By Jim Kaler

Two great celestial triangles accent northern seasons. The summer sky gives us the eponymous Summer Triangle of three white stars -- Vega, Deneb, and Altair -- while the snappy cold of winter gives us more color, with reddish Betelgeuse contrasting against white Procyon and Sirius. Lesser known of these cross- constellation asterisms is the Great Diamond of spring, which I discovered in one of the seminal books of my astronomical life, "Introducing the Constellations" by R. H. Baker, a charming volume in which the pictures are set into a vivid blue background reminiscent of twilight. It is sobering to think that it now sits on my shelf of antique books (which includes the marvelous nineteenth century textbook by Sir John Herschel).

Touring around the Diamond counterclockwise from the north we encounter four marvelous stars that give a strong sense of stellar diversity: Cor Caroli in Canes Venatici, Arcturus in Boötes, Spica in Virgo, and Denebola in Leo. All four have things to say about the stellar condition. Start at the "top" (as northerners see it) with third magnitude Cor Caroli, a binary pair whose brighter member is also called Alpha-2 Canum Venaticorum. (The other, sixth magnitude class F0 Alpha-1, 19 seconds of arc away, is too distant to have shown significant orbital motion.) At first Alpha-1 appears to be yet another white class A (A0) hydrogen-fusing dwarf star like those of the Summer Triangle. But surprise! It is the famed prototype of the "peculiar" (class Ap) magnetic stars. Rotating smartly every 5.47 days, the star brings magnetic patches with weirdly enhanced chemical elements in and out of view, the field strength some 1500 times stronger than Earth's. Indeed the star's full spectral class is "A0pSiEuHg," showing the abundances of silicon, europium, and mercury to be greatly magnified as a result of element diffusion and separation, a problem that can give fits to astronomers trying to establish gross chemical compositions of stars.

Onward to Arcturus. Here is a classic orange, cool K giant that, having begun to die, is already fusing helium into carbon in its deep core. This magnificent "zeroth" magnitude star is the brightest in the northern hemisphere, just topping the Summer Triangle's white Vega. With a relatively high velocity and low metal content, and a motion that causes it to fall behind the Sun, the star has long been believed to be a visitor from the older portions of the Galaxy. Indeed there is even a recent suggestion that Arcturus (along with many other local stars) was brought to us through collision with a dwarf galaxy seven or so billion years ago! Equally intriguing, the star displays evidence of very weak solar-like magnetic activity, which K-type giants are not supposed to do.

South now to Spica. Among the hottest of the first magnitude stars, Spica is not one, but two, in which a luminous (13,000 solar) B1 dwarf with a mass near 11 times that if the Sun and nearing the end of its hydrogen-fusing life mutually orbits a lesser class B4 star every 4.01 days. It was long thought that the stars barely eclipsed each other, providing us with a slight few- hundredths magnitude variation. Not true. Instead, the two are so close that they raise tides in each other, so as they whirl about they present different-sized cross-sections to us. In addition, the more massive is a subtle pulsator with a period of only 0.17 days. Winds from the pair slam together to create X-rays. Spica is regularly covered, or occulted, by the Moon (quite the marvelous sight), which reveals up to three other components, making the whole thing a complex multiple star.

Back up to Denebola, another class A dwarf star, yet one nothing like Cor Caroli. At class A3 a bit cooler than the Hunting Dog's luminary, it (like Vega and Fomalhaut) is surrounded by a dusty disk, which is some kind of leftover of its formation and suggestive of planets. It also seems to be a subtle "Delta Scuti" type variable, such stars being slightly unstable lower-luminosity versions of much more obvious Cepheid variables.

The Great Diamond, which appears flat against the sky, becomes cut when we factor in stellar distances. Those of Denebola and Arcturus are similar, each about three dozen light years, which vividly shows how much brighter evolving giants are than ordinary dwarfs, even those bright ones of class A. Separated by only 23 light years, each would appear about a magnitude brighter from the other than each does from Earth. From there, however, our poor Sun would fade to mere fifth magnitude. At 97 light years, Cor Caroli is nearly three times more distant than Arcturus (explaining its relative faintness), while Spica remains first magnitude even at a respectable 260 light years, the product of its great visual brilliance (the pair shining 2100 times more brightly than our Sun, and far more when its ultraviolet light is factored in). From Spica, our Sun would be only 9th magnitude and require a telescope to see.

And what wonders yet to behold when, in Baker's words, one looks through the Great Diamond. At a distance of 265 light years -- just beyond Spica -- lies one of the great nearby open star clusters, Coma Berenices. Stretching over five degrees of sky (making it 25 light years across), with a few unrelated stars this beautiful naked-eye gathering makes an entire constellation, "Berenices Hair." Seen thorough binoculars this half-billion-year old cluster reminds one of fine lace.

Make now the giant leap far to Diamond's other side. One of the great frustrations of astronomers lies in the pervasive dust of that haunts the Milky Way and that completely hides distant galaxies behind it to create the famed "zone of avoidance." Coma Berenices' constellation, however, is in the direction perpendicular to our Galaxy's plane, and thereby contains the North Galactic Pole, toward which Galactic dust is practically absent. As a result our view can penetrate to enormous distances.

Sprawling across vast amounts of sky is the magnificent Virgo cluster of galaxies. Only 55 million light years away, it is the closest great group (with some 2000 systems), and is home to a veritable sub-listing of the Messier catalogue. Among them is the giant elliptical galaxy M 87, from which emerges a powerful jet from a central black hole that may have a mass over a billion times that of the Sun. One of the key projects of the Hubble Space Telescope was to learn Virgo's distance through the observation of Cepheid variable stars. Then leap some six times farther to visit the rich Coma Cluster of galaxies, which also contains over a thousand members and is pervaded by hot, X-ray-emitting gas. Off in the great distance lie yet more galaxies, first thousands, then millions, stretching into the billions of light years to as far as we can see, to the horizon of the visible Universe itself and back toward the time of the Big Bang, from which the Great Diamond, and all of us, have emerged.

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 May/August 2004 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.