The Hertzsprung-Russell (HR) diagram is a plot of stars' absolute magnitude against their surface temperature (or spectral class).Features of the axes:
• vertical axis: absolute magnitude from about +15 at the bottom to −10 at the top, so brighter stars are higher
• horizontal axis: temperature from about 50 000 K on the left to 2500 K on the right, so temperature increases from right to left, and the scale is not linear (spectral classes O B A F G K M can be shown along the top)
• main sequence: a diagonal band from hot, bright stars (top left) to cool, dim stars (bottom right). Most stars, including the Sun, lie here while fusing hydrogen in their cores. The Sun has M ≈ +5 and T ≈ 5800 K.
• red giants: above and to the right of the main sequence; cool but bright
• red supergiants: across the very top; extremely luminous
• white dwarfs: bottom left; hot but very dimStefan's law, P = σAT4, explains the positions. Two stars with the same temperature but different brightness must differ in surface area. Red giants are cool yet bright, so they must have a very large radius. White dwarfs are hot yet dim, so they must be very small (about the size of the Earth).The HR diagram is also used to track how a star changes during its life.
• vertical axis: absolute magnitude from about +15 at the bottom to −10 at the top, so brighter stars are higher
• horizontal axis: temperature from about 50 000 K on the left to 2500 K on the right, so temperature increases from right to left, and the scale is not linear (spectral classes O B A F G K M can be shown along the top)
• main sequence: a diagonal band from hot, bright stars (top left) to cool, dim stars (bottom right). Most stars, including the Sun, lie here while fusing hydrogen in their cores. The Sun has M ≈ +5 and T ≈ 5800 K.
• red giants: above and to the right of the main sequence; cool but bright
• red supergiants: across the very top; extremely luminous
• white dwarfs: bottom left; hot but very dimStefan's law, P = σAT4, explains the positions. Two stars with the same temperature but different brightness must differ in surface area. Red giants are cool yet bright, so they must have a very large radius. White dwarfs are hot yet dim, so they must be very small (about the size of the Earth).The HR diagram is also used to track how a star changes during its life.