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Brian Albin
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Reged: 08/22/06

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Colour Change of Sirius
      #5717873 - 03/07/13 08:07 AM

In "Southern Stellar Objects for Small Telescopes", by J. E. Gore, 1877; I read:
"Sirius is supposed to have changed in colour since the days of Ptolemy, who is said to have compared it in colour with Antares."

Is it today thought that Ptolemy was referring to a different star? Could an obscuring cloud of some kind have turned a blue or white star apparently red? Or could this star, Sirius actually have changed temperature this much, this rapidly?


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GlennLeDrew
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Re: Colour Change of Sirius new [Re: Brian Albin]
      #5718353 - 03/07/13 12:40 PM

Sirius is quite nearby, at less than 9 light-years. An intervening cloud which could have so greatly reddened Sirius would be obvious in IR imagery, such as the IRAS 100 micron data. I'm not aware of any such cloud adjacent to Sirius on the sky (it would have to still be within arcminutes, after a mere two millennia), and even clouds some tens of degrees distant on the sky are probably no nearer to us than hundreds of light years--let alone less than 9.

And I suspect there is no circumstellar disk which could do this either.

A couple thousand years is nothing at all compared to the age of at least tens of millions of years, with hundreds of millions being likely. Sirius is a rather stable A-type, main-sequence star which theory in no way suggests the possibility of such a radical change in color at any time between settling onto the main sequence and becoming a red giant.

While I'm generally loathe to categorically favor theory over observation, in cases where other phenomena, or mistakes, can be more readily envisaged than departures from theory, I will accord theory with considerable weight. In this case, my instinct is to doubt that Sirius was intrinsically red at that recent epoch, and for what I presume to be a very short time (bluish-white for some untold time, going red for a period, then back to bluish-white soon after and ever since.)


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Jarad
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Re: Colour Change of Sirius new [Re: GlennLeDrew]
      #5718381 - 03/07/13 12:52 PM

Any chance he (or whoever recorded the comment) could have confused Sirius with Betelgeuss? That one does look similar to Antares...

Jarad


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rookie
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Re: Colour Change of Sirius new [Re: Jarad]
      #5719351 - 03/07/13 08:58 PM

Quote:

Any chance he (or whoever recorded the comment) could have confused Sirius with Betelgeuss? That one does look similar to Antares...
Jarad




It's not conceivable that Sirius was confused with any other star, particulary by the Egyptian astronomer and scholar Claudius Ptolomy (85-165 AD). The Egyptians long before Ptolomy worshiped Sothis or Sirius as "herald of the new year and the flood." The quote is taken from a ivory tablet from the First Dynasty (3000-2900BC). The Egyptians observed each year for the first visiblity of Sirius in the morning sky, also called the heliacal rising of Sirius. The event was so important because it marked the anticipated life giving Nile flooding. Egypt was wealthy because it's ability to grow food consistently through the ages and the sighting of Sirius heralded the beginning of the cycle for flooding, planting and harvest. The Egyptians were astute observers of the night sky, developed a 365 day calendar that was used for thousands of years, and were able to tell time during the night by the movement of deacon stars cross the meridian from hour to hour. Their entire economy and worship was based on observing Sirius.

Author, Craig Crossen wrote published this article 10/21/09 in here on Cloudy Nights. The Red Sirius.

It drew much discussion here.


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llanitedave
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Re: Colour Change of Sirius new [Re: rookie]
      #5719367 - 03/07/13 09:06 PM

Wasn't the rising time of Sirius significant in those days?

If they're looking to detect Sirius rising above the horizon, then they'd be looking low, into a dusty haze combining Mediterranean humidity with Saharan aridity.

I think a Sirius low on the horizon, where its appearance is most looked-for, could indeed be described as reddish.

Edit: Following Rookie's link, it appears that my guess is not too far from what a lot of others have already suggested.

Edited by llanitedave (03/07/13 09:38 PM)


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Rick Woods
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Re: Colour Change of Sirius new [Re: llanitedave]
      #5719407 - 03/07/13 09:30 PM

What latitude was Ptolemy viewing from? The lower the star, the redder it would appear.
I've also seen it proposed that the human eye might have been slightly different in its color perception back then. And, any double star observer knows how hard it can be to tell if a faint companion star is blue or red. Maybe some variation of that phenomena is responsible?

I'd consider a lot of options before questioning such a basic observation by a great observer.


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rookie
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Re: Colour Change of Sirius new [Re: Rick Woods]
      #5719626 - 03/07/13 11:22 PM

If the rising color depicted the ruddy description, then why were other white stars not described as red? Certainly they all rise at or just prior to dawn in their season.

