THE EARTH IN SPACE – STARS AND GALAXIES

1. Define and describe “galaxy”.

a. A galaxy is a very large cluster of stars (tens of millions to trillions of stars) gravitationally bound together.
b. There are billions of galaxies in the observable universe.
c. The various types of galaxies include:

* Spiral galaxies, which are typically disk-shaped with a somewhat greater concentration of stars near their centers, often containing arms of stars extending from their central nucleus
*Barred spiral galaxies, a type of spiral galaxy that has the stars arranged in the shape of a bar, which rotates as a rigid system
*Elliptical galaxies, the most abundant type, which have an elliptical shape that ranges to nearly spherical, and lack spiral arms
*Irregular galaxies, which lack symmetry and account for only 10 percent of the known galaxies.
d. Galaxies are not randomly distributed throughout the universe. They are grouped in galactic clusters, some containing thousands of galaxies. Our own, called the Local Group, contains at least 28 galaxies.
2. Locate the Sun’s position in the Milky Way Galaxy.
a. The Sun is one of the 200 billion stars that make up the Milky Way galaxy.
b. The Milky Way galaxy is a large, disk-shaped, spiral galaxy about 100,000 light-years wide and about 10,000 light-years thick at the center (central “bulge”).
c. There are three distinct spiral arms of stars.
d. The Sun is positioned in one of these arms about two-thirds of the way from the galactic center, at a distance of about 30,000 light-years.
e. Scientists suspect that a super massive black hole– an immensely dense area of space that sucks up matter and light – lies at the galaxy’s center
f. It takes about 200 million years for the Sun to revolve around the galactic center.
g. Surrounding the galactic disk is a nearly spherical halo made of gas and numerous globular clusters (nearly spherically shaped groups of densely packed stars).

h. We see the Milky Way as a bright band of stars across the sky. It looks like spilled milk!
i. Most of the points of light in the night sky are stars in the Milky Way
3. Understand why light years are used to measure distances in space.
a. A light-year is a unit of distance (NOT TIME!!!). It is the distance that light can travel in one year.
b. Light moves at a velocity of about 300,000 km each second (in a vacuum). So in one year, it can travel about 10 trillion km. More precisely, one light-year is equal to 9,460,500,000,000 kilometers (5,880,000,000,000 miles).
c. Why would you want such a big unit of distance? In the Universe, the kilometer is just too small to be useful. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km. That is 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance.
d. In our solar system, we tend to describe distances in terms of the Astronomical Unit (AU). The AU is defined as the average distance between the Earth and the Sun. It is approximately 150 million km (93 million miles). Mercury can be said to be about 1/3 of an AU from the Sun and Pluto averages about 40 AU from the Sun. The AU, however, is not big enough of a unit when we start talking about distances to objects outside our Solar System.
e. For distances to other parts of the Milky Way Galaxy (or even further), astronomers use the light-year. Using the light-year, we can say that:

*The Milky Way Galaxy is about 150,000 light-years across.
*The Andromeda Galaxy (one of our nearest neighboring galaxies) is 2.3 million light-years away.
*Proxima Centauri, the closest star, is 4.24 light years away.
*Sirius the “dog star” (the brightest star in the sky) is 8.6 light years away.
*Center of the galaxy is approximately 30,000 light years away.
*The most distant galaxies observed are more than 12 billion light years away.

f. Light minute- the distance light