Page 11 - Curriculum Visions Dynamic Book

Light travels around the Earth seven times a second, so it is not a useful measurement on Earth; it takes 8 minutes for light to reach us from the Sun, so that is not a very useful measurement even
for distances within the Solar System.
By contrast, the time it takes for light to travel from the nearest star outside our Solar System, Proxima Centauri, is 4.3 years. Most stars are vastly farther away. So, while it makes sense to use AUs for the Solar System, it is more manageable to use light-years for interstellar distances.
The time it would take for light
to travel from one side of a galaxy to
another is, on average, 100,000 years, that is, a galaxy is 100,000 light-years across.
Our galaxy has a hundred billion stars in it. The galaxies nearest to our Milky Way are the small ones called the Large and Small Magellanic Clouds (see page 51). They are 200,000 light-years away.
The nearest big galaxy is the Andromeda Galaxy (also called M31). It is two million light-years away. That means light that left the Andromeda Galaxy before the start of the Earth’s Ice Age is only now just reaching us.
Parsec
The parsec is used by professional astronomers to measure the largest distances in the Universe, such as the distances to galaxies.
It is based on calculation of the radiUs of the Earth’s orbit round the Sun. A parsec is the distance from which an observer in space would see the radius of the orbit as making 1 second of arc (1/3600 th of a degree). This distance is about 3.26 light- years. A kiloparsec is a thousand parsecs. The Sun, for example, is 8.5 kiloparsecs from the center of the Milky Way.
A megaparsec is a million parsecs. The distance to the Andromeda Galaxy is 0.7 megaparsec. Some galaxies may be up to 3,000 megaparsecs away.
Lagoon Nebula
(catalogue numbers NGC 6523 and M8)
 This area of hydrogen gas in the constellation Sagittarius is 4,000 light-years away and about 33 light-years across.
astronomical Unit (aU) The average distance from the Earth to the Sun (149,597,870 km).
celestial Relating to the sky above, the “heavens.”
galaxy A system of stars and interstellar matter within the Universe.
interstellar Between the stars.
light-year The distance traveled by light through space in one Earth year, or 63,240 astronomical units.
orbit The path followed by one object as it tracks around another.
parsec A unit used for measuring the largest distances in the Universe.
radiUs (pl. radii) The distance from the center to the outside of a circle or sphere.
11