Sunday, September 27, 2009
Which sparkling star shines brightest in the night sky?
John Goss
John Goss is chairman of the Mid-East Region of the Astronomical League and a former president of the Roanoke Valley Astronomical Society.
Recent columns
- Watch for variable star, meteor shower
- September is the perfect time to discover Jupiter's four secrets
- New telescope looks for Earth-like planets
- What really occurred in July '69? A moon landing
Blog
In an ideal Southwest Virginia fall, summer's humidity and haze are long gone. The season's night air may seem clear, but it is not completely transparent as evidenced by the unsteady appearance of the stars glittering above. The second-, third- and fourth-brightest stars seen from Southwest Virginia are visible at the same time in the early evening sky over the next couple of weeks. All three possess nearly the same brightness.
Sparkling slightly west of overhead lies the blue-white Vega. Low in the northwest sinks yellowish Arcturus. Rising in the northeast is the winter star Capella. Which stands out the most?
Without a doubt, the brightest star in the night sky is Sirius. This winter luminary won't appear in the southeastern sky until 2 a.m. Holding the second spot on our bright star list is Arcturus, which is normally thought of as being a spring and summer star. A tad fainter is Vega, followed very closely by Capella. If Arcturus, Vega, and Capella were all positioned near one another on the starry dome, it would be difficult to identify them by brightness alone.
As with all the stars, the light from Arcturus travels unfettered for many years -- 37 in its case -- before reaching Earth's upper atmosphere. In its final dozen milliseconds of travel time, it zips through 15 miles of a thickening gaseous soup of nitrogen, oxygen and water molecules while glancing off microscopic dust particles. The net effect is that the once steady starlight is diminished, sometimes very noticeably, and is smeared into a colorful "twinkle." The more atmosphere that starlight encounters, the dimmer it becomes and the more twinkling it shows.
Arcturus, hovering not far above the horizon, appears dimmer and twinkles more than Vega, which is nearly overhead. At its low altitude, Arcturus' light passes through at least seven times as much of our distorting atmosphere as Vega's light. Likewise, low-lying Capella also appears much dimmer than Vega. But by 2 a.m. when Vega drops low in the northwest, Capella moves high enough in the sky so that its light travels a much shorter distance through our atmosphere and it grows correspondingly brighter than Vega.
While Vega may appear as the brightest star in early October evenings, the planet Jupiter easily outshines it. Like last month, it is well-placed for careful binocular observations of its tiny disk and its four starlike moons.
Jupiter is the only planet visible until Mars rises in the northeast about 1 a.m. Although it is not nearly as bright as Jupiter, it is well worth observing, especially during the first two weeks of October when it passes Gemini, Castor and Pollux. On Oct. 16, Mars forms a straight line with them creating an unusual bright linear trio.
Early morning skywatchers can catch an interesting two-week planetary dance beginning Monday. At 6:30 a.m., brilliant Venus glows low in the east, and far below it rises the much dimmer Mercury. Then, seemingly out of nowhere, Saturn jumps up to meet Mercury on Oct. 8. Binoculars easily give a better view of this planetary rendezvous. Five mornings more, Saturn leaves Mercury behind and practically bumps into Venus. Again, binoculars easily show the close planetary separation. But keep in mind that in three-dimensional space these planets are always separated by tens, if not hundreds, of millions of miles.
Morning viewers are given two more interesting celestial scenes to start their day. On Oct. 15 and 16, the thin crescent moon, glowing magically with Earthshine, joins Venus and Saturn. Enjoy!
