Key Ideas and Terms

13-1 The Sun is located in the disk of our Galaxy, about 25,000 light-years from the galactic center

• A galaxy is an immense collection of stars and interstellar matter, far larger than a star cluster.
• Our Sun is one of several hundred billion stars in the Milky Way Galaxy, which is observed from Earth as a wispy band of light stretching across the sky.
• Our Galaxy is filled with visible-light-obscuring dust and surrounded by a halo of globular clusters containing RR Lyrae variables, which are similar but smaller than Cepheid variables, allowing distances to be calculated.

13-2 Observations of different types of dust, gas, stars, and star clusters reveal the shape of our Galaxy

• Infrared light passes more easily through dust and gas in our Galaxy, revealing structures.
• Stars are evenly distributed across the disk of our Galaxy, about 160,000 ly (50 kpc) in diameter and about 2000 ly (0.6 kpc) thick.
• The center of the Galaxy is surrounded by a tightly packed distribution of stars, called the central bulge, which is about 6500 ly (2 kpc) in diameter.

13-3 Observations of star-forming regions reveal that our Galaxy has spiral arms

• Dust and gas in the plane of the Milky Way organize into spiral arms, a shape commonly seen in other galaxies.
• Electrons can undergo a spin-flip transition emission of energy, which is seen as a 21-cm radio emission, allowing the Galaxy’s structure to be mapped.

13-4 Measuring the rotation of our Galaxy reveals the presence of dark matter

• From studies of the rotation curves of the Galaxy, astronomers estimate that the total mass of the Galaxy is about 10¹² M_⊙.
• Only about 10% of this mass is in the form of visible stars, gas, and dust. The remaining 90% is in some nonvisible form, called dark matter, that extends beyond the edge of the luminous material in the Galaxy.
• Our Galaxy’s dark matter is unknown, but might be a combination of massive compact halo objects or MACHOs (dim, star-size objects), massive neutrinos, and/or weakly interacting massive particles or WIMPs (relatively massive subatomic particles).

13-5 Spiral arms are caused by density waves that sweep around the Galaxy

• According to the density-wave theory, spiral arms are created by density waves that sweep around the Galaxy. The gravitational field of this spiral pattern compresses the interstellar clouds through which it passes, thereby triggering star formation illuminating the spiral arms.
• According to the theory that arms are caused by star formation, spiral arms are caused by the birth of stars over an extended, stretched region in a galaxy and reveal themselves as grand-design spiral galaxies and flocculent spiral galaxies.

13-6 Infrared and radio observations are used to probe the galactic nucleus

• The innermost part of the Galaxy, or galactic nucleus, has been studied through its radio, infrared, and X-ray emissions (which are able to pass through interstellar dust).
• A strong radio source called Sagittarius A* is located at the galactic center, marking the position of a supermassive black hole with a mass of about 3.7 × 10⁶ M_⊙.