# Keynote lectures/Astrophysics

(Redirected from Astrophysics)
A bubbling cauldron of star birth is highlighted in this new image from NASA's Spitzer Space Telescope. Credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA.

Astrophysics at its simplest is the application of laboratory physics, i.e., physics demonstrated in a laboratory and described with logical laws, to natural astronomical entities. This is done to understand these astronomical entities, their origin, history, and current constitution.

Many of the elementary concepts in physics are introduced to students at or before the secondary level so this resource begins there.

As more of the elementary concepts are introduced and applied to natural astronomical entities, the level of description approaches that of an introductory college level course with details included that are sometimes left out of a more traditional course.

To describe some of the more challenging events that are observed by astronomers, concepts from theoretical physics are modeled to help in the interpretation. This laps into research and allows the presentation of fairly recent results from the scholarly literature.

The natural entities are those observed by astronomers, but the interpretations often require additional trips to the laboratory here on Earth to extend traditional physics.

Stars, for example, are quite large. Putting one in a laboratory for examination has not happened. But, by using computer simulation and creative miniatures, a star can be represented.

## Astronomy

Def. "the mass of the Sun" is called the astronomical unit of mass.[1]

Notation: let the symbol ${\displaystyle M_{\odot }}$ indicate the solar mass.

Astronomical observations do not necessarily need the number of kilograms in the mass of the Sun, but rather use the Sun in proportions or ratios versus another astronomical entity, source, object, or body.

Notation: let the symbol ${\displaystyle L_{\odot }}$ indicate the solar luminosity.

The same is true for luminosity. The total or bolometric luminosity of the Sun is the sum of a spectral distribution from radio through gamma rays. Such a distribution may be compared to other luminous astronomical entities, sources, or objects. Differences in spectral distributions may be used to characterize stars. Each sum also is characteristic. The sum is subject to distance. The farther away a star is the smaller the sum of its total spectral distribution. To compare such a sum to the Sun a standard distance of 10 parsecs is used.

Usually, observational astronomy uses radiation to obtain information about astronomical entites, sources, and objects. The most prevalent observations use optical or visual astronomy.

The electromagnetic spectrum. The red line indicates the room temperature thermal energy. Credit: Opensource Handbook of Nanoscience and Nanotechnology.
This diagram illustrates a special version of a "black body" (instrument), used for defining the luminous intensity unit, before its current scientific International Standard (SI) definition. 1=Radiating cavity 2=Crucible 3=Solidifying platinum (2046 K) Credit: Lex Tollenaar.

Def. an action or process of throwing or sending out a traveling ray in a line, beam, or stream of small cross section is called radiation.

The term radiation is often used to refer to the ray itself.

Radiation comes in many forms and energies.

Notation: let various International System of Units, SI prefixes, occur before the unit of energy, the electronvolt, abbreviated as eV.

For example, PeV denotes 1015 eV.

Cosmic rays may be upwards of a ZeV (1021 eV). Ultra high energy neutrons are around an EeV (1018 eV). But, X-rays only range up to about 120 keV, while the visible (visual) range is around 2 eV.

Astronomy likely started with visual astronomy. Visual refers to that portion of the electromagnetic spectrum called the visible spectrum. Probing the sky with additional portions of this spectrum is difficult as the atmosphere absorbs over many portions.

This has produced fields of observational astronomy based on some portions of the electromagnetic spectrum:

Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body.[2][3][4][5]

Rotational superradiance[6] is associated with the acceleration or motion of a nearby body (which supplies the energy and momentum for the effect). It is also sometimes described as the consequence of an "effective" field differential around the body (e.g. the effect of tidal forces). This allows a body with concentration of angular or linear momentum to move towards a lower energy state, even when there is no obvious classical mechanism for this to happen.

“The rotating body [black hole] produces spontaneous pair production [and] in the case when the body can absorb one of the particles, ... the other (anti)particle goes off to infinity and carries away energy and angular momentum.”[7] Such superradiance is called Zel'dovich radiation.

## Physics

Def. the study of properties and interactions of space, time, matter and energy is called physics.

Def.

1. distance between things,
2. physical extent across two or three dimensions; area, volume (sometimes for or to do something),
3. physical extent in all directions, seen as an attribute of the universe (now usually considered as a part of space-time), or a mathematical model of this,>
4. the physical area one needs within which to live or operate,
5. a (chiefly empty) area or volume with set limits or boundaries, or
6. a set of points, each of which is uniquely specified by a number (the dimensionality) of coordinates

is called a space.

Def.

1. the inevitable progression into the future with the passing of present events into the past,
2. a numerical or general indication of a length of progression,
3. a numerical indication of a particular moment, or
4. an instance or occurrence

is called time, or a time.

Def. the basic structural component of the universe that usually has mass and volume is called matter.

Def.

1. the impetus behind all motion and all activity,
2. the capacity to do work, or
3. the quantity that denotes the ability to do work and is measured in a unit dimensioned in mass × distance²/time² (ML²/T²) or the equivalent

is called energy, or an energy.

Def.

1. the quantity of unbalanced positive or negative ions in or on an object; measured in coulombs or
2. a quantum number of some subatomic particles which determines their electromagnetic interactions

is called an electric charge.

## Astronomical units

Notation: let the symbol ${\displaystyle R_{\oplus }}$ indicate the Earth's radius.

Notation: let the symbol ${\displaystyle R_{J}}$ indicate the radius of Jupiter.

Notation: let the symbol ${\displaystyle R_{\odot }}$ indicate the solar radius.

Both physics and Astronomy/Keynote lecture use units and dimensions to describe observations.

Units of Physics and Astronomy
Dimension Astronomy Symbol Physics Symbol Conversion
time 1 day d 1 second s 1 d = 86,400 s[1]
time 1 "Julian year"[8] J 1 second s 1 J = 31,557,600 s
distance 1 astronomical unit AU 1 meter m 1 AU = 149,597,870.691 km[1]
mass 1 Sun Mʘ 1 kilogram kg 1 Mʘ = 1.9891 x 1030 kg[1]
luminosity 1 Sun Lʘ 1 watt W 1 Lʘ = 3.846 x 1026 W[9]
angular distance 1 parsec pc 1 meter m 1 pc ~ 30.857 x 1012 km[1]

## Times

Many early units of time reflect the astronomical conditions surrounding the observers.

Def. a period of fourteen nights; two weeks is called a fortnight.

Def. a period of seven nights; a week is called a sennight.

Def.

1. any period of seven consecutive days,
2. a period of seven days beginning with Sunday or Monday,
3. a subdivision of the month into longer periods of work days punctuated by shorter weekend periods of days for markets, rest, or religious observation such as a sabbath, or
4. seven days after (sometimes before) a specified date

is called a week.

