# star

Under a star ( ancient Greek ἀστήρ, ἄστρον aster, astron and Latin aster, astrum, stella, sidus for 'star, star'; ahd. Sterno ; astronomical symbol : ✱) is understood in astronomy a massive, self-luminous celestial body of very hot gas and plasma , such as the sun . In addition, a planet in our solar system that is illuminated by the sun is commonly called a star , such as an evening star , although it is not a star like the sun.

In addition to light , a star like the sun also emits radiation in the extreme ultraviolet range (false color representation of the solar emission at 30  nm )

One of the most important findings of modern astronomy is that almost all self-luminous celestial bodies visible to the naked eye are sun-like objects that only appear point-like because of their great distance. About three quarters of the stars are part of a binary or multiple system , many have a planetary system . Stars formed together often form star clusters . Under favorable conditions, several thousand stars can be clearly distinguished. They all belong to the same galaxy as the sun, to the Milky Way , which consists of over a hundred billion stars. This galaxy, along with its neighboring galaxies, belongs to the Local Group , one of thousands upon thousands of galaxy clusters .

Stars can have different sizes, luminosities and colors - like Bellatrix as a blue giant , Algol B as a red giant , the sun and OGLE-TR-122b , a red dwarf (below, next to it the gas planets Jupiter and Saturn )

Stars arise from gas clouds - in certain areas ( H-II area ) from gaseous molecular clouds - through local strong compression in several phases. They are held together by the force of gravity of their own mass and are therefore roughly spherical in shape. While a star inside is several million degrees hot (at the core of the sun just under 16,000,000  Kelvin ), the surface temperature of most of them is between around 2,000 K and 20,000 K (in the photosphere of the sun just under 6,000 K); As exposed star cores, white dwarfs can reach temperatures of up to 100,000 K on their surface. The glowing star surface not only emits intense radiation such as light, but also a stream of charged plasma particles ( stellar wind ) far into space and thus forms an astrosphere .

Stars can differ considerably in mass and volume , as well as in luminosity and color ; these properties change in the course of the evolution of a star. An orienting classification of the stars is already possible with the two characteristics absolute brightness and spectral type . The properties of stars are also important when it comes to the question of whether or not a planet orbiting them could carry life (see habitable zone ).

## etymology

Old High German sterno , Middle High German stern [e] , Swedish stjärna stand next to differently formed Old High German sterro and Middle High German sterre , English star . Extra-Germanic are z. B. Greek astḗr , Latin stella related. The words go back to Indo-European stē̌r- "star".

## Overview

Most stars are made up of 99% hydrogen and helium in the form of hot plasma . Their radiation energy is generated in the star's interior by stellar nuclear fusion and reaches the surface through intense radiation and convection . About 90% of the stars - the main sequence stars - are like the sun in a stable equilibrium between gravitation, radiation and gas pressure, in which they remain for many millions to billions of years.

Celestial bodies in size comparison
1: Mercury < Mars < Venus < Earth
2: Earth < Neptune < Uranus < Saturn < Jupiter
3: Jupiter < Wolf 359 < Sun < Sirius
4: Sirius < Pollux < Arctur < Aldebaran
5: Aldebaran < Rigel < Antares < Betelgeuse
6: Betelgeuse < Granatstern < VV Cephei A < VY Canis Majoris

Afterwards they expand into giant stars and finally shrink into white dwarfs as they slowly cool down. These very compact final stages of stellar evolution and the even denser neutron stars are also counted as stars, although they only emit radiation because of their residual heat.

The closest and best-studied star is the Sun , the center of the solar system . In the Middle Ages it was still unknown that the sun was a “normal star”, but ancient natural philosophers already suspected that it must be hotter than a glowing stone. The sun is the only star on which structures can be clearly seen from Earth: sunspots , sun flares and solar flares .

Only a few relatively close supergiants such as Betelgeuse or Mira are visible in the most modern telescopes as disks that can reveal gross irregularities. All other stars are too far away for that; with the available optical instruments they appear as diffraction disks of point-like light sources.

