ASTR 10227Important textbook chapters:Chapter 21.2 -The H–R Diagram and the Study of Stellar EvolutionChapter 22 – Stars from Adolescence to Old AgeOne of the major goals of astronomy is to understand the detailed nature of stars and theirevolution. In Project #2, you examined stellar spectra which give us information on surfacetemperature and composition. For any given stellar temperature, there can be a wide rangeof possible sizes and luminosities (from a cool dwarf like the Sun to a cool supergianthundreds of times larger and thousands of times brighter than the Sun). In Project #2, youalso learned about the brightness of stars and how these relate to their apparent magnitudes.To measure the brightness of a star we need to measure the amount of light we receive fromit. Astronomers can measure the amount of light from stars using cameras attached to atelescope, information which can then be translated into apparent magnitudes. In this projectyou will use the magnitude of stars measured at different wavelengths to estimate distancesand ages to star clusters.1Why star clusters and not individual stars?Imagine you were an alien anthropologist who arrived on the Earth to study the entire humanlife cycle, but your spaceship was making only a one-hour stop on this planet.One solutionwould be to pick a place crowded with a wide variety of people, say a park on a pleasantsummer day or a popular shopping mall or a busy downtown street corner (during non-pandemic times of course, this wouldn’t work this year).In the course of an hour, you'dobserve little children, middle-aged people, babies pushed past in strollers, senior citizens,teenagers, and so on.With a large enough sample of people, you'd see examples of almostevery major stage in the life-cycle of an average human being – from just after birth to justbefore death.1By taking some basic observable characteristics of all those people (e.g., mass,height, average speed of motion) and plotting them against each other, you'd find that theyfall into recognisable patterns which you can use to describe the evolution of a ‘typical’human life.Astronomers apply the same logic to stars. They take large samples of stars, correlate theirbasic characteristics like luminosity and colour, and study any apparent patterns. In the earlytwentieth century, Ejnar Hertzsprung and Henry Russell independently developed what isnow known as the Hertzsprung-Russell (H-R) diagram – a scatter plot of star’s absolutemagnitudevs.spectral type (you should be able to find several examples of H-R diagrams inyour text, particularly in Chapters 21 and 22).You generated your own basic versions of H-R diagrams in the last project.When we construct an H-R diagram using randomly selected stars with known distances, weexpect to get a distribution of stars along the main sequence, along with scattered giants anddwarf stars. We would expect, if enough stars are observed, to see examples of almost everyspectral type and luminosity class. But, as you saw in the last part of Project #2, when the1And there's a small chance you might even observe instances of those events too.