In physics, a key value on the time axis for collecting physical data is the starting time. Incandescents reach full brightness a fraction of a second after being switched on.

## Distances

Distance along a path is compared in this diagram with displacement. Credit: .

Def. the amount of space between two points, usually geographical points, usually (but not necessarily) measured along a straight line is called a distance.

Distance (or farness) is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, or an estimation based on other criteria (e.g. "two counties over"). In mathematics, a distance function or metric is a generalization of the concept of physical distance. A metric is a function that behaves according to a specific set of rules, and provides a concrete way of describing what it means for elements of some space to be "close to" or "far away from" each other.

Def.

1. a series of interconnected rings or links usually made of metal,
2. a series of interconnected links of known length, used as a measuring device,
3. a long measuring tape,
4. a unit of length equal to 22 yards. The length of a Gunter's surveying chain. The length of a cricket pitch. Equal to 20.12 metres. Equal to 4 rods. Equal to 100 links.,
5. a totally ordered set, especially a totally ordered subset of a poset,
6. iron links bolted to the side of a vessel to bold the dead-eyes connected with the shrouds; also, the channels, or
7. the warp threads of a web

is called a chain.

Def. a unit of length equal to 220 yards or exactly 201.168 meters, now only used in measuring distances in horse racing is called a furlong.

Def.

1. a trench cut in the soil, as when plowed in order to plant a crop or
2. any trench, channel, or groove, as in wood or metal

is called a furrow.

Def. the distance that a person can walk in one hour, commonly taken to be approximately three English miles (about five kilometers) is called a league.

${\displaystyle R_{\odot }(equatorial)=696,342km}$
${\displaystyle R_{J}(equatorial)=71,492km}$
${\displaystyle R_{S}(equatorial)=60,268km}$
${\displaystyle R_{U}(equatorial)=25,559km}$
${\displaystyle R_{N}(equatorial)=24,764km}$

Then,

${\displaystyle R_{J}(equatorial)=F_{J}*R_{\odot }(equatorial),}$
${\displaystyle R_{S}(equatorial)=F_{S}*R_{\odot }(equatorial),}$
${\displaystyle R_{U}(equatorial)=F_{U}*R_{\odot }(equatorial),and}$
${\displaystyle R_{N}(equatorial)=F_{N}*R_{\odot }(equatorial).}$

## Lines of sight

This beautiful galaxy is tilted at an oblique angle on to our line of sight, giving a "birds-eye view" of the spiral structure. Credit: Hubble data: NASA, ESA, and A. Zezas (Harvard-Smithsonian Center for Astrophysics); GALEX data: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics); Spitzer data: NASA/JPL/Caltech/S. Willner (Harvard-Smithsonian Center for Astrophysics.

Def. a straight line along which an observer has a clear view is called line of sight.

In the section on 'senses' above is a demonstration of the principle of 'line of sight'; i.e., "a line from an observer's eye to a distant point toward which [the observer] is looking"[10]. In the image on the left of rain beneath a dark cloud, there is a highway with a vehicle on it. The vehicle is further away from the observer than the right turn onto a side road. Is the blue sky behind the dark cloud? Is the line of trees in the background further away than the dark cloud? Many objects in this image and the others can be layered relative to the observer (some are closer by inspection than others). These layers or strata are strata along the line of sight. The principle of line of sight can be used to make deductions about the relative locations (or positions) of objects from the observer's perspective.

By observing many of the wandering lights in the night sky, an occasional occultation of the light of one astronomical object may occur by the intervention of another along a closer astronomical stratum. On April 25, 1838, an occultation of Mercury by the Moon occurred when Mercury was visible to the unaided eye after sunset.[11] An occultation of Venus by the Moon occurred "on the afternoon of October 14", 1874.[11] An earlier such occultation "occurred on May 23, 1587, and is thus recorded by [Tycho Brahe] in his Historia Celestis"[11]. "Thomas Street, in his Astronomia Carolina (A.D. 1661), mentions three occultations by Venus, being two occasions when the planet covered Regulus, and once when there was an occultation of Mars by Venus."[11] "[Thomas Street] describes [the occultation of Mars by Venus] as follows: "1590,. Oct. 2nd, 16h. 24s. Michael Mœstlin observed ♂ eclipsed by ♀.""[11]

## Coordinates

Cartesian coordinate system with a circle of radius 2 centered at the origin marked in red. The equation of a circle is (x - a)2 + (y - b)2 = r2 where a and b are the coordinates of the center (a, b) and r is the radius. Credit: 345Kai.
By choosing an equal day/night position among the fixed objects in the night sky, the observer can measure equatorial coordinates: declination (Dec) and right ascension (RA). Credit: .
Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left). Description of the relations between axial tilt (or obliquity), rotation axis, plane of orbit, celestial equator and ecliptic. Credit: .

A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length. Each reference line is called a coordinate axis or just axis of the system, and the point where they meet is its origin, usually at ordered pair (0,0). The coordinates can also be defined as the positions of the perpendicular projections of the point onto the two axes, expressed as signed distances from the origin.

The observations require precise measurement and adaptations to the movements of the Earth, especially when and where, for a time, an object or entity is available.

With the creation of a geographical grid, an observer needs to be able to fix a point in the sky. From many observations within a period of stability, an observer notices that patterns of visual objects or entities in the night sky repeat. Further, a choice is available: is the Earth moving or are the star patterns moving? Depending on latitude, the observer may have noticed that the days vary in length and the pattern of variation repeats after some number of days and nights. By choosing an equal day/night position among the fixed objects in the night sky, the observer can measure equatorial coordinates: declination (Dec) and right ascension (RA).

Once these can be determined, the apparent absolute positions of objects or entities are available in a communicable form. The repeat pattern of (day/night)s allows the observer to calculate the RA and Dec at any point during the cycle for a new object, or approximations are made using RA and Dec for recognized objects.

Independent of the choice made (Earth moves or not), the pattern of objects is the same for days or nights of the repeating length once a year. The vernal equinox is a day/night of equal length and the same pattern of objects in the night sky. The autumnal equinox is the other equal length day/night with its own pattern of objects in the night sky.

The projection of the Earth's equator and poles of rotation, or if the observer hasn't concluded as yet that it's the Earth that's rotating, the circulating pattern of stars in ever smaller circles heading in specific directions, is the celestial sphere.

## Emptiness

The universe within 1 billion light-years (307 Mpc) of Earth is shown to contain the local superclusters, galaxy filaments and voids. Credit: Richard Powell.

In set theory, emptiness is symbolized by the empty set: a set that contains no elements.

Def. the state of being devoid of content; containing nothing is called empty.

