AAVSO Alert Notice 690 requests photometry and spectroscopy of alpha Ori (alf Ori, Betelgeuse) during and beyond its current minimum. Please see the notice for details and observing instructions.
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ
>>> At the Alert Notice 690 Zet Ori V-magnituge is equal to 1.79 but at X25131AYL is equal to 2.05. Which one is correct?
The correct combined mean V magnitude for Alnitak's is V= 1.76 and it is actually slightly variable betwen 1.74 and 1.77. 2.05 is the magnitude of Aa (actually V= 2.06) which can't be resolved from a 4.10 companion 0.042" away and a V= 4.02 companion 2.42" away. I have corrected the comparison star database to reflect this in VSP charts. Thanks for noticing it.
Just my opinion, but folks should be avoiding all of the blue stars in Orion as comparisons for Betelgeuse, at least for single-channel photoelectric or imaging sorts of photometry. This includes zeta Ori and Bellatrix (gamma Ori). They are simply too blue and slightly variable, as Sebastian indicated. The various K-type giants in the region are more appropriate, and to be much preferred, especially Aldebaran (V = 0.87) and phi-2 Ori (V = 4.08). Other nearby K-giants could be included in the mix with several comparisons to help deal with zero-point, extinction, and color-term issues. Some high-quality dfferential photometry of 31 Ori = HR 1834 = HD 36167 = CI Ori would also be helpful to confirm the extent to which it is variable at all.
even when Betelgeuse seems to be already rising now, due to all the attention it has been receiving, we considered it a good time to update the 10 Star Tutorial charts.
Since comparison stars are widespread over a large area, observers in the different hemispheres may find it useful to have comparison stars higher in their skies so there are two version of the charts, one for the Southern hemisphere (including eps CMa which at V= 1.50 is very useful now) and another one for the Northern hemisphere (which also works to estimate eta Geminorum).
You can download them from here:
VSP is not useful to create charts for Betelgeuse due to its limited field of view so these charts are recommended.
A list of accurate V magnitudes for the comparison stars follows:
01 bet Ori V= 0.13* (Rigel)
04 alp CMi V= 0.36 (Procyon)
09 alf Tau V= 0.87* (Aldebaran)
11 bet Gem V= 1.14 (Pollux)
15 eps CMa V= 1.50 (Adhara)
16 alp Gem V= 1.58 (Castor)
16 gam Ori V= 1.64 (Bellatrix)
18 zet Ori V= 1.76* (Alnitak)
19 gam Gem V= 1.93 (Alhena)
21 kap Ori V= 2.06* (Saiph)
* only for visual use. Small amplitude variables:
bet Ori - ACYG V= 0.08 - 0.20
alf Tau - LB: V= 0.86 - 0.89
zet Ori (NSV 2553) - VAR V= 1.74 - 1.77
kap Ori (NSV 2641) - ACYG V= 2.04 - 2.09
Remember this chart is only for visual use.
Unfortunately we don't have stars with the same colours of the variable, we have to use what is available.
Good observing and keep an eye to see if Betelgeuse is going to recover smoothly or not from this fading!
What about Betelgeuse chart ID? Robert Stalnaker (thanks Bob!) called our attention regarding this.
People using the 10 Star Tutorial chart have been providing the chart ID "10 Star" or "10 Star Tutorial". That's fine but now that a slight change has been made, it would be a good idea to call the new chart something like "10 Star 2020" so we know the new version is in use.
Another interesting question came up when talking about the rounded labels provided in visuals charts like this one.
For decades visual observers have been using the rounded values provided in the charts to make their estimates. We did not have photometry tables back then (and we don't have one for special charts like these, that's why I provided V values here) . It's been only a few years since photometry tables have been implemented through VSP.
Is there a reason not to use the photometry table values to perform estimates? I don't think so. I would encourage people to do it.
This chart showcases a couple of examples.
Elnath (bet Tau) was removed as a comparison star because it has V= 1.66 while Bellatrix has V= 1.64. They are placed towards the same side of the variable and were thus redundant. Their magnitude difference was 0.02 mag. instead of the 0.1 mag. implied by their rounded values.
Another example: Alnitak (18) has V= 1.76 while Bellatrix (16) has V= 1.64 as mentioned above. The difference is 0.12 and not 0.2 mag.
So rounding off values -as I see it- is just a useful practice for labelling and not filling up the charts with numbers. Also useful to identify comparison stars*. But there is no reason I can think of not to use the accurate V values to reduce the estimates. We haven't talked about using the photometry table values to reduce the visual estimates. Maybe it's time to do it.
*This chart also shows a situation that we have solved in the past by adding an uppercase letter to the lable: when we have two stars with the same rounded magnitude in the sequence.
In this case, especially for Northern observers, having Castor and Pollux so close to each other may make it useful to use Castor when the star is between 1.1 and 1.6. For Southern observers, Adhara and Bellatrix will be much better when the star is 1.5-1.6 (like right now!). So we have two "16" stars.
For now, the solution is labelling Castor as 16N and Bellatrix as 16W.
Some time in the future maybe we could think about giving two decimal places for chart reports to avoid this. But do not complicate things unnecessary right now!
very interesting idea, because the brightness difference of the stars becomes more precisely! And then the estimation of variable stars also might be more exact.
One problem may be, that the comparison to historical data becomes more difficult when using "new defined" visual brightnesses for comparison stars (such as 1.64 mag for Bellatrix instead 1.6...).
(Sorry for my not-so-good English, as I am not a native speaker)
I think the difference won't be apparent because Betelgeuse hadn't been so faint before and the small differences arise from newly added comparison stars like the 18 which wasn't used in the past. The results obtained by using Pollux and Bellatrix in the past will be comparable (the difference between them is rather large, half a magnitude)
Any difference will be as small as to go unnoticed in the long term and compared with the visual scatter. My point is more ideological: not introducing small error sources when there is no need to do it.
Thanks for commenting!
It is most definately on the way up. It is easy to see in the V band plot over the last 20 days. My own measurements form a nice slope even over the least three nights of data. I have it up about 0.1 mag over 13 days.
I normally cannot go that bright, but I developed a new out-of-focus process which seems to work pretty well!