Apparent brightness values of stars vary considerably

In several books, websites , etc. the values of the apparent magnitude
of stars vary considerably of each information source. E.g. Sirius has
mv = -1.58 according to one source and -1.43 to another source a
difference of 0.15 mag which appears to be a difference of only 15%.
For Canopus the values range from -0.63 and -0.86 which is 0.25
magnitudes or factor 1.25 (25%) about two third exposure value. So
specifying stellar magnitudes in hundredths of magnitudes is completely
useless it is only 0.9% difference which is far lower than the
measurement error.

Any ideas ?

Klaas

"skatebiker" wrote in message
ups.com...
In several books, websites , etc. the values of the apparent magnitude
of stars vary considerably of each information source. E.g. Sirius has
mv = -1.58 according to one source and -1.43 to another source a
difference of 0.15 mag which appears to be a difference of only 15%.
For Canopus the values range from -0.63 and -0.86 which is 0.25
magnitudes or factor 1.25 (25%) about two third exposure value. So
specifying stellar magnitudes in hundredths of magnitudes is completely
useless it is only 0.9% difference which is far lower than the
measurement error.

Any ideas ?

Klaas

I had the same question/statement a few months ago. Don't know if this
will help or not but you do realize the mv# is photographic magnitude,
right? Second, are you comparing mv with mv or mv with apparent
magnitudes?

Next, just grab one catalog's set of magnitudes and stick with it.
Don't know what one would be best but SAO catalog would probably still be
known years from now.
--
Michael A. Barlow

For measurements made from Earth, atmospheric conditions are going to
affect the apparent magnitude of a star. So does the measuring
equipment, and the wavelength(s) that you measure the magnitude at.

-Paul W.

On 10 Feb 2005 03:06:13 -0800, "skatebiker"
wrote:

In several books, websites , etc. the values of the apparent magnitude
of stars vary considerably of each information source. E.g. Sirius has
mv = -1.58 according to one source and -1.43 to another source a
difference of 0.15 mag which appears to be a difference of only 15%.
For Canopus the values range from -0.63 and -0.86 which is 0.25
magnitudes or factor 1.25 (25%) about two third exposure value. So
specifying stellar magnitudes in hundredths of magnitudes is completely
useless it is only 0.9% difference which is far lower than the
measurement error.

Any ideas ?

Klaas

----------
Remove 'Z' to reply by email.

On 10 Feb 2005 03:06:13 -0800, "skatebiker"
wrote:

In several books, websites , etc. the values of the apparent
magnitude
of stars vary considerably of each information source. E.g. Sirius
has
mv = -1.58 according to one source and -1.43 to another source a
difference of 0.15 mag which appears to be a difference of only 15%.
For Canopus the values range from -0.63 and -0.86 which is 0.25
magnitudes or factor 1.25 (25%) about two third exposure value. So
specifying stellar magnitudes in hundredths of magnitudes is
completely
useless it is only 0.9% difference which is far lower than the
measurement error.

Any ideas ?

Klaas

----------
Remove 'Z' to reply by email.

Depends on whether the quoted magnitude is in 'V' band of the UBVRI
system or photographic.

All catalogs specify the magnitude band. Google for "UBVRI system
Johnson photometric system"

good luck!!!
Starlight

Michael Barlow wrote:
Klaas "skatebiker" wrote in message
ups.com...
In several books, websites , etc. the values of the apparent
magnitude
of stars vary considerably of each information source.
and Michael followed up with -
I had the same question/statement a few months ago.

The following article may be of help in understanding the historical
(pre-1960s) magnitude systems reported in various catalogues:

Zissell, Ronald E. 1998. Evolution of the "Real" Visual Magnitude
System. JAAVSO 26:151.
NASA ADS link:
http://adsabs.harvard.edu/cgi-bin/np...AVSO..26..151Z

Recently (post-1960s), all charts and catalogues are based on stellar
brightness as seen through a Johnson "V" filter - a filter in the
Johnson UBVRI standardized filter system. E.g. Tirion's Cambridge Star
Atlas, Uranometria 2000.0, etc. By common convention, new catalogues
convert their raw photometric readings into the standard Johnson UBVRI
system. E.g. - the Tycho-2 catalogue.

