Helium flash occurs
a. because helium is very explosive and cannot be controlled when the nuclear reactions occur.
b. because degenerate electrons in the core do not allow the core to expand as it heats up.
c. in Cepheid variables.
d. in stars with masses less than 0.4 M.
e. none of the above
B
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In a plane wave, the wavefronts are
a. circular b. flat c. parallel d. perpendicular to the direction of propagation
Which of the following cannot occur?
A) ?+ ? ?+ + v? B) ?° ? 2? C) ?+ + p ? K+ + ?+ D) ?- + p ? n + ? E) ?- ? ?- + p
The continuous circulation of Earth's water is known as the
a. hydrologic cycle. b. aqua cycle. c. redistribution cycle. d. hydrogeologic cycle.
The force on a current-carrying wire in a magnetic field is equal to zero when
A) the current is parallel to the field lines. B) the current is at a 30° angle with respect to the field lines. C) the current is at a 45° angle with respect to the field lines. D) the current is at a 60° angle with respect to the field lines. E) the current is perpendicular to the field lines.