Astronomers use many different scientific instruments to help them learn about space. One of the more important ones they use is the telescope . Galileo , called the Father of Modern Astronomy, wrote detailed descriptions, made detailed drawings, and kept careful records of observations he made using the telescope. There are two main types of optical telescopes, the refracting and the reflecting telescopes. Refracting telescopes use lenses to bend light coming from distant objects. The lenses focus the light so we can see close up something that is billions of miles a way. A reflecting telescope uses mirrors to bounce the light from some distant object to a focal point. Radio telescopes "listen" to the electrical activity of distant objects, even if the object is so far away that light cannot reach us.

            Astronomers had to make up their own measuring units. Distances are so large in space that they do not measure in miles. Distances in the Solar System are measured in astronomical units . This is the distance from the Earth to the Sun. They measure distances outside the solar system in light-years . A light-year is the distance it takes light to travel in one year. Since light travels at the speed of 186,000 miles per second , this is a very long distance. It takes the light from our own sun over 8 minutes to reach us. The nearest visible star past our sun is Alpha Centauri, and it is about 4 light-years away.

            The brightness of stars depends on how hot they are, how big they are, and how far away from us they are. This measurement of brightness seen from the earth is called the star’s magnitude .

            Astronomers also are careful about the difference between weight and mass . Mass is the amount of stuff that an object has in it. It stays the same no matter where the object is in space. Weight depends on the gravity that is pulling on the object. The more mass an object has, the more it will pull on nearby objects. This will make a larger gravitational pull. We would have the same mass on the earth, the moon, or Jupiter, but we would not weigh the same.

            Our nearest neighbor in space is our own natural satellite, the moon. Because the moon turns on its axis once in 24 hours, we only see one side of the moon from the earth. Our moon does not have enough gravity to hold onto an atmosphere , so meteors strike the surface and make large "holes" in the surface, called craters . Many millions of years ago the moon had active volcanoes. The lava flows from these volcanoes made dark, smooth places called maria , or seas. What looks like the "Old Man in the Moon’ to us is the pattern made by the many craters and seas on the lunar surface. The moon does not make any light of its own. It reflects the light from the sun. As the moon revolves around the earth, the earth’s shadow keeps us from being able to see the entire moon every night. We see the moon change from the New Moon to the Full Moon over the month. These changing shapes we see are called the phases of the moon. The time it takes the phase to change from one Full Moon to the next Full Moon is one month .

            Sometimes the shadow of the moon keeps us from being able to see all the sun. This is called a solar eclipse . If there is a total eclipse of the sun we can see the outer part of the sun, called the corona . Sometimes the earth gets between the moon and the sun, and we can’t see the moon. This is called a lunar eclipse .

            Our planet is one of nine planets in the Solar System. Mercury, Venus, Earth, and Mars are the Terrestrial Planets . Jupiter, Saturn, Uranus, and Neptune are the Gas Giants . Most of the planets have several moons orbiting them, and several have layers of rings, too. The rings are made of rocks, dust particles, and ice. Pluto is so far away that we don’t know very much about it. Most of what we do know about Pluto was discovered as the space probe Voyager traveled past Pluto. There are many bits of large and small rocks, called asteroids , between the orbits of Mars and Jupiter. This is called the Asteroid Belt .

            Some visitors from deep space also come into our Solar System. Meteors are constantly being pulled into our atmosphere by the earth’s gravity. Most burn up from the friction of falling through the atmosphere. We call them shooting stars , but they are not stars. If they are so large that part of the rock makes it to the earth, we call these rocks meteorites . Comets travel in and out of the Solar System in huge elliptical orbits . Comets are made of dust and rock and have a tail made of gases, dust particles, and ice. The radiation from the sun, called the solar wind , makes the tail glow. One of the more famous comets is Halley’s Comet . It can be seen every 77 years when its long orbit brings it close to the earth.

            All the planets rotate, or spin, on their axis. Most are tilted, like the earth. Uranus spins on a horizontal axis. The time it takes to make one complete spin is one day . The time is takes a planet to revolve, or travel in a complete orbit around the sun, is called a year . The farther a planet is from the sun, the longer the year is on that planet. Pluto’s orbit is a very long, narrow elliptical orbit. Because of that, there are times when Neptune is farther from the sun that Pluto.

            At the center of the Solar System is our star, the Sun. By using a solar telescope and other instruments astronomers have learned much about our sun. They can study the core, the photosphere, the chronosphere, the sunspots, solar flares, solar prominences, solar winds, and the corona.

            Stars are made of hydrogen gas packed together with so much gravity that a nuclear reaction, called fusion , takes place. This reaction gives off heat and light. Even though we are 93,000,000 miles away from the sun, the radiation it gives off can still burn us. Our sun is a medium sized yellow star. The hydrogen gas is changed to helium gas in the fusion reaction. As a star gets old the helium gas expands, the star gets very large, and it cools to a red color. When this happens it is called a Red Giant . Eventually the Red Giant collapses in on itself to make a White Dwarf . The collapse of a very large dying star causes a huge explosion called a Supernova . When the largest stars collapse, they do not turn into a Supernova. They become what we call a Black Hole . The gravity of a Black Hole is so great that not even light can escape, so it looks black to us.

            Other bodies in deep space that astronomers have discovered are the nebula and quasars . Nebula are large areas of gases that may be what’s left after a star dies, or may be the gas cloud that stars will be born in. Quasars give off an electromagnetic signal that radio telescopes can detect. Astronomers do not agree on what quasars are, but they do know that they are about 14 billion light-years away! They are the farthest objects we are now able to detect.