What does the Hubble diagram plot?

What does the Hubble diagram plot?

• A. The relationship between a galaxy's brightness and its color
• B. Brightness (or distance) versus redshift (or apparent recessional velocity)
• C. The distribution of stars in a galaxy
• D. The surface temperature of stars vs their age

Answer: (B)

The idea being tested is that the expansion of the universe creates a link between how far away a galaxy is and how fast it appears to be moving away. A Hubble diagram shows distance (often found from a galaxy’s brightness using standard candles) plotted against redshift (which tracks the recessional velocity due to expansion). Because Hubble’s law says velocity grows with distance, closer galaxies have smaller redshifts and distant ones have larger redshifts, producing a clearer trend that the universe is expanding. This is the purpose of the diagram. The other options describe diagrams used for stellar properties within galaxies or stellar evolution, not the extragalactic distance–redshift relationship.

The idea being tested is that the expansion of the universe creates a link between how far away a galaxy is and how fast it appears to be moving away. A Hubble diagram shows distance (often found from a galaxy’s brightness using standard candles) plotted against redshift (which tracks the recessional velocity due to expansion). Because Hubble’s law says velocity grows with distance, closer galaxies have smaller redshifts and distant ones have larger redshifts, producing a clearer trend that the universe is expanding. This is the purpose of the diagram. The other options describe diagrams used for stellar properties within galaxies or stellar evolution, not the extragalactic distance–redshift relationship.