Why are stars no longer forming in the galactic halo?
A) Frequent supernovae blew all the halo gas out into intergalactic space.
B) There was never star formation in the halo; its stars were ejected from the disk.
C) Only low-mass stars formed in the halo, so there were no stellar winds or supernovae to recycle gas.
D) The cold molecular gas required for star formation is in the galactic disk.
D) The cold molecular gas required for star formation is in the galactic disk.
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The Sun's location in the Milky Way Galaxy is
A) near the galactic center. B) in the galactic disk, roughly halfway between the center and the outer edge of the disk. C) in the halo of the galaxy, about 27,000 light-years above the galactic disk. D) at the outer edge of the galactic disk.
According to the density-wave theory, spiral arms are blue because they
A. contain only young stars, whose formation was triggered by the explosion of older red stars just in front of the spiral arm. B. contain a lot of young stars formed from clouds compressed by the density wave. C. have too much metal content to form red stars. D. contain many reflection nebulae from sites of young star formation.
The force fo static friction between two surfaces is parallel to the surface of contact, and in in the direction that opposes relative motion
Indicate whether the statement is true or false
In the context of plate tectonics, what is a subduction zone?
A) a place where two plates are slipping sideways against one another B) a place where a seafloor plate is sliding under a continental plate C) a place where two continental plates are colliding D) a place where two plates are pulling apart