In the past, the term fixed stars was used to distinguish it from tail stars ( comets ) and wandering stars ( planets ) . But the positions of stars in the sky are not fixed, but their star locations slowly shift against each other. The measurable proper motion varies in size and can be around ten arc seconds per year (10.3 ″ / a) for a comparatively close star like Barnard's Arrow Star . In ten thousand years, therefore, some of today's constellations will be significantly changed.

With the naked eye , depending on the darkness and atmospheric conditions, around 2000 to 6000 stars can be seen across the sky , but less than 1000 near a city the ranges of variation of temperatures , luminosity , mass density , volume and service life span immense value ranges. So one would call the outermost layers of red giant stars vacuum according to the criteria of earthly technology , while neutron stars can be denser than atomic nuclei ; With a mass density of 4 · 10 15  kg / m³, a spoon with 12 cm³ would weigh  about as much as the entire water in Lake Constance (48  km³ ). The very different forms of appearance of stars correspond to considerable differences in their internal structure; Turbulent exchange processes often take place between the depth-dependently structured zones. This article provides a rough overview and refers to further articles.

## Stars from the human point of view

Stars have played an important role in all cultures and have inspired human imagination. They were interpreted religiously and used to set the calendar, later also as navigation stars. In ancient times , natural philosophers imagined that the fixed stars could consist of glowing rock , because normal coal fire did not seem to be sufficient for the heat that worked at such a great distance. The fact that stars consist only of gas, on the other hand, was only recognized around 300 years ago - including through various interpretations of sunspots  - and confirmed by the spectral analysis that emerged in the 19th century . The first physically founded hypotheses about the formation of stars come from Kant and Laplace . Both assumed a primordial fog, but their postulated formation processes differed. Often, however, both theories are combined as the Kant-Laplace theory .

### Constellations and star names

Some of the constellations known in western cultures go back to the Babylonians and ancient Greece . The twelve constellations of the zodiac formed the basis of astrology . Due to the precession, however , the visible constellations today are shifted by about one sign compared to the astrological signs of the zodiac . Many of the proper names known today such as Algol , Deneb or Regulus come from Arabic and Latin .

From around 1600, astronomy used the constellations to identify the objects by name in the respective regions of the sky. A system for naming the brightest stars in a constellation, which is still widely used today, goes back to the star maps of the German astronomer Johann Bayer . The Bayer designation of a star consists of a Greek letter followed by the genitive of the Latin name of the constellation in which the star is located; for example γ Lyrae refers to the third brightest star in the constellation Lyra . A similar system was introduced by the British astronomer John Flamsteed : The Flamsteed designation of a star consists of a preceding number in ascending order of right ascension and, in turn, the genitive of the Latin name of the constellation, such as 13 Lyrae. The Flamsteed designation is often chosen when there is no Bayer designation for a star. Most stars are only identified by their number in a star catalog . The most common for this is the SAO catalog with around 250,000 stars. In book form (100 stars per page) it comprises around 2,500 pages in 4 volumes, but is also available as a database .

There are a number of companies and even some observatories that offer paying customers the option of naming stars after them. However, these names are not recognized by anyone except the registering company and the customer. The International Astronomical Union , the official body responsible for naming stars , has clearly distanced itself from this practice.

### Apparent movement of the starry sky

Since the earth revolves around itself once in the course of a day and orbits the sun once in the course of a year, the view of the sky with stars and constellations changes for the observer on earth both during the night and with the seasons .

Facing north (click for animation)

For the observer in the northern hemisphere of the earth (north of the earth's equator) the following applies: When looking north, the starry sky rotates counterclockwise around the pole star during the night . When looking to the south, the apparent star orbits run the other way around (because the observer is standing the other way around): the stars and the starry sky move clockwise from left (east) to right (west). Over the course of a year, the same movement, only 365 times slower, applies if one always looks at the sky at the same time : in the north counterclockwise, in the south from left to right. The starry sky can show very similar images - with the exception of the positions of the planets and the moon: For example, the view on October 31st at 4:00 a.m. is almost the same as on December 31st at 12:00 p.m. or on March 2nd at 8:00 p.m. This means that a time change of four hours (one sixth day) corresponds to a change in the calendar of around 60 days (one sixth year).

