A star is a massive, luminous ball of plasma. Stars group together to form galaxies, and they dominate the visible universe. A star shines because nuclear fusion in its core releases energy which radiates into outer space. Almost all elements heavier than hydrogen and helium were created inside the cores of stars. Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. A star begins as a collapsing cloud of material that is composed primarily of hydrogen along with some helium and heavier trace elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. Once the hydrogen fuel at the core is exhausted, those stars having at least 0.4 times the mass of the Sun expand to become a red giant. The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.
- Astrometric binary star: It is a binary star for which only one of the component stars can be visually observed. The visible star's position is carefully measured and detected to have a wobble, due to the gravitational influence from its counterpart. The position of the star is repeatedly measured relative to more distant stars, and then checked for periodic shifts in position. Typically this type of measurement can only be performed on nearby stars, such as those within 10 parsecs. Nearby stars often have a relatively high proper motion, so astrometric binaries will appear to follow a sinusoidal path across the sky. If the companion is sufficiently massive to cause an observable shift in position of the star, then its presence can be deduced. From precise astrometric measurements of the movement of the visible star over a sufficiently long period of time, information about the mass of the companion and its orbital period can be determined
- Asymptotic Giant Branch: It is the name given to a region of the Hertzsprung-Russell Diagram populated by evolving low to medium-mass stars. When a star exhausts the supply of hydrogen in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star's luminosity increases greatly, and it becomes a red giant. Once the temperature in the core has reached approximately 3x108K, helium burning begins. The onset of helium burning in the core halts the star's cooling and increase in luminosity.
- Big Dipper: The seven brightest stars of the constellation Ursa Major, the Great Bear, form a well-known asterism that has been recognized as a distinct grouping in many cultures from time immemorial.
- Binary and multi-star systems: They consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.
- Black dwarf: It is a hypothetical star, created when a white dwarf becomes sufficiently cool to no longer emit significant heat or light. Since the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe, 13.7 billion years, no black dwarfs are expected to exist in the Universe yet, and the temperature of the coolest white dwarfs is one observational limit on the age of the universe.
- Blue dwarf: It is a hypothetical class of star that develops from a red dwarf star after it has exhausted much of its hydrogen fuel supply. Since red dwarf stars fuse their hydrogen slowly and are fully convective (allowing a larger percentage of their total hydrogen supply to be fused), the lifespan of the universe is not sufficient for any blue dwarfs to have formed yet. Their existence is predicted based on theoretical models. Blue dwarfs eventually evolve into white dwarfs once their hydrogen fuel is completely exhausted.
- Brown dwarfs: They are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores but which have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest mass stars; this upper limit is between 75 and 80 Jupiter masses. Brown dwarfs heavier than 13 MJ do fuse deuterium and those above ~65 MJ also fuse lithium.
- Carbon star: It is a late type giant star similar to the red giants (or occasionally red dwarf) star whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere, and a strikingly red appearance to human observers.
- Cepheid variable or Cepheid: It is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. Because of this correlation a Cepheid variable can be used as a standard candle to determine the distance to its host cluster or galaxy. Since the period-luminosity relation can be calibrated with great precision using the nearest Cepheid stars, the distances found with this method are among the most accurate available.
- Crab Pulsar: It is a relatively young neutron star located in the Crab Nebula. The optical pulsar is roughly 25 km in diameter and the pulsar "beams" rotate once every 33 milliseconds, or 30 times each second. The outflowing relativistic wind from the neutron star generates synchrotron emission, which produces the bulk of the emission from the nebula, seen from radio waves through to gamma rays.
- Eclipsing binary star: It is a binary star in which the orbit plane of the two stars lies so nearly in the line of sight of the observer that the components undergo mutual eclipses. Eclipsing binaries are variable stars, not because the light of the individual components vary but because of the eclipses. The light curve of an eclipsing binary is characterized by periods of practically constant light, with periodic drops in intensity. If one of the stars is larger than the other, one will be obscured by a total eclipse while the other will be obscured by an annular eclipse.
- Giant star: It is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. A hot, luminous main sequence star may also be referred to as a giant. Apart from this, because of their large radii and luminosities, giant stars lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung-Russell diagram and correspond to luminosity classes II or III.
- Globular cluster: It is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centres. Globular clusters, which are found in the halo of a galaxy, contain considerably more stars and are much older than the less dense galactic, or open clusters, which are found in the disk. Globular clusters are fairly common; there are about 158 currently known globular clusters in the Milky Way, with perhaps 10-20 more undiscovered. Large galaxies can have more: Andromeda, for instance, may have as many as 500.
- Hypergiant Star It is a star with tremendous mass and luminosity :(luminosity class 0). Hypergiants are more massive than supergiants, stars which have masses up to 100 times that of the Sun. Hypergiants are not necessarily larger (in terms of volume) than supergiants, but are usually more massive. This approaches the Eddington limit, a theoretical upper limit of stellar mass (about 120 solar masses) at which a star generates so much radiation that it throws off its outer layers. Some hypergiants appear to be more than 100 solar masses and may have initially been 200 to 250 solar masses, challenging current theories of star formation and stellar evolution. Hypergiants are the most luminous stars, thousands to millions of solar luminosities; however, their temperatures vary widely between 3,500 K and 35,000 K. Almost all hypergiants exhibit variations in luminosity over time due to instabilities within their interiors at moderate temperatures and high pressures.
