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galaxy are surrounded by invisible dark matter.
The whole of the galaxy contains several million million stars,
of which our Sun is an above- average-sized member on one of the outer spiral arms.
Set within the galaxy are stars in many forms of their evolution. Some are new stars set far out on the spiral arms; others, nearer the center, are up to 14 billion years old; some have even collapsed and produced black holes.
Nearby, in astronomical terms, there are 20 large galaxies that together make a cluster. The largest and most prominent of these is called the Andromeda Galaxy, more than 2 million light-years away. What we see of this galaxy in the sky is, therefore, what it looked like 2 million years ago!
Many galaxies are visible in the night sky and were classified, along with all other celestial objects,
by C. J. Messier in 1771. All of these objects have M numbers, such as M31, and this is often how astronauts still refer to them.
Gas giants
the more he become convinced that the traditional view of the Universe (known and accepted since the time of Aristotle) was incorrect. Instead, he believed that Nicolas Copernicus held the correct view, and the Earth was not the center of the Universe.
Unlike Copernicus, whose ideas were not published until after his death, Galileo wrote two books that were published during his lifetime. They were unpopular with the church, and he was tried for heresy. Galileo was eventually put under house arrest for the rest of his life and forced to say that what he had seen was untrue.
Galileo probe
A space probe launched in 1989 by NASA and designed to explore Jupiter. Besides photographing the surface, it sent a small probe on a parachute into the Jovian atmosphere.
Gas giants
The name for Jupiter, Saturn, Neptune, and Uranus. These planets contrast with the smaller, rocky planets such as the Earth and Mars.
It is believed that the warmth of the Sun boiled many gases off the surface of the rocky planets. But the outer planets are cold worlds, so far from the Sun that they have never lost their light gases. That
is why (except for Pluto, which
is only the size of a moon) they are such giant worlds, made of hydrogen and helium gas or frozen water, methane, and ammonia.
 Gas giants—The largest gas giant, Jupiter, seen with its largest moon, Ganymede.
Galilean moons
Jupiter’s four largest moons: Io, Europa, Ganymede, and Callisto. They were discovered independently by Galileo and Marius.
Galileo
A pioneering Italian astronomer, Galileo Galilei (1564-1642) was famous as one of the first people ever to make a telescope. With it he saw what he described as mountains on the Moon. He also saw four of the satellites of Jupiter.
The more Galileo looked at the Universe through his telescope,
Ganymede
The largest moon of Jupiter and one of the Galilean moons. It is also the largest moon in the Solar System, being 5,262km across. It orbits at one million kilometers from the planet’s surface. It has an icy surface, but also shows signs of past geological activity (see: Moons).
 Ganymede—(Inset) The surface of Ganymede, showing intriguing signs of geological activity.
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