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.
Using this scale, astronomers now define five magnitudes' difference as having a brightness ratio of 100. Vega was used as the reference star for the scale. Initially it had a magnitude of 0, but more precise instrumentation changed that to 0.3.