Until then, early access to the data is being restricted to the research groups involved in carrying out the Hipparcos mission. The HR diagram is a simply a plot of intrinsic luminosity against temperature for a sample of stars. Each star is represented by a point on the diagram, determined by observation. Plotting data in this way has proved to be an immensely valuable tool for astronomers, providing insight into the physical nature of stars and the way they evolve. On the HR diagram, temperature runs from hot on the left to cool on the right.
Luminosity increases up the vertical axis.
Points representing ordinary stars, such as the Sun, fall on a diagonal band from the upper left to lower right, known to astronomers as 'the main sequence'. The position of a star on the main sequence is primarily determined by its mass, the most massive stars being the hottest and brightest, and vice versa. White dwarf stars, which are hot but very small, fall at the lower left, while red giants and supergiants, which are cool but very large, plot at the upper right. The diagram is named after the Danish astronomer Ejnar Hertzsprung, who first drew up such a diagram in 1911, and the American Henry Norris Rusell, who independently realized its significance in 1913.