For the observer in the southern hemisphere of the earth (south of the earth's equator), the following applies: When looking south, the starry sky rotates clockwise around the south celestial pole. When looking north, the apparent star orbits run the other way around: The stars move counterclockwise from right (east) to left (west). Even over the course of a year, looking south, the same movement results, only more slowly, in a clockwise direction. Looking north, the apparent movement is again counterclockwise from right to left.

### Distribution of stars in the sky

The closest star to earth is the sun. The closest fixed star in the classical sense is Proxima Centauri , it is located at a distance of 4.22  light years (ly). The star that appears brightest after the Sun is Sirius with an apparent magnitude of −1.46 m , followed by around 20 stars of the first magnitude . The luminosity of Sirius, 8.6 light years away, is about 25 times stronger than that of the sun, and over a thousand times weaker than that of Deneb . All stars that can be seen with the naked eye belong to the Milky Way . They are concentrated - together with over 100 billion weaker stars, invisible to the naked eye - in a band across the night sky that marks the plane of the Milky Way.

Image of a star at high magnification (here R Leonis about 330 ly.
Away ). In addition to the unresolved image of the star, the diffraction disks of the point source can also be seen.

Because of their huge distance, stars appear only as points of light in the sky, which when viewed through the eye or telescope smear into diffraction disks. The larger the aperture , the smaller the diffraction rings (see picture). Only the two very close giant stars Betelgeuse and Mira lie with an apparent diameter of approx. 0.03 "at the limit of resolution of the Hubble space telescope and appear there as an unstructured surface.

The flickering of the stars, the scintillation , which is mostly visible when observing with the naked eye, is based on turbulence in the earth's atmosphere . It has nothing to do with the luminous properties of the stars.

Stars of the sixth magnitude can be seen with the naked eye under optimal conditions . In the earthly night sky this is a maximum of 5000, i.e. That is, around 2000 on the visible half of the sky. This number applies to completely clear air and often drops to just 300–500 stars due to industrial and urban light pollution, and even to 50–100 stars in city centers.

## Occurrence and characteristics

In the last hundred years, astronomy has increasingly resorted to methods of physics . A large part of the knowledge about stars is based on theoretical star models, the quality of which is measured by their agreement with astronomical observations. Conversely, because of the enormous variety of phenomena and the range of parameters involved, the study of the stars is also of great importance for basic physical research .

### Spatial distribution and dynamics of the stars

The Milky Way . In this spectacular field alone, the 2MASS analysis software has identified almost 10 million stars and measured their properties.

Almost all stars are found in galaxies . Galaxies consist of a few million to hundreds of billions of stars and are themselves arranged in galaxy clusters . According to estimates by astronomers, there are around 100 billion such galaxies in the entire visible universe with a total of around 70 quintillion (7 × 10 22 ) stars. Due to gravity , stars orbit the center of their galaxy at speeds in the range of a few dozen km / s and typically require several 100,000 to 200 million years for one orbit (cf. Galactic year ). However, turnaround times are significantly shorter towards the center. The stars are not completely evenly distributed within a galaxy, but sometimes form open star clusters such as the Pleiades , also called seven stars , or globular clusters , which are located in the halo of galaxies. In addition, they are much closer in the galactic center than in the peripheral areas.

The longest list of known stars, the Tycho catalog, counts 2,539,913 stars (as of 2015) and lists their position, movement and photometric information. Up to magnitude +11.0, the catalog is considered to be 99.9% complete. It is the result of the Hipparcos satellite mission and its systematic survey of the sky. The successor mission to Hipparcos is the Gaia satellite mission. This satellite has been collecting data since 2013 and is intended to expand the existing data set considerably.

### State variables of the stars

Color-brightness diagram, schematic. The logarithmic brightness scale extends over more than four powers of ten. On the left is the blue and on the right the red spectral range. The drawn line field marks spectral classes B0 to M0 and brightness classes Ia to V.

