query: a low mass star that converts helium to carbon is a

pos: Red giant red giants are the red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process; and the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen burning shell just beyond that. Red giants are stars that have exhausted the supply of hydrogen in their cores and have begun thermonuclear fusion of hydrogen in a shell surrounding the core. They have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is lower in temperature, giving them a

pos: Red giant Red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The most common red giants are stars on the red-giant branch (RGB) that are still fusing hydrogen into helium in a shell surrounding an inert helium core. Other

pos: Circumstellar habitable zone by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants. However, around such a helium-burning star, important life processes like photosynthesis could only happen around planets where the atmosphere has carbon dioxide, as by the time a solar-mass star becomes a red giant, planetary-mass bodies would have already absorbed much of their free carbon dioxide. Moreover, as Ramirez and Kaltenegger (2016) showed, intense stellar winds would completely remove the atmospheres of such smaller planetary bodies, rendering them uninhabitable anyway. Thus, Titan would not

pos: La Superba wavelengths, resulting in a spectrum with even less blue and violet compared to ordinary red giants, giving the star its distinguished red color. La Superba is most likely in the final stages of fusing its remaining secondary fuel (helium) into carbon and shedding its mass at the rate of about a million times that of the Sun's solar wind. It is also surrounded by a 2.5 light year-wide shell of previously ejected material, implying that at one point it must have been losing mass as much as 50 times faster than it is now. La Superba thus appears almost ready

neg: Extreme helium star Extreme helium star An extreme helium star (abbreviated EHe), or a PV Telescopii Variable, is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the Universe. Since there are no conditions where stars devoid of hydrogen can be formed from molecular clouds, it is theorized that they are the product of the mergers of helium-core and carbon-oxygen core white dwarfs. Extreme helium stars form a sub-group within the broader category of hydrogen-deficient stars. The latter includes cool carbon stars like R Coronae Borealis, helium-rich spectral class O or B stars, population I Wolf-Rayet stars,

neg: Helium flash helium shell is consumed. Thermal pulses may cause a star to shed circumstellar shells of gas and dust. Helium flash A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses () and 2.0 ) during their red giant phase (the Sun is predicted to experience a flash 1.2 billion years after it leaves the main sequence). A much rarer runaway helium fusion process can also occur on the surface of accreting white dwarf stars. Low mass stars

neg: Extreme helium star proposed to explain the composition of extreme helium stars. Examination of element abundances from seven EHe stars agreed with those predicted by the DD model. Extreme helium star An extreme helium star (abbreviated EHe), or a PV Telescopii Variable, is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the Universe. Since there are no conditions where stars devoid of hydrogen can be formed from molecular clouds, it is theorized that they are the product of the mergers of helium-core and carbon-oxygen core white dwarfs. Extreme helium stars form a sub-group within the broader

neg: Giant star mass, when on the main sequence, was below approximately , it will never reach the central temperatures necessary to fuse helium. It will therefore remain a hydrogen-fusing red giant until it runs out of hydrogen, at which point it will become a helium white dwarf. According to stellar evolution theory, no star of such low mass can have evolved to that stage within the age of the Universe. In stars above about the core temperature eventually reaches 10 K and helium will begin to fuse to carbon and oxygen in the core by the triple-alpha process. When the core is

neg: DY Centauri that the star in a single-lined spectroscopic binary in an eccentric orbit ("e" = 0.44) with a period of 39.67 days. The companion star has an estimated minimum mass of , so it can be a low mass white dwarf or main sequence star. With an estimated separation of only at periastron, the system must have interacted in the past when the primary had larger dimensions, forming a common envelope. DY Centauri has a peculiar chemical composition and is poor in hydrogen and rich in helium and carbon, being identified as an extreme helium star (EHe). In comparison to other

