The Hertzsprung-Russell Diagram - NMSU Astronomy

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The Hertzsprung-Russell Diagram

  • If you plot the brightness versus color, or spectral type, of stars you get a scatter plot – these quantities are not correlated, and they do not form a pattern together. If you look at this diagram, you will see points scattered all over the place.

    Plot of apparent brightness (how bright a star appears when viewed from Earth) as a function of stellar color for around fifty points. There are points in all four quadrants (upper-left, lower-left, upper-right, lower-right), and no discernible pattern - the points appear to be distributed fairly randomly.
    [NMSU, N. Vogt]

  • But, if we use a sample of stars for which we have known distances, we can calculate their intrinsic luminosities. A plot of luminosity vs spectral type (or color or temperature) is called a Hertzsprung-Russell (H-R) Diagram, and there are some definite patterns within it.
    • What patterns do you see when you look at the H-R diagram below?
    • Do you find stars everywhere in the diagram, or are there regions which are sparsely populated and regions which are heavily populated?
    • Is there a characteristic pattern which most of the stars follow?
    • Do you see a trend between spectral type and luminosity?

      Plot of luminosity as a function of spectral class (which can also be represented as temperature or as stellar color). On the y-axis, luminosity runs from low to high. On the x-axis, spectral class is designated O, B, A, F, G, K, and finally M, and also temperature running from hot to cool, and finally stellar color running from blue through pale blue, white, pale red, and finally red. In the upper-right quadrant (high luminosity, K or M type, cool, and red stars) is shown two large red points. In the lower-left quadrant (low luminosity, O or B type, hot, and blue stars) is shown a sequence of small white points slightly scattered along a straight line. The line is oriented downwards, so that the hotter, bluer O stars have the higher luminosities and the cooler, lighter blue B stars have the lower luminosities. A gently curving line wends its way from the upper-left quadrant (high luminosity, O or B type, hot blue stars) down to the lower-right quadrant (low luminosity, K or M type, cool red stars), dotted with intermediate size yellow points. Most of the points shown lie along this line, which designates the Main Sequence upon which stars like our Sun are found.
      [NMSU, N. Vogt]

    • The H-R diagram has played a crucial role in developing our understanding of stellar structure and evolution. A major focus of astronomy during the 1940's and 1950's was constructing stellar models that would accurately predict the luminosity - temperature relation seen in the H-R diagram.

    • For now, we will note some interesting things about the differences between the nearby stars and the brightest stars and use the H-R diagram to demonstrate one more property of stars.

    • Most stars fall along a sequence that is called the Main Sequence. For this sequence, there is a strong correlation, in the sense that hotter stars are also more luminous.

  • Here is another version of the H-R diagram, showing the distribution of many nearby stars. A dot is placed on the diagram for each observed star, and the color of the dot corresponds to the stellar temperature. Note how the bulk of the stars lie along the Main Sequence, while the giant and dwarf populations are sparser (more rare). Where does the Sun lie on the H-R diagram?

    Distribution of stars on the Hertzsprung-Russell Diagram. The y-axis shows solar luminosity (running from 0.00001 to 100,000) and absolute magnitude (running from +18 to -8) and the x-axis show color (B-V, meaning B filter light minus V filter light, running from -0.3 to 2.2) and spectral class (running from O, B, A, F, G, K, and finally to M). This figure is simply a more detailed form of the previous figure. Giant stars are found at high luminosities with red colors, though the distribution of stars trails down to the Main Sequence below in a tail that is successively fainter and bluer. There is a sprinkling of stars of all colors at the highest luminosities, above those labeled Giants and labeled Bright Giants and Supergiants, and the region where the giants meld into the Main Sequence (colors around 1 with luminosities from 3 to 70) is labeled Subgiants. The Main Sequence again runs from white high luminosity stars through yellow solar type stars down to low luminosity red stars. At much lower luminosities (and clearly segregated) we find the line labeled White Dwarfs, running from colors around zero with luminosities around 0.01 to colors around 1.5 with luminosities around 0.00005.

  • Measure the masses for as many stars as we can find (using the techniques we discussed for binary systems) and we will discover the mass-luminosity relation for Main Sequence stars.

    Plot of luminosity as a function of mass, with luminosity running from 0.01 to 10,000 solar luminosities (log scale) and mass running from 0.2 to 50 solar masses (linear scale). A single line gently wends its way from the lower-left quadrant (0.2 solar mass, 0.01 solar luminosity) towards the upper-right quadrant (50 solar masses, 10,000 solar luminosity), garlanded with yellow points.
    [NMSU, N. Vogt]

    L = M3.5

  • Mass and luminosity are proportional to either – intrinsically bright stars are also very massive, and intrinsically faint stars tend to be rather low mass. This tells us that the Main Sequence in the H-R Diagram is not only a luminosity sequence, but also a mass sequence!

    Plot of luminosity as a function of temperature, with luminosity running from 0.01 to 10,000 solar luminosities (log scale) and temperature running from 40,000 to 2,500 kelvin. The Main Sequence is shown running from the upper-left quadrant (40,000 K, 10,000 solar luminosity) down to the lower-right quadrant (2,500 K, 0.01 solar luminosity). At the high luminosity end are labels for 50 and 20 solar masses, followed by 3 solar masses around 100 solar luminosity, 1 solar mass around 1 solar luminosity, and 0.5 and 0.2 solar mass at the low luminosity end.
    [NMSU, N. Vogt]

  • A more massive star generates a stronger gravitational field than a less massive one. This creates higher pressures in the stellar core, which means that the star converts its fuel into energy faster and more efficiently. Though high mass stars have larger fuel supplies than low mass stars, they burn through it more quickly (hence they are much brighter), so their Main Sequence lifetimes are far shorter.

    Thanks to Mike Bolte (UC Santa Cruz) for the base contents of this slide.

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