Stars can be almost completely characterized with just a few state variables . The most important are called fundamental parameters . These include:

and, depending on the context:

The surface temperature, the acceleration due to gravity and the frequency of the chemical elements on the star's surface can be determined directly from the star's spectrum . If the distance of a star is known, for example by measuring its parallax , the luminosity can be calculated using the apparent brightness , which is measured by photometry . From this information, the radius and mass of the star can finally be calculated. The speed of rotation v at the equator cannot be determined directly, only the projected component with the inclination i , which describes the orientation of the axis of rotation. ${\ displaystyle v \ cdot \ sin i}$

More than 99 percent of all stars can be clearly assigned to a spectral class and a luminosity class. These fall within the Hertzsprung-Russell diagram (HRD) or the related color-brightness diagram in relatively small areas, the most important of which is the main series . A calibration using the known state variables of some stars gives the possibility of estimating the state variables of other stars directly from their position in this diagram. The fact that almost all stars are so easy to classify means that the appearance of stars is determined by relatively few physical principles.

In the course of its evolution, the star moves in the Hertzsprung-Russell diagram. The associated orbit of a star in this diagram is largely determined by a single quantity, namely its initial mass . The stars remain on the main sequence for most of the time, develop into red giants in the late stages and sometimes end as white dwarfs . These stages are described in more detail in the section on stellar evolution.

The range of values ​​of some state variables covers many orders of magnitude . The surface temperatures of main sequence stars range from about 2200  K to 45,000 K, their masses from 0.07 to 120 solar masses and their radii from 0.1 to 25 solar radii . Red giants are significantly cooler and can become so large that the entire earth's orbit could fit into them. White dwarfs have temperatures of up to 100,000 K, but are only as small as the earth, although their mass is comparable to that of the sun . The mass of stars in the main sequence can be estimated using the mass-luminosity relation .

Finally, the proper motion of a star is the velocity vector in relation to the position of the sun. Typical self-movements are between 10 and 100 kilometers per second. This is usually also a property of the star's environment, i.e. H. Stars are mostly at rest in their own environment. This is because stars form in groups from large gas clouds. Random processes such as star encounters in dense globular clusters or possible supernova explosions in their surroundings can give stars above-average speeds (so-called runaway stars or hyper - fast runners ). However, the respective speed never goes beyond values ​​of a few hundred kilometers per second. The first discovery of stars leaving the Milky Way due to their own motion was made in the last few years. At the moment eleven of these stars are known, most of which got their impulse from close encounters with the black hole in the galactic center.

## Star evolution

### Emergence

A large proportion of the stars were formed in the early stages of the universe over 10 billion years ago. But stars are still forming today. Typical star formation proceeds according to the following scheme:

Images of an emerging star: above a shining jet 12 light years long in an optical image, in the lower infrared image the dust disc, the edge of which can be seen as a bar in the middle of a dark double cone.
Schematic overview of the life phases of a star
1. The starting point for star formation is a gas cloud (usually a molecular cloud ), which consists mainly of hydrogen , and which collapses due to its own gravity . This happens when gravity dominates the gas pressure and the Jeans criterion is met. Triggers can e.g. B. the pressure wave of a nearby supernova , density waves in the interstellar matter or the radiation pressure of already formed young stars.
2. The further compression of the gas cloud creates individual globules (spatially narrowly limited dust and gas clouds), from which the stars then emerge: The stars are seldom formed in isolation, but rather in groups. The period of contraction lasts about 10 to 15 million years in total.
3. With the further contraction of the globules the density increases and because of the released gravitational energy (like the increased gravitational pressure) the temperature rises further ( virial law ; the kinetic energy of the particles corresponds to the temperature). The free collapse comes to a standstill when the cloud in the color-brightness diagram reaches the so-called Hayashi line , which delimits the area within which stable stars are possible. Then the star in the color-brightness diagram first moves along this Hayashi line before moving towards the main sequence, where the so-called hydrogen burning begins, i.e. the stellar nuclear fusion of hydrogen to helium by the Bethe-Weizsäcker cycle or the Proton-proton reaction . As a result of the angular momentum of the globules, a disk is formed which orbits the young star and from which it accretes further mass . One or more stars and planets can arise from this accretion disk . However, this phase of stellar evolution is not yet well understood. The plane of the disk becomes the ecliptic . During accretion from the disk, jets of matter form in both directions of the polar axes (see picture), which can reach a length of over 10 light years.

