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Introduction to the HR Diagram Now that we have learned there is a method to measure distances to stars we are in the position of being to able to measure two physical properties about stars: Luminosity Surface Temperature These represent external attributes. Whatever the physics which occurs inside of the star, what we observe are its external attributes. There is a good analogy here with the external attributes of weight and height for humans: 1) Weight and height are in some sense the external manifestation of the internal metabolic process. 2) If one were to plot height against weight for humans there would be a general relation which would define the locus of normality for humans in this diagram. 3) As humans age, their height and weight do change, so the position of an individual person in this diagram will evolve with time. 4) Over most of the lifetime of an individual, their height and weight will not change much, so there will be a region in this diagram where a human spends most of their lifetime. The following visualization emphasizes these concepts: http://opencourse.uoregon.edu/pub/astr122/pod4.pps (click to start the animated sequence) The HR diagram (named after astronomers Hertzsprung and Russell) is a plot of the surface temperature of a star versus its total energy output - which astronomers call Luminosity. When such a plot is made (see example below) it becomes apparent that there is a relation between Luminosity and surface temperature as most all of the stars fall in a diagonal band known as the main sequence: http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/Image31.gif For now, just ignore the words in the diagram that say Giants, Supergiants, White Dwarfs – these represent other evolutionary stages in the life of a star that we will get to later. The important present point is simply that a main sequence exists. A color coded version of this diagram appears below and maybe more helpful in understanding the overall description. http://rst.gsfc.nasa.gov/Sect20/hertzsprung.jpg Important features of the HR diagram: The Y-axis is the total energy output of the star, called the Luminosity. The luminosity of stars is measured in units of the luminosity of the Sun or one solar luminosity. Thus a star that has a luminosity of 10 solar luminosities outputs 10 times more energy than the sun. An alternative way of measuring energy output is with the units called Absolute Magnitude. We will not use those units in this course – only solar luminosities. Inspecting the graph shows that main sequence stars have a very large range in luminosity. The brightest ones have luminosities as high as 10,000 solar luminosities and the faintest ones are at 1/1000 of a solar luminosity. Thus, empirically, we know now that whatever the energy source of stars is, it has a huge dynamic range. This is an important clue as to the nature of the energy source itself. The X-axis is a plot of the surface temperature of the star. Sometimes the X-axis is the B-V or V-R color index and sometimes it’s the spectral type of the star. All of these are equivalent and interchangeable measures of surface temperature. The X-axis is plotted backwards (for historical reasons) in that zero (zero temperature) is at the far right of the graph. So qualitatively the HR diagram can be summarized as follows: Again, an important thing to notice in the above diagram is the tremendous range in luminosities (energy output) that stars can have relative to the Sun. This diagram is therefore an external manifestation of internal characteristics of a star. As a star evolves, its internal properties change and this triggers changes in the externally measured properties of surface temperature and luminosity. However, just like humans spend the majority of their evolutionary lifetimes at one position in the height vs weight diagram – so to, stars spend most of their evolutionary lifetimes at their main sequence locations. Only when something internal changes in the star will there be a corresponding change in the external properties of surface temperature and luminosity. The reasons for such changes will be the subject of the next few lectures. However, as long as the star is stable, then its luminosity and surface temperature will remain constant.