By Mike Simonsen AAVSO
There are some things about U Gem that are fairly
predictable. Its position is (2000) 07 55 05.26 +22 00 05.4. It is normally around 14th magnitude in quiescence,
and every 100+ days or so it will flare up to around 9th magnitude. When not in outburst, U Gem exhibits eclipses
every 4.2 hours. These eclipses vary in depth from about a half magnitude to nearly a full magnitude. During outbursts the
eclipse depth becomes negligible.
Other than those facts, everything else about
U Gem is unpredictable. Outbursts cannot be predicted reliably in spite of the average time between them quoted above. There
are such wide deviations from this average that U Gem needs to be monitored every clear night to be sure the beginning of
the next outburst is caught, particularly since it can rise from minimum to maximum in 30 hours or less.
Outbursts come in two flavors; "longs" that
generally last about 16 days, and "shorts" lasting about 10 days. Usually they alternate, but at times this pattern is broken
by two longs or two shorts in succession.
Just to keep us on our toes, U Gem underwent
a period of unusual activity in 1985. The first long outburst lasted 42 days, twice as long as the longest outburst on record.
This was then followed by the shortest short outburst on record, a mere 6 days. Two successive shorts then followed these
outbursts. (It is possible a long outburst was missed during the summer of 1986.) Clearly, there are no rules for U Gem.
U Gem can be a restless soul in quiescence.
I have witnessed flickering on time scales of seconds while observing visually. At times this can be quite dramatic.
One of the challenges to observing U Gem is
its placement near the ecliptic. This results in a seasonal gap in the data from early June until late August when it is too
close to the Sun to observe. Undoubtedly, outbursts have been missed during this period in years past, making our recorded
history of activity less than complete. Observations made in early September when U Gem is a morning object are important
for filling in this seasonal gap. For a few days each month it is difficult or impossible to observe as the Moon passes through
Gemini and Cancer.
The February 2004 outburst will always be memorable
for me. There wasn’t anything particularly unusual about it, in fact we had been anticipating the next outburst and
had an AAVSO campaign planned to run in conjunction with satellite observations. But, the chart team was putting the final
touches on a brand new sequence and new charts for U Gem. It had taken an inordinate amount of effort to sort out the suspected
variables and a new variable was discovered in the field, which had historically been used as a comparison star. We finally
had the final drafts of the charts checked and dated Feb. 25, 2004 and were preparing to release them prior to the rest of
the batch of charts so they could be used for the upcoming outburst. That night, Feb. 26 UT, U Gem went into outburst. It
was too late to release new charts without causing confusion and scatter in the data, so they were eventually released with
the rest of Batch 6 in March.