- Little dipper: This is the name given to the seven brightest stars of the Ursa Minor constellation because they seem to form a ladle, or dipper shape.
- Magnetar: It is a neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma-rays. The first recorded burst of gamma rays thought to have been from a magnetar was on March 5, 1979. During the following decade, the magnetar hypothesis has become widely accepted as a likely explanation for soft gamma repeaters and anomalous X-ray pulsars.
- Main sequence: It is the name for a continuous sequence of stars that appear on a plot of colour versus brightness for groups of stars. These colour-magnitude plots are known as Hertzsprung-Russell diagrams. Stars on this band are known as main-sequence stars or dwarf stars. After a star has formed, it generates energy at the hot, dense core region through the nuclear fusion of hydrogen atoms into helium. During this stage of the star's lifetime, it is located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors. In general, the more massive the star the shorter its lifespan on the main sequence. After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence.
- Multiple star: It consists of three or more stars which appear from the Earth to be close to one another. Physical multiple stars are multiple star systems. Multiple stars may be called triple if they have three stars, quadruple with four, and so on. A physical triple star is also called trinary, ternary or a triple star system. In a triple star system, each star orbits the centre of mass of the system. Usually, two of the stars form a close binary star and the third is further away; this configuration is called hierarchical. Multiple star systems containing more than three stars are also usually hierarchical.
- Neutron star: It is formed from the collapsed remnant of a massive star; i.e. a Type II, Type Ib, or Type Ic supernova. Neutron stars consist mostly of neutrons. Such stars are very hot, as supported by the Pauli Exclusion Principle indicating repulsion between neutrons. A neutron star is one of the few possible conclusions of stellar evolution. A typical neutron star has a mass between 1.35 and about 2.1 solar masses, with a corresponding radius between 20 and 10 km, respectively -30,000 to 70,000 times smaller than the Sun. Thus, neutron stars have overall densities of 8.4×1016 to 1×1018 kg/m3, which compares with the approximate density of an atomic nucleus of 3×1017 kg/m³.
- Open or Galactic cluster: It is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring. They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic centre, as well as losing cluster members through internal close encounters.
- Optical binaries or optical doubles: They are two stars that only appear to be close together, and are actually separated by a great distance in space and are not gravitationally bound to each other. Optical doubles are distinguished from binary stars by observing them for a long period of time, usually years. If the relative motion looks linear, it may be safely assumed that the motion is due to proper motion alone and that they are an optical double.
- Protostar: It is an object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a solar-mass star it lasts about 100,000 years. It starts with a core of increased density in a molecular cloud and ends with the formation of a T Tauri star, which then develops into a main sequence star. This is heralded by the T Tauri wind, a type of super solar wind that marks the change from the star accreting mass into radiating energy. The protostar, at first, only has about 1% of its final mass. But the envelope of the star continues to grow as infalling material is accreted. After 10,000 - 100,000 years thermonuclear fusion begins in its core, then a strong stellar wind is produced which stops the infall of new mass. The protostar is now considered a young star since its mass is fixed, and its future evolution is now set.
- Pulsars: They are highly magnetized rotating neutron stars which emit a beam of detectable electromagnetic radiation in the form of radio waves. Their observed periods range from 1.5 ms to 8.5 s. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name. Because neutron stars are very dense objects, the rotation period and thus the interval between observed pulses are very regular. For some pulsars, the regularity of pulsation is as precise as an atomic clock. Pulsars are known to have planets orbiting them,
- Quark star or strange star: It is a hypothetical type of exotic star composed of quark matter, or strange matter. These are ultra-dense phases of degenerate matter theorized to form inside particularly massive neutron stars. It is theorized that when the neutron-degenerate matter which makes up a neutron star is put under sufficient pressure due to the star's gravity, the individual neutrons break down into their constituent quarks, up quarks and down quarks. Some of these quarks may then become strange quarks and form strange matter. The star then becomes known as a "quark star" or "strange star", similar to a single gigantic hadron (but bound by gravity rather than the colour force). Quark matter/strange matter is one candidate for the theoretical dark matter that is a feature of several cosmological theories.
- Red dwarf star: According to the Hertzsprung-Russell diagram, it is a small and relatively cool star, of the main sequence, either late K or M spectral type. They constitute the vast majority of stars and have a mass of less than one-half that of the Sun (down to about 0.075 solar masses, which are brown dwarfs) and a surface temperature of less than 3,500 K.
- Red giant: It is a luminous giant star of low or intermediate mass that is in a late phase of its evolution, with nuclear fusion going on in a shell outside the core but not in the core itself. The core matter is electron degenerate and extremely compressed, so the outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The most common red giants are the so-called Red Giant Branch stars (RGB stars) whose shells are still fusing hydrogen, while the core is inactive helium. Another case of red giants of interest are the Asymptotic Giant Branch stars (AGB) that produces carbon by the triple-alpha process from helium. Prominent bright red giants in the night sky include Aldebaran (Alpha Tauri), Gamma Crucis and Alpha Vulpeculae (Lucida Anseris).
