The Sun, Stars, & Planets
Day and night would not be the same without stars! Our sun which
is so essential to life on Earth . That's only an average-sized star! Nuclear fusion in the sun's core produces light energy: hydrogen atoms, which have light atomic nuclei, join together to form a heavier helium nucleus under extremely hot temperatures of the sun, releasing energy in the process. Although the sun is almost 93 million miles from Earth, its light travels at about 186,000 miles per second and it takes only eight minutes to reach Earth through space .
The sun does not just provide light, though. Our whole solar system revolves around it. The earth, other planets, and satellites--like the moon--are all part of this solar system. Each of the planets orbit the sun, at varying speeds. Although the solar system itself seems pretty vast in size, it's only a tiny part of our Milky Way galaxy. A galaxy is a collection of stars, gas, and dust, perhaps 1,500 to 300,000 light-years across (a light-year is the distance light can travel in a year, almost 6 trillion miles). And there are countless galaxies in the universe.
Like the other stars, the sun is formed by gases: it is about 92% hydrogen and 8% helium, with some other elements. Scientists use a spectroscope to break up the light emitting from the star into the spectral colors, each a unique pattern of colors that occurs only from a certain element or combination of elements.
Not all stars have the same brightness, as you can see if you go out at night and look up at the sky. Some appear much brighter than others. Scientists measure brightness in apparent magnitude and absolute magnitude. Apparent magnitude is how bright a star appears from Earth. Sometimes one star looks brighter than others because it is close to Earth. Absolute magnitude, on the other hand, measures the inherent brightness of a star--how bright it appears from a set distance. Thus, one star's inherent brightness can be compared to that of another. About 2,000 years ago, the Greek Hipparchus classified stars by brightness, with the lowest numbers indicating the greatest magnitude in brightness. Now the brightest stars are assigned apparent magnitudes with negative values: the sun's magnitude is -26.7, the moon's is -12.7 when full, Sirius (the brightest star) has -1.4 and Betelguese has +0.4. With unaided eyes you should be able to see stars up to 5-6 magnitude, and with binoculars you should see up to eight or nine. A good telescope will allow you to view much dimmer stars, with magnitudes of around 13.
If you look at a star chart, you'll see that stars are often shown in patterns, or constellations. Many of the constellations that are visible in the Northern Hemisphere of the earth cannot be seen in the Southern Hemisphere, and vice versa. Which constellations are visible also varies with season. This is because the earth is daily spinning on its axis inside the "celestial sphere," the stars and night sky objects surrounding it. The Southern Hemisphere of Earth sees mostly stars in the southern part of the celestial sphere. Then, since Earth is orbiting the sun yearly, the view changes with the seasons. The stars visible in the night sky seem to travel east to west as the earth itself travels around the sun.