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 relative brightness of the planet at each opposition is indicated by the colour of the date label, from orange (faintest) to yellow (brightest). The distances between Mars and the Earth, and between the Earth and the Sun, are also shown, in Astronomical Units (AU) .