TRIANGLE TRIO
Mixed into the birds, beasts, and mythical figures of the classical
ancient constellations is a decidedly
practical figure, Triangulum, the
eponymous Triangle that Hipparchus -- the possible inventor of
trigonometry -- perhaps found inspiring. Look for the pretty
pattern in northern autumn evenings between Aries the Ram and the graceful curves of stars that
represent Andromeda, the focus of
the Perseus myth. While Triangulum is
wholly visible in the southern hemisphere down to a latitude of 55
degrees south -- which takes in all but the southern tip of South
America and Antarctica -- southerners have their own they can
admire, Triangulum Australe, the Southern
Triangle. Southern sky-watchers rather keep their version to
themselves, as "TrA" (as opposed to "Tri") can be seen fully only
below 20 degrees north latitude. One of the 38 accepted "modern constellations," TrA was invented
(discovered?) by a pair of southern explorers and navigators,
Pieter Keyser and Frederick de Houtman around 1600, placed on the
globe of the northern astronomer Petrus Plancius, and finally cast
into permanent celestial memory by Johannes Bayer in his famed Uranometria.
On nearly opposite sides of the sky, the two triangles -- other
than each being three-sided -- have a couple things in common.
Both are small, near the bottom of the size list (Tri ranking 78th,
TrA 83rd), and each figure's Alpha star has a proper name, Alpha
Tri called "Mothallah" (meaning "the
triangle"), while Alpha TrA is the obviously manufactured "Atria." Then the resemblance rather skids to
a halt. Triangulum is rather isosceles, while TrA is close to
equilateral. Triangulum's stars range from 64 light years away for
Alpha to 124 for Beta, while TrA's span
from closer (40 for Beta) to farther, a
hefty 415 for Atria. More obviously, the southern Triangle is much
the brighter of the two. With respective visual magnitudes of
1.92, 2.85, and 2.89, Alpha, Beta, and Gamma (in proper order) all make the top
150 brightest-stars list (coming in at 42nd, 138th, and 149th).
Tri's stars, on the other hand, are out of order, third magnitude
Beta (which at 3.00 closely defines third magnitude) the brightest,
followed by Mothallah at 3.41 and Gamma
at 4.01, allowing one to see the effect of a full magnitude at one
glance.
Five of the sextet are common white class A or F dwarfs or giants. Odd man out is Atria, an
orange class K2 giant that is also the sky's brightest "barium star." Such stars are all evolving
giants that are believed to have been contaminated by heavy
elements when a more-massive companion was itself dying as a giant
and transferring matter to the star we now see. Unfortunately, the
companion -- which should now be a white dwarf -- is nowhere to
be seen. Atria also is distinguished as a "hybrid" star that has
both a cool wind and a hot corona at the same time (which may
belong to yet another companion; we don't know). The northern
triangle responds to this sort of fame with a couple very close
binaries. Mothallah has a companion that orbits in a mere 1.7
days, while Beta Tri has a sunlike neighbor that takes just 32 days
to make a turn.
Of more interest is that both constellations have good naked-eye
(providing you have dark skies and good eyes) variables. Up north,
eastern Triangulum hosts R Trianguli (the first variable to be
found in the figure), long-period Mira-type variable that starts
at bright sixth magnitude (5.7 or so) and then plunges to a
miserable 12.5, when it is visible only in a decently sized
telescope, returning to maximum visibility every 266 days. Though
a luminous class M giant, its relative apparent faintness comes
from its great distance of about 1300 light years the distance
having a large uncertainty). Like all Miras, R Tri is entering its
final death throes. Nearly the size of Earth's orbit, several
thousand times more luminous than the Sun, unstable, and
brightening with a dead core made of carbon and oxygen, the star
will shortly turn itself into a planetary nebula and then a white
dwarf. If you can catch it, you might admire the color of as it
goes from class M4 at its brightest to a deeply cool M8 at the
bottom.
Triangulum Australe responds with "X." Far redder than R Tri, X
TrA is an irregularly-variable class C5 carbon star some 1500 or so light
years off that wobbles between sixth and seventh magnitude. Carbon
stars have lofted to their surfaces fresh carbon made in the fusion
of helium atoms in their nuclear-burning cores. Carbon molecules
and metal atoms depress the stars' short-wave light, leaving little
but red to come through, such stars often startling to see against the
blackness of night. They are a major contributor to the carbon in
the Universe.
