Astronomy found that its mass determines nearly every aspect of a star’s physics; in the end, mass matters most.  Its mass and lifespan primarily determine the star’s place on the HR diagram.  The star’s position on the HR diagram determines its absolute brightness and color. The problem was that astronomers did not understand why this was so at the beginning of the twentieth century. First, they had thought that maybe younger stars would burn brighter, but this was not necessarily so.  Then astronomers hypothesized that larger stars would be brighter, which is also not necessarily so. The reason why so… Read More »

Stars are not uniformly distributed throughout the night sky but are often clumped together in star clusters.  These star clusters were of some nuisance to astronomers trying to discover comets or deep sky objects because they appeared similar to the casual observer. Charles Messier gained some fame in cataloging some of these annoying clusters so they could not be confused with other objects, especially comets.  His occupation as the Astronomer of the Navy for the government of France afforded him some opportunity to gain familiarity with the night sky.  He made a catalog of star clusters so that he and… Read More »

The next mystery to be addressed by astronomy concerned stellar mass.  Henry Russel proposed in 1914 that the hotter, more luminous stars had greater mass, while the cooler stars had a lower mass.  This means that the stars on the upper left of the Hertzsprung-Russell diagram were had a greater stellar mass and had a blue color, while those on the lower right corner of the Hertzsprung-Russell diagram were red stars with much lower masses. Unfortunately, early in the twentieth century, there was no way astronomers could measure stellar mass. How astronomers figured this out represents one of the greatest… Read More »

The Hertzsprung-Russell diagram is foundational to an understanding of stars and virtually everything astronomers have learned about stars, galaxies, and the universe.  The diagram plots the absolute magnitude of a star against its spectrum. Thus, to put any star on the diagram, we only need to know its surface temperature (which relates to its spectral type) and the absolute brightness. Astronomers can determine the spectral type for visible stars if the telescope can gather enough light.  The amount of light collected from a star can be increased with a larger telescope, longer collections time, or more sensitive equipment. In addition,… Read More »