Massive stars are less common than low-mass stars. This is described by the original mass function . Depending on the mass, there are different scenarios for star formation:

• Above a certain limit mass, stars cannot possibly form through the accretion process, since these stars would already produce such a strong stellar wind in the accretion stage that the loss of mass would exceed the accretion rate. Stars of this size, such as the blue stragglers (Engl. Blue stragglers ), probably caused by star collisions .
• Massive and thus hot stars with more than 8 solar masses contract comparatively quickly. After the ignition of the nuclear fusion, the UV -rich radiation quickly drives the surrounding globules apart and the star does not accrete any further mass. You can therefore very quickly get to the main series in the Hertzsprung-Russell diagram . At 265 solar masses, the heaviest star discovered to date with the abbreviation R136a1 is a little over a million years old and is located in a star cluster in the tarantula nebula of the Large Magellanic Cloud . When it was formed, the star could have had up to 320 solar masses.
• Stars between about 3 and 8 solar masses go through a phase in which they are called Herbig-Ae / Be stars . In this phase the star is already on the main sequence, but accretes mass for some time.
• Stars with lower mass between 0.07 and 3 solar masses remain embedded in the globules for some time after the ignition of the nuclear fusion and continue to accrete mass. During this time they can only be seen in the infrared spectral range. As they approach the main sequence, they go through the T-Tauri star stage .
• Objects between 13 and 75 masses of Jupiter (or 0.07 solar masses) also reach the temperature required to ignite a nuclear fusion , but not the fusion of hydrogen, but only that of primordial deuterium , which is present in small amounts , and from 65 Jupiter masses also of lithium . These objects are called brown dwarfs and their mass is between the planetary gas giants (up to 13 M J ) and stars. Since there is not enough fuel to stop the contraction significantly, brown dwarfs are called substellar objects.
Active star formation region NGC604 with a diameter of 1,300 light years in the triangular nebula M33

A double or multiple star system as well as a single star can arise from a globule . When stars form in groups, stars that have formed independently of one another can also form double or multiple star systems through mutual capture. It is estimated that around two thirds of all stars are part of a double or multiple star system.

In the early stages of the universe, only hydrogen and helium were available for star formation. These stars are part of the so-called Population III, they were too massive and therefore too short-lived to exist to this day. The next generation, known as Population II stars, still exist today, they are mainly found in the halo of the Milky Way, but they have also been detected near the Sun. Stars that were formed later contain a certain proportion of heavy elements from the start, which were created in earlier star generations by nuclear reactions and, for example, enriched interstellar matter with heavy elements via supernova explosions. Most of the stars in the disk of the Milky Way are one of them. They are known as Population I stars.

An example of an active star forming region is NGC 3603 in the constellation Kiel of the ship at a distance of 20,000 light years . Star formation processes are observed in the infrared and in the X-ray range, since these spectral ranges are hardly absorbed by the surrounding dust clouds, unlike visible light. Satellites such as the Chandra X-ray telescope are used for this purpose .

### Main sequence phase

The color-brightness diagram of simultaneously formed stars of different masses contains a junction point that reflects the age of the group. Above this point the stars have already evolved from the main sequence.

The further course of the stellar evolution is essentially determined by the mass. The greater the mass of a star, the shorter its burning time. The most massive stars use up all their fuel in just a few hundred thousand years. Their radiation output exceeds that of the sun by a hundred thousand times or more. The sun, on the other hand, has not even completed half of its main sequence phase after 4.6 billion years. The low-mass red dwarfs develop much more slowly. Since the red dwarfs can reach an age of tens of billions to trillions of years and the universe is only about 14 billion years old, not a single one of the stars with the lowest mass has been able to leave the main sequence.

