus about 11 degrees east of the bright star Altair in Aquila.

Shining at magnitude 3.6, Rotanev is the brightest star in Delphinus, which lies at a distance of 101 light-years. It was discovered to be a binary star in 1873 by American astronomer S. W. Burnham. Both stellar components are white in color, one classified a giant, the other a subgiant. The system is estimated to be about 1.8 billion years old.

The star Gamma Delphini, which represents the nose of the dolphin, is regarded by many observers to be one of the finest double stars in the heavens. First noted by astronomer F.G.W. Struve in 1830, the pair is separated by 8.9 seconds-of-arc and lies at a distance of about 115 light-years. Color estimates for this pair has ranged from reddish-yellow and greyish-lilac to yellow and blue-green. Modern observers generally find the primary star to be a pale yellow while the secondary appears yellow-orange.

Although the Milky Way lies nearby, Delphinus contains only four deep-sky objects: two planetary nebulae and two globular clusters, all of them noteworthy. The brightest of these is NGC 6934, a globular cluster consisting of primary ancient stars located 52,000 light-years away in the halo of our galaxy. The cluster is estimated to be approximately 10 billion years old.

A tale from Greek lore describes the romantic interests of Poseidon, the sea god, in his pursuit of the beautiful nymph Amphitrite. Initially the mermaid resisted his advances by fleeing to be among her sisters. Poseidon then sent messengers to find her and bring her to him, among them a dolphin. Delphinus found Amphitrite and convinced her to return to the sea god. After the two were married, Poseidon decided to honor the dolphin by placing his image among the stars.

For those of us who live in the Northern Hemisphere, autumn will arrive at 12:21 p.m. on Wednesday September 22. At that time the sun will stand directly above the equator as it passes from north to south. As a result, the length of day and night will be nearly equal across the globe.

Four of the five naked-eye planets are evening objects this month. Brilliant Venus and the much dimmer Mercury hover above the western horizon just after sunset. Although Mercury will lie a full 27 degrees from the sun on September 13 when it achieves greatest eastern elongation, it will stand only four degrees above the horizon. The best chance to see it will probably be on September 8 when the waxing crescent moon will be five degrees north of the speedy orb. Mercury fades from view in the later part of the month.

Saturn and Jupiter in the southeastern sky will put on the best evening show, particularly to those who have access to telescopes. Mars is lost in the glare of the sun during September.

The best time for stargazing will be early in the month when no moon will appear in the evening sky. The Full Harvest Moon will arrive on September 20.

This month’s dark-sky awareness tip to help fight light pollution here in Sisters is this: If possible, switch out lights with poor light shielding with light fixtures that provide proper shielding to reduce sky glow.