How do we know that there is much more mass in the halo of our galaxy than in the disk?
A) There are so many globular clusters in the halo that their total mass is greater than the mass of stars in the disk.
B) Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk.
C) Although the question of mass in the halo was long mysterious, we now know it exists because we see so many brown dwarfs in the halo.
D) The recent discovery of photinos, combined with theoretical predictions, tells us that there must be a huge mass of photinos in the halo.
E) We don't know that there is more mass in the halo; it is only a guess based on theory.
B
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Fill in the blank(s) with the appropriate word(s).
In size, from largest to smallest, the correct order for the Galilean moons is:
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A 2.5-cm-diameter cylindrical refractory crucible for melting lead is to be built for thermocouple calibration. An electrical heater immersed in the metal is shut off at some temperature above the melting point. The fusion-cooling curve is obtained by observing the thermocouple emf as a function of time. Neglecting heat losses through the wall of the crucible, estimate the cooling rate (W) for the molten lead surface (melting point 327.3°C, surface emissivity 0.8) if the crucible depth above the lead surface is (a) 2.5 cm, (b) 17 cm. Assume that the emissivity of the refractory surface is unity and the surroundings are at 21°C, (c) Noting that the crucible would hold about 0.09 kg of lead for which the heat of fusion is 23,260 J/kg, comment on the suitability of the crucible for the
purpose intended.
GIVEN
A cylindrical refractory crucible filled with molten lead
Cylinder diameter (D) = 2.5 cm
Melting point of lead (T1) = 327.2°C = 600.3 K
Surface emissivity of lead (?1) = 0.8
Mass of lead in crucible (m) = 0.09
Heat of fusion of lead (hfg) = 23,260 J/kg
FIND
The cooling rate (q) if the crucible depth above the lead surface (L) is
(a) 2.5 cm = 0.025 m
(b) 17 cm = 0.17 m
(c) Comment on the suitability of the crucible for thermocouple calibration
ASSUMPTIONS
Heat loss through the wall of the crucible is negligible
The emissivity of the refractory surface (crucible wall above the lead) is unity (?2 =1)
The surroundings behave as a blackbody enclosure
The temperature of the refractory surface is uniform at TR
SKETCH
PROPERTIES AND CONSTANTS
the Stephan-Boltzmann constant
for air at the film temperature of
Thermal expansion coefficient
Thermal conductivity
Kinematic viscosity
Prandtl number (Pr) = 0.71