In addition to the mass, the proportion of heavy elements is important. In addition to its influence on the burning time, it determines whether, for example, magnetic fields can form or how strong the stellar wind becomes, which can lead to a considerable loss of mass in the course of star evolution. The following evolutionary scenarios relate to stars with solar element abundances, as are common for most stars in the disk of the Milky Way. In the magellanic clouds, for example, two dwarf galaxies in the vicinity of the Milky Way, the stars have a significantly lower proportion of heavy elements.

After their formation, stars spend most of their burning time (about 90 percent of their lifetime) on the main sequence. During this period, hydrogen is evenly fused to helium in the star's core . The heavier stars are hotter and brighter and are located at the top left in the color-brightness diagram, the lighter ones at the bottom right with the cooler ones with lower luminosity. During this main sequence phase, the stars slowly get bigger, hotter and brighter and move in the direction of the giant stars. This also applies to the sun, which is around 40 percent brighter today than when it was first created.

The nuclear fusion of hydrogen to helium takes place in a central area of ​​the star, which only takes up a few percent of its total volume, but contains around half of its mass. The temperature there is over 10 million Kelvin. The fusion products also accumulate there. The transport of energy to the star's surface takes several hundred thousand years. It takes place via radiation transport , heat conduction or convection . The area that emits radiation into space is called the stellar atmosphere . Their temperature is several thousand to several tens of thousands of Kelvin. For example, a star with 30 solar masses has a typical surface temperature of 40,000 K. It therefore emits almost exclusively UV radiation and only about 3% visible light.

### Late stages

#### Last burn phases

Planetary nebula Messier 57 ( Ring Nebula ) with a diameter of about one light year
Nebula around the extremely massive star Eta Carinae with a longitudinal diameter of about 0.5 light years, formed by eruptions 100 to 150 years ago

When the temperature and pressure are high enough, the helium nuclei produced during hydrogen burning begin to fuse in the core of the star. The hydrogen burning is not stopped, but continues in a shell around the helium burning core. This goes hand in hand with the fact that the star leaves the main sequence in the Hertzsprung-Russell diagram. The ignition of the helium burning is only possible for stars of sufficient mass (from 0.3 solar masses, see below), lighter stars glow out after the hydrogen burning. The further development is clearly different for low-mass and high-mass stars. Stars up to 2.3 solar masses are called low-mass.

• Low-mass stars of up to 0.3 solar masses continue the fusion of hydrogen in a growing shell around the extinct core. They go out completely after the end of this so-called bowl burning . Due to the decrease in temperature in the center, they give in to gravity and contract into white dwarfs with a diameter of a few thousand kilometers. As a result, the surface temperature initially rises sharply. In the further course the white dwarfs cool down and finally end up as black dwarfs .
• Low-mass stars between 0.3 and 2.3 solar masses such as the sun itself, through further contraction, reach the temperature and density necessary for burning helium in their core. When the helium burner is ignited, dramatic processes take place within seconds, in which the power conversion in the center can increase to 100 billion times the current solar power without anything being visible on the surface. These processes up to the stabilization of the helium burning are called helium flash . When burning helium, elements up to oxygen are created . At the same time, hydrogen burning takes place in a shell around the core. As a result of the rise in temperature and power, the stars expand into red giants with diameters typically 100 times that of the sun. Often the outer shells of the stars are repelled and form planetary nebulae . Eventually the helium burn will also go out and the stars will turn into white dwarfs as described above.
• Massive stars between 2.3 and 3 solar masses reach the stage of carbon burning after the helium burn , in which elements up to iron are formed. In a certain sense, iron is the star ash, as no further energy can be obtained from it through fusion. However, due to stellar winds or the formation of planetary nebulae , these stars lose a considerable part of their mass. They get below the critical limit for a supernova explosion and also become white dwarfs.
• Massive stars over 3 solar masses burn practically all lighter elements in their core to iron in the last millennia of their life cycle. These stars also repel a large part of the mass in their outer layers as stellar winds. The resulting nebulae are often bipolar structures, such as the Homunculus Nebula around η Carinae . At the same time, onion-like shells form around the core inside the star, in which various fusion processes take place. The states in these bowls differ dramatically. This is illustrated using the example of a star with 18 solar masses, which has 40,000 times the solar power and 50 times the solar diameter:
Overview of the fusion processes within massive stars
Fusion
material
Fusion process
(nucleosynthesis)
Temperature
(mill. K )
Density
(kg / cm³)
merger
period
H Hydrogen burning 40 0.006   10 million years
Hey Helium burning 190 1.1 1 million years
C. Carbon burning 740 240 10,000 years
No Burning neon 1,600 7,400 ten years
O Oxygen burn 2,100 16,000 5 years
Si Silicon firing 3,400 50,000 1 week
Fe core Nuclear fusion of the heaviest elements 10,000   10,000,000   -
The boundary between the helium and carbon zones is comparable with the earth's atmosphere over a lava lake in terms of the relative temperature and density jump. A significant part of the total stellar mass is concentrated in the iron core with a diameter of only about 10,000 km. As soon as it exceeds the Chandrasekhar limit of 1.44 solar masses, it collapses within fractions of a second, while the outer layers are repelled by released energy in the form of neutrinos and radiation and form an expanding cloud of explosion. It is not yet known exactly under which circumstances a neutron star or a black hole is formed as the end product of such a type II supernova . In addition to the mass, the rotation of the precursor star and its magnetic field should also play a special role. The formation of a quark star would also be possible , the existence of which, however, is only hypothetical so far. If the supernova occurs in a binary star system in which mass transfer takes place from a red giant to a white dwarf ( type Ia ), carbon fusion processes can even tear the star apart completely.

