# Stars

While natural objects in the sky, especially at night, may be sensed by sight, sound, smell, taste, or touch (vibration), many have been seen from the light they emit, absorb, reflect, transmit, or fluoresce.
Some of the natural emitters are stars.

## Theoretical stars

A star is a massive, luminous sphere of plasma held together by gravity. This is a traditional definition of a star. The term "luminous" relates to light, specifically visible light, as it is perceived by the human eye.
Def.
    1.a: "any natural luminous body visible in the sky [especially] at night",
    1.b: "a self-luminous gaseous celestial body of great mass whose shape is [usually] spheroidal and whose size may be as small as the earth or larger than the earth's orbit".
is called a star.
Def. "any object forming on a dynamical timescale, by gravitational instability", is called a star.
Def. a star that exists alone, is secluded or isolated from other stars, a reclusive or hermitary star, is called a solitary star.
Def. a separate, distinct, or individual star from others in a group is called a single star.
A solitary star differs from a single star in that the former exists alone, secluded or isolated from other stars. For example, Psi$^2$ Aquarii (93 Aquarii) is a solitary star. Radial velocity measurements have not yet revealed the presence of planets orbiting it.
18 Scorpii is another solitary star.
Def. a star that shares a barycenter with one or more astronomical substellar objects is called a unary star.
A unary star contrasts with a binary star, trinary (three stars), and a multiple star. It is not necessarily alone like the solitary star.
Def. a star moving faster than 65 km/s to 100 km/s relative to the average motion of the stars in the Sun's neighbourhood is called a high-velocity star.
Def. a high-velocity star moving through space with an abnormally high velocity relative to the surrounding interstellar medium is called a runaway star.
Def. a star whose elliptical orbit takes it well outside the plane of its galaxy at steep angles is called a halo star.
Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models.

## Luminescences

Def. "the natural medium emanating from the sun and other very hot sources (now recognised as electromagnetic radiation with a wavelength of 400-750 nm), within which vision is possible" is called light.
Def. "to shine light on something" is called illuminate.
Def. "emitting light" is called luminous.
Def. any "emission of light that cannot be attributed merely to the temperature of the emitting body" is called luminescence.
Def.
* "the state of being luminous or a luminous object",
* "the ratio of luminous flux to radiant flux at the same wavelength",
* "the rate at which a star radiates energy in all directions"
is called luminosity.

## Sun

Def. the "star at the center of our solar system" is called the Sun.
Def. a "luminous celestial body, made up of plasma (particularly hydrogen and helium) and having a spherical shape" is called a star.
After the above definition for a star is the comment, "Depending on context the sun may or may not be included."

## Astrognosy

Stellar astrognosy, or perhaps stellagnosy as Latin for "star" is stella, deals with the materials of stars and their general exterior and interior constitution.
At right is a diagram of the internal rotation in the Sun, showing differential rotation in the outer convective region and almost uniform rotation in the central radiative region. The transition between these regions is called the tachocline.
Until the advent of helioseismology, the study of wave oscillations in the Sun, very little was known about the internal rotation of the Sun. The differential profile of the surface was thought to extend into the solar interior as rotating cylinders of constant angular momentum. Through helioseismology this is now known not to be the case and the rotation profile of the Sun has been found. On the surface the Sun rotates slowly at the poles and quickly at the equator. This profile extends on roughly radial lines through the solar convection zone to the interior. At the tachocline the rotation abruptly changes to solid body rotation in the solar radiation zone.

## Radiative zones

From the experimentally derived diagram at the upper right using helioseismology, the apparent radiative zone begins at about 0.64 R$_ʘ$ and continues inward.

## Tachoclines

With reference to the above helioseismology diagram, the tachocline extends outward from the radiative zone to at most 0.70 R$_ʘ$.

## Convection zones

The zone or spherical shell between the tachocline and the photosphere appears to consist of two shells: an inner apparent convective sphere and an outer shell beneath the photosphere of which the photosphere may be a part.

## Photospheres

Def. a "visible surface layer of a star, and especially that of a sun" is called a photosphere.
"When we speak of the surface of the Sun, we normally mean the photosphere." "[T]he photosphere may be thought of as the imaginary surface from which the solar light that we see appears to be emitted. The diameter quoted for the Sun usually refers to the diameter of the photosphere." The photosphere emits visual, or visible, radiation.

