Page 6 - Curriculum Visions Dynamic Book

The Milky Way
The universe is made of clusters of stars called galaxies. The region of space containing the Sun is known as
the Milky Way Galaxy. The Milky Way itself is a large collection of perhaps 200 billion stars, some of which are far bigger than our Sun.
The Milky Way is about 1,000,000,000,000,000,000 km (about 100,000 light-years or about 30 kiloparsecs) across. The space probe that has given us the most information about the Solar System is called Voyager. Voyager travels at 17.3 km/s. It would take Voyager over 1,700,000,000 years to cross the Milky Way Galaxy.
Another way to think about the galaxy’s size is to know that it takes about 8 minutes for light from the Sun to reach the Earth. In comparison, the Sun’s nearest known stellar neighbor is a dwarf star called Proxima Centauri, which is about 4.3 light-years away.
The Milky Way is a spiral galaxy—it has a number of spiraling arms that lead away from the main mass of the galaxy, which is contained in a central bulge.
A spiral galaxy contains stars at all stages of development. In the center the stars are all of different ages. But on the spiral arms we only find new, or relatively young, stars.
The Sun lies about 8 kiloparsecs from the center on what is known as the Sagittarius (spiral) arm of the Milky Way.
 We cannot see the whole Milky Way because we are inside it. But this shows roughly where the Sun lies within it.
dwarf star A star that shines with a brightness that is average or below.
galaxy A system of stars and interstellar matter within the universe.
kiloparsec A unit of a thousand parsecs. A parsec is the unit used for measuring the largest distances in the universe.
light-year The distance traveled by light through space in one Earth year, or 63,240 astronomical units.
local group The Milky Way, the Magellanic Clouds, the Andromeda Galaxy, and over 20 other relatively near galaxies.
milky way The spiral galaxy in which our star and Solar System are situated.
probe An unmanned spacecraft designed to explore our Solar System and beyond.
spiral galaxy A galaxy that has a core of stars at the center of long curved arms made of even more stars arranged in a spiral shape.
universe The entirety of everything there is; the cosmos.
voyager A pair of U.S. space probes designed to provide detailed information about the outer regions of the Solar System.
For more on Voyager see “Outer worlds” in Volume 6: Journey into space.
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