Concrete is the most common artificial composite material of all and typically consists of loose stones (aggregate) held with a matrix of cement. Concrete is a very robust material, much more robust than cement, and will not compress or shatter even under quite a large compressive force.
2) Y-axis contains absolute brightness Why do astronomers use the H-R Diagram? Astronomers use H-R Diagrams to classify stars and to understand how stars change over time.
The Relation Between Brightness and Luminosity. The brightness of a star is proportional to its luminosity divided by its distance from the observer squared. For this example at one unit of distance from the star the brightness is one, but at three units of distance from the star the brightness is nine times smaller.