## Chromospheres

The chromosphere (literally, "sphere of color") is the second of the three main layers in the Sun's atmosphere and is roughly 2,000 kilometers deep. It sits just above the photosphere and just below the solar transition region.
The density of the chromosphere is very small, it being only 10$^{−4}$ times that of the photosphere, the layer just below it, and 10$^{−8}$ times that of the atmosphere of Earth. This makes the chromosphere normally invisible and it can only be seen during a total eclipse, where its reddish color is revealed. The color hues are anywhere between pink and red. However, without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere.
The density of the chromosphere decreases with distance from the center of the sun. This decreases logarithmically from 10$^{17}$ particles per cubic centimeter, or approximately 2×10$^{−4}$ kg/m$^3$ to under 1.6×10$^{−11}$ kg/m$^3$ at the outer boundary.
The temperature begins to decrease from the inner boundary of about 6,000 K to a minimum of approximately 3,800 K, before increasing to upwards of 35,000 K at the outer boundary with the transition layer of the corona.

## Transition regions

The solar transition region is a region of the Sun's atmosphere, between the chromosphere and corona. It is visible from space using telescopes that can sense ultraviolet. It is important because it is the site of several unrelated but important transitions in the physics of the solar atmosphere:
* Below, most of the helium is not fully ionized, so that it radiates energy very effectively; above, it is fully ionized.
* Below, gas pressure and fluid dynamics dominate the motion and shape of structures; above, magnetic forces dominate the motion and shape of structures, giving rise to different simplifications of magnetohydrodynamics.

## Coronas

A coronal cloud is a cloud, or cloud-like, natural astronomical entity, composed of plasmas and usually associated with a star or other astronomical object where the temperature is such that X-rays are emitted. Small coronal clouds are above the photosphere of many different visual spectral type stars extending sometimes millions of kilometers into space.
"This energy [10$^{32}$ to 10$^{33}$ ergs] appears in the form of electromagnetic radiation over the entire spectrum from γ-rays to radio burst, in fast electrons and nuclei up to relativistic energies, in the creation of a hot coronal cloud, and in large-scale mass motions including the ejections of material from the Sun."
Although a coronal cloud is usually "filled with high-temperature plasma at temperatures of T ≈ 1–2 (MK), ... [h]ot active regions and postflare loops have plasma temperatures of T ≈ 2–40 MK."

## Stellar sciences

A division of astronomical objects between rocky objects, liquid objects, gas objects (including gas giants and stars), and plasma objects may be natural and informative. This division allows moons like Io to be viewed as rocky objects like Earth as part of planetary science rather than as a satellite around a star like Jupiter.
A further benefit is the view of gaseous objects as potential stars, failed stars, or stars radiant over peak radiation bands. These objects may be best studied as a part of stellar science.
Each of the gas objects described are by approximate radius, increasing from apparent gas dwarfs, through gas giants, to large stars with examples.
Viewing a gaseous object with multiple radiation astronomy detectors may uncover what the object looks like beneath the gas. In some instances the gaseous object turns out to have a detectable rocky interior.
Accompanying higher temperatures is usually plasma with its ionized atoms. Around a gaseous object this plasma may be a coronal cloud.
Objects with parallax measurements available are especially helpful as such measurements allow the determination of the object's radius.

## Stellar astronomy

"Our Sun itself frequently has sunspots, relatively cool dark magnetic depressions that move across its surface. HD 12545, however, exhibits the largest starspots yet observed. Doppler imaging - the use of slight changes in color caused by the rotation of the star - was used to create this false-color image. The vertical bar on the right gives a temperature scale in kelvins. This giant, binary, RS CVn star, also known as XX Trianguli, is visible with binoculars in the constellation of Triangulum. The starspot is thought to be caused by large magnetic fields that inhibit hot matter from flowing to the surface."
"A giant starspot was revealed on the K0 giant star XX Triangulum, also known as HD12545, using Doppler imaging on the Kitt Peak National Observatory's 0.9m Coude Feed telescope. This picture shows a series of views around the star".
"To observe spots on the surfaces of other stars, astronomers need to "resolve" the stellar disk. This cannot be done directly with the largest telescopes even planned, but Doppler imaging can be used to obtain a map of inhomogeneities on a star's surface. The principle is similar to medical tomography, but instead of a scanner rotating around a fixed object, a rotating star is observed with a fixed telescope. A cool starspot rotating into view at the preceding limb of the star causes a blue-shifted asymmetry in each spectral line profile. This asymmetry moves into the line center at the time of meridian passage, and turns into a red-shifted asymmetry after meridian passage. The asymmetry fades away when the spot disappears at the receding limb. The higher the latitude of the spot, the shorter will be its visible path across the projected disk of the star, or the spot may even be circumpolar if the stellar rotation axis is inclined. All this information is hidden in the variation of the spectral line profiles and is reconstructed by mathematical inversion to create a true picture of the stellar surface. For a successful application, the telescope needs to "see" the entire stellar surface during at least one stellar rotation."
"XX Triangulum is an active K0 giant binary star, approximately 10 times larger and twice as massive as the Sun. Its rotation period is 24 days, so that 24 consecutive (clear) nights of telescope time with an excellent high-resolution optical spectrograph are needed to obtain a good Doppler image. Because starspots vary on the same (short) time scales as Sunspots do (they are stable for about one stellar rotation), all the observations must be made on one rotation cycle. NSF's Kitt Peak National Observatory is one of the few facilities worldwide that offers this capability with the 0.9-m coude feed telescope."
"During the observations, XX Triangulum had its brightest magnitude since the discovery of its light variability in 1985 and showed the largest photometric amplitude so far (0.63 magnitudes in V). The large photometric amplitude was explained when the Doppler-imaging inversion algorithm also recovered a not-quite-as-large equatorial warm spot (350 K above the photospheric temperature) in the hemisphere opposite the dark spot. Strassmeier speculates that the warm spot harbors the same magnetic field as the cool spot but opposite polarity. Surprisingly, the fact that the star was brighter at a time of high spot activity is in agreement with the solar analogy despite the "unsolar" dimension of the gigantic spot."