TrA also boasts not one, but a pair of naked eye Cepheid variables. R and S TrA
vary between sixth and seventh magnitude over respective periods of
3.39 and 6.32 days. Cepheids are class F and G evolving
supergiants that, like Miras, are in an unstable state that causes
them to pulsate, to change their dimensions and luminosities (which
also range into the thousands of Suns). Smaller than Miras, they
are much more regular. Of supreme importance, the visual
luminosities of Cepheids are tightly correlated with their periods,
which makes them wonderful "standard candles" for measuring
distances to other galaxies. Too far away for parallax, the
period-luminosity relation tells of respective distances for R and
S of 2000 and 2700 light years.
The Crown Jewel, though, belongs to the northern Triangle, as it is
home to one of the nearest galaxies to the Earth, the strikingly
beautiful Triangulum Spiral, Messier 33,
one of only four galaxies visible to the naked eye (the others
being the two Magellanic Clouds of the
southern hemisphere and the famed Andromeda spiral, M 31). While M 31, at a distance of 2.5 million light
years, is usually taken as the farthest thing you can see with the
naked eye, M 33 -- which is a much more difficult naked-eye object
-- is actually a bit farther, 2.7 million light years. Third
ranked in size in our Local Group of galaxies after M 31 and our
own, it is still huge, a full degree in the sky, some 50,000 light
years, across. Buried within it, to the northeast of center, is
one of the most magnificent of diffuse nebulae, NGC 604. Lit by
200 hot young stars, 1300 light years across, this giant, if placed
at the Orion Nebula, would not just
overfill the constellation Orion, but
would extend into Lepus and Taurus. At center is a huge bubble blown
out by the winds and supernova explosions of the massive stars
within.
But wait. The title of this column is a TRIO of Triangles. What
happened to the third one? In the late seventeenth century,
Johannes Hevel (Hevelius) filled in many of the blank areas of sky
seen from the north with another set of modern constellations. To
him we owe Canes Venatici, Lacerta, Leo Minor, and four
others. Another quartet, however, never made it. Among the
rejected were Musca, the Fly, which
unsurprisingly perches on the back of Aries the Ram, and Triangulum
Minus, the "Smaller Triangle," which lies just southeast of
classical Triangulum and was made of 6, 10, and 12 Trianguli before
the numbers were assigned to John Flamsteed's stars.
The brightest of them, fifth magnitude (4.94) 6
Tri, is an outstanding quadruple star. Through the telescope
one sees it as a visual double made of a fifth magnitude mid-class
G evolving giant coupled to a seventh magnitude mid-class F
ordinary dwarf and separated by 3.9 seconds of arc (an easy split),
which at a distance of 305 light years corresponds to a true
separation of at least 360 Astronomical Units. Though the stars
actually are both about the same nearly-white color, juxtaposition
coupled with brightness difference makes the eye see color that is
not there, Admiral Smythe referring to them as "topaz-yellow" and
"green." While there is nothing special about such doubles, each
is also a spectroscopic binary
with a very tight orbit. The brighter, 6 Tri A, has a companion
that goes around in just 14.7 days, while 6 Tri B's mate takes a
mere 2.2 days. The brighter pair has a measured separation of just
0.2 Astronomical Units. The rapid orbital motion has spun up the
rotation of the giant, which in turn produces stellar activity,
including includes spots, which makes the star slightly variable
and a fine example of an "RS Canum Venaticorum" star (the prototype
of such binaries). As a result, 6 Tri A is also known by its variable name, TZ Tri. The
other double just sits there watching the whole thing, knowing that
once one of its own components evolves, it may follow the lead of
6 Tri A and become an RS CVn star itself.
The eye loves patterns, evident in this trio of triangles. We
might also add the two big ones that span constellation boundaries,
the Winter Triangle (made of Betelgeuse in Orion, Procyon in Canis
Minor, and Sirius in Canis Major) and the Summer
Triangle (formed from Vega in Lyra, Deneb in Cygnus, and Altair
in Aquila). Find more on the stars of
the Winter one in Hunting Orion
and "Going to the Dogs, and on the
Summer one in Three White Stars.
Then go out and admire Triangulum, its radiant southern
counterpart, or even the long-forgotten "little one."
Copyright © James B. Kaler, all rights reserved.
These contents are the property of the author and may not be
reproduced in whole or in part without the author's consent
except in fair use for educational purposes. First published in
the January/June 2008 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.