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 brighter an object appears, the lower its magnitude value (i.e. inverse relation). In addition, the magnitude scale is logarithmic: a difference of one in magnitude corresponds to a change in brightness by a factor of about 2.5. Generally, the visible spectrum (vmag) is used as a basis for the apparent magnitude. However, other spectra are also used (e.g. the near-infrared J-band).