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Astronomy 101 - Learning About Stars

Lesson 5: The Universe Has Gas

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Stars are massive shining spheres of hot gas*, the closest of which is our Sun. Those stars which you see with your naked eye in the night sky all belong to the Milky Way Galaxy, the huge system of stars that contains our solar system. There are around 5,000 stars which can be seen with the naked eye, though not all stars are visible at all times and places. With a small telescope, hundreds of thousands of stars can be seen.

Larger telescopes can show millions of galaxies, many of which may each contain over 200 billion stars. Today, scientists believe there are more than 1 x 1022 stars in the universe (10,000,000,000,000,000,000,000). Many of these stars are so large that if they took our Sun's place, they would engulf Earth, Mars, Jupiter, and Saturn. Others, called white dwarf stars, are around the size of Earth, and neutron stars are less than about 10 miles in diameter.

Our Sun, one of the stars in the universe, is about 93 million miles from Earth, 1 Astronomical Unit (AU). The difference in its appearance from the stars visible in the night sky is due to its close proximity. The next closest star is Proxima Centauri, which is more than 20 trillion miles from Earth.

Stars come in a wide variety of colors, ranging from deep red, through orange and yellow to an intense white-blue. The color of a star depends on its temperature. Cooler stars tend to be red, while the hottest ones are blue.

Hipparchus, an astronomer we haven't met yet lived in the second century BC. He divided stars into six brightness groups, which are called magnitudes. First magnitude is the brightest and sixth magnitude is the faintest. This system, though a bit modified, is still in use. The difference in magnitude is measured mathematically. Each star magnitude is 2.5 times brighter than the next lower star. This has expanded our system into more than six levels, with the brightest stars now represented by negative numbers.

Stars - Stars - Stars

* Stars are primarily made of hydrogen, smaller amounts of helium, and trace amounts of other elements. Even the most abundant of the other elements present in stars (oxygen, carbon, neon, and nitrogen) are only present in very small quantities.

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