- Astrophysics
Astrophysics is the branch of
astronomy that deals with thephysics of theuniverse , including the physical properties (luminosity ,density ,temperature , and chemical composition) of celestial objects such asstar s, galaxies, and theinterstellar 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 andparticle 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 inaerospace engineering , physics, or astronomy departments at many universities.History
Although astronomy is as ancient as recorded history itself, it was long separated from the study of physics. In the
Aristotel ian 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 theEarth and all the otherplanet s in theSolar System orbited theSun . Unfortunately, in the geocentric world of the time, Aristarchus'heliocentric theory was 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 ofCopernican heliocentrism in the 16th century AD. This was due to the dominance of thegeocentric model developed byPtolemy (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 throughreasoning in the 2nd century BC. This may have involved the phenomenon oftide s, [Lucio Russo , "Flussi e riflussi", Feltrinelli, Milano, 2003, ISBN 88-07-10349-4.] which he correctly theorized to be caused by attraction to theMoon and 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 [527] .] 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 byAryabhata (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 andcelestial spheres are 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 [116] ] 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 withphysics , and the earliest attempt at applying the experimental method to astronomy and astrophysics. He disproved the universally held opinion that themoon reflectssunlight like amirror and correctly concluded that it "emits light from those portions of its surface which thesun 's light strikes." In order to prove that "light is emitted from every point of the moon's illuminated surface," he built an "ingeniousexperiment al 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 theintensity of the light-spot formed by the projection of themoonlight through two smallaperture s 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-Shatir produced 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 andAli Kuşçu provided the earliest empirical evidence for theEarth's rotation , using the phenomena ofcomet s to refute Ptolemy's claim that a stationery Earth can be determined through observation. Kuşçu further rejectedAristotelian physics andnatural 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 byAl-Birjandi (d. 1528), who in his analysis of what might occur if the Earth were rotating, develops a hypothesis similar toGalileo Galilei 's notion of "circularinertia ", 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 Copernicus in the 16th century,Galileo Galilei discovered the four brightest moons ofJupiter in 1609, and documented their orbits about that planet, which contradicted the geocentric dogma of theCatholic Church of his time, and escaped serious punishment only by maintaining that his astronomy was a work ofmathematic s, 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, onlyempirical rules were discovered, such asKepler's laws of planetary motion , discovered at the start of the 17th century. Later that century,Isaac Newton bridged 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 Newtoniangravity and 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 ", maritimenavigation was transformed. Starting around 1670, the entire world was measured using essentially modernlatitude instruments and the best availableclock s. 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 line s 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 uniquechemical element s. In this way it was proved that the chemical elements found in the Sun (chieflyhydrogen ) were also found on Earth. Indeed, the elementhelium was 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 ofquantum physics that 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]See also:
*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 Observational astrophysics
The majority of astrophysical observations are made using the
electromagnetic spectrum .*
Radio astronomy studies radiation with awavelength greater than a few millimeters.Radio waves are usually emitted by cold objects, includinginterstellar gas and dust clouds. Thecosmic microwave background radiation is theredshift ed light from theBig Bang .Pulsar s were first detected atmicrowave frequencies. The study of these waves requires very largeradio telescope s.
*Infrared astronomy 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 usualoptical telescopes. Objects colder than stars (such as planets) are normally studied at infrared frequencies.
*Optical astronomy is the oldest kind of astronomy. Telescopes paired with acharge-coupled device orspectroscope s are the most common instruments used. The Earth'satmosphere interferes somewhat with optical observations, soadaptive optics andspace telescope s 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 andnebula e.
*Ultraviolet , X-ray andgamma ray astronomy study very energetic processes such asbinary pulsar s,black hole s,magnetar s, 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 telescope s and ground-basedimaging air Cherenkov telescope s (IACT). Observatories of the first type areRXTE , theChandra X-ray Observatory and theCompton Gamma Ray Observatory . IACTs are, for example, theHigh 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 wave observatories have been constructed, but gravitational waves are extremely difficult to detect.Neutrino observatories have also been built, primarily to study our Sun.Cosmic ray s 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 pulsar s) 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 theHertzsprung-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:
*Spectroscopy
*Radio astronomy
*Neutrino astronomy (future prospects)Theoretical astrophysics
Theoretical astrophysicists use a wide variety of tools which include analytical models (for example,
polytrope s to approximate the behaviors of a star) andcomputation al 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 dynamics and evolution; galaxy formation;large-scale structure ofmatter in theUniverse ; origin ofcosmic ray s;general relativity andphysical cosmology , including string cosmology andastroparticle 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 forblack hole ("astro")physics and the study ofgravitational waves .Some widely accepted and studied theories and models in astrophysics, now included in the
Lambda-CDM model are the Big Bang,Cosmic inflation ,dark matter ,dark energy and fundamental theories of physics.ee also
* Astronomical observatories
* Important publications in astrophysics
*List of astrophysicists
*Nucleosynthesis
*Particle accelerator
*Astrodynamics
*Astrochemistry References
External links
* [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.