Astrophysics is the branch of
astronomythat deals with the physicsof the universe, including the physical properties ( luminosity, density, temperature, and chemical composition) of celestial objects such as stars, galaxies, and the interstellar medium, as well as their interactions. The study of cosmology is theoretical astrophysics at scales much larger than the size of particular gravitationally-bound objects in the universe.
Because astrophysics is a very broad subject, "astrophysicists" typically apply many disciplines of physics, including
mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics. In practice, modern astronomical research involves a substantial amount of physics. The name of a university's department ("astrophysics" or "astronomy") often has to do more with the department's history than with the contents of the programs. Astrophysics can be studied at the bachelors, masters, and Ph.D. levels in aerospace engineering, physics, or astronomy departments at many universities.
Although astronomy is as ancient as recorded history itself, it was long separated from the study of physics. In the
Aristotelian worldview, the celestial world tended towards perfection—bodies in the sky seemed to be perfect spheres moving in perfectly circular orbits—while the earthly world seemed destined to imperfection; these two realms were not seen as related. Aristarchus of Samos(c. 310–250 BC) first put forward the notion that the motions of the celestial bodies could be explained by assuming that the Earthand all the other planets in the Solar Systemorbited the Sun. Unfortunately, in the geocentric world of the time, Aristarchus' heliocentric theorywas deemed outlandish and heretical, and for centuries, the apparently common-sense view that the Sun and other planets went round the Earth nearly went unquestioned until the development of Copernican heliocentrismin the 16th century AD. This was due to the dominance of the geocentric modeldeveloped by Ptolemy(c. 83-161 AD), an Hellenized astronomer from Roman Egypt, in his " Almagest" treatise.
The only known supporter of Aristarchus was
Seleucus of Seleucia, a Babylonian astronomer who is said to have proved heliocentrism through reasoningin the 2nd century BC. This may have involved the phenomenon of tides, [ Lucio Russo, "Flussi e riflussi", Feltrinelli, Milano, 2003, ISBN 88-07-10349-4.] which he correctly theorized to be caused by attraction to the Moonand notes that the height of the tides depends on the Moon's position relative to the Sun. [ Bartel Leendert van der Waerden(1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy", "Annals of the New York Academy of Sciences" 500 (1), 525–545  .] Alternatively, he may have determined the constants of a geometric model for the heliocentric theory and developed methods to compute planetary positions using this model, possibly using early trigonometric methods that were available in his time, much like Copernicus. [ Bartel Leendert van der Waerden(1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy", "Annals of the New York Academy of Sciences" 500 (1), 525–545 [527-529] .] Some have also interpreted the planetary models developed by Aryabhata(476-550), an Indian astronomer, [B. L. van der Waerden (1970), "Das heliozentrische System in der griechischen,persischen und indischen Astronomie," Naturforschenden Gesellschaft in Zürich, Zürich: Kommissionsverlag Leeman AG. (cf. Noel Swerdlow (June 1973), "Review: A Lost Monument of Indian Astronomy", "Isis" 64 (2), p. 239-243.)
B. L. van der Waerden (1987), "The heliocentric system in Greek, Persian, and Indian astronomy", in "From deferent to equant: a volume of studies in the history of science in the ancient and medieval near east in honor of E. S. Kennedy", "
New York Academy of Sciences" 500, p. 525-546. (cf. Dennis Duke (2005), "The Equant in India: The Mathematical Basis of Ancient Indian Planetary Models", "Archive for History of Exact Sciences" 59, p. 563–576.).] [Thurston, Hugh (1994), "Early Astronomy", Springer-Verlag, New York. ISBN 0-387-94107-X, p. 188: quote|"Not only did Aryabhata believe that the earth rotates, but there are glimmerings in his system (and other similar systems) of a possible underlying theory in which the earth (and the planets) orbits the sun, rather than the sun orbiting the earth. The evidence is that the basic planetary periods are relative to the sun."] [ Lucio Russo(2004), "The Forgotten Revolution: How Science Was Born in 300 BC and Why It Had To Be Reborn", Springer, Berlin, ISBN 978-3-540-20396-4. (cf. Dennis Duke (2005), "The Equant in India: The Mathematical Basis of Ancient Indian Planetary Models", "Archive for History of Exact Sciences" 59, p. 563–576.)] and Albumasar (787-886), a Persian astronomer, to be heliocentric models. [ Bartel Leendert van der Waerden(1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy", "Annals of the New York Academy of Sciences" 500 (1), 525–545 [534-537] .]
