Stars are not spread uniformly through space but are instead grouped into what Immanuel Kant referred to as "island universes" and what we today call "galaxies." As we look beyond stars to the galaxies which they belong to, we will find that

• Galaxies are classified into different types according to their shapes by the properties of the orbits of the stars they contain.
• The arms of spiral galaxies, which form whenever the disk of a spiral galaxy is disturbed, are sites of star formation.
• Stars and gas account for only a small fraction of the mass of a galaxy.
• Galaxies are composed mostly of dark matter which interacts through gravity but does not emit or absorb noticeable amounts of electromagnetic radiation.
• Most-perhaps all-large galaxies have supermassive black holes at their centers.
• When these supermassive black holes are "fed," as during encounters between galaxies, they may blaze forth with the light of thousands of normal galaxies.
• The light emerging from active galactic nuclei comes from a region no larger than the orbit of Neptune in our own Solar System.

Organize

• Read Chapter 19 in your textbook or ebook.
• Take the Diagnostic Quiz to assess your understanding of the basic concepts and identify gaps in your understanding of the assigned reading.

Learn

• Use the FlashCards to test your memory for new terms.
• Work the "Thinking about the Concepts" and "Applying the Concepts" questions in the end of chapter review materials.
• If your instructor has assigned SmartWork online homework, login from the right navigation pane for additional practice and review.

Connect

• Periodic Connections boxes and the Seeing the Forest through the Trees section at the chapter's end draw your attention to recurring themes and help prepare you for concepts explored more fully in subsequent chapters.
• Visit Astronomy in the News for breaking news of new discoveries and the opportunity to apply what you've learned in this chapter to real-world (and real Universe) events.

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