The Hertzsprung-Russell (H-R) diagram is a graph of stars' luminosity against their surface temperature. Plotting many stars shows they fall into distinct groups.
• luminosity on the vertical axis, usually relative to the Sun (LSun = 1), on a logarithmic scale from about 10−4 to 106
• surface temperature on the horizontal axis, also logarithmic, decreasing from left to right (about 40 000 K down to 2500 K)The main regions:
• Main sequence: a diagonal band from hot and luminous (top left) to cool and dim (bottom right). About 90% of stars, including the Sun (5800 K, 1 LSun), are here, fusing hydrogen in their cores.
• Red giants: above the main sequence on the right. They are cool but luminous, so from L = 4πr2σT4 they must be large.
• Red supergiants: across the top right, even larger and more luminous.
• White dwarfs: bottom left. They are hot but dim, so they must be very small.Moving up the diagram at a fixed temperature means a larger star; moving left at a fixed luminosity means a smaller, hotter one. A star's position changes as it ages, so the diagram is also used to follow a star's evolution.
• luminosity on the vertical axis, usually relative to the Sun (LSun = 1), on a logarithmic scale from about 10−4 to 106
• surface temperature on the horizontal axis, also logarithmic, decreasing from left to right (about 40 000 K down to 2500 K)The main regions:
• Main sequence: a diagonal band from hot and luminous (top left) to cool and dim (bottom right). About 90% of stars, including the Sun (5800 K, 1 LSun), are here, fusing hydrogen in their cores.
• Red giants: above the main sequence on the right. They are cool but luminous, so from L = 4πr2σT4 they must be large.
• Red supergiants: across the top right, even larger and more luminous.
• White dwarfs: bottom left. They are hot but dim, so they must be very small.Moving up the diagram at a fixed temperature means a larger star; moving left at a fixed luminosity means a smaller, hotter one. A star's position changes as it ages, so the diagram is also used to follow a star's evolution.