[vsnet-alert 23870] Re: Betelgeuse still dimming

[Brian Skiff]

> On Jan 19, 2020, at 11:50 AM, P. Clay Sherrod <drclay at tcworks.net> wrote:
> 
> I am sure that all of us are keeping up with the dimming of Betelgeuse; last 
> evening (Jan. 19 @ 2:00 UT) the star is as dim as I have ever seen it; on the 
> previous time I was able to assess (Jan. 11), it was around magnitude 1.5-1.6; 
> on Jan. 19 I assigned a visual magnitude of +1.9, using the stars Alnitak (1.9) 
> and Bellatrix (1.7). Betelgeuse appears significantly red at this time and I 
> warn observers about the well known Perkinje Effect, whereas staring at a 
> reddish star for a prolonged period will result in its appearing brighter than 
> it actually is, something that I noticed last evening.


     One also has to be cognizant of the significant color term in the visual photometry, which is roughly:  V = mv - 0.2(B-V).  Thus red things are brighter in V than the straight visual magnitude.  This may not be quite right in this case, since Betelgeuse and its available comps (all fairly blue) are bright enough that one is in the `mesopic’ range of color sensitivity — and the coefficient just quoted would be smaller, and the Purkinje effect tend to dominate.
     Luckily there are several observers getting standard B,V photometry using photomutiplers and CCDs, plus J,H photometry in the near-IR.  It continues to fade about 0.01 mag per day in V.  (See the AAVSO V-band lightcurve.)  The professional community is also getting resolved images and spectra of the star in various ways across the spectrum.  So it is not being overlooked.
     I have keyed-in several long series of visual and photoelectric photometry (about 1800 lines of data; I’ll be pleased to share the file), which indicates that the star has never been so faint since 1840.  There apparently was a fairly deep minimum about 1945, but do not have those data to hand yet.
     I doubt Betelgeuse is `vsnet-alert’ worthy, but it is good to keep an eye on it for now!


\Brian