What does Hubble's law describe?

• A. The energy emitted by distant galaxies.
• B. The speed at which a galaxy appears to be moving away from Earth is proportional to its distance from Earth.
• C. The formation of stars in nebulae.
• D. The orbits of planets around the Sun.

Answer: (B)

Hubble's law describes how the speed at which a galaxy is moving away from us depends on its distance. In simple terms, more distant galaxies recede faster, with velocity proportional to distance (v = H0 × d). This relation comes from observing galaxy redshifts and measuring distances to those galaxies, showing a clear trend that faraway galaxies have greater redshifts. The constant H0, the Hubble constant, sets the expansion rate of the universe and supports the idea that space itself is expanding, not that galaxies are all flying apart from a single center in a local system.

This focus on the large-scale motion of galaxies due to cosmic expansion makes it the correct interpretation. The other options describe different astrophysical ideas—energy emitted by distant galaxies relates to luminosity, star formation in nebulae to stellar birth processes, and the orbits of planets around the Sun to gravitational dynamics within a solar system—none of which capture the observed velocity–distance relationship that Hubble's law expresses.