For a pair of stars to be classified as a visual binary,which condition must be TRUE?
A) The stars must be orbiting around each other, and both stars must be visible through telescopes from Earth.
B) The stars must be orbiting around each other, and absorption or emission lines from both stars must be visible in the spectrum.
C) The stars must be orbiting around each other, and one must periodically cross in front of the other (i.e., it must eclipse the other) as seen from Earth.
D) The stars must lie in almost the same direction from Earth but must not be orbiting around each other.

Question

Quizplus · Accepted Answer

For a pair of stars to be classified as a visual binary, they must be orbiting around each other, and both stars must be visible through telescopes from Earth. This is what makes them "visual" binaries, as opposed to spectroscopic binaries, where absorption or emission lines from both stars are visible in the spectrum, or eclipsing binaries, where one star periodically eclipses the other as seen from Earth. Option D is wrong because, for stars to be considered a binary, they must orbit each other, and not just be aligned in the same direction.

For a Pair of Stars to Be Classified as a Visual