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Thread: Simple Question

  1. #1 Simple Question 
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    If an astronomer sees an object in the sky, how can he tell
    if it is a LARGE star very FAR away or if it is a
    SMALL star much CLOSER to us ??


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    Moderator Moderator Dishmaster's Avatar
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    That's not a simple question at all. The main answer lies in the so called Hertzsprung-Russell diagram. There is an empirical relationship between the intrinsic brightness of a star and its surface temperature. The temperature can be measured in different ways, the easiest being imaging the star at different wavelengths (filters). When you have determined its temperature, you can derive its intrinsic brightness. The comparison with the measured brightness (aka apparent brightness) then gives you the distance of that star.



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    It can be a serious problem. We have found two quasars connected to each other, yet they have very different redshifts (thanks to the fact that quasars have huge redshift making black holes at their centre) so they seem to be millions, even billions of light years from each other. I suspect that may be so with GRB's too in that they all seem to be very distant and unbelievably powerful but if close would be more reasonably powerful.

    Type 1A supernovae were said to be a standard brightness so were used for measurement, then they found one twice as bright as normal (so it would be thought to be very close) simply because it was rotating faster than normal, so could hold more surface mass before collapsing. There are other reasons why a Type 1A supernova may be brighter or duller than "normal", so not quite the standard candle they once were.

    As Dishmaster shows, apparent and absolute magnitudes of stars can be helpful over "small distances" like in our galaxy.
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    Moderator Moderator Dishmaster's Avatar
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    Quote Originally Posted by Cyberia
    It can be a serious problem. We have found two quasars connected to each other, yet they have very different redshifts
    No, you haven't. This is highly debated and cannot be considered a fact.
    Quote Originally Posted by Cyberia
    Type 1A supernovae were said to be a standard brightness so were used for measurement, then they found one twice as bright as normal (so it would be thought to be very close) simply because it was rotating faster than normal, so could hold more surface mass before collapsing.
    It was always questioned, whether Type 1A supernovae are indeed always the stereotypical standard candles. At least among scientists.
    Quote Originally Posted by Cyberia
    As Dishmaster shows, apparent and absolute magnitudes of stars can be helpful over "small distances" like in our galaxy.
    No, it also works for large distances, if the telescope is still capable of detecting the object. The physics do not suddenly change outside the Milky Way. Individual stars have been isolated in the Andromeda Galaxy. This is very important to create the ladder of different distance measures.
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    Highly debated as in: "We don't believe it. There must be another explanation." Some quasars are clearly connected and clearly have hugely different redshifts. Fact.

    Every book I've looked in, every paper I have read, even on any forum, type 1A supernovae are considered infallible.

    I'm not saying the physics changes. Just that we need sufficient telescope power to detect single stars and to know that there are no other factors involved (dust or gas clouds for instance).
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    Moderator Moderator Dishmaster's Avatar
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    Quote Originally Posted by Cyberia
    Highly debated as in: "We don't believe it. There must be another explanation." Some quasars are clearly connected and clearly have hugely different redshifts. Fact.
    I haven't found a convincing example for this yet. As far as I can say, all these claims are based on interpretation of pictures. Nothing concrete like a physical analysis of chemical abundances or velocity fields hinting to a physical connection. Line of sight effects are up to now the most probable explanations.

    Quote Originally Posted by Cyberia
    Every book I've looked in, every paper I have read, even on any forum, type 1A supernovae are considered infallible.
    Under certain assumptions, yes. But these assumptions are often not touched or questioned in popular articles.
    Quote Originally Posted by Cyberia
    I'm not saying the physics changes. Just that we need sufficient telescope power to detect single stars and to know that there are no other factors involved (dust or gas clouds for instance).
    The influence of interstellar matter can be easily disentangled from distance effects by multicolour photometry, i.e. measuring the brightness at different wavelengths. This is a standard technique. Furthermore, additional observations at millimetre and centimetre wavelengths are very diagnostic in locating interfering clouds. The results are typically not based on only one measurement or data point, but they rely on a considerable amount of data that is evaluated by cross referencing.
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