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Thread: Standard candles

  1. #1 Standard candles 
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    Type 1A supernovae are said to be standard candles because all are believed to be the same so we can use their apparent brightness as a measure of distance. But:

    The researchers have revealed that in about a quarter of the cases in spiral galaxies, and possibly more, the companion star that 'donates' its mass to the white dwarf is probably a regular, medium-sized star, largely similar to our own Sun.

    Are stellar explosions created equal? Scientists recheck a standard model for supernovae on which cosmic distance measurement is based


    Just 1 in 4 "donor" stars are of a certain type.

    Stars do vary quite a lot, especially according to their size and so their makeup, and of course, their age. You have a bigger star, it can dump mass onto a white dwarf more quickly which cannot be accommodated with fast enough and cause it to become unstable and to go supernova more quickly, so with less material, so not as bright.

    Or a "donor" star can be old or young which would then decide on the matter taken by the dwarf star and an old star dumping heavier elements onto it would cause a different explosion to a younger star dumping lighter elements onto it.

    Or a star can go through a very active period. We have seen the surface of stars where most of it is one or more absolutely massive sun spots, which would means it would naturally send a lot of material into space compared to a normal star.

    It also depends on the orientation of the "donor" star as in a nearby dwarf star will take more from the equator of another star than from it's pole.

    And then there is the dwarf star itself. One supernova a while back which was twice as bright as it should be (so seemingly much closer) was found to be spinning faster than normal so it could hold far more mass before going supernova.

    There is also the element of how close these two bodies are to each other in that the closer they get, the slower both would tend to spin as they have a gravitational pull on each other.

    So Type 1A supernovae are maybe more of a guide to cosmic distance than a yardstick?


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  3. #2  
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    Type Ia supernovae are not, at least as a class of events, standard candles. This is a matter of some confusion in the popular literature. However, despite the inherent variability of these events, they can be used as standard candles because of an established relationship between how their brightness grows and fades over time and their absolute brightness.

    We do not even know if these events are caused by the explosion of white dwarf stars. It is highly probable that the light we see from these events is due to a large region of metals that are heated over a series of weeks from their interior. This fits with the explosion of a white dwarf star, which would distribute an outer shell of material that would be heated by the expansion of the interior shells. However, no simulations of such explosions match the available data well enough to really convince.

    There is a certain amount of error remaining in the use of SNeIa, and this might lay in the differences in different origins of these explosions. There does not seem to be reason to increase the error expected in the use of these events based on an investigation of their origins.


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    Wouldn't the fading be down to the type of supernova since not all supernovae are the same, as causes in this case can vary (as opposed to an old star going supernova naturally)?

    Absolute brightness would also be down to their cause, the size of the dwarf star, the amount and quality of the material it has to "play with"?

    It is generally accepted that these are white dwarfs but as you say, there is some doubt.
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    Quote Originally Posted by Cyberia View Post
    Wouldn't the fading be down to the type of supernova since not all supernovae are the same, as causes in this case can vary (as opposed to an old star going supernova naturally)? Absolute brightness would also be down to their cause, the size of the dwarf star, the amount and quality of the material it has to "play with"?
    Type Ia supernovae are a group of events characterized by certain features. These features are probably caused by something, but currently our use of these features doesn't depend on whatever the cause of these features is. We might be able to use them better if we knew the causes.
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  6. #5  
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    PhysBang. You make it sound worse than I do since if we cannot be certain of the cause (to some extent), we cannot in any way be certain that the results follow a distinct pattern we can use with any accuracy.
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    Here is the paper, and unlike the link at the end of the ScienceDaily article, this one is not behind a paywall.

    [1108.3664] Circumstellar Material in Type Ia Supernovae via Sodium Absorption Features

    Type Ia supernovae are key tools for measuring distances on a cosmic scale. They are generally thought to be the thermonuclear explosion of an accreting white dwarf in a close binary system. The nature of the mass donor is still uncertain. In the single-degenerate model it is a main-sequence star or an evolved star, whereas in the double-degenerate model it is another white dwarf. We show that the velocity structure of absorbing material along the line of sight to 35 type Ia supernovae tends to be blueshifted. These structures are likely signatures of gas outflows from the supernova progenitor systems. Thus many type Ia supernovae in nearby spiral galaxies may originate in single-degenerate systems.
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  8. #7  
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    Quote Originally Posted by Cyberia View Post
    PhysBang. You make it sound worse than I do since if we cannot be certain of the cause (to some extent), we cannot in any way be certain that the results follow a distinct pattern we can use with any accuracy.
    Really? This just isn't how physics gas worked for centuries now. Newton very specifically pointed out that he could find now cause for gravity, he just pointed out how it worked systematically.The same thing with type Ia supernovae: we don't know their cause, but we know many of their systematic regularities.
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