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Alert Notice 690: Rare very faint minimum of Betelgeuse (alpha Ori)

January 6, 2020

AAVSO Forum threads (scroll to the bottom of a thread for latest posts):
- Campaigns and Observation Reports: https://www.aavso.org/alpha-ori-campaign-2020
- General AAVSO Discussion: https://www.aavso.org/alpha-ori-0
- Long Period Variables: https://www.aavso.org/question-about-betelgeuse-article-re-comparison-stars
- Spectroscopy: https://www.aavso.org/betelgeuse

The semiregular variable alpha Ori (alf Ori, Betelgeuse) is undergoing a very rare, very faint minimim. On behalf of numerous astronomers, the AAVSO is requesting that AAVSO observers monitor alf Ori in multiple wavelengths and obtain spectroscopy.

Of particular interest are observations in V, R, H, J, and B, along with spectroscopy. Time-resolved observations (at least 2-3 observations per night) are requested. Visual observations (not more than one per night) are also welcome.

PEP observers should contact AAVSO PEP Section Leader Tom Calderwood; his email address is "tjc at cantordust dot net".

Alf Ori typically has a minimum magnitude that may be 0.7 - 1.0 V, with occasional cycles brighter or slightly fainter. The previous minimum (Dec 2018-Jan 2019) was ~ 0.9 V (PEP, T. Calderwood, Bend, OR; PEP, E. van Ballegoij, Heesch, Netherlands; DSLR, W. Vollmann, Vienna, Austria). Recent observations submitted to the AAVSO International Database show the current minimum at 1.407 V +/- 0.014 on 2020 Jan. 1 8462 UT (DSLR, Vollmann), 1.4 visual on Jan. 6.8374 UT (V. della Vecchia, Italy).

The extreme brightness of alf Ori presents a problem for choosing suitable comparison stars for DSLR and visual observing. Nearby bright stars and their V magnitudes include:

Rigel (bet Ori) - 0.12 V mean mag - note that color is blue; use with caution
Procyon (alf CMi) - 0.36 V
Aldebaran (alf Tau) - 0.87 V
Pollux (bet Gem) - 1.14 V
Castor (alf Gem) - 1.58 V - not recommended due to color of star
Bellatrix (gam Ori) - 1.64 V - not recommended due to color of star
Alnitak (zet Ori) - 1.79 V
Alhena (gam Gem) - 1.92 V
Saiph (kap Ori) - 2.06 V

Finder charts for alf Ori may be created using the AAVSO Variable Star Plotter (VSP). However, the most practical finder chart for alf Ori (showing Orion, comp stars, and nearby constellations) is the one given in the AAVSO's 10-Star Tutorial (https://www.aavso.org/sites/default/files/10startutorial-2013.pdf). Comp stars included on it (V magnitudes rounded for visual observing) are: Rigel at 0.1, Procyon at 0.4, Aldebaran at 0.9, Pollux at 1.2, Alhena at 1.9, and Saiph at 2.1, as well as others more relevant to other targets on the chart.

Visual observers need to be aware of the Purkinje effect and use the quick-glance method to make their observations; do not stare at alf Ori or the comparison stars, as doing so may make them become artificially bright.

Coordinates (J2000): R.A. 05 55 10.30  Dec. +07 24 25.4

Please report all photometry observations to the AAVSO International Database using the name ALF ORI.

Spectroscopy reports should be submitted to the AAVSO Spectroscopy Database (https://www.aavso.org/apps/avspec/).

This Alert Notice was compiled by Elizabeth O. Waagen.

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