- Red Giant Branch stars (RGB): They are the most common red giants whose shells are still fusing hydrogen, while the core is inactive helium.
- Spectroscopic binary star: It is a binary star in which the separation between the stars is usually very small, and the orbital velocity very high. Unless the plane of the orbit happens to be perpendicular to the line of sight, the orbital velocities will have components in the line of sight and the observed radial velocity of the system will vary periodically. Since radial velocity can be measured with a spectrometer by observing the Doppler shift of the stars' spectral lines, the binaries detected in this manner are known as spectroscopic binaries. In some spectroscopic binaries, spectral lines from both stars are visible and the lines are alternately double and single. Such a system is known as a double-lined spectroscopic binary. In other systems, the spectrum of only one of the stars is seen and the lines in the spectrum shift periodically towards the blue, then towards red and back again. Such stars are known as single-lined spectroscopic binaries.
- Star clusters: They are groups of stars which are gravitationally bound. One distinguishes: globular clusters or tight groups of hundreds of thousands of very old stars; open clusters, generally contain less than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association.
- Star system or stellar system: It is a small number of stars orbiting each other and bound by gravitational attraction. A large number of stars bound by gravitation are generally called a star cluster or galaxy, although, broadly speaking, they are also star systems. Star system is occasionally also used to refer to a system of a single star together with a planetary system of orbiting smaller bodies.
- Sun: The Sun is the star at the centre of the Solar System. The Earth, other planets, asteroids, meteoroids, comets and dust orbit the Sun, which by itself accounts for about 99.8% of the solar systems mass. Energy from the Sun, in the form of sunlight, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather. The Sun is composed of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 25% of mass, 7% of volume), and trace quantities of other elements. The Sun has a surface temperature of approximately 5,780 K, giving it a white colour which, because of atmospheric scattering, appears yellow as seen from the surface of the Earth.)
- Supergiant Stars: They are among the most massive stars. In the Hertzsprung-Russell diagram they occupy the top region of the diagram. In the Yerkes spectral classification supergiants are class Ia (most luminous supergiants) or Ib (less luminous supergiants). They typically have bolometric absolute magnitudes between -5 and -12. The most luminous supergiants are often classified as hypergiants of class 0. Supergiants can have masses from 10 to 70 solar masses and brightness from 30,000 up to hundreds of thousands times the solar luminosity. They vary greatly in radii, usually from 30 to 500, or even in excess of 1000 solar radii. Because of their extreme masses they have short lifespans of only 10 to 50 million years and are mainly observed in young cosmic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies.
- T Tauri stars: They are pre-main sequence. Their surface temperatures are similar to those of main sequence stars of the same mass, but they are significantly more luminous because their radii are larger. Their central temperatures are too low for hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract towards the main sequence, which they reach after about 100 million years. They typically rotate with a period between one and twelve days, compared to a month for the Sun, and are very active and variable.
- Variable star: It is a star that undergoes significant variation in its
luminosity. In contrast, most stars have little variation in luminosity;
the sun, for instance, undergoes relatively little variation in brightness
(usually about 0.1% over an 11 year solar cycle). Variable stars are of
two types:
- stars that are intrinsically variable, that is, their luminosity actually
changes, for example because the star periodically swells and shrinks;
- eclipsing and rotating variables, where the apparent changes in brightness
are a perspective effect.
- Visual binary star: It is a binary star for which the angular separation between the two components is great enough to permit them to be observed as a double star in a telescope. The brightness of the two stars is an important factor, as brighter stars are harder to separate due to their glare than dimmer ones are. The brighter star of a visual binary is the primary star, and the dimmer is considered the secondary.
- White dwarf or degenerate dwarf: It is a small star composed mostly of electron-degenerate matter. As white dwarfs have mass comparable to the Sun's and their volume is comparable to the Earth's, they are very dense. Their faint luminosity comes from the emission of stored heat. They comprise roughly 6% of all known stars in the solar neighbourhood. White dwarfs are thought to be the final evolutionary state of all stars whose mass is not too high -over 97% of the stars in our Galaxy. After the hydrogen-fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core by the triple-alpha process. Usually white dwarfs are composed of carbon and oxygen. It is also possible that core temperatures suffice to fuse carbon but not neon, in which case an oxygen-neon-magnesium white dwarf may be formed. Also, some helium white dwarfs appear to have been formed by mass loss in binary systems.
- Wolf-Rayet stars (WR stars): They are evolved, massive stars (over 20
solar masses), and are losing their mass rapidly by means of a very strong
stellar wind, with speeds up to 2000 km/s. While our own sun loses 10?16
of its own mass every year, a Wolf-Rayet star loses 10?5 solar masses a
year. These stars are also very hot: their surface temperatures are in the
range of 25,000 K to 50,000 K.