What key event marks the transition of a protostar into a true (main-sequence) star?
A) the onset of sustained nuclear fusion in its core
B) the clearing of the gas from the surrounding gas cloud into interstellar space
C) the moment when the luminosity first matches the luminosity of the lowest-mass main sequence stars
D) the onset of a strong stellar wind
A) the onset of sustained nuclear fusion in its core
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The Bohr model pictures a hydrogen atom in its ground state as a proton and an electron separated by the distance a0 = 0.529 × 10^?10 m. The electric potential created by the proton at the position of the electron is
a. ?13.6 V. b. +13.6 V. c. ?27.2 V. d. +27.2 V. e. +5.12 × 10^9 V.
What is the minimum distance between two points on the Moon that could be just resolved by the Keck telescope in Hawaii using a wavelength of 600 nm and having perfect optics? The diameter of this telescope mirror system is 10 m, and the distance to
the Moon is 380,000 km.
A solar eclipse that occurs when the Moon's umbra does not reach Earth's surface is called
a. a total solar eclipse. b. a partial solar eclipse. c. an annular solar eclipse. d. a penumbral solar eclipse. e. an umbral solar eclipse.
What kind of light can be detected from an airplane but not from the ground?
A) most infrared light B) only the shortest-wavelength infrared light C) radio waves D) x-rays