Page 50 - Curriculum Visions Dynamic Book

structures called galaxies.
We are part of the Solar System. The Earth moves
around the star we called the Sun. The Sun is on the outer part of an arm of a moderate-sized galaxy that we call the Milky Way.
If we use powerful telescopes, we can see that there are other galaxies. The more we look at stars and clusters of stars, the more we see them as parts of an even greater group. Some are far away, others much closer to us.
The Local Group
Astronomers now call the galaxies that are our near neighbors the Local GroUp. They include our galaxy, the Milky Way, the neighboring smaller satellite galaxies of the Magellanic CloUds, the Andromeda Galaxy, our nearest independent galaxy, and over 20 other relatively near galaxies. The Milky Way and Andromeda are the biggest members of the Local Group.
Our galaxy is made up of a number of arms that spiral to the central hub. This is called a spiral galaxy (see pages 42–45). The Andromeda Galaxy is another spiral galaxy.
Looking farther afield, we see that many galaxies in our Local Group are not, in fact, spirals but form great football-shaped masses called elliptical galaxies. They are bigger and brighter, and are not flattened into a disk like the spirals.
Groups of galaxies, like our Local Group, cluster together throughout the cosmos, but many contain vastly more galaxies, often numbering thousands rather than the few that make up the Local Group. But on a large scale the size range and spacing of galaxies are much the same in all directions.
Light as a time machine
We have already said that we use the speed of light as a measure for space. Some galaxies are several billion light- years from the Earth. Light reaching us from these galaxies is so old that it left the galaxy at just about the time that the
elliptical galaxy A galaxy that has an oval shape rather like a football, and that has no spiral arms.
local groUp The Milky Way, the Magellanic Clouds, the Andromeda Galaxy, and over 20 other relatively near galaxies.
magellanic cloUd Either of two small galaxies that are companions to the Milky Way Galaxy.
milky Way The spiral galaxy in which our star and Solar System are situated.
nebUla (pl. nebUlae) Clouds of gas and dust that exist in the space between stars.
radiation The transfer of energy in the form of waves (such as light and heat) or particles (such as from radioactive decay of a material).
satellite An object that is in an orbit around another object.
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.
stellar Wind The flow of tiny charged particles (called plasma) outward from
a star.
Ultraviolet A form of radiation that
is just beyond the violet end of the visible spectrum and so is called “ultra” (more than) violet. At the other end of the visible spectrum is “infra” (less than) red.
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