Anonymous
Anonymous asked in Science & MathematicsAstronomy & Space · 3 weeks ago

What information does the H-R Diagram tell us about stars?

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  • 3 weeks ago

    It primarily tells us that their luminosity and spectral class is not a random distribution.

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  • Clive
    Lv 7
    3 weeks ago

    It is a graph of brightness on the vertical axis and temperature (which directly corresponds to colour) on the horizontal axis.  The important part is if you actually plot this out and put individual stars in their place on the graph, you find it's not random.  Most stars lie on a diagonal line from the top left to bottom right, which has been called the Main Sequence, and there are areas of the graph that are completely empty.

    So that got astronomers thinking about the life cycle of stars and why most of them are on this line but some aren't.  It turns out that stars, for most of their lives, are shining by nuclear fusion of hydrogen into helium and while they do, they sit nicely on the Main Sequence, with the biggest ones being the brightest, hottest and most blue in the top left corner, and the smallest being the dimmest, coolest and reddest in the bottom right corner.  And many others all along the line, totally depending on their size.

    If a star is on the Main Sequence, you know it's a bog-standard ordinary star just shining away steadily by hydrogen fusion.  The Sun, fortunately for us, is an average sized yellow-green star sitting bang in the middle of it.

    Now what about what's not on the Main Sequence?  The main groups are red giants at the top right and white dwarfs at the bottom left.  Clearly there's something different going on in those.  And so it has been deduced from observations of these oddballs that this is what you get when stars get old and move off the Main Sequence.

    Which gives us the theory that an average star like the Sun will form, maybe with planets, it pulls itself together by its own gravity and gets hot enough for hydrogen fusion to start, and it will continue to shine like that for billions of years just sitting there on the Main Sequence.  But eventually it will run out of hydrogen, get unstable, start fusing bigger atoms into even bigger atoms, swell up, cool down because it has expanded and become a red giant.  Once it can't fuse any more, it will shrink down to a tiny white dwarf.

    So red giants are old and unsteady.  Take Betelgeuse - it has definitely dimmed recently and that's made astronomical news, so what's going on?  Very hard to tell, but it's big enough to go off bang as a supernova rather than just shrinking down to a white dwarf as we think the Sun eventually will, so there is a lot of speculation as to whether it will do that soon.  (The Sun won't go supernova, it's too small, but Betelgeuse is definitely big and heavy enough to explode.)

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  • The Hertzsprung-Russell Diagram is a graphical tool that astronomers use to classify stars according to their luminosity, spectral type, color, temperature and evolutionary stage. Stars in the stable phase of hydrogen burning lie along the Main Sequence according to their mass.

    Source(s): The internet
    • Clive
      Lv 7
      3 weeks agoReport

      You can classify them without the diagram.  So what does the DIAGRAM do?

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  • 3 weeks ago

    The surface and sometimes core temperatures and masses of the spectral classes of stars.

    • Clive
      Lv 7
      3 weeks agoReport

      No, that is the information you need to plot it.

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