# Stellarium 2 - The H-R Diagram.pdf - Astronomy 11 Name: _...

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1Danish astronomer Ejnar Hertzsprung(left) and American astronomer HenryNorris Russell (right)….co-developers ofthe H–R diagram.Astronomy 11Name: _____________________________Date: ______________________________Stellar Properties and the H–R Diagram(Stellarium Exercise #2)Introduction:If we want to learn about a star, understand what a star is and how a star works, a good place to start iswith theproperties of the star.The primary properties of a star include itssize, temperature, luminosity, mass, ageand distance(from Earth).There are a couple of ideas to keep in mind as you do this exercise and learn about theproperties of the stars:First of all, these properties will vary from one star to the next.For instance, some stars are quitelarge and others are small, some stars are very hot and others are relatively cool, etc.Secondly, for a given star, many ofits properties are related to each other.For instance, a massive star is usuallylarge, bright and hot, while a low mass star is often small, dim and cool.(There are important exceptions to these ‘rules’ which we will discuss as we goalong.)A useful way to display some of the properties of a star, and to see therelationships that may exist between these properties, is with a graph.In the1910’s, Danish astronomer Ejnar Hertzsprung and American astronomer HenryNorris Russell together developed a graph that displays the luminosity of a star(vertical axis) versus its temperature (horizontal axis).To this day an H–Rdiagram, as it is called, is one of the most valuable tools astronomers have forunderstanding the properties of a star.In this exercise you will create an H–R diagram for a group of starsand use the diagram to identify and understand the relationships between theproperties of the stars.Principles:Let’s start by reviewing a couple of important laws of physics that are relevant for understanding theproperties of the stars.A key idea is that the light emitted by a star is due to the heat of the star (much like a piece ofmetal will glow when heated).The light given off by hot objects (like stars) is called thermal radiation, and objects thatemit thermal radiation obey theblackbody radiation laws.These laws includeWien's LawandStefan’s Law(seesection 3.4 of our text).Wien’s Law says:Tmm2.9max=λEquation 1whereλmaxis the wavelength of light being emitted most intensely by the star, and T is the surface temperature of the star.Stefan’s Law says:L = 4πσR2T4Equation 2In this equation, L is the luminosity (light energy per second) emitted by the star, R is the radius of the star, T is thesurface temperature of the star, andσis Stefan’s constant).We’ll refer to Equation 2 a little later in the exercise.The other primary idea to discuss here is the Hertzsprung – Russell diagram.As mentioned above, an H–Rdiagram graphs a star'sLuminosityvs.Temperature.A star’s luminosity is how much total energy it gives off persecond.The luminosity is directly related to the

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