Stars over Sisters

As the autumnal equinox approaches, the nights come sooner giving us more time to go out and enjoy the night sky. The sun stands directly above the earth's equator at 7:21 a.m. PDT on September 22 on its journey into the southern hemisphere, marking the first day of the new season. At this time, the axis of Earth's rotation is tilted neither toward nor away from the sun but instead is perpendicular to the sun's rays. This results in the length of days and nights being nearly equal at most locations on the earth.

Fall is a great time to look for the zodiacal light. The zodiacal light is a cone-shaped light source that can be seen near the eastern and western horizons. It is caused by sunlight reflecting off dust and ice particles that exist in the plane of the ecliptic and is best seen before sunrise in the autumn. Look for a dim patch of light extending upward from the eastern horizon just before dawn.

Something else to search for this September is the Helix Nebula, also referred to as NGC 7293. Located in the constellation of Aquarius (between Capricornus and Pisces), this object is the largest and nearest of a class of bodies known as planetary nebulae.

A planetary nebula actually has nothing to do with planets but is, instead, a shell of gas, usually round in shape, that has been expelled by a low-to-medium-mass dying star. The term was coined by William Herschel who thought these dim, ghost-like objects resembled the planet Uranus, which he discovered in 1781.

The Helix Nebula was the first planetary nebula found to have cometary knots. Cometary knots are structures that resemble comets. They all seem to extend out radially from the nebula's center and appear to have "tails." The sizes of the knots are estimated to be as large as our solar system, a distance extending from the sun to the orbit of Pluto. These knots give NGC 7293 a look resembling a giant eye in space that some observers refer to as the "Eye of God." The Helix Nebula lies at a distance of 695 light-years.

After lying less than 0.1 of a degree apart in the sky on August 27, the planets Jupiter and Venus will spend the month of September moving away from each other. Jupiter continues its plunge toward the sun, disappearing behind our star on September 26. Venus, meanwhile, climbs ever higher in the western sky. Mars and Saturn, both in southern Ophiuchus early in the month, still show prominently in the evening sky. By month's end the faster-moving Mars will have entered Sagittarius.

The dates to watch this month to keep up with the moon's phases are September 1, new moon; September 16, full moon; then a second new moon on September 30.

To learn more about the planets, constellations and deep-sky objects, come out to the Sisters Astronomy Club's next Stars Over Sisters starwatch on Saturday, September 24. A slide presentation will begin at 7:30 p.m. inside the Sisters Park & Recreation District building located at 1750 W. McKinney Butte Rd. The event is free.