#### Nucleosynthesis and Metallicity

Elements heavier than helium are produced almost exclusively by nuclear reactions in the late course of star evolution, known as nucleosynthesis . During the fusion reactions taking place in the plasma in thermal equilibrium , all elements up to the atomic number of iron can arise. Heavier elements, in which the binding energy per nucleon increases again, are formed by trapping nuclear particles in non-thermal nuclear reactions. Heavy elements are mainly formed by neutron capture with subsequent β-decay in carbon-burning giant stars in the s-process or in the first, explosive phase of a supernova in the r-process . Here s stands for slow and r for rapid . In addition to these two most common processes, which ultimately lead to clearly distinguishable signatures in the element frequencies, proton capture and spallation also take place.

The resulting elements are largely fed back into the interstellar medium, from which further generations of stars are created. The more often this process has already been carried out, the more the elements that are heavier than helium are enriched. For these elements, the collective term metals has become common in astronomy . Since these metals accumulate fairly evenly, it is often sufficient to specify the metallicity instead of the individual element abundances . Stars whose relative abundance patterns deviate from this scheme are called chemically peculiar . Later generations of stars consequently have a higher metallicity. Metallicity is therefore a measure of the age of a star's formation.

## Double stars

A binary star or binary star system consists of two stars that appear or actually stand close together in the sky. When they are gravitationally bound to each other, they periodically move around their common center of gravity .

A distinction is made between the following types of double stars or pairs of stars:

• Optical double stars (apparent double stars): two stars that appear in the sky from earth in almost the same direction, but which do not influence each other gravitationally.
• Geometric double stars (spatial double stars): Stars that are spatially close to one another, but are not bound to one another due to their high relative speeds.
• Physical binary stars or binary star systems are two stars that are gravitationally bound due to their spatial proximity and move around a common center of gravity according to Kepler's laws . Over half of all stars in the universe are part of a binary star system.
• A multiple star system consists of more than two physically bound stars.

