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Thread: Sirius Color Change Mystery

  1. #1 Sirius Color Change Mystery 
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    In ancient times, people always said that Sirius was red, but its color is blue-white now. Sirius can be seen with nacked eye. For stellar body itself, it can't change color within thousands of years in any evolving stage. Some common explanations of color change can be eliminated:


    1. In main-sequence evolving course, the time that Sirius appeared to change color in thousands of years was too short, which has already been overthrown by astronomers;
    2. The assumed third star Sirius C in the system, whose induced mass limit is approximately 0.1-0.25 solar mass, as a brown dwarf or red dwarf, is not capable of directly relevant to abnormal red phenomenon of Sirius;
    3. The theory that red giant Sirius B once appeared accounting for red Sirius is also overthrown, for most astronomers think that it would last at least 100,000 years for red giant evolving into white dwarf;
    4. The time scale which Sirius changes from red to blue itself is also greatly larger than thousands of years (even if for the situation of red giant or supernova progenitor), so it is not possible that the color change of Sirius is caused by stellar color completely.


    In the late period of a massive star, forming iron and iron-peak elements would absorb tremendous energy rather than produce energy, thus its lead to stellar body sharply cooling, so does it account for color change mystery?


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  3. #2  
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    What if there was a giant red star near Sirius and we thought they were talking about sirius when they actually were talking about the other star?


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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by siriusnova View Post
    In ancient times, people always said that Sirius was red
    That doesn't appear to be true:
    However, not all ancient observers saw Sirius as red. The 1st century AD poet Marcus Manilius described it as "sea-blue", as did the 4th century Avienus.[56] It is the standard star for the color white in ancient China, and multiple records from the 2nd century BC up to the 7th century AD all describe Sirius as white in hue.[57][58]
    Sirius - Wikipedia, the free encyclopedia

    But, only loosely related... what a great photo: Siriusly twinkling | Bad Astronomy | Discover Magazine
    ei incumbit probatio qui dicit, non qui negat
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    Sirius appeared mainly red in ancient times. Some recorded its blue-white color, especially in low latitude places. And some even recorded its color change for naked eye.
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    One suggestion I have seen is that the red description was based on observations made when the star was low in the sky (near the horizon). Although that doesn't explain why the colour would be mentioned then and not when it is higher in the sky. Perhaps the effect of the atmosphere is more noticeable because the star is so bright....
    ei incumbit probatio qui dicit, non qui negat
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    The star near horizon would appear redder when lights pass through thicker atmosphere(as the sun). However, Sirius near the horizon in ancient times was mainly red, while Sirius near the horizon is blue-white now.
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    I don't buy the story about the historic reception of Sirius being red. As a physicist and a sceptic, I first would have to be convinced that the references are reliable. It would not be the first time that someone had made a mistake then it propagated through various generations, who simply copied the entries of target lists. You must realise that especially the Romans were not particularly skilled in Astronomy. If the report of Ptolemy is being considered a credible source for such a statement now, then contemporary poets, politicians and soldiers may have believed it as well without questioning. Note that even until the 17th century, astronomical "knowledge" was mainly deduced from philosophical tracts instead of empirical verification. "If Ptolemy said it is red, then it is red - even if I see it is blue."

    Today, it is very difficult to judge, why this idea came up. A true change in colour would be the last reason, I'd believe in.

    Nevertheless, since the companion is a white dwarf: there is a class of binary stars - cataclysmic binaries - in which the orbit of the companion is so close to the primary host star that matter is transferred and accumulated onto the white dwarf. As soon as a critical mass is accumulated, a runaway reaction produces an outburst (a nova, or supernova Type Ia). But then, we nowadays should be able to observe a remnant of such an outburst, which apparently we don't in the case of Sirius.
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    Brassica oleracea Strange's Avatar
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    The other thing to consider is the use of colour words in language. A famous example is Homer's description of the sea as "wine" coloured. It would need a careful analysis of the texts in question to determine if translations as "red" are actually what was meant.

    Note that in many languages, there is only one colour word (beyond light and dark). When there is only a single colour word it is always ... red.
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    Forum Freshman overthelight's Avatar
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    Is the color made by voting?
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    Quote Originally Posted by Dishmaster View Post
    I don't buy the story about the historic reception of Sirius being red. As a physicist and a sceptic, I first would have to be convinced that the references are reliable. It would not be the first time that someone had made a mistake then it propagated through various generations, who simply copied the entries of target lists. You must realise that especially the Romans were not particularly skilled in Astronomy. If the report of Ptolemy is being considered a credible source for such a statement now, then contemporary poets, politicians and soldiers may have believed it as well without questioning. Note that even until the 17th century, astronomical "knowledge" was mainly deduced from philosophical tracts instead of empirical verification. "If Ptolemy said it is red, then it is red - even if I see it is blue."

    Today, it is very difficult to judge, why this idea came up. A true change in colour would be the last reason, I'd believe in.

    Nevertheless, since the companion is a white dwarf: there is a class of binary stars - cataclysmic binaries - in which the orbit of the companion is so close to the primary host star that matter is transferred and accumulated onto the white dwarf. As soon as a critical mass is accumulated, a runaway reaction produces an outburst (a nova, or supernova Type Ia). But then, we nowadays should be able to observe a remnant of such an outburst, which apparently we don't in the case of Sirius.
    Well said. And good to see you again!!!
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    Quote Originally Posted by overthelight View Post
    Is the color made by voting?
    Huh? No, by physics.

    Human perception is another matter.
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    Astronomer Fred Schaaf writes about the many hues of Sirius as its light passes through our atmosphere.
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    Figure Blue-white star surrounded with low temperature red stellar wind gas.

    The essence of Sirius color change mystery possibly is forming of iron. (Subject Leader: Possible)

    The nuclear fusion of forming iron and elements heavier than iron require absorbing tremendous energy, rather than produce energy, so the sharp decline of temperature directly leads to sharp decline of stellar luminosity and stellar wind intensity.

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