The stellar rating also usually indicates the absolute brightness of the stars. Although it’s important to nuance this, we’ll see a couple of exceptions later. Blue stars tend to be the brightest; and the red ones, the less bright. But more experienced observers will encounter brighter red stars overnight than other white or blue: How is that possible? Keep in mind that, watching from Earth, stars are at different distances. Thus, a red star that shines little but is very close, will apparently shine brighter than a blue that this much farther away.
Apart from temperature and brightness, color also tends — with the same hue — to indicate the size of a star: the hotter and hotter blues are usually larger and the red ones smaller.
A STAR DIAGRAM
Knowing that by the color of a star its surface temperature can be estimated and its brightness and size are often known, astronomers Arena Hertzsprung and Henry Norris Russell put these characteristics on a chart known as Diagrams Hertzsprung-Russell—which they both developed independently around 1910.
In this H-R diagram, the vertical axis sorts the types of stars from least brighter to brighter, while on the horizontal axis they are sorted from warmer to less, taking into account their temperature, color and stellar classification. The result is a diagonal that sorts the stars by these characteristics: this part of the diagram is called the main sequence.
Most of the stars are found in this main sequence. They shine thanks to the fusion nuclear reactions in its core,which convert hydrogen into helium. These stars are in the stage where they mature and spend most of their active lives. Compared to people, the main sequence would be the stage from adolescence to retirement. The Sun is close to the midpoint of that stellar life: it is a medium G-type yellow star.
THE PERIODIC TABLE OF STARS
Stars can shine for billions of years. But nothing is forever. The fuel they use for nuclear reactions is limited and depleted. When there is no hydrogen left to burn, the fusion of helium takes over, but unlike the previous one it is much more energetic. This causes the stars, towards the end of their life, to swell up to thousands of times their original size, becoming giants. Expansion also causes them to lose heat on their surface, having to spread more energy in a larger area, and that’s why they turn red. These giant and red stars are an exception and are located at the top right of the diagram, known as the giant zone.
Red giants don’t last long (on a stellar scale), they quickly deplete the little fuel they have left. When this happens, the star runs out of nuclear reactions inside that hold the star: then gravity pulls its entire surface and shrinks the star until there is a dwarf left. Due to this brutal compression the energy is concentrated and its surface increases in temperature, radically changing its brightness to white. The corpse of a star is a white dwarf. These stellar corpses are another exception to the main sequence and are located at the lower left of the diagram.
Latest achievements of science is the periodic table. We can say that the Hertzsprung-Russell diagram is the periodic table of stars. In its full form, it may seem somewhat a strange and deformed rainbow; but knowing how to read it, we will see how the different colors of the stars relate to their temperatures, sizes, brightness and phases of stellar life, distributed in an orderly and elegant way. It is a map of the colors of the stars: the key to jumping to another level, both in the knowledge of the cosmos and in the ability to enjoy the beauty of the firmament.