## Variable stars

The apparent and often also the absolute brightness of some stars is subject to fluctuations over time, which can be seen in the light curves . A distinction is made between the following types of variable stars:

• Coverage variable . These are double stars that temporarily cover themselves from an earthly perspective during their orbit.
• Rotationally variable . The observed change can be attributed to the rotation of the star, since it does not radiate equally brightly in all directions (e.g. pulsars ).
• Pulsation variable . The state variables change more or less periodically and thus also the luminosity. Most stars go through such unstable phases during their evolution, but usually only after the main sequence stage. Important types are:
• Cepheids - your period can be precisely assigned a certain luminosity. They are therefore important as so-called standard candles when determining the distance.
• Mira Stars - Their periods are longer and more irregular than those of the Cepheids.
• RR Lyrae stars - They pulsate very regularly with a comparatively short period and have about 90 times the luminosity of the sun.
• Cataclysmically Mutable . These are usually binary star systems in which a mass transfer takes place from a red giant to a white dwarf. They show eruptions at intervals of a few hours up to several million years.
Supernova remnant Cassiopeia A
• Supernovae . There are several types of supernovae, of which type Ia is also a binary star phenomenon. Only types Ib, Ic and II mark the end of the evolution of a massive star.
• Eruptive mutable . You suffer outbreaks for short periods of time, which are often repeated at more or less irregular intervals. Examples are (e.g. UV Ceti stars , T Tauri stars ):
• X-ray binary stars are binary star systems that emit X-rays. A compact partner receives matter from another star through accretion . As a result, the X-ray binary stars resemble the cataclysmic variables.

## literature

• SW Stahler & F. Palla: The Formation of Stars. WILEY-VCH, Weinheim 2004, ISBN 3-527-40559-3
• HH Voigt: Outline of Astronomy. 4th edition. Bibliographisches Institut, Mannheim 1988, ISBN 3-411-03148-4 .
• H. Scheffler, Hans Elsässer : Physics of the stars and the sun. 2nd Edition. BI-Wiss.-Verl., Mannheim 1990, ISBN 3-411-14172-7 .
• Rudolf Kippenhahn , A. Weigert: Stellar structure and evolution. Springer, Berlin 1990, ISBN 3-540-50211-4 (English).
• N. Langer: Life and Death of the Stars. Beck's series. Beck, Munich 1995, ISBN 3-406-39720-4 .
• D. Prialnik: An Introduction to the Theory of Stellar Structure and Evolution . Cambridge University Press, Cambridge 2000, ISBN 0-521-65065-8 .
• J.Bennett, M.Donahue, N.Schneider, M.Voith: Astronomie (Chapters 14-16) , eds. Harald Lesch, 5th edition (1170 pages), Pearson-Studienverlag, Munich-Boston-Harlow-Sydney-Madrid 2010
• Thassilo von Scheffer , The Legends of the Stars , 1939.

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## supporting documents

1. ^ The dictionary of origin (=  Der Duden in twelve volumes . Volume 7 ). 2nd Edition. Dudenverlag, Mannheim 1989, p. 709 . See also DWDS ( "Stern" ) and Friedrich Kluge : Etymological dictionary of the German language . 7th edition. Trübner, Strasbourg 1910 ( p. 442 ).
2. E. Høg, C. Fabricius, VV Makarov, S. Urban, T. Corbin, G. Wycoff, U. Bastian, P. Schwekendiek and a .: The Tycho-2 Catalog of the 2.5 million brightest stars . In: Astronomy & Astrophysics . 355, 2000, pp. L27..L30. bibcode : 2000A & A ... 355L..27H .
3. Norbert Przybilla et al .: HD 271791: An Extreme Supernova Runaway B Star Escaping from the Galaxy. arxiv : 0811.0576v1 , doi: 10.1086 / 592245 .
4. ^ Brown et al .: MMT Hypervelocity Star Survey. arxiv : 0808.2469v2 .
5. Most massive stars double the previously assumed maximum
6. Carolin Liefke: Record star far larger than expected: star with 300 solar masses discovered. Max Planck Institute for Astronomy, press release from July 21, 2010 from the Science Information Service (idw-online.de), accessed on December 23, 2014.
7. ^ V. Joergens: Origins of Brown Dwarfs . In: Reviews in Modern Astronomy . 18, 2005, pp. 216-239. arxiv : astro-ph / 0501220v2 . bibcode : 2005RvMA ... 18..216J .
 This article was added to the list of excellent articles on May 6, 2004 in this version .