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.
If the star was at 10 parsecs distance from us, then its apparent magnitude would be equal to its absolute magnitude. The absolute magnitude is a measure of the star's luminosity---the total amount of energy radiated by the star every second. If you measure a star's apparent magnitude and know its absolute magnitude, you can find the star's distance (using the inverse square law of light brightness). If you know a star's apparent magnitude and distance, you can find the star's luminosity (see the table below).