Free space, a perfect vacuum ais expressed in the classical physics model. Vacuum state is a perfect vacuum based on the quantum mechanical model. In mathematical physics, the homogeneous equation may correspond to a physical theory formulated in empty space are disambiguations for "empty space".

In astronomy, voids are the empty spaces between filaments (the largest-scale structures in the Universe), which contain very few, or no, galaxies. ... Voids located in high-density environments are smaller than voids situated in low-density spaces of the universe.[12]

## Forces

Def. a physical quantity that denotes ability to push, pull, twist or accelerate a body which is measured in a unit dimensioned in mass × distance/time² (ML/T²): SI: newton (N); CGS: dyne (dyn) is called force.

Def. a force associated with nuclear decay is called the weak nuclear force.

Def. a fundamental force that is associated with the strong bonds is called the strong nuclear force.

The key values to determine in both force and energy are ${\displaystyle {\frac {L}{T^{2}}}}$ and ${\displaystyle {\frac {L^{2}}{T^{2}}}}$. Force (F) x distance (L) = energy (E), ${\displaystyle {\frac {L}{T^{2}}}\times L={\frac {L^{2}}{T^{2}}}}$. Force and energy are related to distance and time using proportionality constants.

### Sources of the forces

One theoretical source of a force is the presence of mass.

Def. the basic structural component of the universe [that] usually has mass and volume is called matter.

In physics, mass, more specifically inertial mass, can be defined as a quantitative measure of an object's resistance to the change of its speed. In addition to this, gravitational mass can be described as a measure of magnitude of the gravitational force which is

1. exerted by an object (active gravitational mass), or
2. experienced by an object (passive gravitational force)

when interacting with a second object. The SI unit of mass is the kilogram (kg).

Newton's second law of motion is that ${\displaystyle F=ma}$, where ${\displaystyle F}$ is the force applied, ${\displaystyle m}$ is the mass of the object receiving the force, and ${\displaystyle a}$ is the acceleration observed for the astronomical object. The newton is therefore:[13]

${\displaystyle {1~{\rm {N}}=1~{\rm {kg}}{\frac {\rm {m}}{{\rm {s}}^{2}}}}}$

where:

N: newton
kg: kilogram
m: metre
s: second.
 Every point mass attracts every single other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between them:[14] ${\displaystyle F=G{\frac {m_{1}m_{2}}{r^{2}}}\ }$, where: F is the force between the masses, G is the gravitational constant, m1 is the first mass, m2 is the second mass, and r is the distance between the centers of the masses. The diagram shows two masses attracting one another. Credit: .

In the International System of Units (SI) units, F is measured in newtons (N), m1 and m2 in kilograms (kg), r in meters (m), and the constant G is approximately equal to 6.674×1011
N m2 kg−2
.[15]

Another theoretical source of a force is charge.

Coulomb's law states that the electrostatic force ${\displaystyle F}$ experienced by a charge, ${\displaystyle q}$ at position ${\displaystyle r_{q}}$, in the vicinity of another charge, ${\displaystyle Q}$ at position ${\displaystyle r_{Q}}$, in vacuum is equal to:

${\displaystyle F={qQ \over 4\pi \varepsilon _{0}}{1 \over {r^{2}}},}$

where ${\displaystyle \varepsilon _{0}}$ is the electric constant or the permittivity of free space and ${\displaystyle r}$ is the distance between the two charges.

Coulomb's constant is

${\displaystyle k_{e}=1/(4\pi \varepsilon _{0}\varepsilon _{r}),}$

where the constant ${\displaystyle \varepsilon _{0}}$ is in SI units of C2 m−2 N−1.

For reality, ${\displaystyle \varepsilon _{r}}$ is the relative (dimensionless) permittivity of the substance in which the charges may exist.

## Densities

${\displaystyle \rho _{\odot }=1.408kg/m^{3}}$
${\displaystyle \rho _{J}=1.326g/cm^{3}}$
${\displaystyle \rho _{S}=0.687g/cm^{3}}$
${\displaystyle \rho _{U}=1.27g/cm^{3}}$
${\displaystyle \rho _{N}=1.638g/cm^{3}}$
${\displaystyle \rho _{H_{2}}(0^{\circ }C,101.325kPa)=0.08988g/L}$

## Energies

Def. a quantity that denotes the ability to do work and is measured in a unit dimensioned in mass × distance²/time² (ML²/T²) or the equivalent is called energy.

The energy ${\displaystyle E}$ for the system in the section about forces is

${\displaystyle E=F\cdot D,}$

where ${\displaystyle D}$ is the displacement.

## Fields

Surface magnetic field of Tau Scorpii is reconstructed by means of Zeeman–Doppler imaging. Credit: Pascalou petit.

Def. "[a] region affected by a particular force" is called a field.

Def. a region of space around a charged particle, or between two voltages; it exerts a force on charged objects in its vicinity is called an electric field.

Def. a condition in the space around a magnet or electric current in which there is a detectable magnetic force and two magnetic poles are present is called a magnetic field.

Def. The fundamental force of attraction that exists between all particles with mass in the universe is called gravitation.

## Temperatures

The photograph of various lamps illustrates the effect of color temperature differences (left to right): (1) Compact Fluorescent: General Electric, 13 watt, 6500 K (2) Incandescent: Sylvania 60-Watt Extra Soft White (3) Compact Fluorescent: Bright Effects, 15 watts, 2644 K, and (4) Compact Fluorescent: Sylvania, 14 watts, 3000 K. Credit: Ramjar.

Laboratory conditions are often expressed in terms of standard temperature and pressure.

Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.

In chemistry, IUPAC established standard temperature and pressure (informally abbreviated as STP) as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm, 1 bar),[16] An unofficial, but commonly used standard is standard ambient temperature and pressure (SATP) as a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm). The STP and the SATP should not be confused with the standard state commonly used in thermodynamic evaluations of the Gibbs free energy of a reaction.

"Standard conditions for gases: Temperature, 273.15 K [...] and pressure of 105 pascals. The previous standard absolute pressure of 1 atm (equivalent to 1.01325 × 105 Pa) was changed to 100 kPa in 1982. IUPAC recommends that the former pressure should be discontinued."[16]

NIST uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 101.325 kPa (14.696 psi, 1 atm). The International Standard Metric Conditions for natural gas and similar fluids are 288.15 K (59.00 °F, 15.00 °C) and 101.325 kPa.[17]

## Measurements

A typical tape measure with both metric and US units is shown to measure two US pennies. Credit: .

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the meter, second or degree Celsius.

## Detectors

This is an image of a real X-ray detector. The instrument is called the Proportional Counter Array and it is on the Rossi X-ray Timing Explorer (RXTE) satellite. Credit: .

