Interstellar gas and dust pervade the Galaxy

• Interstellar gas and dust, which make up the interstellar medium, are concentrated in the disk of the Galaxy

• Clouds within the interstellar medium are called nebulae

• Dark nebulae are so dense that they are opaque

• They appear as dark blots against a background of distant stars

• Emission nebulae, or H II regions, are glowing, ionized clouds of gas

• Emission nebulae are powered by ultraviolet light that they absorb from nearby hot stars

• Reflection nebulae are produced when starlight is reflected from dust grains in the interstellar medium, producing a characteristic bluish glow