The solar system is made up of the sun and its family of eight planets. Pluto, formerly the ninth planet, was reclassified as a dwarf planet in August of 2006 when similar-sized objects were discovered. Scientists decided to finally define "planet" when they found Eris to be larger than Pluto. Planets and stars are different from one another. A planet is a body that does not give off light of its own. It reflects the light of the sun. Stars produce and give off their own heat and light. Our sun is a star. Our sun is a center of our solar system and the planets revolve around it. It is also the largest member. The word solar means “of the sun". Whenever the sun goes in space, the rest of the family or system goes too.
The solar system is party of an even larger family in space. The sun is only one of a huge group of stars called the Milky Way Galaxy. Compared to the Milky Way Galaxy, our solar system is an extremely tiny speck. If you look through a powerful telescope, you would see millions and millions of other galaxies, each containing billions of stars of their own. Our solar system, the Milky Way and all the other galaxies make up the Universe. Everything in space is part of the Universe.
The invention of the optical telescope is surrounded by controversy. One story puts it in a shop of a Dutch Len maker, Hans Lippershey, in October of 1608. As the story goes, two children were playing with his lenses, put two together, peered through them at a distant church tower and saw it wonderfully magnified. This was the first known optical telescope. In July of 1609, Galileo Galilei developed the first astronomical optical telescope. With his simple telescope, he made important early discoveries in astronomy. His telescope is formed by the combination of a concave and a convex lens, today known as the Galileo telescope system. The front convex lens is called an objective because it is close to the object being viewed and the rear concave lens is called an eyepiece because it is closest to the observer's eye. Binoculars based on Galileo telescope design, known as opera glasses, were also invented in the same period. In 1611, Johannes Kepler invented another type of longer telescope comprising two convex lenses, known as the Kepler telescope system. The Kepler telescope forms and upside down image, but with a slightly larger field of view. Telescope for personal use with the eye, including binoculars, are called afocal optical systems because rays of light from a distant object that are parallel when they enter the objective are also parallel when they exit the eyepiece. For afocal telescopes, an important parameter is angular magnification. The angular magnification or magnification factor is the ratio between the angle subtended by the output image and the angle subtended by the input object. Kepler telescopes usually have larger magnification than Galileo ones.
The Sun is our nearest star. This is why it looks so much bigger and brighter than other stars. It is close enough to be our main source of heat and light and all life on Earth depends on it. Not all stars are like our Sun. The Sun is one of about 200 billion stars held together in a huge collection of stars, gas and dust called the Milky Way. There are hundreds of billions of galaxies in the universe. It's not just astronauts who are interested in stars and galaxies. Humans have been gazing at the night sky for thousands of years. Our knowledge of stars has changed over time and so have the methods used to explore them.
Galaxies appear on the sky as huge clouds of light, thousands of light years across. Each contains anywhere from a million stars up to a million million; gravity binds the stars together, so they do not wander freely through space. Almost all the light of galaxies comes from their stars. One of the great discoveries of the twentieth century is that the Universe is not static, but expanding; the galaxies all recede from each other and from us. Our Universe appears to have had a beginning, the Big Bang, that was not so far in the past: the cosmos is only about three times older than the Earth.