Also known as BL Ceti/UV
Ceti, GCTP 343.10 A/B, LHS 9/10, GJ 65 A/B
I don’t know about you, but I was
expecting to arrive at the last of the uber-faint red dwarfs on our pilgrimage
with a sense of “Good Riddance”. Instead, I feel that I will miss these
uncooperative dots, alternately cursed at or exclaimed over. Admit it now – the
joy at finally capturing one of these elusive little stars has more than made
up for the (very real) frustration encountered along the way in your search for
them. But it is altogether fitting that we end with Luyten 726-8. For one
member of this double star system is the prototype of nearly all the red dwarfs
that we have tracked down over the course of three seasons, and has (under the
designation UV Ceti) given its name to the entire class of red dwarf flare
stars which make up the majority of suns in our Milky Way Galaxy, and probably
of the universe as a whole.
The observational history of this star
system is full of surprise and irony. It was the ultimately less-interesting of
this stellar pair that in 1948 first caught the attention of Willem Jacob
Luyten, in his process of cataloguing stars with high proper motion. Luyten
726-8’s true motion is mostly perpendicular to our line of sight, which results
in an unusually high apparent movement of 3.37 arcseconds per year.
Additionally, it is receding from our solar system at a velocity of slightly
more than 18 miles per second. At a distance of 8.73 ly, it is presently the
6th closest star to the Sun. But less than 29,000 years ago, it made its
closest approach to the Earth, then being separated from us by only 7.2 ly. (It
will make another far more dramatic close encounter with a neighboring star
only a little more than 30,000 years from now, but more on that later.) Luyten
immediately made note of the star’s variability, and it was given the variable
star designation of BL Ceti.
But Luyten, who after all is credited with
making the first ever confirmed observation of activity on a flare star as
early as 1924, apparently missed BL Ceti’s companion star, Luyten 726-8 B (the
first-detected BL Ceti grabbing the “A” designation, despite being the lesser
of the two components). Only months after BL Ceti’s discovery, astronomers
Alfred Harrison Joy and Milton L. Humason of the Mount Wilson Observatory
discovered the companion star Luyten 726-8 B in the midst of an unprecedented
flare episode. The brightness of this system’s B component had increased in the
very first observation of it by more than 4 magnitudes, with its surface
temperature shooting up to more than 10,000º (an increase of nearly 400%). The
star was given the designation UV Ceti, and became widely known as the
prototypical red dwarf flare star. Such stars to this day are most often
referred to as UV Ceti Variables.
I absolutely cannot help but digress a bit
at this point. Co-discoverer of the star UV Ceti, Milton Humason, is a most
amazing figure in the history of astronomy. A high school dropout, he worked as
a mule team driver, transporting equipment and supplies up the mountain trails
to California’s Mount Wilson Observatory during its construction in 1917. He
stayed on after completion of the facility as janitor. He became fascinated by
the work being done there, and despite his complete lack of education several
of the astronomers willingly passed off to him the more mundane aspects of
their research, eventually entrusting the 26-year old who had dropped out of
school at age 14 with taking images and even spectrographs of faint objects.
After only two years of this, no less than Mount Wilson’s director, George
Ellery Hale, made Humason a full-time staff member. (It probably helped that he
had meanwhile married the daughter of the observatory’s chief engineer, Helen
Dowd.) He stayed on at the observatory until his retirement in 1957, having
more than justified the faith put in him by Hale. In addition to many
observational discoveries (such as UV Ceti), Humason is most famous for his
work with Edwin Hubble, being responsible for measuring the redshifts of more
than 620 distant galaxies. It was these painstaking observations that enabled
Hubble to determine that the universe was expanding, one of the cornerstones of
modern cosmology.
UV Ceti is even today the object of intense observation. The American Association of Variable Star Observers (AAVSO) dates its involvement with the star to January 1950, or only 18 months after its discovery. AAVSO records show that UV Ceti can become 75 percent brighter than normal in as little as 20 seconds. Unfortunately, flare episodes for any star are (as yet) completely unpredictable. So if you yourself wish to observe one, you’d better be prepared to stick with a single candidate star for an entire evening, checking in every minute or so to see whether anything’s going on. UV Ceti’s magnitude can spike within seconds, and then rapidly (taking from just a few minutes to hours) return to its baseline quiescent state.
As already pointed out several times in
previous sections, these eruptions occur across the spectrum to include quite
lethal levels of X-rays, and most likely make the chances for life in such a
disturbed environment very slim indeed.
Luyten 726-8 A and B are of nearly
identical mass (0.01 solar) and radius (0.14 solar). Their surface temperatures
are (when quiescent) about 2,670º. Don’t expect to split this double in the
eyepiece! They are separated from each other by a very tight 2.1 to 8.8 AU, and
orbit about a common center of gravity every 26.5 years. (Compare to 2,600
years for the components of Groombridge 34.)
Of interest to a (far) future generation
of astronomers, beginning a little more 29,000 years from now, Luyten 726-8
will begin an extremely close encounter with nearby Epsilon Eridani (the subject
of a future posting), passing within 0.93 ly of that star. The near-miss will
take more than 4000 years to complete. It may not have much effect on us, but
the potential is there for huge
consequences to any planetary system about Epsilon Eridani. (And such a system
is not hypothetical. Astronomers agree that its existence has been confirmed.)
If we assume that other solar systems are surrounded by an Oort Cloud of
comets, as is our own, then Luyten 726-8’s passage through such would undoubtedly
perturb the orbits of countless such bodies, likely resulting in something
analogous to the Late Heavy Bombardment endured by terrestrial bodies in our
own solar system. Good question – was the Late Heavy Bombardment the result of
a near miss billions of years ago with another sun? If so, the evidence would
be long gone by now, and the culprit tens of thousands of light years away.
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