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 |