49 47 Tucanae
Johann Elbert Bode logged this particular object 47th
(in order of right ascension) in the Toucan in his 1801
catalog of stars, although he didn’t observe it himself.
He instead picked it up from Nicolas Louis de Lacaille’s
catalog. At magnitude 4.1, 47 Tucanae is visible to the
naked eye. Considered second only to Omega Centauri
in richness and brightness, this globular cluster is a
true gem of the far southern sky. It is also designated
NGC 104.
As amazing as this globular appears, it is outshone
by its celestial neighbor, the Small Magellanic Cloud
(see #63). The dwarf galaxy lies some 15 times more
distant than 47 Tucanae, which is roughly 13,000
light-years away. With a wide-field telescope, it is
easy to get both in the same low-power field — a truly
awesome sight!
About the same diameter in the sky as the Full Moon
(30.9'), this cluster is ideal for a small telescope with
low and moderate magnification. With larger optics,
the colors of the brightest stars become conspicuous.
This is true for most bright globular clusters.
Observers who have seen a handful of globulars
are aware that while these are all ancient spherical
aggregates of stars, they don’t all look the same. Some
have sharply concentrated cores, some are gradually
concentrated, and some are more evenly illuminated
across their width. Astronomers suspect some clusters
with dense cores have central black holes, just like
most galaxies harbor central supermassive black holes.
In 1999, astronomers targeted 47 Tucanae’s dense
core with the Hubble Space Telescope in a survey
to monitor stars, seeking a slight dip in brightness
as planets pass in front of them. They didn’t find any
here, suggesting planets must be rarer in globular
clusters than in open clusters. Researchers believe
this is due to the composition of a globular’s stars —
mostly hydrogen, and lacking in the heavier elements
necessary to birth planets.
But 47 Tucanae does have a plethora of other
interesting targets: It contains 27 fast-rotating pulsars,
the spinning remnants of dead stars. Only Terzan 5 in
Sagittarius has more, with 39. — A.G.
30 ASTRONOMY • JANUARY 2022 KFIR SIMON
48 NGC 4319 and Markarian 205
An observer’s current technology can limit the ability to answer scientific questions. Film replaced visual observ-
ing and CCD chips have now replaced film. And the Hubble Space Telescope and other modern observatories
have allowed discoveries far surpassing those achievable with the once all-important Palomar 200-inch telescope.
Halton C. Arp’s research into peculiar galaxies highlights this march of science. In the mid-20th century, he
observed with Palomar’s scope that in many cases, two galaxies appeared close to one another on the sky but
had discordant redshifts. Arp hypothesized that these conflicting redshifts arose because, although the galax-
ies truly were close, one of the two was made from material being ejected by the other.
NGC 4319 and Markarian 205 were one of Arp’s most prominent examples. Though Arp reported the two
galaxies were connected by a “luminous bridge,” redshifts put NGC 4319 at 80 million light-years away and
Markarian 205 at 1 billion — 14 times more distant! Today, Hubble shows Arp’s purported bridge is actually part
of a larger envelope surrounding the foreground galaxy.
NGC 4319 is the brighter member of Arp’s duo at magnitude 12.8. It is 3' by 2' across and classified as an
SBab barred spiral with a large central hub. The arms at the end of the bar include counter-arms, giving it a
ring-shaped appearance. Markarian 205 is considered one of the closest quasars. At magnitude 14.5, it is vis-
ible with a 10- to 12-inch scope under skies free of light pollution. This object appears starlike, as the galaxy in
which the bright quasar is embedded only became visible with Hubble. When you look at Markarian 205, the
photons your eyes are receiving were emitted at a time when simple multicellular plant life began appearing
in terrestrial sedimentary rocks during the late Precambrian.
Faint galaxies are always a challenge to find using the star-hop method. This duo is located one-third of
the way between Kappa (κ) Draconis and Polaris, forming the apex of a large right triangle with Kappa and
ANTHONY AYIOMAMITISLambda (λ) Draconis. There’s also a 5th-magnitude star located about 0.2° southeast of this galactic pair. — A.G.