Astronomy and Astrophysics

The following is a metanode of topics, terms, concepts, and ideas related to the fields of astronomy and astrophysics. It is by no means complete, but its organization will hopefully give the casual reader an idea of what the science of astronomy entails. The nodes are outlined in similar order to what one might find in a college-level, introductory textbook on astronomy, beginning with the most basic phenomenology of astronomy (what is visible to the naked eye) and a description of the tools and fields of observational astronomy. From there, we travel outwards from the Earth to cover the solar system, the stars, the Milky Way Galaxy, other galaxies and extragalactic astronomy, and finally cosmology. We have combined "astronomy" (the observational science) with "astrophysics" in a single node, since there is substantial crossover between the two in modern times, and "astronomy" is a logical place to look for astrophysical information.

Note that a metanode of famous astronomers exists separately, under astronomer.

Also note that planetary science is given very short shrift here. This is partly because it is still distinct from classical astronomy and astrophysics -- it is the only field of astronomy where we have actually visited our objects of study -- and partly because it encompasses elements of geology, chemistry, meteorology, and biology. There is currently little organized information about planetary science on e2; as more writeups on this field are added, we may consider making a separate index. Noders familiar with planetary science are urged to contact E2_Science if they are interested in organizing this.

This index is incomplete and will be for some time, so please contribute! Dead links will be noted with a (*) after the node title -- if you see one, please fill it! If you would like to have a writeup or topic added to this node, please /msg E2_Science.

Observational Astronomy

Astronomy is one of the oldest sciences, primarily because the apparent motions of objects in the heavens relate to one of our most fundamental concepts: the passage of time. Nearly every living thing on Earth is affected by celestial cycles, especially the diurnal motion of the Sun, the tides of the moon, and the seasons of the year. So it is unsurprising that our distant ancestors were curious about the sky. Furthermore, since it was (and still largely is) utterly remote from us, "those lights in the sky" produced endless speculations on their nature, and their role in our lives.

Most of the fundamentals of astronomy relate to three things: position and time, both of which are deeply linked, and brightness. To the human eye, most stars appeared to lie on a giant sphere -- The Celestial Sphere -- which rotated endlessly about our world, changing our night sky during the course of the year. Most human cultures picked out patterns on this celestial sphere (since pattern recognition is one thing humans are especially good at), and grouped these stars into constellations. The apparent motions of the Sun and planets were superimposed on this sphere, with the Sun's daily and yearly motions, and the moon's monthly apparitions having special importance. Thus we began to understand and measure such fundamental concepts as solstice and equinox, ecliptic (and of course, the Zodiac), analemma, sidereal period and synodic period, and so on. Later, as our understanding of the heavens became more extensive and sophisticated, we began to quantify things like time and position, giving rise to concepts like equatorial coordinates. And of course it was known to the ancients that stars had different apparent brightnesses, leading again to a desire to quantify this, as with magnitude.

The links listed below will lead you to several fundamental concepts in astronomy. Reading and understanding each will give you a good idea of how we understand the heavens to be organized, but will also give you a glimpse of what our remote ancestors were capable of understanding about the universe, without benefit of telescope or computer.

Telescopic Astronomy

The telescope was "invented" (according to historical record) around 1608. Since that time, the field of optical and telescopic astronomy -- astronomy done with the aid of optical technology -- has made fantastic advances. At first, substantial knowledge was gained with simple refracting telescopes which made use of lenses. Eventually, astronomers, engineers, and tinkerers realized one could use curved mirrors to increase the light gathering power of telescopes by orders of magnitude without having to create monstrously huge glass and metal structures. Thus the reflecting telescope and its many variants were born: the Newtonian telescope, the Schmidt telescope, the Cassegrain telescope, and the Dobsonian, to name a few.

Along with instrumentation to view the heavens, we began to develop means for permanently storing or otherwise quantifying our observations. Beginning with the early photographic plates of the early 19th century, we have since developed technology for performing photometry and spectroscopy of celestial objects. The links below list a few of the important ones.

Fields of Astronomy

With the incredible advances of both technology and knowledge of physics since the early 17th century have come similar advances in astronomical technology and understanding. We now know that visible light is but a small portion of the energy emitted by astronomical objects in the universe, and that observing different wavelengths of light can provide a wealth of physical information about objects in the universe. The first hints that there was more to the universe than meets the eye came when William Herschel placed thermometers in sunlight dispersed by prisms, and found temperature changes even when the thermometer lay beyond the reddest light visible. Now, we know astronomical objects can emit light from very low frequency radio waves to gamma rays so energetic that each photon has the energy of a speeding bullet.

