WATERING THE GOAT
Given the glories of the deep southern Zodiac, where we admire the
striking figures of Scorpius and Sagittarius, as well as their settings in the
Milky Way, the next two, Capricornus (the
"Water Goat") and Aquarius (the Waterman,
with Pisces the trio making the "wet
quarter"), tend to be paid little heed. To the naked eye, we might
be drawn to the star Algedi (Alpha
Capricorni), which at first seems to be a naked-eye double rather
like Mizar and Alcor. Alas, the two fourth magnitude stars
are not related, but merely lie along the same line of sight, the
fainter (Alpha-1 because of its westerly position) over six times
farther than brighter Alpha-2, which is 109 light years away. Over
in Aquarius, our eye falls naturally to one of the best-known of
the sky's many informal "asterisms," the Y-shaped Water Jar. From
there, look to the east to find brighter Alpha and Beta Aquarii (Sadalmelik and Sadalsuud). Both are highly luminous
"yellow" supergiants two to three thousand times the true
brightness of the Sun that were probably
born more or less together as hot class B stars within a now-
dissipated, huge, loose "OB association."
Each of these constellations holds a wondrous object to bring
into focus. Capricornus is home to the somewhat neglected globular
cluster M 30, which lives in the shadow of Sagittarius's M 22 (not
to mention the other showpieces of the sky,
M 13, Omega Centauri,
and 47 Tucanae). Globular clusters are the oldest objects known in
the Galaxy. The brightest of them contain over a million stars
packed into a tiny volume only a few light years across. Because
the processes of stellar evolution had not yet had time to create
heavy elements from the Big Bang's hydrogen and helium, Galactic
globulars are metal-poor. M 30 falls right in line, indeed is one
of the more metal-deficient, various estimates placing the iron
abundance anywhere between 0.008 and 0.012 times that of the Sun.
Its age estimated at 11.9 billion years, the cluster was born only
1.7 billion years after the Big Bang itself. At a distance approaching
30,000 light years and some five times less luminous than Omega
Centauri, the cluster still contains well over 100,000 stars, half
of them packed into a volume only 20 light years across, just five
times the distance between here and Alpha
Centauri. Imagine a view of a thousand first magnitude stars!
Globular clusters are held tightly together by their own gravity.
As the more massive stars fall to the center, the less massive are
moved to the outside and can "evaporate" away while the remaining
cluster "collapses." Like M 15 in Pegasus, M 30 appears about as centrally concentrated
as it can get, something to recommend it to the telescopic
viewer.
Move now to Aquarius, and an object dear to the hearts of the
English. In 1785, the great William Herschel wrote "I shall
conclude this paper with an account of a few heavenly bodies that
from their singular appearance leave me almost in doubt where to
class them. The first precedes Nu Aquarii 5'.4 in time, and is 1'
north..." Sir William had discovered the "planetary nebulae,"
epitomized for us by NGC 7009, which appears as an elongated disk
with two projecting "ansae" that look like jug handles and that
give NGC 7009 the popular name "the Saturn Nebula." (The Ring
Nebula in Lyra was already known, but was
not placed into Herschel's new class until later.) Unlike globular
clusters, planetary nebulae are just-formed "newcomers." They are
created when bloated, dying giant stars expel their outer
envelopes, leaving behind dense carbon-oxygen cores that will
ultimately make white dwarfs. For a time the cores are hot enough
to ionize and illuminate the fleeing gases, resulting in wondrously
complex rings and shells around hot blue stars.
While the distances of planetary nebulae are generally not
well known, the Saturn Nebula seems to be about 4000 light years
away, and is illuminated by a very hot star estimated at 90,000
Celsius (compared with the Sun's 6000) that heats the surrounding
gas to a temperature of 10,000 degrees C. The nebula is deceptive.
Appearing almost "solid" in the telescope, it is in reality a very
thin gas with a density of just 3000 atoms (almost all hydrogen)
per cubic centimeter, which is more vacuous than the best
laboratory vacuums on Earth. The main body of the nebula has a
diameter of about half a light year, while the odd ansae are
somewhat over that distance from the central star. Expanding at
the rate of 20 kilometers per second, the nebula was formed roughly
5000 years ago, and will dissipate into the cosmic gloom in about
ten times that, carrying several tenths of a solar mass back into
interstellar space. The shells of planetary nebulae are created
when fast, hot winds from the central stars sweep up gas ejected by
slower winds from earlier evolutionary stages. NGC 7009's ansae
tell of high-speed oppositely-directed flows whose collective
origin is not understood. Such structures appear in a good
fraction of planetaries.
The circle of discovery begins to close, as Aquarius is now
home to the planet Uranus, which Herschel discovered in 1781 in the
constellation Gemini, and whose blue-
green disk (along with those of the other planets) caused Herschel
to call NGC 7009 a "planetary nebula" (meaning "disk-like"). The
ill fit between the observed position of
Uranus and that predicted under Newtonian gravitational theory
led England's John Couch Adams and France's Urbain Leverrier to
calculate the position of a purported transuranian planet, which in
turn led to the discovery of Neptune in 1846. Tightening the
circle, Neptune is slowly orbiting through our other constellation
of attention, Capricornus. It will complete its first full orbit
of the Sun since discovery in the year 2010, still among the stars
of the Water Goat.
The other large planets of the Solar System, Jupiter and
Saturn, are made mostly of hydrogen and helium. The relatively
small sizes of Uranus and Neptune (the two planets remarkably
similar to each other) indicate that they contain much less
hydrogen and much more heavy stuff, of which water is a leading
candidate, perfectly appropriate given their two current wet
constellations of residence.
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/April 2004 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.