The brightest star in Aquila is Altair, "the eagle," a white star that is about 17 light-years from Earth. Altair is the southern point of a pattern of three bright stars called the Summer Triangle. (Deneb, in the constellation Cygnus, forms the triangle's northeastern point. Vega, in Lyra, is in the northwest. Altair is nice and bright and easy to find right up to the beginning of winter.)
Of all the star patterns in the sky, the Big Dipper is the most universally recognized. The dipper's seven bright stars form a portion of the great bear. It's hard to see the rest of the bear, especially from light-polluted cities.
After you locate the dipper, look at the two stars that mark the outer edge of its bowl. Now connect these two stars, then extend the line above the dipper's bowl. Polaris, the north star, lies along this line, about five times the distance between the two pointers. No matter where the Big Dipper is in our sky, those two stars always point to Polaris.
Leo's brightest star is blue-white Regulus, one of the brightest stars in the night sky. It is about 79 light-years away. Regulus rises almost due east, with the body of the lion following it into the sky over the next couple of hours. Once Regulus climbs into the sky, look to its left — toward the north — for the backwards question mark, known as the Sickle, that outlines his head and mane.
About two hours later, look low in the east for Leo's tail — a white star named Denebola, which comes from an Arabic name that, appropriately enough, means "tail of the lion."
Arcturus, the fourth-brightest star in Earth's night sky, is about 25 times larger than the Sun, and it produces about a hundred times as much energy. But Arcturus is nearing the end of its life. In astronomical parlance, Arcturus has moved off the main sequence and entered the "giant" phase of its life.
When we look at a rainbow, we see sunlight spread out into red, orange, yellow, green, blue, and violet. But this familiar palette of colors represents just a small part of a much larger "spectrum" of energy, which includes radio waves, X-rays, and many other kinds of energy that our eyes cannot see. Over the past century, using new types of telescopes that can detect these previously invisible waves, astronomers have discovered a wide variety of new objects and gained important new insights into the workings of the universe.
One of the most stupendous discoveries in astronomy came with the recent application of supernovae -- the explosions of massive stars -- to study the shape and fate of the universe. The shocking results: The universe seems to be pervaded with a previously unknown, and still unexplained, "dark energy" that works against gravity and is apparently accelerating the universe to a bleak fate of eternal expansion.
All of the stars, galaxies, and nebulae seen by telescopes make up only four percent of the contents of the universe. Scientists are unsure about the nature of the rest.
A large part of our universe is made up of so-called "dark matter," which emits no detectable energy, such as visible light, X-rays, or radio waves. However, it reveals itself by its gravity, just like a magnet underneath a table betrays its presence by attracting paperclips and pins.
Astronomers use several different methods to date the universe. In recent years, results from these differing methods have been coming into closer agreement.
One method of determining the universe's age involves finding the oldest stars and deciphering their ages based on knowledge of how stars are born, evolve, and die. White dwarf stars are particularly good candidates for these studies. They are the burned-out cinders of stars that were once like the Sun.
Cosmology deals with one of mankind's earliest questions: what is the nature of the world? Ancient civilizations believed that Earth was flat, and covered with a spherical dome on which the Sun, Moon, planets, and stars resided. In the second century A.D., the Ptolemaic system moved Earth to the universe's center. In the mid 1500s, the Copernican system placed the Sun at the universe's center.