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DarkSkys
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Re: Colour Change of Sirius new [Re: Rick Woods]
      #5719820 - 03/08/13 03:28 AM

Quote:

What latitude was Ptolemy viewing from? The lower the star, the redder it would appear.
I've also seen it proposed that the human eye might have been slightly different in its color perception back then. And, any double star observer knows how hard it can be to tell if a faint companion star is blue or red. Maybe some variation of that phenomena is responsible?

I'd consider a lot of options before questioning such a basic observation by a great observer.




Ptolemy Lived in Alexandria,egypt. So 30~*N

I've thought this was an interesting subject from the time I first read about it.

This paper cites Ancient chinese texts that talk of a red sirus.
http://adsabs.harvard.edu/full/1991A%26A...252..193B

Edited by DarkSkys (03/08/13 03:35 AM)


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GlennLeDrew
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Re: Colour Change of Sirius new [Re: DarkSkys]
      #5719883 - 03/08/13 06:05 AM

To be sure, the presence of the white dwarf in orbit about Sirius is a most compelling factor. We expect this companion must once have been a red giant. But as other investigators have rightly noted, after such a short time we should expect to be still seeing some indication of the transition, be it an outright planetary nebula or just some ejected gas and dust/soot, neither of which has had a trace been found by many means.

If it was Sirius itself which either flared up for some period of time, or was formerly red since before Man gazed skyward, that its brightness before and after the color change seems to not have changed much is curious. Significant color change like this, for not-so-massive stars like Sirius, is expected to be accompanied by a significant change in brightness.

To be honest, I have not looked at all closely at this case. I therefore wonder how many independent observations we are confident of having, and how much reportage is merely repeating what has been read or heard. And it seems no small amount of interpretation and 'reading between the lines' is required when deciphering these old documents. In some cases I wonder if arcane meaning related to mythological importance is the more correct interpretation, not literal fact.

It sure makes for a sleuth's dream!


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Mister T
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Reged: 02/01/08

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Re: Colour Change of Sirius new [Re: GlennLeDrew]
      #5719886 - 03/08/13 06:13 AM

Perhaps Sirius has a verrrry slow rotation rate.

Ptolemy may have been looking at it's tail lights.


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Brian Albin
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Reged: 08/22/06

Loc: Western Oregon
Re: Colour Change of Sirius new [Re: Mister T]
      #5722114 - 03/09/13 11:33 AM

I wonder if during the dark ages we started calling the white star Sirius not realizing that the classical era people were referring to one of the red stars when they used that name.

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star drop
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Loc: Snow Plop, WNY
Re: Colour Change of Sirius new [Re: Brian Albin]
      #5723271 - 03/10/13 12:41 AM

There is a simple explanation. Ptolemy was viewing through warm toned glass.

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hm insulators
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Re: Colour Change of Sirius new [Re: rookie]
      #5728288 - 03/12/13 02:33 PM

Quote:

If the rising color depicted the ruddy description, then why were other white stars not described as red? Certainly they all rise at or just prior to dawn in their season.




Probably because Sirius is such a bright star and especially from more-northern latitudes is close to the southern horizon where the light from the star has to penetrate through more atomsphere, especially if it is also rising or setting.

A month or so back, I was coming from a predawn trip to the supermarket and saw Sirius very low in the western sky and I thought at first it was a distant airplane landing light as it was in just the right spot for planes coming into Phoenix Sky Harbor. But when it didn't move, I realized I was looking at Sirius and it was twinkling like mad and changing color. At one point, Sirius actually shone a bright orange in color for a second or two. It was really lovely!


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Brian Albin
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Reged: 08/22/06

Loc: Western Oregon
Re: Colour Change of Sirius new [Re: hm insulators]
      #5728968 - 03/12/13 08:53 PM

"And as the fiery Sirius alters hue,
And bickers into red and emerald,"

Tennyson - The Princess


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GlennLeDrew
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Re: Colour Change of Sirius new [Re: Brian Albin]
      #5729252 - 03/12/13 10:48 PM

Reminds me of one if the more memorable observations of all time..,

From a weather station at 70N, in the depth of winter, where Surius rises barely above 3 degrees above the southern horizon, one really cold night when the wind was calm and there was a pronounced temperature inversion, I saw this star go through the most astonishing variation in color and brightness.

This was while the star was still creepingly ascending toward culmination. over a cycle of some 10 seconds, the color ran the entire gamut from bluish to ruby red, like one if those slowly changing colored decorative lights. And simultaneously, on the same slow cycle, the brightness varied over several magnitudes.

What made the spectacle especially remarkable was the serene pace of variation. Not a single quick flicker whatsoever. Just that inexorable and drawn out, smooth change in color and brightness. I watched for the better part of half an hour, and it was still going on when I went indoors. Through my 70mm binos it was simply mesmeric! A memory for a lifetime.


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