Astronomy is the scientific study of the Universe and its contents.
The Wikiversity Astronomy textbook is divided into several units typical of introductory texts at the university level. These units can be presented out of order, or some may be omitted:
This is the study of constellations, asterisms, apparent diurnal and annual motions of the sun and stars, motion and appearance of the moon, lunar and solar eclipses, lunar and solar calendars, and stellar coordinate systems. Experts in this field display a solid understanding of the three-dimensional character of the solar system; the typical size and speed scales involved; the relationship between angular size, actual size, and distance; and the vocabulary needed to describe positions of astronomical objects. This unit provides the background required to understand the historical revolutions in astronomical knowledge described in the next unit.
This is the study of the history of astronomy, in particular ancient (typically cross-cultural) astronomy and its transition to the modern viewpoint. The works of Copernicus, Galileo, Kepler, and Newton are studied with an emphasis on the evolution from the geocentric to the heliocentric viewpoint. The collection study of light is introduced, both its intensity and its color content, allowing later units to rely on the crucially important concept of spectroscopy. Historical and modern telescopes are described. Experts in this field are able to discuss not only that the earth revolves around the sun, but also how we know this to be true.
This is the study of the formation, life, and death of stars. We describe the balance between the inward gravitational forces acting on a star with the outward forces that characterize that star. With this model we study the nebulae in which stars are born, main-sequence stars, red giant and supergiant stars, white dwarfs, neutron stars, black holes, supernovae, and the nebulae produced in stellar death. Experts in this field typically see the stellar life and death sequence as a recycling process in which galaxies become more deeply enriched with heavier elements.
This is the study of the largest observed systems in the universe and their contents. Discussion of systems much larger than the solar system immediately requires the introduction of the topic of dark matter, the (apparently) unaccounted for material that binds galaxies together gravitationally. The evolution of galaxies is treated and the quasar introduced. Hubble's discovery of the expansion of the universe is combined with Einstein's theory of gravity to describe the past (and predict the future) of the universe as a whole. Experts in this field juggle unconfirmed theories (Inflation), tantalizing data (accelerating expansion), and spectacular observations (relic heat from the big bang) in an attempt to produce a grand narrative of the universe.
This is the study of the small bodies and systems that orbit stars. Planets display enormous diversity in their composition, thermodynamic properties, geological activity, and atmospheres due to a number of factors we'll study here. We will study the planets and moons in our solar system in depth before moving on to the multitudes of planets discovered orbiting other stars.
In addition to these particular units of study, several overarching themes present themselves in the study of astronomy. First, that the universe is describable and understandable . Second, that astronomy as a science is driven through observation rather than experiment, and is therefore vulnerable to a different set of systematic faults than physics or chemistry. Third, that astronomy is a wonderful adventure for humankind, filled with joy and promise.
[Would recommend the original text, below, be moved to subchapters.]
Note: Contains theoretical ideas and conjectures. --Eoin O Liathain
99.9% of the visible Universe is in the form of Astrophysical plasma, the same stuff that makes up the stars fills interstellar space. Visible matter makes up only 4% of the mass / gravity budget of the Universe; the rest is in the form of Dark matter 23% and a mysterious force that is pulling the Universe apart at an ever increasing pace called Dark energy 73%. Though we cannot directly detect it, and we do not know what it is made of, thought there are competing theories (see Axions(probably the most likely, though it could be a combination of theories) Super symmetry, and Strange matter) we can detect the effects of Dark matter on ordinary matter through gravitational interactions; galaxies seem to be built around pockets of dark matter like bricks of a house. At this point, if you have done the math you probably realize that the stuff you see around, regular, non plasma, baryonic matter is extremely rare in the Universe. This matter makes up about 0.0004% of the matter in the Universe. The Universe is about 13.8 billion years old and may be finite or infinite, but has been estimated by a group of leading astophysists to be 151 light years wide, though this is debated; the most commonly accepted theory for how the Universe was created is the big bang, or more particularly the hot big bang it also fits nicely with nuclear physics and particle physics by explaining why no elements heavier than Beryllium were created during the big bang, and also explains the abundance of Deuterium (an isotope of Hydrogen with a neutron in the nucleus, accounts for approximately 1% of the Hydrogen) in the Universe, since it fuses easily into Helium 4; the reason is that the universe started off at an infinite temperature and density and expanded and cooled very quickly. --Bone Spencer 01:04, 17 May 2006 (UTC)
Stars are made mostly of hydrogen and helium and give off energy in the form of electromagnetic radiation by fusing elements into progressively heavier elements until it reaches iron (which has one of the highest binding energies) and no more energy can be obtained through nuclear fusion.
--Bone Spencer 01:04, 17 May 2006 (UTC)