Capricornus is one of the oldest recognized constellations, dating back at least as far as the ancient Sumerians. Despite its proximity to the ecliptic, the celestial equator, and the center of the Milky Way, Capricornus is actually one of the smaller and fainter zodiacal constellations and has few exciting targets for amateur astronomers.
Mythology of Capricornus
The earliest mentions of the constellation Capricornus date back over four thousand years, and are from the Early Bronze Age. It was also depicted on Babylonian star charts before 1000 BC. With such a long heritage, it is surprising that even the earliest depictions of the constellation show it as a goat-fish hybrid – an image it retained through various mythologies over the millennia.
Despite its proximity to much brighter constellations, Capricornus’ importance in ancient times came not from its visual splendor, but rather its location in the night sky. It once marked the location of the Winter Solstice, a distinction which, due to the precession of the equinoxes, is now held by the constellation of Sagittarius.
In its earliest forms, Capricornus was thought to represent the Sumerian god Enki (later, “Ea”), who was often depicted as a goat-fish chimera. Indeed, early star charts even explicitly named the constellation, “The goat – fish”.
Despite the likeness, the representation used by the ancient Greeks is less clear, and there are a number of mythological goats the constellation could represent.
Foremost among these is Pan, a goat-headed God that saved himself from a monster by diving into a river and growing the tail of a fish. It is also possible that Capricornus represented Amalthea, the goat whose milk nourished the infant god, Zeus, after his father, Cronos, attempted to kill him.
The Stars of Capricornus
Despite being one of the fainter constellations in the zodiac, Capricornus is still relatively easy to identify. It lies adjacent to one of the brightest in the night sky, Sagittarius, which marks the center of the Milky Way galaxy.
The brighter stars of Capricornus also form a distinct triangle which, it must be said, doesn’t much resemble a goat with the tail of a fish.
The brightest star in the constellation is Delta Capricorni (δ Cap), otherwise known as “Deneb Algedi”. This literally translates as “The tail of the goat”. No prizes for guessing at which end of the constellation it can be found!
With an apparent magnitude of only 2.9, Deneb Algedi is not the most impressive of sights. It is, however, a white giant star roughly 40 light years away from us, and were it not for the intervening distance, it would appear to outshine our own Sun by almost a factor of ten.
The star also fluctuates in brightness over the course of a day or so – a sign of its hidden nature as a Beta Lyrae variable. This means that, rather than being a lone star, δ Capricorni is actually a close, binary pair of stars that orbit each other along our line-of-sight. From our perspective, the stars occasionally obscure each other, causing the fluctuation in total brightness of the system.
Alpha and Beta Capricorni
At the far vertex of the triangular Capricornus “goat” lie Alpha and Beta Capricorni. Both of these are double-star pairings visible to the naked eye, though the systems are very different from one another.
Alpha Capricorni shares part of its name with Deneb Algedi, though as “Deneb” translates as “tail” (and because goats generally only have one of these – fish-like or not), you might correctly guess that the common name is “Algedi”, meaning “The kid”.
The Alpha stars are both yellow giants, and they are VERY widely spaced. The separation is over 500 light years, in fact, so the two stars are not gravitationally bound to each other, but are just a coincidental alignment from our perspective.
On the other hand, the Beta Capricorni stars (the brighter of which is known as “Dabih”) are a true, gravitationally-bound, binary system. They are much closer to each other, astronomically speaking, but the separation is still around 21,000 times the separation of the Earth and our Sun.
It is also interesting to note that, at 330 light years from us, the Beta Capricorni system is closer to us than the Alpha Capricorni stars are to each other.
Deep Sky Objects in Capricornus
Messier 30 (NGC 7099) is a relatively bright globular cluster that is around 93 light years across and 27,000 light years from Earth.
Through a pair of binoculars, the cluster will appear as a hazy patch around four arc-minutes across, while an eight-inch or larger telescope will increase the size of the cluster and begin to resolve individual, faint stars.
An interesting feature of M30 is its increasing brightness towards the center. This is because the cluster has undergone a process known as core collapse, and the innermost stars are now packed so close together that the region has one of the highest stellar densities in the entire Milky Way galaxy.
Messier 30 is also in a retrograde orbit (i.e.: it circles the galaxy in the opposite direction to that of the stars that comprise the galaxy itself). This probably indicates that the cluster is likely to have originated from outside the Milky Way, rather than being made by the same processes as the rest of our galaxy.
The most likely scenario is that, early in its history, a small galaxy was gravitationally captured by the larger Milky Way, and entered a retrograde orbit. Gradually, the stars of that smaller galaxy died or were dispersed across the Milky Way.
The stars in M30 are likely to be the remaining, ancient remnant core region of that galaxy. Indeed, analysis of the stars that comprise M30 shows the globular cluster to be almost 13 billion years old.