But the Johnson V brightness is not what is seen with the naked-eye
because the naked-eye is more sensitive to red light than when a star
is viewed through a Johnson V filter. Although O,B and A stars
generally have the same apparent brightness to the naked-eye and when
seen through a Johnson V filter, color more-red-colored F,G,K and M
stars appear brighter to the naked-eye than when seen through a Johnson
V filter.

Read the following for more background on the difference between
naked-eye and Johnson V magnitude -

Stanton, Richard H. 1999. Visual magnitudes and the "average observer":
The SS Cygni field experiment. JAAVSO 27:97
NASA ADS link:
http://adsabs.harvard.edu/cgi-bin/np...AVSO..27...97S

Although the Johnson V magnitude does not exactly reflect the apparent
brightness of star as seen with the naked-eye, by convention the
Johnson V filter reading is used anyway. It provides a common objective
basis for reporting stellar brightnesses.

The UBVRI in the Johnson UBVRI filter system means Ultraviolet, Blue,
Visual (green), Red and Infrared (also red in color). The Johnson
"Visual" (green) filter refers to the fact that the filter's passband
is concentrated in the green-yellow spectrum of light through which the
human eye principally perceives the daylight terresterial world.
Physically, Johnson BVR filters look something like a planetary no. 80
Blue (analogous to Johnson B), no. 58 Green (analogous to Johnson V),
and no. 25 Red (analoguous to Johnson R) -

http://www.telescope.com/shopping/pr...&iProductID=83

- except, on gross inspection, Johnson BVR filters are much darker in
color.

The difference between Johnson BRV filters and common planetary filters
is Johnson filters are precisely engineered and tested to pass an
agreed spectral band and intensity of light.

http://www.andcorp.com/Web_store/UBVRI/Johnson.html

- Peace Canopus56

P.S. -

Also of interest - related Usenet sci.astro.amateur discussions:

Brian Skiff. 4/2/1996. Star magnitudes, catalogues, etc.

Bill Ferris. 2/15/2002. 18:34:55 PST. UNSO-A2.0 vs. GSC

David Knisley can tell the difference visually so will straighten it all
out.


skatebiker wrote:

In several books, websites , etc. the values of the apparent magnitude
of stars vary considerably of each information source. E.g. Sirius has
mv = -1.58 according to one source and -1.43 to another source a
difference of 0.15 mag which appears to be a difference of only 15%.
For Canopus the values range from -0.63 and -0.86 which is 0.25
magnitudes or factor 1.25 (25%) about two third exposure value. So
specifying stellar magnitudes in hundredths of magnitudes is completely
useless it is only 0.9% difference which is far lower than the
measurement error.

Any ideas ?

Klaas

In article . com,
"skatebiker" writes:
In several books, websites , etc. the values of the apparent magnitude
of stars vary considerably of each information source. E.g. Sirius has
mv = -1.58 according to one source and -1.43 to another source a
difference of 0.15 mag which appears to be a difference of only 15%.

As other people have said, a variety of magnitude systems have been
used over the years, many of them -- especially older ones -- poorly
defined. For precision work, one needs to be very careful about the
source of the magnitudes and how they were corrected to put them on a
standard system.

Very bright stars have special problems. Most modern photometric
systems are defined by standard stars in the magnitude range of
roughly 7 to 14. It is hard to measure stars a thousand or so times
brighter than the brightest standards using the same equipment; there
can easily be problems with non-linearity. Accurate measurements
could be made if someone wanted to make the effort to do it, but
nobody will unless there is a scientific reason.

Stars with extreme colors also present problems, by the way.

specifying stellar magnitudes in hundredths of magnitudes is completely
useless it is only 0.9% difference which is far lower than the
measurement error.

For stars within the usual range of color and magnitude, 2%
photometry is not terribly hard, and better than 1% is certainly
possible. I was at a lunch talk not long ago where the speaker
talked about looking for variability at the 0.0003 mag level.
(That's about a third of a milli-magnitude.) Of course this is
asking whether an individual star is _constant_ to this level, not at
all the same thing as saying what one star's brightness is relative
to some other star.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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