A visual, optical telescope itself, together with the astronomer, various detectors and accompanying computers, plus necessary hardware to move the telescope and the dome to keep the telescope aimed at a specific astronomical object or entity are often all housed inside an observatory.

Detectors such as the X-ray detector at right collect individual X-rays (photons of X-ray light), count them, discern the energy or wavelength, or how fast they are detected. The detector and telescope system can be designed to yield temporal, spatial, or spectral information.

A technique called wavelength dispersive X-ray spectroscopy (WDS), is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. The wavelength of the impinging X-ray and the crystal's lattice spacings are related by Bragg's law where the detector counts only [X]-rays of a single wavelength. Many elements emit or fluoresce specific wavelengths of X-rays which in turn allow their identification.

## Clocks

This is a sundial from Ai Khanoum, Afghanistan. Credit: Musee Guimet, World Imaging.
This image shows another sundial from Ai Khanoum, Afghanistan. Credit: Musee Guimet, World Imaging.
This chart shows the increasing accuracy of NIST (formerly NBS) atomic clocks. Credit: National Institutes of Standards and Technology (NIST), USA.
The FOCS 1 is a continuous cold caesium fountain atomic clock in Switzerland. Credit: .

The image at right shows a sun dial from Ai Khanoum, Afghanistan, dated to the 3rd century BCE, ~2300 b2k. The image at left is also from Ai Khanoum, Afghanistan, showing its workings.

"[T]he earliest known sundial [is] from an Egyptian burial dated in the fifteenth century B.C. Sometimes called a shadow clock, or an L-board because of its shape [with] relatively crude performance."[18] A "[f]ragment of a late Egyptian sundial [from] about 3000 B.C." exists.[18]

An atomic clock is a clock device that uses an electronic transition frequency in the microwave, optical, or ultraviolet region[19] of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS.

The FOCS 1 continuous cold cesium fountain atomic clock started operating in 2004 at an uncertainty of one second in 30 million years. The clock is in Switzerland.

## Motion calibrators

POA CALFOS is the improved Post Operational Archive version of the Faint Object Spectrograph (FOS) calibration pipeline ... The current version corrects for image motion problems that have led to significant wavelength scale uncertainties in the FOS data archive. The improvements in the calibration enhance the scientific value of the data in the FOS archive, making it a more homogeneous and reliable resource.

## Computers

This image is of the large astrolabe made by Gualterus Arsenius in 1569. Credit: David Monniaux.

The astrolabe was effectively an analog calculator capable of working out several different kinds of problems in spherical astronomy.

Some form of an "astrolabe" may have been in use by the third millennium BC.[20]

Def. a programmable electronic device that performs mathematical calculations and logical operations, especially one that can process, store and retrieve large amounts of data very quickly; now especially, a small one for personal or home use employed for manipulating text or graphics, accessing the Internet, or playing games or media is called a computer.

A computer is a general purpose device that can be programmed to carry out a finite set of arithmetic or logical operations. Since a sequence of operations can be readily changed, the computer can solve more than one kind of problem.

## Fusions

Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy. Fusion is the process that powers active stars, the hydrogen bomb and some experimental devices examining fusion power for electrical generation. The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy, while the fusion of nuclei heavier than iron absorbs energy. The opposite is true for the reverse process, nuclear fission. This means that fusion generally occurs for lighter elements only, and likewise, that fission normally occurs only for heavier elements. There are extreme [[astrophysical events that can lead to short periods of fusion with heavier nuclei. This is the process that gives rise to nucleosynthesis, the creation of the heavy elements during events such as supernovas.

## Fission

In nuclear physics and nuclear chemistry, nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of a particle splits into smaller parts (lighter nuclei). The fission process often produces free neutrons and photons (in the form of gamma rays), and releases a very large amount of energy, even by the energetic standards of radioactive decay.

The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissile isotopes.[21][22] Most fissions are binary fissions (producing two charged fragments), but occasionally (2 to 4 times per 1000 events), three positively charged fragments are produced, in a ternary fission. The smallest of these fragments in ternary processes ranges in size from a proton to an argon nucleus.

## Planetary physics

Def. a process in which fragments of material (spall) are ejected from a body due to impact or stress is called spallation.

In planetary physics, spallation describes meteoritic impacts on a planetary surface and the effects of a stellar wind on a planetary atmosphere.

## Atomic physics

Atomic astrophysics is concerned with performing atomic physics calculations that will be useful to astronomers and using atomic data to interpret astronomical observations. Atomic physics plays a key role in astrophysics as astronomers' only information about a particular object comes through the light that it emits, and this light arises through atomic transitions.

"[B]oth evolved and main sequence stars" have abundance anomalies which "involve most chemical elements ... [These] abundances can vary by many orders of magnitude."[23] For "nonmagnetic stars with 6000 < Teff < 10000 K", "[s]ome 20% of the stars" have abundance anomalies that "are apparently caused by chemical separation" from "[t]he competition between gravity and selective radiative acceleration".[23] This "leads to the appearance of either overabundances or underabundances."[23]

## Biophysics

"[T]he presence of an astrophysical water maser attests to the large abundance in the interstellar medium of water molecules with inversely populated rotational levels.(20-22) In biophysics, rare gas−H2O pair potentials are used to model hydrofobic interactions,(23-26) which are key to the conformational stability of proteins and nucleic acids, as well as to the stability of micelles and biological membranes.27"[24]

## Theoretical astrophysics

The stream lines on this computer simulation of a supernova show the flow of matter behind the shock wave giving clues as to the origin of pulsars. Credit: Blondin and Mezzacappa.

[A]nalytical models [such as] polytropes to approximate the behaviors of a star ... and computational numerical simulations ... [give] insight into the heart of what is going on [or] can reveal the existence of phenomena and effects that would otherwise not be seen.[25][26]

At right is a computer simulation of a supernova explosion.

"Our primary scientific and computational focus is on tera- to exa-scale simulation of supernovae of both classes in the Universe."[27]

"The stream lines in this image [at right] show the two counter rotating flows that may be established below the supernova shock wave (the surface in the image) by the instability of the shock in a core collapse supernova explosion. The innermost flow accretes onto the central object, known as the proto-neutron star, spinning it up. This may be the mechanism whereby pulsars (spinning neutron stars) are born."[27]

"[T]he core collapse supernova shock wave is likely reenergized to initiate an explosion at much later times than previously anticipated. The shock wave must exit the iron core and enter the oxygen layer before shock revival can occur. In the oxygen layer, the density of the star drops off dramatically, which gives the shock less to plow through. In addition, in the oxygen layer, nuclear burning can occur, aiding the shock energetically. The delay to explosion is naturally set by the time it takes for the shock to reach the oxygen layer. The previously discovered stationary accretion shock instability (SASI) causes large-scale distortions of the shock, causing it to reach the oxygen layer sooner in certain directions, thereby precipitating the onset of explosion. In this new picture, we have obtained explosions over a range of stellar progenitors, between 10 and 20 Solar masses."[27]

Def. the branch of astronomy or physics that deals with the physical properties of celestial bodies and with the interaction between matter and radiation in celestial bodies and in interstellar space is called astrophysics.

