I have just been looking at the Washington Double Star Catalog's data for Capella (Alpha Aurigae), whose visual magnitude is usually quoted as 0.08. But in the following table, the magnitude for the A component (Capella itself) varies from 0.08 in 1999 and 0.24 in 1999 and 2003 to 2.10 in the late 19th or early 20th century!

Capella is a variable star (RS CVn type), but the General Catalog of Variable Stars gives its range as 0.03 to 0.16.

So how could observers in 1878, 1895, 1898 and 1908 measure Capella A's magnitude as 2.10?


Aa = components
1999 = year of observation
0.1 = separation in arcseconds
230 = Position Angle in degrees
0.08 0.18 = visual magnitudes of the two components
ANJ 1 = discoverer code

WDS

__________________
- Learn a lot teaching others.

Hum,
interesting.
Perhaps the orbit was aligned then and has since, due to precession, moved to a more a perpendicular aspect.
Or, perhaps it was just an exceptional outburst that was observe.
The spectroscopic binary of yellow G-class giants stars, that have luminosities of around 50 and 80 times that of the Sun and lie less than 100 million km apart with an orbital period of 104.02 days, are in the process of becoming red giants...
Such an outburst or long term variability would be expected as they near the red giant stage.

__________________
`Irony` actually does mean `metal like`...

Perhaps the change is an artifact; in other words, maybe our photometry has gotten better. Or maybe the stated range is within the error bars of the observations.

__________________
"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State

2.10 is almost 6.5 times dimmer than 0.08.

Maybe the 2.10 is just a typo for 0.10?

The components B, C, D, E, F and G are all unrelated optical doubles, so orbits certainly don't come into it.

__________________
- Learn a lot teaching others.

I wouldn't think that the situation is that the record is a typo, or because photometry wasn't as good 100 years ago. There are plenty of zero, first, and second magnitude stars nearby to compare it to. A factor of six in brightness doesn't seem all that amazing. There is a star which completes the square in Cassioepia (on the side closest to Pegasus) which I've seen change in magnitude that much. It will be interesting to see more complete records about its brightness. You show nothing between 1908 and 1999. I have a strong memory of it being about magnitude zero since 1971 when it was one of my favorite stars to look at. If it brightened, I have no idea when.

__________________
Forming opinions as we speak

Per Wikipedia, http://en.wikipedia.org/wiki/Capella_%28star%29, Capella is made up of two binary stars that are in the process of turning into red giants (which would explain the yellowish color). Such a phase in a star's life is filled with fluctuations which are short to the star but long to us, so that's probably what causes the variability.

__________________
Yes, I have a life. It's quite different from yours.

Looks about as bright as it did when I first recognized it back in the 1950s. And that's over a half century ago.

Then, the sky was a lot clearer and darker, so perhaps it's gotten brighter, but conditions make it appear dimmer.

Still trying to figure out why Betelgeuse is Alpha Orionis.

Time to give Michael Keaton a call.

__________________
A person's name, or a mark representing it, as signed personally or by deputy, as in subscribing a letter or other document.

Other strange things in this data

Capella H is a 10th magnitude pair of red dwarfs just about resolvable by my ETX - 11.7/13.7 is strange.

Aa cannot be 0.08 and 0.18 separated by 0.1 seconds. Aa and Ab are the two components of a spectroscopic binary which we see as A.


I suspect the answer to why the 1870 - 1905 data is 2.1 is something to do with how the measurements were made. My guess is that this would be an early photographic plate magnitude (not visual). It would fit as the measurement is repeatable.

The data for Hh came from Burnham's Celestial Handbook. That particular double is not in the WDS. On www.alcyone.de they call HL.

I don't think that's right. Aa is the spectroscopic binary. It was discovered with the spectroscope at Lick Observatory in 1899. ANJ 1 refers to J. A. Anderson, who first measured the separation with the interferometer on the 100-inch telescope at Mt Wilson in December 1919. According to Burnham's Celestial Handbook the separation is never greater than 0.05'', so the WDS's 0.1 is rounded up! (Is 1999 a typo for 1919?)

So A is the primary (which is about 3 solar masses, luminosity = 90 Suns), a is the comes (2.8 solar masses, luminosity = 70 suns). Hh (or HL), a close double, is also part of the Capella system, which is thus a quadruple system. The other components are unrelated field stars.

As good a guess as we've had! On Alcyone, they give a "photovisual" magnitude of 0.2, which may or may not be relevant.

