| In structure, our Milky Way is most similar to: | M-31, the Andromeda Galaxy |
| From the Sun, the distance to the Galactic Center is about: | 8,000 pc |
| What two observations allow us to calculate the Galaxy’s mass? | the Sun’s orbital velocity and its distance from the Galactic Center |
| The circular but relatively flat portion of the galaxy is the ____ | disk |
| The first portion of the galaxy to form was the ___ | halo |
| A tightly packed group of a few hundred thousand very old stars is a | globular cluster |
| A ___ stands out in a photo of a galaxy because it shines brightly with light from massive young stars and glowing clouds of gas and dust. | spiral arm |
| Our Milky Way galaxy is a | spiral galaxy |
| Stars orbiting in the ___ near the galaxy’s center can have orbits highly inclined to the galactic plane. | bulge |
| Why are ionization nebulae so bright? | They are regions where gas is ionized by hot, young stars. |
| Galaxy | gravitationally bound collection of a large number of stars. The Sun is a star in the Milky Way Galaxy |
| Milky Way Galaxy | the spiral galaxy in which the Sun resides. The disk of our Galaxy is visible in the night sky as the faint band of light known as the Milky Way |
| Galactic disk | flattened region of gas and dust that bisects the galactic halo in a spiral galaxy. This is the region of active star formation. |
| Galactic bulge | thick distribution of warm gas and stars around the center of a galaxy |
| Galactic halo | region of a galaxy extending far above and below the galactic disk, where globular clusters and other old stars reside |
| Spiral nebula | historical name for spiral galaxies, describing their appearance |
| Spiral galaxy | galaxy composed of a flattened, star-forming disk component which may have spiral arms and a large central galactic bulge |
| Variable star | a star whose luminosity changes with time |
| Cataclysmic variable | collective name for novae and supernovae |
| Intrinsic variable | star that varies in appearance due to internal processes (rather than, say, interaction with another star) |
| Pulsating variable star | a star whose luminosity varies in a predictable, periodic way |
| RR Lyrae | variable star whose luminosity changes in a characteristic way. All RR Lyrae stars have more or less the same average luminosity |
| Cepheid variable | star whose luminosity varies in a characteristic way, with a rapid rise in brightness followed by a slower decline |
| Instability strip | region of the H-R diagram where pulsating post-main-sequence stars are found |
| Period-luminosity relation | a relation between the pulsation period of a Cepheid variable and its absolute brightness |
| Galactic center | the center of the Milky Way, or any other galaxy. The point about which the disk of a spiral galaxy rotates |
| Population I and II stars | classification scheme for stars based on the abundance of heavy elements. Within the Milky Way, Population I refers to young disk stars and Population II refers to old halo stars |
| Galactic year | time taken for objects at the distance of the Sun (about 8 kpc) to orbit the center of the Galaxy, roughly 225 million years |
| Tidal streams | groups of stars thought to be the remnants of globular clusters and even small satellite galaxies torn apart by our Galaxy’s tidal field |
| Thick disk | region of a spiral galaxy where an intermediate population of stars resides, younger than the halo stars but older than stars in the disk |
| Spiral arm | distribution of material in a galaxy forming a pinwheel-shaped design, beginning near the galactic center |
| Spiral density wave | Proposed explanation for the existence of galactic spiral arms, in which coiled waves of gas compression move through the galactic disk, triggering star formation |
| Self-propagating star formation | mode of star formation in which shock waves produced by the formation and evolution of one generation of stars triggers the formation of the next |
