First, let's talk about stars and planets. Stars are balls of gas that are so massive that the pressure inside of them causes a nuclear reaction. That makes them bright and hot (like the sun, sometimes even hotter).

Planets can vary a bit, and I don't know the exact definition. Planets like the earth and Mars are rocky with a metallic center -- in other words, they're solid. But planets like Jupiter and Saturn are mostly gas, like a star. The difference is that they aren't big enough to cause the nuclear reactions, so they aren't so hot and don't emit much light (certainly none visible to the eye, what we see is the reflection of the sun).

What's interesting is that there are things called "brown dwarfs" that are bigger than planets like Jupiter and seem almost to be stars but aren't quite.

About the galaxy: no human being has been past the Earth's moon. So we're stuck in one corner of our solar system (our sun and it's planets). The solar system is just a tiny part of the galaxy (which has billions of stars). And there are many galaxies in the known universe. The universe is a very big place!

A planet is usually defined as an object (which might be solid, liquid, gas, or a combination of all three) which is too small to produce nuclear fusion (which is the process by which the sun burns) and which orbits a star. The Earth is an example of a planet and orbits the sun, which is a star.

A star is usually defined as a body of gas which is large enough and dense enough that the heat and crushing pressure at its center produces nuclear fusion. This is a fancy way of saying that it glows or burns, like our sun. Some stars are known to have planets orbiting around them (our sun is one example!), but not all stars have planets.

A solar system is a star with its planets. Our solar system consists of the sun, the Earth and eight other planets. There are smaller objects in the solar system, like the comets, asteroids, and each planet's moons.

A galaxy is a collection of stars (some of which have planets) which clump together, held to each other by their combined gravity. The galaxy we live in is called The Milky Way. There are 200 billion stars in it, and they aren't very closely packed together, so it is quite a big place.

No one from the Earth has ever left the galaxy -it is just too far to go. Even if you traveled at the speed of light, it would take about ten thousand years to get there. The farthest that any object launched from Earth has traveled is eight billion miles, which is to say it is just on the outer edge of our solar system. The farthest any person has ever traveled is to the Moon - just 240,000 miles away.

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A star is basically a giant ball of very high temperature gas...it is made up of > 99.9 % of Hydrogen(H) and Helium( He). The critical part of a star is that the temperature at its center is high enough to enable H to slam into another H and make He and also generate a LOT of LIGHT ENERGY.

A planet on the other hand comes in two flavors: There are the Giant planets that are MORE or LESS the same COMPOSITION as a star (H plus He); however, because they are smaller than a star, a planet never reaches an internal temperature high enough to ignite the thermonuclear reaction of H+H= He plus ENERGY. Hence planets don't give off light by internal generation. (They do reflect light of course) These are called gas giant planets and JUPITER is a good example. There is another type of planet called a TERRESTRIAL planet which is made up of ROCK and Metals. The Earth or Mars is an example. In the case of Earth, it is 30 % Fe metal and 70 % rock.

A star is generally defined by its ability to generate light via nuclear (fusion) reactions that convert hydrogen into helium. Planets are formed from the collection of gas and dust that surrounds a star. They do not have the necessary mass, when sufficiently compressed by its own gravity, to support the hydrogen-to-helium fusion reactions. On a side note, there are "in between" sized objects that aren't quite massive enough to become full blown stars, but can support other types of fusion (protons into deuterium). These are called brown dwarfs and they can emit some light early in their life. They are not quite stars, but also not really planets.

Nobody has ventured out of our galaxy yet (at least from Earth!). Actually, nobody has been further away than the moon!

The formation of planets and stars begins with one large event. Scientists believe the process begins when a large cloud of dust and gas collapses. The material that collapses to the center forms the star and the rest of the material is left to rotate as a cloud around the star. Eventually, after tens of thousands of years, the dust in the cloud clumps together, forming planetesimals. These bodies collide and become larger bodies known as planets. Stars are sources of energy; they emit light and heat created through nuclear fusion of hydrogen and helium in their cores. In contrast, planets have no renewable source of energy and cool off slowly through time. There are observational differences between stars and planets, too. When you look up into the night sky, stars are the bodies that appear to twinkle and have fixed locations relative to each other. Planets, on the other hand, don't twinkle and they move relative to the stars.

No, no one has ever been out of our galaxy. Only unmanned spacecraft have been to the outer planets of our solar system, far from out of our galaxy. NASA currently has 3 interstellar probes in space, and you can read about two of them at voyager.