Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties ( luminosity , density , temperature , chemical composition) of astronomical objects such as stars , galaxies , and the interstellar medium , as well as their interactions. The study of cosmology is theoretical astrophysics at its largest scale; conversely, since the energies involved in cosmology, especially the Big Bang , are thelargest known, the observations of the cosmos also serve as the laboratory for physics at its smallest scales as well.
In practice, virtually all modern astronomical research involves a substantial amount of physics. The name of a school'sdoctoral program ("Astrophysics" or "Astronomy") in many places like the United States often has to do more with the department'shistory than with the contents of the programs.
Although astronomy is as old as recorded history, it was long separated from the study of physics. In the Aristotelian worldview, the celestial pertained to perfection—bodies in the skybeing perfect spheres moving in perfectly circular orbits—while the earthly pertained to imperfection; these two realmswere seen as unrelated.
For centuries, the apparently common-sense view that the Sun and other planets went round the Earth went unquestioned, until Nicolaus Copernicus suggested in the 16th century that the Earth and all theother planets in the Solar System orbited the Sun. Galileo Galilei made quantitative measurements central to physics, but inastronomy his observation didn't have astrophysical significance.
The availability of accurate observational data led to research into theoretical explanations for the observed behavior. Atfirst, only ad-hoc rules were discovered, such as Kepler'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 lawsthat rule the dynamics of objects on earth rules the motion of planets and the moon. Celestial mechanics , the application of Newtonian gravity 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 Principia , maritime navigation was transformed. Starting around 1670, the entire world was measured usingessentially modern latitude instruments and the best available clocks . The needs of navigation provided a drive for progressively more accurate astronomicalobservations and instruments, providing a background for ever more available data for scientists.
At the end of the 19th century it was discovered that, when decomposingthe 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 thespectra of gases, specific lines corresponding to unique chemicalelements . In this way it was proved that the chemical elements found in the Sun (chiefly hydrogen ) were also found on Earth. Indeed, the element helium was firstdiscovered in the spectrum of the sun and only later on earth, hence its name.During the 20th century , spectrometry (the study of these spectral lines) advanced, particularly as a result of the advent of quantum physics that was necessary to understand the astronomical andexperimental observations.
Most astrophysical processes cannot be reproduced in laboratories on Earth. However, there is a huge variety of astronomicalobjects visible all over the electromagnetic spectrum. The study of these objects through passive collection of data is the goalof observational astrophysics.
The equipment and techniques required to study an astrophysical phenomenon can vary widely. Many astrophysical phenomena thatare of current interest can only be studied by using very advanced technology and were simply not known until very recently.
The majority of astrophysical observations are made using the electromagnetic spectrum .
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, butgravitational waves are extremely difficult to detect. Neutrino observatories havealso been built, primarily to study our Sun. Cosmic rays consisting of veryhigh 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 changefaster than this cannot readily be observed. However, historical data on some objects is available spanning centuries ormillenia. On the other hand, radio observations may look at events on a millisecond timescale ( millisecond pulsars ) orcombine years of data ( pulsar deceleration studies). The information obtained from these different timescales is verydifferent.
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 toour understanding of other stars.
The topic of how stars change, or stellar evolution , is oftenmodelled 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 stellarobject, from birth to destruction. The material composition of the astronomical objects can often be examined using:
Main article: Theoreticalastrophysics
Theoretical astrophysicists create and evaluate models to reproduce and predict observations. They use a wide variety of toolswhich include analytical models (for example, polytropes to approximate thebehaviors of a star ) and computational numerical simulations .
A few examples of this process:
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