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1.Protostars difficult to observe becauseA.the protostar stage very short.B.they surrounded

Question : 1.Protostars difficult to observe becauseA.the protostar stage very short.B.they surrounded : 1777236

1.Protostars are difficult to observe because
A.the protostar stage is very short.
B.they are surrounded by cocoons of gas and dust.
C.they radiate mainly in the infrared.
D.all of the above
E.they are all so far away that the light hasn't reached us yet.

2.The nuclear reactions in a star's core remain under control so long as
A.luminosity depends on mass.
B.pressure depends on temperature.
C.density depends on mass.
D.weight depends on temperature.
E.temperature depends on mass.

3.Interstellar gas clouds may collapse to form stars if they
A.have very high temperatures.
B.encounter a shock wave.
C.rotate rapidly.
D.are located near main sequence spectral type K and M stars.
E.all of the above.

4.____ are small luminous nebulae excited by nearby young stars.
A.T Tauri stars
B.Herbig-Haro objects
C.O associations
D.Bok Globule
E.Giant molecular clouds

5.Opacity is
A.the balance between the pressure and force of gravity inside a star.
B.the force that binds protons and neutrons together to form a nucleus.
C.the force that binds an electron to the nucleus in an atom.
D.a measure of the resistance to the flow of radiation (photons) through a gas.
E.the temperature and density at which a gas will undergo thermonuclear fusion.

6.The visual pink glow of the Great Nebula in Orion
A.is a Herbig-Haro object.
B.is a reflection nebula of dust.
C.is an emission nebula of thin gas.
D.contains only young low mass stars.
E.is believed to be about 5 billion years old.

7.The ____ is the nuclear fusion of three helium nuclei to form a carbon nucleus.
A.proton-proton chain
B.Carbon-Nitrogen-Oxygen cycle (CNO cycle)
C.triple-alpha process
D.Coulomb barrier
E.infrared cirrus

8.Emission nebulae are also called ____ because they are composed of ionized hydrogen.
A.HI regions
B.HII regions
C.Bok globules
D.HeI regions
E.reflection nebulae

9.____ are star-like objects that contain less than 0.08 solar masses an will never raise their core temperatures high enough that the proton-proton chain can begin. Other minor fusion reactions do occur in these objects. They fall in a gap between the low-mass M dwarf stars and the massive planets in which nuclear fusion never occurs.
A.Brown dwarfs
B.Herbig-Haro objects
C.Bok globules
D.T-Tauri star
E.Main sequence stars

10.The average star spends ____ of its lifetime on the main sequence.
A.1%
B.2%
C.10%
D.20%
E.90%

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