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.
Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. The apparent magnitude (m, or vmag for the visible spectrum) is the brightness of an object as it appears in the night sky from Earth, while the absolute magnitude (M v, V and H) describes the intrinsic brightness of an object as it would appear if it were placed at a certain distance from Earth, 10 parsecs for stars.