-To know the temperature of a star, you first need to know its distance and to account for the dust between us and the star. You can guess the effect of distance. A star farther away will be fainter (lower y-axis value) than the same star closer (think of car headlights as they approach). Dust will remove light from the star's path to our telescopes, which makes the star seem like it has less energy than it otherwise would have, which makes it do two things on this diagram: 1) look fainter (lower y-axis value) and 2) look cooler (higher x-axis value). The stars spread throughout the diagram are all stars bright enough to be detected in our Milky Way between GD-1 and us but made fainter and redder (spread to the lower-right) by their spread in distance from us and the amount dust in the line of sight.
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