Sky & Telescope - March 2024 - 36

Variable Stars
kindling temperature for hydrogen. This triggers a runaway
thermonuclear reaction (basically, a huge H-bomb) that
blows up the surface layer, resulting in the star's sudden
brightening. As the ejecta expand outward and the white
dwarf cools, the star slowly fades to its pre-eruption state over
months or even years.
Once the white dwarf returns to its quiescent state, the
whole process begins again, creating a new eruption cycle. For
most novae, a complete cycle takes up to a million years. But for
a small fraction of these systems, the recurrence time is much
shorter - from one year to one century. Those with recurrence
times of less than 100 years are classed as recurrent novae (RNe).
Only a handful of RNe have two or more known eruptions, and
of those only 10 are known to exist in the Milky Way.
1866
Late on the evening of May 12, 1866, Julius Schmidt,
director of the National Observatory of Athens in
Greece, scanned the skies around the Northern
Crown but saw no new star down to 5th magnitude.
Less than three hours later, observing from western
Ireland, John Birmingham spotted a new 2nd-magnitude
star in Corona Borealis and knew that he'd found
something remarkable. News of Birmingham's discovery
travelled fast, so many astronomers were able to
contribute observations, including British astronomer
William Huggins, who became the first person to
examine a nova's light spectroscopically. Before T
CrB, there was no recognition of the phenomenon that
today we call a " nova. " After T CrB, the existence of
novae became well known, though no one had any
idea about the forces at play.
T CrB is the prototype RNe and consists of a red giant star
and a 1.35-solar-mass white dwarf with an unusually long
orbital period of 227 days. Most novae have orbital periods
of only 3 to 8 hours! T CrB famously erupted in 1866 and
1946 - data from those observations tell us something about
its behavior. Its light curve (a plot of brightness against time)
has a rise and fall that is fast - very fast: Its rise time is only
a few hours long, and it remains near its peak brightness
of magnitude 2.0 only for half a day or so. In total, the star
remains naked-eye visible for only one week.
Time to Blow?
When I was a teenager, I noted that 80 years had elapsed
between the T CrB eruptions of 1866 and 1946 and figured
the next eruption should occur some time around 2026. Of
course, many others made that same calculation. But were
there additional eruption episodes predating the one in 1866?
A few months ago, I found a long-lost, eyewitness report of
one that occurred around Christmastime in 1787. Taking this
episode into account, the average recurrence time turns out
to be closer to 79 years, indicating the next eruption is due
sooner - perhaps in 2025.
In addition, over the last few years I compiled detailed
light curves for T CrB's two most recent eruptions and found
them to closely match. I also examined the light curves for
the other nine Milky Way RNe, finding that they always
reach the same peak each time. I now feel confident that
the upcoming T CrB eruption will once again reach a peak
magnitude of 2.0. But were there additional clues that would
allow me to further refine the timeframe for this event?
A LONG HISTORY The light curve of T Coronae Borealis stretches
along the bottom of this page and the next three. The vertical axis is the
same on each page, giving the visual (V) magnitude in orange and the
blue (B) magnitude in blue. The horizontal axis provides the date. In total,
124,935 magnitude estimates were used to create this plot.
1
2
3
4
5
6
7
8
9
10
11
12
13
1855
36 MARCH 2024 * SKY & TELESCOPE
Secondary
eruption
Argelander
star catalog
estimate
1865
1875
1885
1895
B or V magnitude
BEATRIZ INGLESSIS / S&T, SOURCE: BRADLEY E. SCHAEFER / AAVSO
Sky & Telescope - March 2024

Table of Contents for the Digital Edition of Sky & Telescope - March 2024