Below are links to the various fields of astronomy: most of these fields are considered distinct, since each requires its own unique forms of telescope and detectors. Some even require satellites in space. In addition to the formal research fields, we also mention the work of amateurs in Amateur Astronomy and Astrophotography. The work of amateur astronomers and astronomy enthusiasts has always been an important part of scientific astronomy, from the discovery of variable stars and early advances in radio astronomy to rapid observations of transient events like gamma-ray bursts. The line between "amateur" astronomers and professionals has always been blurry, and is growing ever more so.

Famous Observatories

Though one can easily conduct astronomy from one's backyard or rooftop, most societies have created special places for conducting observations of the heavens. Since ancient times, many cultures consider astronomical observations sacred and vital to society, placing great importance in the construction of observatories. In modern times, scientific observatories are placed in increasingly remote locations out of necessity, as cities and towns become ever more light-polluted and crowded. And some forms of astronomy require observations in space, necessitating orbiting observatories and even robotic explorers of our solar system. Below is an incomplete list of famous observatories and satellites in the history of astronomy, many of which are still in existence today.

The Solar System and Planetary Science

The solar system is comprised of everything contained within the gravitational and heliospheric influence of The Sun. It consists of the Sun itself, the nine major planets and their moons, the asteroids, the comets, dust, gas, and everything else contained within a sphere of (roughly) one light-year in radius about the Sun.

The Sun, the closest star to Earth, is the most important part of our solar system. Because it is the most massive object in the solar system, it acts as the gravitational focus of all other matter in the solar system. It also provides essentially all of the luminous energy of the solar system, and all of the planets and other objects shine by reflecting light from the Sun to our eyes. The Sun also forms the basis for our understanding of all other stars in the universe, and it is by detailed study of the Sun that we began to understand the inner workings of stars. The Sun also provided some early indications that not all things in the heavens were truly "perfect". For example, the discovery of sunspots showed that the surface of the Sun was not unblemished, and soon other examples of "solar activity" were discovered.

The planets are also an important part of our solar system. The planets are important from a physical standpoint -- for example, while the Sun contains most of the mass and emits most of the luminous energy in our solar system, the orbiting planets carry over 99 percent of the angular momentum of the solar system. And obviously, the planets (at least a few of them) are important as known or potential abodes for life. However, the planets are also important from a philosophical standpoint. They showed our early ancestors the "stars" were not "fixed" in space, and could indeed wander about the Celestial Sphere. From this, they obtained their name planet from the Greek planetes, meaning "wanderers". Eventually, study of the planets led to a deeper understanding of how the universe works, though an understanding of gravitation.

And of course, other members of our solar system continue to have profound scientific and cultural impacts. For example, comets have been considered important phenomena -- both good and bad -- since ancient times, and they remain an important cultural and artistic influence today. The discovery of asteroids beginning in the 19th century continues to be an important scientific task, and we now know this task may have profound implications for human civilization.

Stellar Astronomy and Astrophysics

Stellar Astronomy and Astrophysics is an important (and very broad) topic. The Stars may rightly be considered the most important objects in the universe. After the big bang, it was the stars that generated all of the atoms of all of the elements more complex than helium (including most of the atoms that make up you). These elements were distributed in supernova explosions and the dusty winds of dying stars. And it is the stars that generate most of the visible light in the universe; when you look at distant galaxies, most of the light you see isn't the "galaxy" itself, but the stars it is made of. Because stars are so fundamentally important, it is worth spending more time reading about them.

Stars are like giant and complex physical laboratories, and we first spend some time talking about the physics of stellar astronomy. Below are listed several nodes on physics relevant to stellar astronomy, including: a primer on electromagnetic radiation, and nodes on radiative transfer and various emission processes important in stars; nodes on the behavior of the gas that makes up the stars, and how it behaves; and discussions of fundamental physics important in stars, like gravitation and nuclear physics. While some of these topics are somewhat advanced, browsing them should give you an idea of how complex the stars are.

After the preliminaries, we discuss what we know about the stars, and how we know it. Most important of all of these nodes are the discussions of stellar classification in stellar spectra and Hertzsprung-Russell diagram. These two topics summarize best what we know about stars, and why. From there, we spend time talking about the details of stars and stellar populations, as in Populations I, II, and III stars, the various evolutionary stages of stars, and finally, the very diverse field of variable stars and variable star research. We conclude with nodes about some of the most extreme stars known: neutron stars, black holes, and their associated phenomena -- including the (no longer so) mysterious gamma ray bursts.

