SUMMARY OF KEY IDEAS

The Milky Way

The Structure of Our Galaxy

Types of Galaxies

Clusters and Superclusters

Superclusters in Motion

Quasars and Other Active Galaxies

Supermassive Engines

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WHAT DID YOU THINK?

  • What is the shape of the Milky Way Galaxy? The Milky Way is a barred spiral galaxy. A bar of stars, gas, and dust runs through its central region. It has two major spiral arms, several minor arms, and is surrounded by a complex spherical halo system of stars and dark matter.

  • Where is our solar system located in the Milky Way Galaxy? The solar system is between the Sagittarius and Perseus spiral arms, about 26,000 ly from the center of the Galaxy (about halfway out to the visible edge of the galactic disk).

  • Is the Sun moving through the Milky Way Galaxy and, if so, about how fast? Yes. The Sun orbits the center of the Milky Way Galaxy at a speed of 878,000 km/h (550,000 mi/h).

  • Are most of the stars in spiral galaxies located in their spiral arms? No. The spiral arms contain only 5% more stars than the regions between the arms.

  • Do all galaxies have spiral arms? No. Galaxies may be spiral, barred spiral, lenticular, elliptical, or irregular. Only spirals and barred spirals have arms.

  • Are galaxies isolated objects? No. Galaxies are grouped in clusters, and clusters are grouped in superclusters.

  • Is the universe contracting, unchanging in size, or expanding? The universe is expanding.

  • What does “quasar” stand for? The term quasar stands for quasi-stellar radio source.

  • What do quasars look like? They look like stars, but they emit much more energy than any star.

  • Where do quasars get their energy? A quasar is powered by a supermassive black hole with millions or billions of solar masses at the center of a galaxy.