The ancient pole star known as Thuban turns out to be an
eclipsing binary. If you're up for a challenge, you can see the eclipses
for yourself.
The
star Thuban, also known as Alpha Draconis, has long been known to be a
binary system. Now data from NASA's TESS show its two stars undergo
mutual eclipses. NASA / MIT / TESS
NASA's TESS mission has a primary goal of looking for exoplanets. But
as it's examining stars for the subtle dips that mark a planet's
passage in front of its star, it can catch lots of other stellar
goings-on, too. At the recent American Astronomical Society meeting in
Honolulu, astronomers announced that TESS had uncovered something rather
surprising about the well-known double star in Draco named Thuban (also
called Alpha Draconis). Turns out, the two stars eclipse each other as
seen from Earth.
“The first question that comes to mind is ‘how did we miss this?’” says Angela Kochoska (Villanova University).
When two stars in a binary system eclipse each other, their overall
brightness as seen from Earth fades and increases again in a cyclic way.
Thuban's eclipses occur twice every 51.4 days. So even though Thuban
isn't exactly the brightest star in the sky — at magnitude 3.7 it's the
eighth-brightest star in Draco — the discovery at first took astronomers
by surprise.
However, the primary eclipse causes a change in brightness of only
about 0.1 magnitude; the secondary eclipse is marked by an even smaller
dip, 0.02 magnitude. The variations are small in part because the stars
don't fully eclipse each other. What's more, the eclipses are only six
hours long. So perhaps it's no surprise that the small, brief dips in
brightness were missed until now.
Even the space-based Kepler mission didn't catch the star's
variations. Thuban was actually too bright for Kepler to look at without
saturating its detector.
TESS, though, is designed to look at bright, nearby stars. The
satellite monitors large swaths of sky for 27 days at a time, splitting
each celestial hemispheres into 13 sectors each. Near the poles, these
sectors overlap in what's termed the continuous viewing zone. These long observations with very precise measurements enabled TESS to find the subtle variations in Thuban's brightness.
Thuban, the Ancient Pole Star
Thuban is famous not for its brightness, but for the role it played in the Egyptian sky some 4,700 years ago: North Star.
Earth precesses, wobbling like a top as it spins, though its
wobble takes 26,000 years to come full circle. So back when the
Egyptians were just starting to build their pyramids, it was Thuban, not
Polaris, that aligned with Earth's rotation axis.
As such, all the stars of the northern sky would have appeared to
revolve around Thuban, giving it a place of celestial importance for
ancient Egyptians, who may have aligned the Great Pyramids based on its
position.
See for Yourself
Stellarium, with additions.
While Thuban is known to be a binary, if you observe the system,
you're really just observing the primary. This giant star is several
hundred times brighter than the Sun, though dim in our view for being
300 light-years away.
The changes in Thuban's magnitude as it eclipses (and is eclipsed by)
its smaller companion are small, as noted above. But if you're up for
the observing challenge, Kochoska has calculated the times of the
upcoming eclipses visible throughout 2020.
The table below gives the primary and secondary eclipses that occur
in 2020. The bolded lines are observable from the North American Central
Time Zone (CT) and/or the Central European Time Zone (CET); the lines
in italics mark eclipses where an observer would miss the beginning but
could still catch the end.
Primary Eclipses
UTC
CST/CDT
CET/CEST
February 22, 2020 4:44
February 21, 2020 22:44
February 22, 2020 5:44
April 13, 2020 14:48
April 13, 2020 9:48
April 13, 2020 16:48
June 4, 2020 0:52
June 3, 2020 19:52
June 4, 2020 2:52
July 25, 2020 10:56
July 25, 2020 5:56
July 25, 2020 12:56
September 14, 2020 21:01
September 14, 2020 16:01
September 14, 2020 23:01
November 5, 2020 7:05
November 5, 2020 1:05
November 5, 2020 8:05
December 26, 2020 17:09
December 26, 2020 11:09
December 26, 2020 18:09
Secondary Eclipses
UTC
CT/CDT
CET/CEST
February 9, 2020 5h:59m
February 8, 2020 23:59
February 9, 2020 6:59
March 31, 2020 16h:3m
March 31, 2020 11:03
March 31, 2020 18:03
May 22, 2020 2h:7m
May 21, 2020 21:07
May 22, 2020 4:07
July 12, 2020 12h:11m
July 12, 2020 7:11
July 12, 2020 14:11
September 1, 2020 22h:15m
September 1, 2020 17:15
September 2, 2020 0:15
October 23, 2020 8h:19m
October 23, 2020 3:19
October 23, 2020 10:19
December 13, 2020 18h:23m
December 13, 2020 12:23
December 13, 2020 19:23
If you catch an eclipse, let us know in the comments!
0 Comments