## Radiative dynamos

A radiative dynamo is "a dynamo taking place in the radiative layers" of a star.
It is a theoretical construction to explain the magnetohydrodynamic properties of plasma occurring in the outer atmospheric layers of astronomical objects including stars. As such it is a part of theoretical stellar science and theoretical astrophysics.

## Disc dynamos

A disk generator is a DC electrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim (or ends of the cylinder), the electrical polarity depending on the direction of rotation and the orientation of the field.
Large research generators can produce hundreds of volts, and some systems have multiple generators in series to produce an even larger voltage. They are unusual in that they can source tremendous electric current, some more than a million amperes, because the homopolar generator can be made to have very low internal resistance.
Then, in reverse, more than a million amperes as a current between the rim of a disc and the center creates a potential difference and rotates an electrically conductive disc in a plane perpendicular to form a uniform magnetic field.
"Since cosmical clouds of ionized gas are generally magnetized, their motion produces induced electric fields [..] For example the motion of the magnetized interplanetary plasma produces electric fields that are essential for the production of aurora and magnetic storms".
The "rotation of a conductor in a magnetic field produces an electric field in the system at rest. This phenomenon is well known from laboratory experiments and is usually called 'homopolar ' or 'unipolar' induction."

## Stellar active regions

A stellar active region is "[a] localized, transient volume of [a stellar] atmosphere in which plages, [star]spots, faculae, flares, etc., may be observed. Active regions are the result of enhanced magnetic fields; they are bipolar and may be complex if the region contains two or more bipolar groups."
A stellar active region on a star's surface can form a bright spot which intensifies and grows. An active region may have a coronal portion.
Most stellar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars are commonly called starspots and both light and dark spots have been measured.

## Starspots

Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness. Observed starspots are in general much larger than those on the Sun, up to about 30 % of the stellar surface may be covered, corresponding to sizes 100 times greater than those on the Sun.
The distribution of starspots across the stellar surface varies analogous to the solar case, but differs for different types of stars, e.g., depending on whether the star is a binary or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle. Some stars have longer cycles, possibly analogous to the Maunder minima for the Sun.

## Carbon stars

The accretion disk can become thermally stable in systems with high mass-transfer rates (Ṁ). Such systems are called nova-like (NL) stars, because they lack outbursts characteristic of dwarf novae.
"A bright star [in the image at left] is surrounded by a tenuous shell of gas in this unusual image from the NASA/ESA Hubble Space Telescope. U Camelopardalis, or U Cam for short, is a star nearing the end of its life. As it begins to run low on fuel, it is becoming unstable. Every few thousand years, it coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse. The gas ejected in the star’s latest eruption is clearly visible in this picture as a faint bubble of gas surrounding the star."
"U Cam is an example of a carbon star. This is a rare type of star whose atmosphere contains more carbon than oxygen. Due to its low surface gravity, typically as much as half of the total mass of a carbon star may be lost by way of powerful stellar winds."
"Located in the constellation of Camelopardalis (The Giraffe), near the North Celestial Pole, U Cam itself is actually much smaller than it appears in Hubble’s picture. In fact, the star would easily fit within a single pixel at the centre of the image. Its brightness, however, is enough to overwhelm the capability of Hubble’s Advanced Camera for Surveys making the star look much bigger than it really is. The shell of gas, which is both much larger and much fainter than its parent star, is visible in intricate detail in Hubble’s portrait. While phenomena that occur at the ends of stars’ lives are often quite irregular and unstable (see for example Hubble’s images of Eta Carinae, potw1208a), the shell of gas expelled from U Cam is almost perfectly spherical."
"The image was produced with the High Resolution Channel of the Advanced Camera for Surveys [using the 606 nm and 814 nm filters]."

