The stars, our galaxy and the universe

A star like our Sun is a huge ball of plasma that fuses hydrogen into helium thereby creating light and heat. Stars in later stages of their lives fuse helium into heavier elements and the really hot stars can eventually fuse elements into iron. The Sun is about 93,000,000 miles away from us. The earth is about 7,920 miles in diameter while the Sun is about 860,000 miles in diameter. The diameter of the Sun is about 110 earths and the interior of the Sun could contain about a million earths. Our Sun, the earth and all of the other moons and planets in the Solar System are about 4.5 billion years old. When our Sun dies, it will swell into a red giant and incinerate the earth, but that won't happen for another 4.5 billion years.

Our solar system has eight planets, six of which are easily visible to the naked eye (Uranus is visible but it requires dark skies and a good finder chart - Neptune requires binoculars or a telescope). The average distance from the earth to the moon is about 237,150 miles. The eight planets and their approximate distance from the Sun are as follows:

Mercury - 35 million miles
Venus - 65 million miles
Earth - 93 million miles
Mars - 137 million miles
Jupiter - 467 million miles
Saturn - 850 million miles
Uranus - 1.7 billion miles
Neptune - 2.7 billion miles

Our distance from the other planets varies because we travel around the Sun each year and other planets have orbits around the Sun that are different from ours. For example, Mercury orbits the Sun in just 88 days, but Saturn takes about 30 years. To find out what planets are in the sky tonight, you can visit the Sky and Telescope website.

Number Crunching

A light year is about 6 trillion miles. To travel one light year in a space shuttle at 30,000 mph would take about 25,000 years.

Our galaxy, the Milky Way, is approximately 100,000 light years in diameter but only about 1000 light years thick and we are about 25,000 light years from the center. The Milky Way has approximately 200 billion stars of which about 3,000 are visible to the naked eye from a dark site. Most of the visible stars are several hundred light years away and the most distance stars we see with the naked eye are no more than about 2,000 light years away. The Milky Way is one of about 50 billion galaxies in the universe.

How much is a billion? If you earned $1.00 per second, 24 hours per day, you would earn a million dollars in less than 12 days. You would have to wait over 31.5 years before you would earn a billion dollars!

At the mid latitudes of the U.S. and Europe, we are spinning at 700-900 mph. The earth is traveling around the Sun at 67,000 mph or about 1.6 million miles every day and the Sun is traveling around the galactic center of the Milky Way at 500,000 mph. It takes about 200 million years for the Sun to orbit the center of the Milky Way.

Getting Around the Sky

The two pointer stars in the Big Dipper (the two stars that make up the far side of the bowl opposite the handle) point to Polaris, which is the North Star. The altitude of Polaris is the same as your latitude (at 37 degrees North, Polaris is 37 degrees above the horizon). Your index finger held at arms length is about 1/2 degree (the size of the Sun and the moon) and your closed fist held vertically at arms length is about 10 degrees . The zenith (straight overhead) is 90 degrees above the horizon.

You can download free monthly star charts from SkyMaps.com and use these guidelines to find planets, star clusters and other deep sky objects or you can make your own planisphere to see the sky on any day or time of the year. From a dark site, it is possible to see many beautiful deep sky objects including galaxies millions of light years away with binoculars or a small telescope. When searching for stars or deep sky objects in binoculars, it is helpful to look at the area you want to scan with your unaided eyes and slowly bring the binoculars up to your eyes.

To learn more about getting started in astronomy and see more of what's in the sky tonight, visit the Sky and Telescope website or contact me directly if you need help in choosing a telescope, learning how to operate your telescope or learning more about the night sky.