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.
Apparent brightness is how we see the star's brightness from Earth. The absolute brightness is what a star would look like if there was a standard distance that we were looking at all of the stars from. Light Year. The distance that light travels in one year.