In the 9th century AD, the Persian physicist and astronomer,
Ja'far Muhammad ibn Mūsā ibn Shākir, hypothesized that the heavenly bodies and celestial spheresare subject to the same laws of physics as Earth, unlike the ancients who believed that the celestial spheres followed their own set of physical laws different from that of Earth. [Harvard reference |last=Saliba |first=George |authorlink=George Saliba |year=1994a |title=Early Arabic Critique of Ptolemaic Cosmology: A Ninth-Century Text on the Motion of the Celestial Spheres |journal=Journal for the History of Astronomy |volume=25 |pages=115-141  ] He also proposed that there is a force of attraction between "heavenly bodies", [citation|first=K. A.|last=Waheed|year=1978|title=Islam and The Origins of Modern Science|page=27|publisher=Islamic Publication Ltd., Lahore] vaguely foreshadowing the law of gravity. [Harvard reference |last=Briffault |first=Robert |authorlink=Robert Briffault |year=1938 |title=The Making of Humanity |page=191]
In the early 11th century,
Ibn al-Haytham(Alhazen) wrote the "Maqala fi daw al-qamar" ("On the Light of the Moon") some time before 1021. This was the first successful attempt at combining mathematical astronomy with physics, and the earliest attempt at applying the experimental method to astronomy and astrophysics. He disproved the universally held opinion that the moonreflects sunlightlike a mirrorand correctly concluded that it "emits light from those portions of its surface which the sun's light strikes." In order to prove that "light is emitted from every point of the moon's illuminated surface," he built an "ingenious experimental device." Ibn al-Haytham had "formulated a clear conception of the relationship between an ideal mathematical model and the complex of observable phenomena; in particular, he was the first to make a systematic use of the method of varying the experimental conditions in a constant and uniform manner, in an experiment showing that the intensityof the light-spot formed by the projection of the moonlightthrough two small apertures onto a screen diminishes constantly as one of the apertures is gradually blocked up."citation|first=G. J.|last=Toomer|title=Review: "Ibn al-Haythams Weg zur Physik" by Matthias Schramm|journal=Isis|volume=55|issue=4|date=December 1964|pages=463–465 [463–4] |doi=10.1086/349914]
In the 14th century,
Ibn al-Shatirproduced the first model of lunar motion which matched physical observations, and which was later used by Copernicus. George Saliba(2007), [http://youtube.com/watch?v=GfissgPCgfM Lecture at SOAS, London - Part 4/7] and [http://youtube.com/watch?v=0VMBRAd6YBU Lecture at SOAS, London - Part 5/7] ] In the 13th to 15th centuries, Tusi and Ali Kuşçuprovided the earliest empirical evidence for the Earth's rotation, using the phenomena of comets to refute Ptolemy's claim that a stationery Earth can be determined through observation. Kuşçu further rejected Aristotelian physicsand natural philosophy, allowing astronomy and physics to become empirical and mathematical instead of philosophical. In the early 16th century, the debate on the Earth's motion was continued by Al-Birjandi(d. 1528), who in his analysis of what might occur if the Earth were rotating, develops a hypothesis similar to Galileo Galilei's notion of "circular inertia", which he described in the following observational test: [Harvard reference |last=Ragep |first=F. Jamil |year=2001a |title=Tusi and Copernicus: The Earth's Motion in Context |journal=Science in Context |volume=14 |issue=1-2 |pages=145–163 |publisher= Cambridge University Press] [Harvard reference |last=Ragep |first=F. Jamil |year=2001b |title=Freeing Astronomy from Philosophy: An Aspect of Islamic Influence on Science |journal=Osiris, 2nd Series |volume=16 |issue=Science in Theistic Contexts: Cognitive Dimensions |pages=49-64 & 66-71 ]
After heliocentrism was revived by
Nicolaus Copernicusin the 16th century, Galileo Galileidiscovered the four brightest moons of Jupiterin 1609, and documented their orbits about that planet, which contradicted the geocentric dogma of the Catholic Churchof his time, and escaped serious punishment only by maintaining that his astronomy was a work of mathematics, not of natural philosophy (physics), and therefore purely abstract.
The availability of accurate observational data (mainly from the observatory of
Tycho Brahe) led to research into theoretical explanations for the observed behavior. At first, only empiricalrules were discovered, such as Kepler's laws of planetary motion, discovered at the start of the 17th century. Later that century, Isaac Newtonbridged the gap between Kepler's laws and Galileo's dynamics, discovering that the same laws that rule the dynamics of objects on Earth rule the motion of planets and the moon. Celestial mechanics, the application of Newtonian gravityand Newton's laws to explain Kepler's laws of planetary motion, was the first unification of astronomy and physics.
After Isaac Newton published his book, "
Philosophiae Naturalis Principia Mathematica", maritime navigationwas transformed. Starting around 1670, the entire world was measured using essentially modern latitudeinstruments and the best available clocks. The needs of navigation provided a drive for progressively more accurate astronomical observations and instruments, providing a background for ever more available data for scientists.
