Recently, astronomers have begun using these using these next-generation detectors on telescopes to explore how stars look at these wavelengths. One result has been the discovery of a brand-new class of stars, L stars. L-stars are not easily visible in the optical because are they so very, very cool, but thanks to infrared detectors, we now know they exist. So far, we have discussed how to derive the intrinsic luminosity of a star and the star's temperature. This is the Hertzprung-Russel diagram, otherwise known as the HR diagram.
Bright stars are at the top of the diagram, cool stars are near the bottom.
Several patterns immediately emerge when we plot stellar positions in the HR diagram. This band is called the main sequence and it makes sense. So, it is reasonable that cools stars are faint and hot stars are bright. However, there are some peculiar stars that do not fall on the main sequence. In particular, some stars are both very red and very bright, while others are very blue and very faint.