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B Cepheid Variable Stars and Space-Time Warping

  1. May 12, 2017 #1
    After going through the starting parts of Astrophysics, (and excuse me if I completely mess this up), that if the apparent brightness of the star decreases due to the loss in energy and contracts then expands again over a certain period.

    Then as the energy released has a mass equivalence, then the mass of the overall system of the star will decrease therefore the curvature created by the star will also decrease, and will be the opposite as the star expands.

    Therefore anything in orbit around these Cepheid stars should have a change in their orbits relative to the period of change?

    This is more of a question to see if my thinking is correct, so please correct me!

    Thank you in advance
  2. jcsd
  3. May 12, 2017 #2

    Vanadium 50

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    None of your suppositions are correct, I'm afraid.
  4. May 12, 2017 #3


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    I'm sorry but I don't understand what you're trying to say here.

    The mass of a star only ever decreases except in very rare circumstances, such as white dwarfs accumulating mass by stealing matter from a companion star.
  5. May 12, 2017 #4
    I'm sorry if I'm not being clear but a Cepheid Star decreases in brightness over time before increasing due to the fact that it can no longer contain the energy.


    As the graph shows, which should mean the energy lost causes the star to cool, and then contract. And as the energy being released in one form or another has a mass equivalence should mean the mass of the star decreases.

    Or am I going completely off course with this?
  6. May 12, 2017 #5


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    Its mass decreases, because it's shining - just as any other star. It never increases, though, which is what your opening post suggested. As stars lose mass this way (as well as due to solar wind), bodies orbiting them spiral out. However, the fraction of stellar mass converted to radiation is minuscule - fusion converts less than 1% of initial mass over its lifetime, so the rate of orbital changes is mostly negligible.
    In case of variable stars, the rate of mass loss due to varying radiative output fluctuates, so the rate at which orbits spiral out also fluctuates (but they don't spiral in).
    Last edited: May 12, 2017
  7. May 12, 2017 #6


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    The accepted model for the variation in the Cepheid star works like this:
    The outer layers of the atmosphere contains ionized helium. During the dim stage of the star, it is highly ionized, which makes it opaque to visible light. As it heats from the energy it is absorbing, it expands, becomes less ionized and becomes less opaque, allowing more of the star's light to pass through and we see the star as brightening. But this also means that the gas is absorbing less energy and begins to cool again, contract, becoming more opaque, and once again blocking the light we see. So the total energy output of the star doesn't vary, just the amount of visible amount that is allowed to pass through the helium gas layer. It is just that outer gas layer that expands and contracts, and not the entire star.
  8. May 15, 2017 #7


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    Yes, well put. The OP has got his orders of magnitude wrong, which accounts for his ideas about what could be happening. The numbers count, as usual.
  9. May 15, 2017 #8

    stefan r

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    At low altitude high humidity air is transparent to visible light. When it rises water vapor goes through a phase change and becomes cloud droplets. The cloud changes the appearance of earth's surface from space. Plants, soil, and rock adsorb a lot of light. Clouds are more reflective. Helium is very different from water. Water is convenient to observe at home and it switches from clear to opaque under some circumstances.

    The helium phase change is from double ionized to single ionized and then back to double.
    The star is hot enough to double ionize when it is compact. Any black body radiates more light when it has a larger surface. The opacity amplifies the oscillation. When compact and at high temperature the helium retains light and the extra energy helps drive the expansion. At large radius the surface emits more light and the helium stops retaining so the star starts to cool.
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