UNDER THE HANDLE
When I was young, I would visit my cousins in southern New York
State, where it was insanely dark: to outsiders' surprise, New
York, home of New York City, is mostly rural and home also to
mountains, farms, cows, and good cheese. Directly south of the
Big Dipper's handle (in Britain, the
Plough), on one trip I first saw this amazing spattering of
stars that looked like lace affixed to the firmament. It was
hard to look away. Coma Berenices,
Berenices Hair, honors an ancient Queen who vowed to cut off her
hair and offer it to Aphrodite if her husband returned safely
from battle. He did, she did, and the gods put it into the sky.
As a constellation, Coma is something of a hybrid. Though honored in
ancient times, Ptolemy did not include it in his list. It was
later added back by Tycho when modern astronomers (1600-1800)
began filling in the sky, Flamsteed
making use of it in his great catalogue. Though the really
modern (1930) boundaries spread out farther, the constellation's
lacy core is a modest open cluster
more formally called Melotte 111 (after the British astronomer
P.J. Melotte). Leaving aside the Ursa
Major Cluster (dominated by the middle five stars of the
Dipper), with a distance of around 300 light years, twice as far
as the Hyades, Mel 111 is the second
closest cluster, and is
just over two-thirds of the way to the Pleiades and halfway to the Beehive. Some 500 million years old, the combined
gravity of its stars is not enough to keep it together. Coma is
particularly known for its lack low mass stars, many of which
have already departed.
Just a few degrees to the east of the cluster lies the North Galactic Pole, which stands
perpendicular to the midline of the Milky
Way. Looking away from our Galaxy's dusty disk gives us a
wonderfully clear view of distant galaxies, which are absent
from the Galactic plane, the old astronomers -- not knowing the
natures of galaxies nor of interstellar dust -- referring to
it as the "zone of avoidance."
Coma Berenices, the broader constellation that includes the
cluster, has few other bright stars of interest. For these, look
between Coma and the Dipper's handle for a pair that run
parallel to the latter. Here is a real "modern" constellation,
Canes Venatici, the Hunting Dogs,
invented by Hevelius in the seventeenth century from the
"unformed" stars of Ursa Major. The
luminary, third magnitude Cor
Caroli ("Charles Heart," named by Edmund Halley in honor of
Charles II), 115 light years away, is a binary made of a dominant
magnitude 2.90 class A0 dwarf called Alpha-2 (as it's
the eastern of the two) coupled with a magnitude 5.60 F0 dwarf
(Alpha-1) 19 seconds of arc away. They have been tracking each
other since Herschel first measured their separation in 1777.
With a weird surface composition powerfully enhanced by silicon,
mercury, europium, and various other elements, Alpha-2 is the
prototype of the class A magnetic stars that slop over into the
B and F dwarfs. In the outer layers of slowly rotating (hence
quiescent) "metallic" (Am etc.) stars, some elements are raised
by radiative lofting, while gravity causes others to settle
downward. In the magnetic (Ap) stars, strong fields up to tens
of thousands of times that of Earth (Alpha-2 at about 2000)
concentrate the enhanced elements into patches, "starspots." As
a magnetic star rotates, its field axis, which is tilted against
the rotation axis, flops around, and the star varies slightly in
brightness as its starspots swing in and out of view, Alpha-2
going around every 5.469 days. Analysis of the Doppler shifts
in the various spectrum lines allows the construction of a movie
of the rotating star, spots and all. If that is not enough,
Alpha-1 seems to be a metallic star.
The other star of the configuration, fourth magnitude Beta CVn
(or Chara, meaning "Joy"), a mere 27.3
light years away, is a G0 dwarf a lot like the Sun. Almost the same temperature, it's 19
percent more luminous, possibly as a result of more advanced age
and/or slightly higher mass. And like the Sun, it radiates X-rays from a hot corona.
