Page 63 - Reading Success B5

The Sun, the Earth, the Moon, and all the other planets are located in the galaxy.
In fact, the whole Solar System is in the galaxy. All objects in a galaxy are bound
together by gravity. There are several different types of galaxies. The three main

types of galaxies are: elliptical galaxy, spiral galaxy, and irregular galaxy.
An elliptical galaxy has stars that are closely bound together. That is why they form
an elliptical shape. Based on research, the shape of an elliptical galaxy is not
conducive to new star formation. It is because of this that an elliptical galaxy is
usually made up of old and evolved stars.

A spiral galaxy has a core, and has groups of stars that stretch out in spiral
formation.
Other galaxies are neither elliptical nor spiral. They take on unusual forms due to

special circumstances like interactions with other galaxies.
It is observed that there are many galaxies that exist in the Universe. However,
when referring to the Galaxy, there really is only one. This is the Milky Way. The Solar
System is in the Milky Way. It has the Sun as its primary star. The Solar System is
only one of the many star clusters in the Galaxy. The Milky Way, or the Galaxy, is a

spiral galaxy with stars in the middle appearing in the shape of a bar. This bar is
estimated to have a length of 27,000 light years. Around this bar-shaped core is a
disk of dust, gas, and stars. This cluster of stars, gas, and dust forms four “arms” that

spiral around the Milky Way’s core. The cluster of stars of the Milky Way Galaxy
takes up an area with a diameter of 110,000 light years. Within this area, the Galaxy
is believed to have an estimated two hundred to four hundred billion stars.
To have an idea of how huge the Galaxy is, one light year is equivalent to 9,460
billion kilometers. It is very difficult to be sure of the Milky Way’s age. However,

according to expert studies, the oldest star that is found in the Milky Way is around
13.2 billion years old. Scientific research also shows that there is a large mass in the
middle of the Milky Way. Many experts believe that this very tight center is actually a

huge black hole. Scientists believe that most galaxies have black holes at their center.

Main Idea

What is the main idea of this story?

a. black holes

b. the Solar System
c. spiral galaxy

d. the galaxy

66_Reading Success B 5