Ch 23: The Milky Way Galaxy

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

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