Alas, there seems to be no planet, perhaps
the result of a notably lower metal content. Largely unsung is
"La Superba," Y CVn, a red carbon star, though one not
nearly so red as beautiful R Leporis, "Hind's Crimson Star." Carbon stars have
molecular absorptions of cyanogen (CN) and other features that
wipe out the blue parts of their spectra, leaving them as glowing coals.
In an advanced state of evolution, they are brightening as giants for the second time with shrinking
dead carbon/oxygen cores, and have through convection shoveled
up carbon manufactured deep down by helium fusion.
Near the southern border of Canes Venatici, between Cor Caroli
and Arcturus, lies one of the
Galaxy's great stellar systems, the globular cluster Messier 3.
Just over thirty thousand light years away, 100 times the
distance of the Coma cluster, exceeded in absolute brightness by
only 47 Tucanae, Omega Centauri, and perhaps M5 in Serpens, it may contain as many as a million stars.
Flipping to the northern border, 3.5 degrees southeast of Alkaid (Eta Ursae Majoris) at the end of
the Dipper's handle, find an even more spectacular system, M51,
the Whirlpool Nebula, a classic spiral galaxy some 25 million
light years away (the distance uncertain), nearly a thousand
times farther than M3. In 1845, William Parsons, third Earl of
Rosse, used his giant 72-inch telescope to discover the galaxy's
spiral arms. Until Hubble figured out they were galaxies, M51
and a cohort of other "spiral nebulae" were commonly thought to
be small and in our own large Galaxy. Rosse's spiral arm
drawing was so dramatic that it showed up in van Gogh's "Starry
Night." At M51's north edge is a small satellite galaxy, NGC
5195, that is tidally disrupting M 51 and is likely responsible
for the beautiful spiral structure and a high star formation
rate. At the center of M51 is a modest supermassive black hole similar to the
one at the center of our Galaxy. M51 seems to be a
gravitational partner with M63 six degrees to the south-
Up in Canes Venatici's northwest corner, the nucleus of the
spiral galaxy M106 (NGC 4258) contains a water "megamaser" (a
powerful natural microwave laser) from gas in orbit around an
active supermassive black hole that is devouring its
surroundings. The velocity of the maser from its maximum
Doppler shift coupled to the motion across the line of sight
yields a distance of 23.5 million light years, accurate to a
couple tenths, making the galaxy a foundation stone in the
cosmic distance ladder from which we derive the expansion and
acceleration rates of the Universe.
With galaxies in mind, drop back to Coma Berenices. At the
constellation's southern edge lies the spillover of the vast
Virgo Cluster of galaxies.
With a distance of around 55 million light years, it's the
nearest large cluster, splashing some 15 degrees across the sky.
We are closely related to it as part of the Virgo supercluster.
Of its 16 Messier galaxies (the most famous the giant
elliptical M87 with its relativistic jet), six fall into Coma
Berenices: M84, 88, 91, 98, 99, and 100, all spirals of one sort
or another. Of the six, M100 is notable for its role in the
Hubble Space Telescope's Key Project to use its Cepheid variables to help
establish the distance scale of the Universe.
There are enough Virgo cluster galaxies within the bounds of
Coma Berenices that it is sometimes called the Virgo-Coma
cluster. That can be confusing, as near the constellation's
northeastern corner we find a separate Coma Berenices galaxy
cluster that lies 300 million light years away, another 10 of so
times farther than M51. Two huge ellipticals dominate it, NGC
4874 and 4889. In the 1930s, Fritz Zwicky measured the radial
(line of sight) velocities of several of its members relative to
the cluster's average recession speed. He found
that the individual galaxies were orbiting within the cluster
much faster than expected given the members' estimated masses,
and surmised that some kind of pervasive "dark matter" must pervade the
system. While part of it is hot intergalactic gas, the rest
remains mysterious. The existence of the stuff is bolstered by
galaxy rotation speeds (derived from such beauties as edge-on
NGC 4565 in Coma), gravitational lensing, and the cosmic microwave
background. Dark matter constitutes a quarter the mass-
energy of the Universe and we still do not know what it is,
Coma's legacy extending far beyond her beautiful hair.
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 August 2014 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.