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The Difference Between a Solar System & a Galaxy

Outer space, the region beginning at the upper reaches of Earth's thin atmosphere, is unimaginably vast. Even the most powerful telescopes have not revealed the edges of the universe, even though they have seen more than 80 billion trillion miles (that's 22 zeros!). The huge reaches of space have created a problem: Everything out there is so big and so far away that it's difficult for the human mind to understand and categorize the distances involved. Solar systems and galaxies are both huge objects, but they are of vastly different size and character.

Our Solar System
- The solar system is the collection of objects bound to the sun by gravitational attraction.
  
  The solar system is roughly defined as the group of objects that are trapped by the gravitational field of the sun. This includes the major and minor planets, but it also includes comets and planet-like objects well beyond the orbit of Pluto. The farthest region of the solar system, an area called the Oort Cloud, is more than 11 trillion miles away. It would take the fastest airplane 179,000 years to get to the edge of the solar system.
Other Solar Systems
- Our solar system is not the only one out there. Looking at the uncountable stars in the night sky, it's easy to imagine that at least some of them must have planets. It has only been in the last 20 years or so that technology has become capable of detecting solar systems beyond our own. Now there are more than 460 known planetary systems orbiting other stars --- technically, they aren't solar systems, because "sol" refers to the sun. There is no reason to imagine that these systems are drastically different from our solar system --- they consist of a number of bodies captured by the gravitational field of their stars.
Milky Way Galaxy
- Galaxies are made of stellar systems bound to each other by gravitational attraction.
  
  Although planetary systems are huge, galaxies are vastly larger. All the known planetary systems are within the Milky Way galaxy. Galaxies are held together with gravitational attraction, just like planetary systems, but the attraction is not between a star and its planets, but among stars. Gravitational attraction pulls stars into the centers of their galaxies, while rotational velocity keeps the stars from falling straight in. The Milky Way is almost a million trillion miles across. The fastest airplane, the one that could make it to the edge of the galaxy in 179,000 years, would take nearly 10 billion years to get across the galaxy. Put another way, if the solar system was a ball as big around as a football field, then the galaxy would be the size of Earth.
Other Galaxies
- Within a tiny slice of sky, thousands of galaxies are revealed.
  
  The Hubble space telescope has taken a number of "deep field" images -- pictures of tiny regions of the sky that have few nearby stars. By taking long, repeated exposures of these areas, the telescope was able to collect light from distant galaxies. Each of these pictures of just a tiny region of sky has shown thousands of galaxies. Using those images, and data from other telescopes, astronomers have estimated there are hundreds of billions of galaxies in the universe.

Astronomy