Def. the "branch of astronomy or physics that deals with the physical properties of celestial bodies and with the interaction between matter and radiation in celestial bodies and in the space between them"[28] is called astrophysics.

Here's a theoretical definition:

Def. laboratory physics and its logical laws applied to astronomical phenomena and astronomical observations applied to those logical laws through laboratory experimentation are called astrophysics.

## Entities

This image is an aerial view of the Goddard Space Flight Center in Greenbelt, Maryland USA. Credit: NASA.

"Superfluid vacuum theory is an approach in theoretical physics and quantum mechanics where the physical vacuum is viewed as superfluid. The ultimate goal of the approach is to develop scientific models that unify quantum mechanics (describing three of the four known fundamental interactions) with gravity. This makes SVT a candidate for the theory of quantum gravity and an extension of the Standard Model. It is hoped that development of such theory would unify into a single consistent model of all fundamental interactions, and to describe all known interactions and elementary particles as different manifestations of the same entity, superfluid vacuum.

EDP Sciences is a publishing group gathering several entities:

• EDP Sciences, the publishing partner of the scientific communities;
• EDP Santé, the is the medical branch of the company;
• EDP Open, the platform for open access journals.

Based on existing knowledge accumulated through previous missions, new science questions are articulated. Missions are developed in the same way an experiment would be developed using the scientific method. In this context, Goddard does not work as an independent entity but rather as one of the 10 NASA centers working together to find answers to these scientific questions.

## Sources

Astrophysical X-ray sources are astronomical objects with physical properties which result in the emission of X-rays.

There are a number of types of astrophysical objects which emit X-rays, from galaxy clusters, through black holes in active galactic nuclei (AGN) to galactic objects such as supernova remnants, stars, and binary stars containing a white dwarf (cataclysmic variable stars and super soft X-ray sources), neutron star or black hole (X-ray binaries). Some solar system bodies emit X-rays, the most notable being the Moon, although most of the X-ray brightness of the Moon arises from reflected solar X-rays. A combination of many unresolved X-ray sources is thought to produce the observed X-ray background. The X-ray continuum can arise from bremsstrahlung, either magnetic or ordinary Coulomb, black-body radiation, synchrotron radiation, inverse Compton scattering of lower-energy photons be relativistic electrons, knock-on collisions of fast protons with atomic electrons, and atomic recombination, with or without additional electron transitions.[29]

The origin of all observed astronomical X-ray sources is in, near to, or associated with a coronal cloud or gas at coronal cloud temperatures for however long or brief a period.

## Objects

Massive astrophysical compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar space unassociated with any planetary system. Since MACHOs would not emit any light of their own, they would be very hard to detect. MACHOs may sometimes be black holes or neutron stars as well as brown dwarfs or unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs.

Radio Objects with Continuous Optical Spectra, (abbr. ROCOS, also referred to as ROCOSes) is a group of about 80 astrophysical objects characterized by optical spectra anomalously devoid of emission or absorption features, which makes it impossible to determine their distances and locations in relation to our galaxy.[30][31][32] They are considered to be a subclass of blazars, and are similar in their spectral characteristics to DC-dwarfs and single stellar-mass black holes.[33]

## Fine-structure constants

The fine-structure constant (usually denoted α is a fundamental physical constant, namely the coupling constant characterizing the strength of the electromagnetic interaction. Being a dimensionless quantity, it has constant numerical value in all systems of units.

## Continuum

"Spectrometer throughputs were determined from an extensive laboratory calibration and then were adjusted slightly based on in-flight calibration spectra of known astrophysical continuum sources (hot DA white dwarf stars)."[34]

## Emissions

This diagram illustrates part of the concept behind Bremsstrahlung electromagnetic radiation. Credit: Trex2001.

Alpha decay is characterized by the emission of an alpha particle, a 4He nucleus. The mode of this decay causes the parent nucleus to decrease by two protons and two neutrons. This type of decay follows the relation:

${\displaystyle {}_{Z}^{A}\!X\to {}_{Z-2}^{A-4}\!Y+{}_{4}^{2}\alpha }$ [35]

Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting particles such as neutrons, protons, or alpha particles. The neutron capture, even after any intermediate decay, often results in the formation of an unstable activation product. Such radioactive nuclei can exhibit half-lives ranging from small fractions of a second to many years.

Proton emission (also known as proton radioactivity) is a type of radioactive decay in which a proton is ejected from a nucleus. Proton emission can occur from high-lying excited states in a nucleus following a beta decay, in which case the process is known as beta-delayed proton emission, or can occur from the ground state (or a low-lying [nuclear isomer] isomer) of very proton-rich nuclei, in which case the process is very similar to alpha decay.

Beta decay is characterized by the emission of a neutrino and a negatron which is equivalent to an electron. This process occurs when a nucleus has an excess of neutrons with respect to protons, as compared to the stable isobar. This type of transition converts a neutron into a proton; similarly, a positron is released when a proton is converted into a neutron. These decays follows the relation:

${\displaystyle {}_{Z}^{A}\!X\to {}_{Z+1}^{A}\!Y+{\bar {\nu }}+\beta ^{-}}$
${\displaystyle {}_{Z}^{A}\!X\to {}_{Z-1}^{A}\!Y+\nu +\beta ^{+}}$ [36]

Gamma ray emission is follows the previously discussed modes of decay when the decay leaves a daughter nucleus in an excited state. This nucleus is capable of further de-excitation to a lower energy state by the release of a photon. This decay follows the relation:

${\displaystyle {}^{A}\!X^{*}\to {}^{A}\!Y+\gamma }$[37]

Generation of electromagnetic radiation can occur whenever charged particles pass within certain distances of each other without being in fixed orbits, the accelerations (or decelerations) may give off the radiation. This is partly illustrated by the diagram at right where an electron has its course altered by near passage by a positive particle. Bremsstrahlung radiation also occurs when two electrons or other similarly charged particles pass close enough to deflect, slow down, or speed up at least one of the particles.

Bremsstrahlung includes synchrotron and cyclotron radiation.

When high-energy radiation bombards materials, the excited atoms within emit characteristic "secondary" (or fluorescent) radiation.

## Backgrounds

There are several astrophysical components contributing to the sky background: these could be sets of point sources like faint asteroids, Galactic stars and far away galaxies, as well as diffuse sources like dust in the Solar System, in the Milky Way, and in the intergalactic space.

