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# Planet Earth Help (page 3)

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## Earth’s Place In The Galaxy

Even though our Sun seems to be the center of our Universe, it is really just one of the kids on the block. Our solar system is found on one of the spiral arms, Orion , of the spiral galaxy known as the Milky Way .

The Milky Way is one of millions of galaxies in the Universe. The Andromeda galaxy is the nearest major galaxy to the Milky Way.

Think of the Milky Way galaxy as one “continent” among billions of other continents in a world called the Universe. Its spiraling arms or “countries” are called Centaurus , Sagittarius , Orion , Perseus , and Cygnus . The Milky Way galaxy is around 100,000 light years across. The center of the Milky Way is made up of a dense molecular cloud that rotates slowly clockwise throwing off solar systems and cosmic debris. It contains roughly 200 billion (2 × 10 12 ) stars.

Although Andromeda is the closest full-size galaxy to the Milky Way, the Sagittarius Dwarf , discovered in 1994, is the closest Galaxy. It is 80,000 light years away or nearly 24 kiloparsec. (A parsec is 3.26 light-years away.)

A light-year is a unit of distance, which measures the distance that light travels in one year.

Light moves at a velocity of about 300,000 km/sec. So in one year, it can travel about 10 trillion km. More precisely, one light-year is equal to 9,500,000,000,000 km.

Orion, our “country” within the Milky Way, has many different star systems or “cities.” Each star solar system is like a “city” with buildings, parks, and homes. Our solar system is located on the outer edge of the Orion arm. The planets of the solar systems are the “buildings and homes.”

Figure 1-3 shows an edge view of the local Milky Way galaxy and our place in it.

Fig. 1-3. The solar system is at the edge of the Milky Way galaxy.

Practice problems of this concept can be found at: Planet Earth Practice Test

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