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Re: (meteorobs) Is Sirius Getting Dimmer, or Moving Closer to Us?

A while back Mark Fox wrote:

> With that said, I have a starry question.  In one of my old star books (a
Golden Nature Guide, copyrighted 1951) lists the bright star Sirius with a
visual magnitude of -1.58.  In the Encyclopedia Britannica (1969) they list
it as -1.45 mag.  However, on the Heavens Above web site (which I frequently
visit) the star is now listed as -1.44 mag.  Now Sirius, being a relatively
close star, may cause someone to conclude that the differences in magnitude
result from the actual movement of the star--- that is, Sirius has
noticeably moved away in the past 50 or so years. However, this may not be
so if its differences in distance are correct.  In the Encyclopedia
Britannica  a distance of 8.70 lightyears is given while on the Heavens
Above web site a value of 8.60 is present. From this, the star is obviously
moving closer.
*******
    I am a beginning variable star observer, and am fortunate to number
among my circle of friends a few variable personalities who are far more
advanced than myself. So I decided to forward this question to them for
feedback, adding
the following question of my own:
*******
> Siriusly, Burnham's does have an answer for this -- the oldest value of
> mag -1.58 was clearly in error -- but would its approach of ~7 km per
second
> be enough to result in a change from -1.42 in Burnham's to -1.44 in the
> current Handbook. Or would that just be in improved measuring techniques?
*******
    I received the following response from Rick Huziak, whom I would
consider an expert on the subject, and am forwarding it with his permission,
warts and all. *******
Siriusly, from what I've heard discussed on the AAVSO web, as much as I'd
like to believe Bruce's hope that Sirius may be changing mag, what the top
photometrists always whine about is stars that are 'too bright' to avoid
saturation problems with the detectors.  Typically, the photom guys are
using 16", 24" or 36" scopes, and stars brighter than 12th are considered
unreliable, even in a 16", since they cannot take short enough exposure to
avoid the saturation, but retain a statistical amount of photons to work
with.  They keep trying to convince amateur photometrists with very small
scopes to take on the roll of monitoring the 'spotlights' brighter than
12th.  (Remember the article of the planet transit that was observed thru a
2" scope - they detected a 0.02 mag drop as the planet went across an 8th or
9th mag? star - can't remember the real mag of the star offhand).  However,
we were all amazed at the success of such a small scope, but those who
understand the nature of photometrybetter will realize that this is the
*only* scope that could have been successful.  (Alternately, you can stop a
4-meter scope down to 2-inches, but no one will allow that!).  At any rate -
to answer the question - Sirius's mag is not well known because a)
adjustments to the base scale over time, but b) mostly cuz it's so dang
bright that most scopes can't touch it.  There is also not a whole lot to
compare it to.  Differential photometry is more accurate than absolute
photometry, but with differential, it's nice to have stars nearer to the mag
of the target, and a star brighter and one dimmer is always nice.  Only the
sun is brighter than Sirius, but even it's tru mag is not known to any great
degree of accuracy.  Yada, yada......

Rick
--
* * * * * * * * * * * * *
Richard Huziak
Manufacturing Engineering
SED Systems, Saskatoon
tel. (306) 933-1676
<huziak@SEDSystemsdot ca>
* * * * * * * * * * * * *
Hope this answers your question, Mark. I'll have a little more to say about
variable stars in a separate post.  regards, Bruce

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