## Degenerate stars

EG 5 is a yellow degenerate. EG 5 is another designation for Van Maanen's star.
Van Maanen's star (van Maanen 2) is a white dwarf star. Out of the white dwarfs known, it is the third closest to the Sun, after Sirius B and Procyon B, in that order, and the closest known solitary white dwarf.
Van Maanen's star has a radius of 9,000 ± 1,400 km. It's effective surface temperature is 6,220 ± 240 K.
Degenerate stars are white dwarfs of spectral luminosity class VII.
Some yellow degenerate stars are of white dwarf spectral type DC (which show no detectable lines) mostly below T$_{eff}$ < 10,000 K.

## F stars

"The bright southern hemisphere star RS Puppis, at the center of the image, is swaddled in a gossamer cocoon of reflective dust illuminated by the glittering star. The super star is ten times more massive than our sun and 200 times larger."
"RS Puppis rhythmically brightens and dims over a six-week cycle. It is one of the most luminous in the class of so-called Cepheid variable stars. Its average intrinsic brightness is 15,000 times greater than our sun’s luminosity."
"The nebula flickers in brightness as pulses of light from the Cepheid propagate outwards. Hubble took a series of photos of light flashes rippling across the nebula in a phenomenon known as a "light echo." Even though light travels through space fast enough to span the gap between Earth and the moon in a little over a second, the nebula is so large that reflected light can actually be photographed traversing the nebula."
"By observing the fluctuation of light in RS Puppis itself, as well as recording the faint reflections of light pulses moving across the nebula, astronomers are able to measure these light echoes and pin down a very accurate distance. The distance to RS Puppis has been narrowed down to 6,500 light-years (with a margin of error of only one percent)."

## Novas

“For many years this object [Nova Vulpecula 1670 on the right] was thought to be a nova, but the more it was studied the less it looked like an ordinary nova — or indeed any other kind of exploding star.”
Its designations in addition to Nova Vulpecula 1670 are HR 7539, CK Vulpeculae, and AAVSO 1943+27, according to SIMBAD.
“We have now probed the area with submillimetre and radio wavelengths. We have found that the surroundings of the remnant are bathed in a cool gas rich in molecules, with a very unusual chemical composition.”
"As well as APEX, the team also used the Submillimeter Array (SMA) and the Effelsberg radio telescope to discover the chemical composition and measure the ratios of different isotopes in the gas. Together, this created an extremely detailed account of the makeup of the area, which allowed an evaluation of where this material might have come from."
The "mass of the cool material was too great to be the product of a nova explosion, and in addition the isotope ratios [...] measured around Nova Vul 1670 were different to those expected from a nova."
"But if it wasn’t a nova, then what was it? The answer is a spectacular collision between two stars, more brilliant than a nova, but less so than a supernova, which produces something called a red transient. These are a very rare events in which stars explode due to a merger with another star, spewing material from the stellar interiors into space, eventually leaving behind only a faint remnant embedded in a cool environment, rich in molecules and dust. This newly recognised class of eruptive stars fits the profile of Nova Vul 1670 almost exactly."
“This kind of discovery is the most fun: something that is completely unexpected!”

## Stellar groupings

Def. two "stars that appear to be one when seen with the naked eye" is called a double star.
Def. a "star that appears as a double due to an optical illusion; in reality, the stars may be far apart from each other" is called an optical double.
Def. two and only two stars orbiting around their apparent barycenter is called a binary star.
Def. a "binary star whose components can be visually resolved" is called a visual binary.
Def. a "group of gravitationally bound stars" is called a star cluster.
Def. "a more or less irregular star cluster containing tens to thousands of stars" is called an open cluster.
Def. "a spherical star cluster containing thousands to millions of stars" is called a globular cluster.
Def. a "large group of many stars spread over a very many light-years of space [a region of greater than average stellar density]" is called a star cloud.
Def. a group of stars moving together through space is called a stellar association.
Def. an association of stars stretched out along its orbit of a galaxy is called a stellar stream.
Def. a stellar association drifting through the galaxy as a somewhat coherent assemblage is called a moving group.
Def. any "of the collections of many millions of stars ... existing as independent and coherent systems" is called a galaxy.
Def. any "galaxy, considerably smaller than the Milky Way, that has only several billions of stars" is called a dwarf galaxy.
Def. a "small group of stars that forms a visible pattern but is not an official constellation" is called an asterism.
Def. any "of the [89] officially recognized regions of the sky, including all stars" is called a constellation.

## Observatories

The Navy Optical Precision Interferometer shown at right has been used for over 20 years to create and refine star catalogs that help navigate deep space ventures and determine the location of satellite arrays used by GPS systems across the world.

## Hypotheses

* A star is any astronomical gaseous object that exhibits a photosphere.