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.
1The ~ is a logarithmic scale of brightness. As the magnitude of a star increases, its brightness decreases. The apparent magnitude m is the brightness of a star measured at Earth, and it depends both on the luminosity and the distance of the star.