Jul 01

July 2006

Whats Up in the Sky July, 2006

The Milky Way Galaxy

This is the season of the summer vacation. Many of us are probably planning a camping trip up north, a beach vacation, or perhaps a tour of western national parks. Whatever your destination, some moonless night you may want to take a side trip to a dark location far from the glare of city lights where you can enjoy one of the most spectacular objects in the night sky, our own galaxy, the Milky Way.

Our sun, along with about 200 billion other stars, is a member of this galaxy, a giant spinning pinwheel of stars, gas, and dust. In fact, every star we can see is a member of the Milky Way. We see the Milky Way edge-on from the inside, so it appears as a faint band of diffuse light which, at this time of year, stretches from Sagittarius and Scorpius in the south high across the eastern sky to Cassiopeia in the north.

Its name dates back to antiquity, long before its true nature was known, and is purely descriptive. In fact, the word “galaxy” comes from the Greek “gala”, which means “milk”. It is associated with many ancient legends and has been described over the centuries as a great celestial river, a path to the stars, a giant serpent, and the “backbone of the night”. It wasn’t until the winter of 1609-1610 when Galileo turned his telescope to the sky that this flowing stream of milk was resolved into stars.

The number of stars in the Galaxy is so enormous as to be beyond our comprehension but the following may help. It’s been estimated that a typical box of salt contains about a million grains, so 200,000 boxes contain as many salt grains as there are stars in the Milky Way. If you tried to model the Galaxy by spreading all those grains out over a giant circle, the average distance between grains would be 7 miles!

The Milky Way is about 100,000 light years across, which means it would take a beam of light 100,000 years to traverse its entire diameter. We are located about 30,000 light years from the center, or nucleus, where the concentration of stars is the greatest. If you look toward the constellation Sagittarius (known as the “teapot”), located near the southern horizon, you are looking directly toward the nucleus of the Galaxy. But dense clouds of gas and dust in the plane of the Galaxy obscure our view and create great dark regions seemingly devoid of stars. These dark dust lanes add beauty and structure and are themselves interesting to observe.

The region above the spout of the teapot is one of my favorite parts of the sky. Scan this area with binoculars or a small telescope and you can find many interesting objects including huge clouds of glowing gas, open clusters of stars, and very distant globular clusters. With binoculars or the naked eye, observing the Milky Way is fascinating and fun. It’s easy to imagine the wonder and awe of the ancients and Galileo as they viewed this part of what’s up in the sky.

Here are this month’s viewing highlights:
Planets this month: Saturn and Mars are fading in west at dusk. Jupiter dominates southern evening sky. Venus is brilliant morning “star” in east before dawn.
July 2: Venus is 4° north of the star Aldebaran.
July 3: First-quarter Moon.
July 9: Month’s southernmost moonset in SW before dawn.
July 10: Full Moon.
July 16-22: Use binoculars to see Mars close in on the star Regulus low in west at dusk.
July 17: Last-quarter Moon.
July 20: Crescent moon passes very close to the Pleiades, covering some stars in the cluster in the predawn sky (3-5 am).
July 25: New Moon.
July 28: Peak of Southern Delta Aquarid meteor shower before dawn.

Peter Burkey – SAAA President