In this topic you will learn about how with tools and ideas people are learning more about the universe every day.

Stars are grouped together throughout the universe. They are in groups called galaxies. A galaxy is a large group of stars held together by gravity. The stars in a galaxy are moving. They orbit the center of their galaxy.

A galaxy may contain around 200 billion stars. There are about 100 billion galaxies in the universe. Galaxies differ in size and structure. Astronomers classify galaxies into three groups based on their shapes. A spiral galaxy is a whirlpool-like galaxy. An elliptical galaxy is football or basketball shaped. An irregular galaxy does not have a recognizable shape.

The Milky Way is our home galaxy. The Milky way is a spiral galaxy. Our solar system is located in one of the spiral arms.

The heated gases of stars produce waves of light. The light produces a spectrum. A spectrum is a band of colors made when white light is broken up. You also see a pattern of dark lines. Each star or galaxy has its own pattern of lines.

Galaxies move away from each other. As space expands, the wavelength of light traveling through space also expands. This increase in wavelength of light due to space expanding is called an expansion redshift. It is called a redshift because the light's spectrum has been shifted to longer (redder) wavelengths.

Astronomers say that in the past the galaxies must have been closer to each other. The beginning moment of the universe, when the density of the universe was very high is called the big bang. According to the big bang theory, at some point in the past, all matter was in the form of a huge, compact ball. The universe has expanded ever since the big bang.

Astronomers have discovered a form of electromagnetic radiation coming from all directions in space. This radiation is left over from the beginning of the universe. It is called background radiation. Some objects in the universe shine brighter than trillions of Suns. They have a size about that of the solar system. They are quasars. A quasar is an extremely bright, extremely distant, high-energy source.