On a Hertzsprung-Russell diagram, where is the brightness (luminosity) represented?

• A. To the right on the horizontal axis
• B. Along the diagonal line from bottom-left to top-right
• C. Higher on the vertical (Y) axis
• D. At the bottom of the diagram

Answer: (C)

Brightness is shown by how high a star is plotted on the diagram. In a Hertzsprung-Russell diagram, the vertical (Y) axis represents luminosity, so a more luminous star appears higher up, while a dimmer star sits lower. The horizontal axis represents surface temperature (hot to cold, usually left to right), which is why hot stars cluster on the left and cool ones on the right. The diagonal line running from bottom-left to top-right is the main sequence, illustrating the relationship between temperature and luminosity for many stars, not where brightness itself is located. So the correct idea is that brightness corresponds to higher positions on the vertical axis.