Galactic and Extragalactic Astronomy

From the stars, we move on to nodes about galaxies. Galaxies are huge aggregations of matter that formed from faint ripples of density in the early universe. Since that time, the matter in galaxies has slowly collected into stars and other types of objects that make up the "modern universe", including vast clouds of gas and dust, clusters of stars, and even more exotic things like emission line nebulae and supermassive black holes.

First we begin with Galaxy, Milky Way, and The Great Debate, nodes which explain what galaxies are, and how we know this. In particular, The Great Debate is an important node to read. The fact that the "spiral nebulae" are "island universes" separate from our own Milky Way Galaxy is a relatively recent discovery in astronomy, and the history of this discovery in an important one.

From there we move on to talk about the building blocks of galaxies -- the star clusters, the nebulae, and the interstellar medium -- in some detail, before finally discussing the different kinds of galaxies, and their classification systems. Finally, we end with discussions of the physics of galaxies, the nature of active galaxies and supermassive black holes, and finally the greatest building blocks of the universe -- the galaxy clusters.


Finally, we end this metanode with a discussion of cosmology -- the study of the universe as a whole. Cosmology is perhaps the oldest of intellectual pursuits, as two of its fundamental questions are how was the universe created?, and why is it the way it is?. As scientific knowledge developed over time, our understanding of the universe expanded as well. Today, the general scientific consensus is that the universe is a "relic" of a great event -- the big bang -- which created all of the great structures we see in the universe today. The universe is filled with billions and billions of galaxies, some beyond the reach of even our largest telescopes. These galaxies are gathered together into great clusters and sheets, surrounding vast, empty voids of space. The redshifts of external galaxies are believed (again, by general consensus) to be due to the expansion of space itself -- another result of the big bang. Cosmology entails the study of these great structures and the underlying physics of matter and energy to explain how the universe came to be and why it looks the way it does. One of the fascinating things about cosmology is that it merges studies of the greatest structures in the universe with the most infinitesimal particles and fundamental physics. We do not yet know how the universe was created or how it will end, because we do not yet fully understand the most basic physics of matter and energy.

Below are a few links covering this very broad topic, beginning with a discussion of how scientists believe the universe began and why. This is followed by nodes on how we measure distances in astronomy. This is a very important topic in cosmology; the universe appears very two-dimensional to our eyes, so it isn't easy to estimate how far away things are. Finally, we end with a discussion of some of the physics of cosmology, including the expanding universe, the newly-resurrected concept of the cosmological constant, and the ultimate fate of... everything.

In the classical Liberal Arts, the ancients considered music the extension of mathematics in time, and geometry the extension of number in space. Observing the clockwork motions of the moon, planets, and other heavenly bodies, they synthesized mathematical notions in space and time into astronomy, or "the music of the spheres". This colorful attribution is often credited to Pythagoras, although the first reference in literature appears near the end of Plato's Republic. Since the classical age, physics has informed astronomy so as to form a new field, astrophysics.

Astronomy is also a song by Blue Öyster Cult, first appearing on the 1974 Secret Treaties release, and reworked (in my opinion, much improved, if shorter), for the 1988 Imaginos release. Metallica also covered BÖC's Astronomy on their 1998 Garage Inc. album. BÖC fans might wonder why Burnin' for You and Fear the Reaper get so much more airplay; Astronomy is critically acclaimed, but lacks a mainstream pop theme.

An Advance in Sid Meier's Civilization.

Perhaps the oldest of the sciences, astronomy originated as simple observation and recording of regular celestial movements. Ancient Greek students of astronomy understood that the Sun was the center of the solar system, made reasonable estimates for the sizes and distances of the Sun and the Moon, and made a close estimate of the size of the Earth.

Prerequisites: Mysticism and Mathematics.
Allows for: Navigation and Theory of Gravity.

XVII. Astronomy

The Wain upon the northern steep
   Descends and lifts away.
Oh I will sit me down and weep
   For bones in Africa.

For pay and medals, name and rank,
   Things that he has not found,
He hove the Cross to heaven and sank
   The pole-star underground.

And now he does not even see
   Signs of the nadir roll
At night over the ground where he
   Is buried with the pole.

A.E. Housman, Last Poems

Public domain: first published in 1922.

As*tron"o*my (#), n. [OE. astronomie, F. astronomie, L. astronomia, fr. Gr. , fr. astronomer; star + to distribute, regulate. See Star, and Nomad.]




Not from the stars do I my judgment pluck; And yet methinks I have astronomy. Shak.


The science which treats of the celestial bodies, of their magnitudes, motions, distances, periods of revolution, eclipses, constitution, physical condition, and of the causes of their various phenomena.


A treatise on, or text-book of, the science.

Physical astronomy. See under Physical.


© Webster 1913.

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