## Acoustics

"Acoustical measurements are being increasingly used in exploring the properties of matter; the interaction between sound fields and electromagnetic waves is an important part of plasma physics; and magneto-hydrodynamic wave motion is a phenomenon of growing importance in the sciences of meteorology and of astrophysics."[38]

## Accelerator physics

Beam of electrons are moving in a circle in a magnetic field (cyclotron motion). Lighting is caused by excitation of atoms of gas in a bulb. Credit: Marcin Białek.

Accelerator physics consists of two basic parts: particle acceleration and beam dynamics.[39] "Accelerator physics relates primarily to the interaction of charged particles with electromagnetic fields."[39]

Def. "[t]he interplay between particles and fields is called beam dynamics."[39] Boldening is added.

"α particles escaping the Coulomb barrier of Ra and Th nuclei" is a natural radioactive accelerator.[40] "Cosmic rays arise from galactic source accelerators."[40]

A cyclotron is a compact type of particle accelerator in which charged particles in a static magnetic field are travelling outwards from the center along a spiral path and get accelerated by radio frequency electromagnetic fields. ... Cyclotrons accelerate charged particle beams using a high frequency alternating voltage which is applied between two "D"-shaped electrodes (also called "dees"). An additional static magnetic field ${\displaystyle B}$ is applied in perpendicular direction to the electrode plane, enabling particles to re-encounter the accelerating voltage many times at the same phase. To achieve this, the voltage frequency must match the particle's cyclotron resonance frequency

${\displaystyle f={\frac {qB}{2\pi m}}}$,

with the relativistic mass m and its charge q. This frequency is given by equality of centripetal force and magnetic Lorentz force. The particles, injected near the center of the magnetic field, increase their kinetic energy only when recirculating through the gap between the electrodes; thus they travel outwards along a spiral path.

Cyclotron radiation is electromagnetic radiation emitted by moving charged particles deflected by a magnetic field. The Lorentz force on the particles acts perpendicular to both the magnetic field lines and the particles' motion through them, creating an acceleration of charged particles that causes them to emit radiation (and to spiral around the magnetic field lines). Cyclotron radiation is emitted by all charged particles travelling through magnetic fields, however, not just those in cyclotrons. Cyclotron radiation from plasma in the interstellar medium or around black holes and other astronomical phenomena is an important source of information about distant magnetic fields. The power (energy per unit time) of the emission of each electron can be calculated using:

${\displaystyle {-dE \over dt}={\sigma _{t}B^{2}V^{2} \over c\mu _{o}}}$

where E is energy, t is time, ${\displaystyle \sigma _{t}}$ is the Thomson cross section (total, not differential), B is the magnetic field strength, V is the velocity perpendicular to the magnetic field, c is the speed of light and ${\displaystyle \mu _{o}}$ is the permeability of free space.

A synchrotron is a particular type of cyclic particle accelerator originating from the cyclotron in which the guiding magnetic field (bending the particles into a closed path) is time-dependent, being synchronized to a particle beam of increasing kinetic energy. The synchrotron is one of the first accelerator concepts that enable the construction of large-scale facilities, since bending, beam focusing and acceleration can be separated into different components. Unlike in a cyclotron, synchrotrons are unable to accelerate particles from zero kinetic energy; one of the obvious reasons for this is that its closed particle path would be cut by a device that emits particles. Thus, schemes were developed to inject pre-accelerated particle beams into a synchrotron. The pre-acceleration can be realized by a chain of other accelerator structures like a linac, a microtron or another synchrotron; all of these in turn need to be fed by a particle source comprising a simple high voltage power supply, typically a Cockcroft-Walton generator.

## X-rays

High-mass X-ray binaries (HMXBs) are composed of OB supergiant companion stars and compact objects, usually neutron stars (NS) or black holes (BH).

## Blues

This image shows a beam of accelerated ions (perhaps protons or deuterons) escaping the accelerator and ionizing the surrounding air causing a blue glow. Credit: Lawrence Berkely National Laboratory.
The image shows the blue glow given off by the synchrotron beam from the National Synchrotron Light Source. Credit: NSLS, Brookhaven National Laboratory.

The image above shows a blue glow in the surrounding air from emitted cyclotron particulate radiation.

At left is an image that shows the blue glow resulting from a beam of relativistic electrons as they slow down. This deceleration produces synchrotron light out of the beam line of the National Synchrotron Light Source.

## Superluminals

Cherenkov radiation glows in the core of the Advanced Test Reactor. Credit: Matt Howard.

"The existence of superluminal energy transfer has not been established so far, and one may ask why. There is the possibility that superluminal quanta just do not exist, the vacuum speed of light being the definitive upper bound. There is another explanation, the interaction of superluminal radiation with matter is very small, the quotient of tachyonic and electric fine-structure constants being q2/e2 ≈ 1.4 x 10-11 [5], and therefore superluminal quanta are hard to detect."[41]

At right is an example of Cherenkov radiation. Cherenkov radiation (also spelled Čerenkov) is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. Cherenkov radiation is an example of medium specific superluminals.

## Electricity

"The techniques used to advance fusion research can be fruitfully applied to some basic problems in astrophysics. ... How is it that a thin, wispy plasma can conduct electricity as well as a thick, concentrated one?"[42]

## Magnetohydrodynamics

"Magnetohydrodynamics (MHD) is the macroscopic theory of electrically conducting fluids, providing a powerful and practical theoretical framework for describing both laboratory and astrophysical plasmas."[43]

"Magnetic field generation by dynamo action is often studied within the theoretical framework of magnetohydrodynamics (MHD)."[44]

## Epicyclic frequency

In astrophysics, particularly the study of accretion disks, the epicyclic frequency is the frequency at which a radially displaced fluid parcel will oscillate. It can be referred to as a "Rayleigh discriminant". When considering an astrophysical disc with differential rotation ${\displaystyle \Omega }$, the epicyclic frequency ${\displaystyle \kappa }$ is given by

${\displaystyle \kappa ^{2}\equiv {\frac {2\Omega }{R}}{\frac {d}{dR}}(R^{2}\Omega ),}$

where R is the radial co-ordinate[45]. This quantity can be used to examine the 'boundaries' of an accretion disc - when ${\displaystyle \kappa ^{2}}$ becomes negative then small perturbations to the (assumed circular) orbit of a fluid parcel will become unstable, and the disc will develop an 'edge' at that point. For example, around a Schwarzchild black hole, the Innermost Stable Circular Orbit (ISCO) occurs at 3x the event horizon - at ${\displaystyle 6GM/c^{2}}$. For a Keplerian disk, ${\displaystyle \kappa =\Omega }$.

