Galaxy

Definition

A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one trillion stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers. The Milky Way is the galaxy that contains our Solar System, and is just one of the hundreds of billions of galaxies in the observable universe.

5 Must Know Facts For Your Next Test

1. Galaxies are classified into three main types: elliptical, spiral, and irregular, based on their visual morphology.
2. The Milky Way galaxy, which contains our solar system, is a spiral galaxy with a diameter of over 100,000 light-years.
3. Supermassive black holes, with masses millions to billions of times that of the Sun, are believed to exist at the center of most, if not all, galaxies.
4. Dark matter, an unknown form of matter that does not interact with electromagnetic radiation, makes up about 85% of the total matter in the universe and is essential for the formation and structure of galaxies.
5. Galaxies are thought to have formed from the gravitational collapse of giant clouds of gas and dust, with dark matter playing a crucial role in this process.

Review Questions

• Describe the general structure and composition of a galaxy.
  • A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one trillion stars, each orbiting the galaxy's center of mass. The Milky Way, the galaxy that contains our Solar System, is a spiral galaxy with a diameter of over 100,000 light-years and is estimated to contain over 200 billion stars.
• Explain the role of supermassive black holes in the center of galaxies and how they influence the galaxy's structure and evolution.
  • Supermassive black holes, with masses millions to billions of times that of the Sun, are believed to exist at the center of most, if not all, galaxies. These supermassive black holes play a crucial role in the formation and evolution of their host galaxies. They act as gravitational anchors, influencing the motion and distribution of stars, gas, and dust within the galaxy. The energy released by matter falling into the supermassive black hole can also impact the surrounding galaxy, driving the formation of new stars or even expelling material from the galaxy.
• Discuss the importance of dark matter in the formation and structure of galaxies, and how its presence has been inferred by astronomers.
  • Dark matter, an unknown form of matter that does not interact with electromagnetic radiation, makes up about 85% of the total matter in the universe. Dark matter is essential for the formation and structure of galaxies. Astronomers have inferred the presence of dark matter by observing its gravitational effects on the motion of stars and gas within galaxies, as well as on the large-scale structure of the universe. Without the gravitational influence of dark matter, galaxies would not have been able to form and maintain their structure as we observe them today. The distribution and behavior of dark matter is a key factor in understanding the evolution and properties of galaxies in the universe.

Related terms

Milky Way: The galaxy that contains our Solar System, comprising over 200 billion stars and stretching over 100,000 light-years in diameter.

Supermassive Black Hole: An extremely dense, massive black hole found at the center of most, if not all, galaxies, including the Milky Way.

Dark Matter: An unknown form of matter that cannot be seen directly but accounts for most of the matter in the universe, and plays a crucial role in the formation and structure of galaxies.