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 apparent brightness of a star as seen from Earth, expressed using the magnitude scale, is known as apparent visual magnitude. The larger the magnitude, the fainter the star. You must remember that this magnitude scale only takes visible light into account.