## Classical mechanics

"In collisions at the low relative energies of the interstellar medium, the de Brogue wavelength of a hydrogen atom exceeds the width of a typical molecular potential well, and a break with classical mechanics is indicated."[46]

## Chemical physics

"The detection of interstellar formaldehyde provides important information about the chemical physics of our galaxy. We now know that polyatomic molecules containing at least two atoms other than hydrogen can form in the interstellar medium."[47] "H2CO is the first organic polyatomic molecule ever detected in the interstellar medium".[47]

"Over the past 30 years, radioastronomy has revealed a rich variety of molecular species in the interstellar medium of our galaxy and even others."[48]

## Atmospheric physics

"The atmospheric flow [of a hot Jupiter] is characterized by a super-rotating equatorial jet, transonic wind speeds, and eastward advection of heat away from the dayside. [There is] a dynamically induced temperature inversion ("stratosphere") on the planetary dayside and ... temperatures at the planetary limb [which] differ systematically from local radiative equilibrium values, a potential source of bias for transit spectroscopic interpretations."[49]

## Molecular astrophysics

Molecular astrophysics concerns the study of emission from molecules in space. There are 110 currently known interstellar molecules. These molecules have large numbers of observable transitions. Lines may also be observed in absorption--for example the highly redshifted lines seen against the gravitationally lensed quasar PKS1830-211. High energy radiation, such as ultraviolet light, can break the molecular bonds which hold atoms in molecules. In general then, molecules are found in cool astrophysical environments. The most massive objects in our Galaxy are giant clouds of molecules and dust, creatively named Giant Molecular Clouds. In these clouds, and smaller versions of them, stars and planets are formed. One of the primary fields of study of molecular astrophysics then, is star and planet formation. Molecules may be found in many environments, however, from stellar atmospheres to those of planetary satellites. Most of these locations are cool, and molecular emission is most easily studied via photons emitted when the molecules make transitions between low rotational energy states. One molecule, composed of the abundant carbon and oxygen atoms, and very stable against dissociation into atoms, is carbon monoxide, CO. The wavelength of the photon emitted when the CO molecule falls from its lowest excited state to its zero energy, or ground, state is 2.6 mm, or 115 gigahertz (billion hertz). This frequency is a thousand times higher than typical FM radio frequencies. At these high frequencies, molecules in the Earth's atmosphere can block transmissions from space, and telescopes must be located in dry (water is an important atmospheric blocker), high sites. Radio telescopes must have very accurate surfaces to produce high fidelity images.

## Solar cycles

"The principal objective of this study is to invent a simple, quantitative, closed kinematical model of the solar cycle which is based as far as possible either on well-accepted physical effects or on observed facts (whether fully understood or not), in the hope that such a model might single out the most important factors for detailed study, and might suggest further directions for investigation."[50] "[T]he model describes ... the time variation of average sunspot latitudes (Spoerer's law) and the width of the eruption zone (Maunder's butterfly diagram)".[50]

### Spöerer's law

"For the greater part of the sun-spot period there is practically but one zone of spots in each hemisphere. The departure from this condition of things near or at the time of minimum, when the spots of the dying cycle are approaching the equator, and the forerunners of the new cycle are beginning to appear in high latitudes, is the only case in which the solar spots are distinctly separated into more than a single zone in each hemisphere."[51] "Spöerer's law ... involves that in a minimum year the zone about 15° should be entirely barren".[51]

"First of all there were only fifteen groups seen during the entire year [1901], north and south put together. Of these, seven were in the north, and the mean latitude for the north was 8.6°, exactly the latitude of one spot of the seven, and this very naturally, seeing that it was by far the greatest group of the year, the celebrated "eclipse group.""[51] Bold added. "Greatest group" and "eclipse group" are both relative synonyms for "dominant group".

"[T]he spot-groups have been carefully examined for cases of return, and where it appeared clear that the same group has returned a second time or more frequently, without any temporary disappearance or subsidence, such a long-continued group has been treated as an entity throughout."[52] Bold added. "It has been forgotten that, whatever the cause which produces this variation of rotation rate with latitude, the causes producing difference of rate within any given latitude are more effective still."[52]

"[T]here is a slight retardation of the rotation period from the first cycle to the second, shown by both northern and southern hemispheres."[52]

## Nuclear physics

Nuclear astrophysics is an interdisciplinary branch of physics involving close collaboration among researchers in various subfields of nuclear physics and astrophysics, with significant emphasis in areas such as stellar modeling, measurement and theoretical estimation of nuclear reaction rates, cosmology, cosmochemistry, gamma ray, optical and X-ray astronomy, and extending our knowledge about nuclear lifetimes and masses. In general terms, nuclear astrophysics aims to understand the origin of the chemical elements and the energy generation in stars.

"Plasma is the medium for magnetically or inertially-confined controlled thermonuclear fusion. A plasma of deuterium and tritium ions heated to a temperature of 108 degrees Kelvin undergoes thermonuclear burn, producing energetic helium ions and neutrons from fusion reactions."[53]

## Plasma physics

"Plasma is the fourth state of matter, consisting of electrons, ions and neutral atoms, usually at temperatures above 104 degrees Kelvin."[53] "The sun and stars are plasmas; the earth's ionosphere, Van Allen belts, magnetosphere, etc., are all plasmas. Indeed, plasma makes up much of the known matter in the universe."[53]

Computer simulation of plasma is performed partly "using fluid models" and partly "using many-particle models (meaning 103 to 106 particles) in order to obtain detailed kinetic behavior; part is done using hybrid models with both fluids and particles."[53]

## Nucleosynthesis

In astrophysics, silicon burning is a very brief[54] sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8–11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the main sequence on the Hertzsprung-Russell diagram. It follows the previous stages of hydrogen, helium (the triple-alpha process), carbon, neon and oxygen burning processes. Silicon burning begins when gravitational contraction raises the star’s core temperature to 2.7–3.5 billion kelvins (GK). The exact temperature depends on mass. When a star has completed the silicon-burning phase, no further fusion is possible. The star catastrophically collapses and may explode in what is known as a Type II supernova.

