The most important item in the astronomy alphabet is astronomy itself. It's not surprising that the word comes from the Greek for star. Apart from the Moon, the most obvious objects in the night sky are stars. In the past, star was a broad term for small heavenly bodies. A planet, for example, was a wandering star, and since a comet's tail resembles long hair, a comet was a hairy star.

Astronomy and astrology
Astronomy is the scientific study of heavenly bodies. This includes the formation, movement and composition of heavenly bodies, their origins, their interactions and the physical laws governing them, and even whether life might exist on them.

Astrology is the study of the movements of heavenly bodies in order to understand their supposed influence on human affairs. Evidence-based science has not been able to validate the claims of astrology nor propose a mechanism by which it might work.

Since most of the sciences have names ending in -logy, astronomy is one of the odd ones out. Long ago, today's astronomy and today's astrology were united in one package called astrology. Although the separation of astronomy and astrology occurred gradually over a period of a few centuries, it was essentially complete by the end of the eighteenth century.

The astrological part of the discipline gradually fell into disrepute. Neither the outlook of the Church nor that of the Enlightenment was compatible with astrology. Both Catholic and Protestant churches saw it as sacrilegious, a kind of magic and an affront to God. And the more rational approach of the thinkers of the Enlightenment applied the rules of evidence to astrology and found it wanting.

The scientific side of the study of the heavens was gradually strengthened by the invention and improvement of the telescope, and by more accurate timing devices. Astronomy was strengthened as a science because it could more accurately predict heavenly events such as eclipses and transits. In addition, the accumulated body of observations enabled astronomers to explore the physical principles underlying what they saw in the heavens. All of this completed its split with astrology, for there was no comparable revolution in astrology's predictive success.

Aspects of astronomy
Thousands of years ago, there were severe limitations on what was observed. And even with the experience of many centuries, it wasn't until the early seventeenth century that any astronomer had made precise observations over a long period of time. This was the unique accomplishment of Tycho Brahe whose data were used by Johannes Kepler to formulate his three laws describing mathematical relationships in the Solar System. Tycho's measurements would now be called astrometry.

Today's astronomy has a number of specialist areas that make use of astrometric data. There are also observations of such diverse things as pulsars, galaxies, water on the Moon, exoplanets and their atmospheres, and the radiation left over from the infancy of the Universe. The main specialist areas relate to the application of the traditional sciences - physics, chemistry and biology - to the study of the heavens.

Astrometry is the most traditional aspect of astronomy, being the measurement of positions and movements of heavenly bodies. Star catalogs are an early form of this kind of astronomy. Today the measurement methods are more sophisticated, and they're used for such varied tasks as measuring distances in space, tracking near-Earth objects, finding distant Solar System objects, and discovering and confirming extrasolar planets.

Astrophysics uses the principles of physics to explore the nature of heavenly objects. For example, the field takes in the formation and evolution of stars, planets and galaxies. It also probes gravity, black holes, dark matter and dark energy. An astrophysical emphasis in astronomy evolved in the nineteenth century when many astronomers became more interested in understanding the heavens than in merely describing them, and had the tools to do so. Modern astrophysicists are using cutting-edge physics to try to make sense of the heavens.

Astrochemistry was eventually made possible by two major developments. The first was spectroscopy which allowed you to identify elements from the light they give off. Applying spectroscopy to heavenly objects tells us about their chemical composition, temperature and many other things. The second development was sensitive telescopes, and telescopes that detect light outside the visible range. Much of the interesting chemistry occurs in places which visible light can't penetrate. Many organic molecules (i.e., ones that contain carbon) have been detected in space by these means, and organic molecules are the building blocks of life.

Astrobiology (also known as exobiology) is the science of extraterrestrial life. Since we don't know of any life other than on Earth, this science is laying the foundations for the possible detection of life elsewhere and the protocols for studying it. One essential part of astrobiology is understanding our own planet, since it's the only example we have of life in the Universe. How might life have begun and evolved on Earth?