Hotter stars would be brighter that cooler stars. This diagram came to be known as the Hertzsprung Russell diagram, or simply the HR diagram. The horizontal scale shows the temperature of the stars. It is by convention reversed so that the hottest stars are located near the origin, and the coolest stars are to the right. Measuring the stars luminosity was more difficult.
Some stars are closer to the earth, while others are extremely distant.
Because light becomes more spread out as it moves away from its source, more distant stars would look dimmer, even if they were very bright up close. It was decided to standardize the brightness of each star so that it appeared to be located at a distance of 10 parsecs from the earth, or about 32.6 light years away. The various boxes on the diagram illustrate the general areas that are studied here in order to gain a greater understanding of how star temperature is measured. All rights reserved.