## Condensed matter physics

"[T]he equation of state of [hot dense matter] in the infalling core of a star undergoing the collapse ... ultimately may lead to a type II supernova (Lattimer, 1981)."[55]

## Hydrostatic equilibrium

In any given layer of a star, there is a hydrostatic equilibrium between the outward thermal pressure from below and the weight of the material above pressing inward. The isotropic gravitational field compresses the star into the most compact shape possible. A rotating star in hydrostatic equilibrium is an oblate spheroid up to a certain (critical) angular velocity. An extreme example of this phenomenon is the star Vega, which has a rotation period of 12.5 hours. Consequently, Vega is about 20% fatter at the equator than at the poles. A star with an angular velocity above the critical angular velocity becomes a Jacobi (scalene) ellipsoid, and at still faster rotation it is no longer ellipsoidal but piriform or oviform, with yet other shapes beyond that, though shapes beyond scalene are not stable.[56] If the star has a massive nearby companion object then tidal forces come into play as well, distorting the star into a scalene shape when rotation alone would make it a spheroid. An example of this is Beta Lyrae. Hydrostatic equilibrium is also important for the intracluster medium, where it restricts the amount of fluid that can be present in the core of a cluster of galaxies.

## Stellar ages

Various methods and tools are involved in stellar age estimation, an attempt to identify within reasonable degrees of confidence what the age of a star is. These methods include stellar evolutionary models, membership in a given star cluster or system, fitting the star with the standard spectral and luminosity classification system, and the presence of a protoplanetary disk, among others. Nearly all of the methods of determining age require knowledge of the mass of the star, which can be known through various methods. No individual method can provide accurate results for all types of stars.[57]

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe.

## Quasars

In astronomical spectroscopy, the Gunn–Peterson trough is a feature of the spectra of quasars due to the presence of neutral hydrogen in the intergalactic medium (IGM). The trough is characterized by suppression of electromagnetic emission from the quasar at wavelengths less than that of the Lyman-alpha line at the redshift of the emitted light.[58]

In 2001, a quasar with a redshift z = 6.28[59] using data from the Sloan Digital Sky Survey, that a Gunn–Peterson trough was finally observed.

## Cygnus X-1

This X-ray image of Cygnus X-1 was taken by a balloon-borne telescope, the High Energy Replicated Optics (HERO) project. Credit: NASA.
This is a Chandra X-ray Observatory image of Cygnus X-1. Credit: NASA/CXC.

Cygnus X-1 (abbreviated Cyg X-1)[60] is a well-known galactic X-ray source.[61] Cygnus X-1 was the first X-ray source widely accepted to be a black hole candidate and it remains among the most studied astronomical objects in its class. It is now estimated to have a mass about 14.8 times the mass of the Sun[62] and has been shown to be too compact to be any known kind of normal star or other likely object besides a black hole. If so, the radius of its event horizon is probably about 26 km.[63]

This system may belong to a stellar association called Cygnus OB3, which would mean that Cygnus X-1 is about five million years old and formed from a progenitor star that had more than 40 solar masses. The majority of the star's mass was shed, most likely as a stellar wind. If this star had then exploded as a supernova, the resulting force would most likely have ejected the remnant from the system. Hence the star may have instead collapsed directly into a black hole.[64]

Measurements of the Doppler shift of the star's spectrum demonstrated the companion's presence and allowed its mass to be estimated from the orbital parameters.[65] Based on the high predicted mass of the object, they surmised that it may be a black hole as the largest possible neutron star cannot exceed three times the mass of the Sun.[66]

With further observations strengthening the evidence, by the end of 1973 the astronomical community generally conceded that Cygnus X-1 was most likely a black hole.[67] More precise measurements of Cygnus X-1 demonstrated variability down to a single millisecond. This interval is consistent with turbulence in a disk of accreted matter surrounding a black hole—the accretion disk. X-ray bursts that last for about a third of a second match the expected time frame of matter falling toward a black hole.[68]

The space-based Chandra X-ray Observatory was used to measure the spectral signature of iron atoms orbiting near the object. A rotating black hole drags the nearby space around with it—a process known as frame-dragging, which allows atoms to orbit closer to the event horizon. In the case of Cygnus X-1, none of the atoms were found orbiting closer than 160 km. Hence, if this object is a black hole, then this data shows it is not rotating to any significant degree.[69][70]

The pulsations from neutron stars are caused by the neutron star's magnetic field and the no hair theorem guarantees that black holes do not have magnetic poles.

## Geography

"Neutrino astrophysics offers new perspectives on the Universe investigation: high energy neutrinos, produced by the most energetic phenomena in our Galaxy and in the Universe, carry complementary (if not exclusive) information about the cosmos with respect to photons. While the small interaction cross section of neutrinos allows them to come from the core of astrophysical objects, it is also a drawback, as their detection requires a large target mass. This is why it is convenient put huge cosmic neutrino detectors in natural locations, like deep underwater or under-ice sites."[71]

## History

"The first written account of astronomical observations south of the Antarctic Circle dates back to 1772 (Bayly & Cook 1782), when the appointed astronomer and navy officer William Bayly made astrometric measurements aboard the ships “Discovery” and “Resolution”. On their voyage, led by Captain James Cook, they circumnavigated Antarctica, crossing the Antarctic Circle three times from 1772 to 1775."[72]

## Mathematics

"[T]he generally invariant line element

${\displaystyle ds^{2}=g_{\mu \nu }dx^{\mu }dx^{\nu }}$

[contains] the spacetime metric tensor ${\displaystyle g_{\mu \nu }(x^{\rho }),\mu ,\nu ,\rho =0,1,2,3,}$ [which] plays a dual role: on the one hand it determines the spacetime geometry, on the other it represents the (ten components of the) gravitational potential, and is thus a dynamical variable."[73]

## Sciences

"The results of strong-field, perturbative calculations of gravitational radiation emitted when finite size, axisymmetric matter distributions fall into a much more massive Schwarzschild black hole are presented. Wave amplitudes and total wave energy outputs for the lowest multipole moments and, in some cases, frequency spectra are calculated. The results are presented in both graphical and tabular form to facilitate the testing of fully relativistic 2 + 1 hydrodynamical codes."[74]

## Technology

This is the Visible and Infrared Survey Telescope for Astronomy. Credit: EDO.

The VISTA (Visible and Infrared Survey Telescope for Astronomy) is a reflecting telescope with a 4.1 metre mirror, located at the Paranal Observatory in Chile.

The scientific goals of the VISTA surveys, which started in 2010, include many of the most exciting problems in astrophysics today, ranging from the nature of dark energy to the threat of near-Earth asteroids.[75]

## Applied physics

Applied physics is a general term for physics which is intended for a particular technological or practical use.[76] It is usually considered as a bridge or a connection between "pure" physics and engineering.[77]

"A number of on-going astrophysical and atmospheric programs are aimed at spectroscopic exploration of the terahertz (THz) frequency range. There is an urgent need here for low-noise mixers for heterodyne receivers."[78]

## Hypotheses

1. The astrophysics of electromagnetism on the scale of galaxies and galaxy clusters is more influential than gravity.
2. A control group for astrophysics may contain those measurables obtained in astronomy that are directly relatable to those of laboratory physics, or physics standards.

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