At the end of the 19th century, it was discovered that, when decomposing the light from the Sun, a multitude of
spectral lines were observed (regions where there was less or no light). Experiments with hot gases showed that the same lines could be observed in the spectra of gases, specific lines corresponding to unique chemical elements. In this way it was proved that the chemical elements found in the Sun (chiefly hydrogen) were also found on Earth. Indeed, the element heliumwas first discovered in the spectrum of the Sun and only later on Earth, hence its name. During the 20th century, spectroscopy(the study of these spectral lines) advanced, particularly as a result of the advent of quantum physicsthat was necessary to understand the astronomical and experimental observations. [ [http://www.arxiv.org/abs/astro-ph/9711066 Frontiers of Astrophysics: Workshop Summary] , H. Falcke, P. L. Biermann]
Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure
Timeline of white dwarfs, neutron stars, and supernovae
Timeline of black hole physics
Timeline of gravitational physics and relativity
The majority of astrophysical observations are made using the
Radio astronomystudies radiation with a wavelengthgreater than a few millimeters. Radio wavesare usually emitted by cold objects, including interstellar gasand dust clouds. The cosmic microwave background radiationis the redshifted light from the Big Bang. Pulsars were first detected at microwavefrequencies. The study of these waves requires very large radio telescopes.
Infraredastronomy studies radiation with a wavelength that is too long to be visible but shorter than radio waves. Infrared observations are usually made with telescopes similar to the usual opticaltelescopes. Objects colder than stars (such as planets) are normally studied at infrared frequencies.
Optical astronomyis the oldest kind of astronomy. Telescopes paired with a charge-coupled deviceor spectroscopes are the most common instruments used. The Earth's atmosphereinterferes somewhat with optical observations, so adaptive opticsand space telescopes are used to obtain the highest possible image quality. In this range, stars are highly visible, and many chemical spectra can be observed to study the chemical composition of stars, galaxies and nebulae.
Ultraviolet, X-ray and gamma ray astronomystudy very energetic processes such as binary pulsars, black holes, magnetars, and many others. These kinds of radiation do not penetrate the Earth's atmosphere well. There are two possibilities to observe this part of the electromagnetic spectrum— space-based telescopes and ground-based imaging air Cherenkov telescopes (IACT). Observatories of the first type are RXTE, the Chandra X-ray Observatoryand the Compton Gamma Ray Observatory. IACTs are, for example, the High Energy Stereoscopic System(H.E.S.S.) and the MAGIC telescope.
Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances. A few
gravitational waveobservatories have been constructed, but gravitational waves are extremely difficult to detect. Neutrinoobservatories have also been built, primarily to study our Sun. Cosmic rays consisting of very high energy particles can be observed hitting the Earth's atmosphere.
Observations can also vary in their time scale. Most optical observations take minutes to hours, so phenomena that change faster than this cannot readily be observed. However, historical data on some objects is available spanning centuries or
millennia. On the other hand, radio observations may look at events on a millisecond timescale ( millisecond pulsars) or combine years of data (pulsar deceleration studies). The information obtained from these different timescales is very different.
The study of our own Sun has a special place in observational astrophysics. Due to the tremendous distance of all other stars, the Sun can be observed in a kind of detail unparalleled by any other star. Our understanding of our own sun serves as a guide to our understanding of other stars.
The topic of how stars change, or
stellar evolution, is often modeled by placing the varieties of star types in their respective positions on the Hertzsprung-Russell diagram, which can be viewed as representing the state of a stellar object, from birth to destruction. The material composition of the astronomical objects can often be examined using:
Neutrino astronomy(future prospects)
Theoretical astrophysicists use a wide variety of tools which include analytical models (for example,
polytropes to approximate the behaviors of a star) and computational numerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen. [H. Roth, "A Slowly Contracting or Expanding Fluid Sphere and its Stability", "Phys. Rev." (39, p;525–529, 1932)] [A.S. Eddington, "Internal Constitution of the Stars"]
Theorists in astrophysics endeavor to create theoretical models and figure out the observational consequences of those models. This helps allow observers to look for data that can refute a model or help in choosing between several alternate or conflicting models.
Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.
Topics studied by theoretical astrophysicists include:
stellar dynamicsand evolution; galaxy formation; large-scale structureof matterin the Universe; origin of cosmic rays; general relativityand physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole("astro") physicsand the study of gravitational waves.
Some widely accepted and studied theories and models in astrophysics, now included in the
Lambda-CDM modelare the Big Bang, Cosmic inflation, dark matter, dark energyand fundamental theories of physics.
* Astronomical observatories
* Important publications in astrophysics
List of astrophysicists
* [http://www.intellecttoday.com/ Scientific Discussion: Astrophysics]
* [http://www.aip.org/history/cosmology/index.htm Cosmic Journey: A History of Scientific Cosmology] from the American Institute of Physics
* [http://www.vega.org.uk/video/subseries/16 Prof. Sir Harry Kroto, NL] , Astrophysical Chemistry Lecture Series. 8 Freeview Lectures provided by the Vega Science Trust.
* [http://home.slac.stanford.edu/ppap.html Stanford Linear Accelerator Center, Stanford, California]
* [http://www.iasfbo.inaf.it Institute for Space Astrophysics and Cosmic Physics]
* [http://www.journals.uchicago.edu/ApJ/ Astrophysical Journal]
* [http://www.aanda.org/ Astronomy and Astrophysics, a European Journal]
* [http://master.obspm.fr/ Master of Science in Astronomy and Astrophysics]
Wikimedia Foundation. 2010.