By Jim Kaler

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.