Perhaps, in order to see the fainter components it was necessary to use some sort of filter to reduce the glare from Capella, so the 2.10 is the latter's magnitude as measured through the filter (in which case the fainter components would actually be brighter than the data suggest?

__________________
- Learn a lot teaching others.

I think that this explains all.

"By the late 19th century astronomers were using photography to record the sky and measure star brightnesses, and a new problem cropped up. Some stars showing the same brightness to the eye showed different brightnesses on film, and vice versa. Compared to the eye, photographic emulsions were more sensitive to blue light and less so to red light. Accordingly, two separate scales were devised. Visual magnitude, or mvis, described how a star looked to the eye. Photographic magnitude, or mpg, referred to star images on blue-sensitive black-and-white film. These are now abbreviated mv and mp, respectively. "

and

"On early 20th-century and older orthochromatic (blue-sensitive) photographic film, the relative brightnesses of the blue supergiant Rigel and the red supergiant Betelgeuse irregular variable star (at maximum) are reversed compared to what our eyes see since this archaic film is more sensitive to blue light than it is to red light. Magnitudes obtained from this method are known as photographic magnitudes, and are now considered obsolete."


This means that Capella at G8 is fainter on a photographic plate than visually. This difference is apparently 1.9 magnitudes on old photographic plates. Interestingly this difference is the basis for establishing a BV colour index. For the modern equivalent a G3 star would have a BV of around 0.7 magnitudes.

Just to ramble on a little more - the initial Pogson scale referenced Polaris as 2.0. This would imply that Capella would be around 0.0 not 0.1 at the time.

What source are you quoting, ozark1?

I considered the possibility that the 2.1 is simply a photographic magnitude, but according to Burnham's Celestial Handbook, Capella's color index is about +0.8, so I would expect it to have a photographic magnitude of about 0.9, not 2.1!

__________________
- Learn a lot teaching others.

The measurements in question, reported in the Washington Double Star catalog, are based on visual measurements by a human looking into the eyepiece of a telescope, not photographic plates. The values labelled "BU" are probably Burnham's, the one labelled "BAR" is probably E. E. Barnard; I don't recognize the observer labelled FRH.

These observers were all much more interested in the positions of the double stars they were measuring than in their magnitudes. There are many reports which omit magnitudes entirely -- I did a little reading in old, old journals yesterday.

I suspect that this entire question, which is driven by these old measurements from double-star catalogs, is pointless. I would pay attention to changes in the magnitude of a star only if the values came from a project which was focusing on photometry.

BU is the discoverer code for S. W. Burnham
BAR is indeed E. E. Barnard
FRH is R. Furuhjelm

I don't really think there has been any significant change in Capella's brightness in the past century, but I am still curious about how its magnitude could be recorded as 2.10.

For the moment I'm sticking with my filter theory. I've noticed that many catalogs (e.g. Hipparcos, GSC, USNO) have no stars in the immediate vicinity of bright stars like Capella. It's probably impossible to see the fainter components let alone measure their magnitudes, separations and position angles without using a filter to reduce the glare from Capella (and remove its diffraction spikes).

__________________
- Learn a lot teaching others.

I just read the original source for E. E. Barnard's measurement of Capella in 1898. It appears in the Washington Double Star Catalog with the label "BAR 25", with magnitude values "MagA = 2.10" and "MagB=17.10".

The original source is a paper published in Astronomical Journal, Vol 19, p 113 (1898). You can see it for yourself in the ADS:

http://articles.adsabs.harvard.edu//...00113.000.html

On the final page are four measurements of Capella, with the separation and position angle listed for each one. There are no individual magnitude measurements listed. Barnard writes in the introduction to this paper, "No magnitudes have been assigned to the known stars."

Evidently, someone added magnitude values to the record when placing it into the Washington Double Star Catalog. They were NOT reported by Barnard. I suspect the same thing happened with the other puzzling magnitude values.

Thanks for that, StupendousMan.

I've tried to track down the original source for S. W. Burnham's records from 1878 and 1908 to see if they can shed light on this conundrum, but so far without success. However, all the double-star measurements of his that I have taken a look at also omit magnitudes, so it looks as though you're right about someone at a later date adding in the magnitudes.

__________________
- Learn a lot teaching others.

I decided to take the bull by the horns and email the WDS about this. Here's the response I got:

__________________
- Learn a lot teaching others.