neg: TX Piscium and cause the hydrogen shell to be extinguished. This creates strong convection and the third dredge-up (TDU) which brings helium fusion products such as carbon to the surface. After several TDUs, the abundance of carbon in the atmosphere begins to exceed that of oxygen and then the star is known as a carbon star. In TX Piscium, the carbon/oxygen ratio is calculated to be 1.03, which is at the low end of the scale of carbon stars. It is thought to be a relatively new carbon star. TX Piscium has a mass of approximately . Modelling of its observed properties

neg: Nucleosynthesis is carbon because its formation from He is a bottleneck in the entire process. Carbon is produced by the triple-alpha process in all stars. Carbon is also the main element that causes the release of free neutrons within stars, giving rise to the s-process, in which the slow absorption of neutrons converts iron into elements heavier than iron and nickel. The products of stellar nucleosynthesis are generally dispersed into the interstellar gas through mass loss episodes and the stellar winds of low mass stars. The mass loss events can be witnessed today in the planetary nebulae phase of low-mass star

neg: Red supergiant star One notable group of low mass high luminosity stars are the RV Tauri variables, AGB or post-AGB stars lying on the instability strip and showing distinctive semi-regular variations. Red supergiants develop from main-sequence stars with masses between about and . Higher-mass stars never cool sufficiently to become red supergiants. Lower-mass stars develop a degenerate helium core during a red giant phase, undergo a helium flash before fusing helium on the horizontal branch, evolve along the AGB while burning helium in a shell around a degenerate carbon-oxygen core, then rapidly lose their outer layers to become a white dwarf with a

neg: Carbon star hydrogen fusion temporarily ceases. In this phase, the star's luminosity rises, and material from the interior of the star (notably carbon) moves up. Since the luminosity rises, the star expands so that the helium fusion ceases, and the hydrogen shell burning restarts. During these "shell helium flashes", the mass loss from the star is significant, and after many shell helium flashes, an AGB star is transformed into a hot white dwarf and its atmosphere becomes material for a planetary nebula. The "non-classical" kinds of carbon stars, belonging to the types C-J and C-H, are believed to be binary stars, where

neg: Hydrogen-deficient star the outer layer of the star. The double degenerate scenario is a binary-star approach in which a smaller degenerate helium white dwarf and a larger carbon-oxygen white dwarf orbit each other so closely that they eventually inspiral due to gravitational wave losses. At the Roche limit, mass transfer takes place from the helium to the carbon-oxygen star. The latter undergoes helium shell burning to form a supergiant and evolve to a hydrogen-deficient star. The double degenerate scenario provides a better fit to the observational data. Hydrogen-deficient star A hydrogen-deficient star is a type of star that has little or no

neg: Telescopium that is the prototype of a class of variables known as PV Telescopii variables. First discovered in 1952, it was found to have a very low level of hydrogen. One theory of its origin is that it is the result of a merger between a helium- and a carbon-oxygen white dwarf. If the combined mass does not exceed the Chandrasekhar limit, the former will accrete onto the latter star and ignite to form a supergiant. Later this will become an extreme helium star before cooling to become a white dwarf. While RR Telescopii, also designated "Nova Telescopii 1948", is often

neg: Helium flash that all released energy is absorbed by the entire core, leaving the degenerate state to become nondegenerate. Earlier computations indicated that a nondisruptive mass loss would be possible in some cases, but later star modeling taking neutrino energy loss into account indicates no such mass loss. In a one solar mass star, the helium flash is estimated to release about , or about 0.3% of the energy release of a Type Ia supernova, which is triggered by an analogous ignition of carbon fusion in a carbon–oxygen white dwarf. When hydrogen gas is accreted onto a "white dwarf" from a binary

neg: Procyon of 8,600 km, versus 5,800 km for Sirius B. The radius agrees with white dwarf models that assume a carbon core. It has a stellar classification of DQZ, having a helium-dominated atmosphere with traces of heavy elements. For reasons that remain unclear, the mass of Procyon B is unusually low for a white dwarf star of its type. With a surface temperature of 7,740 K, it is also much cooler than Sirius B; this is a testament to its lesser mass and greater age. The mass of the progenitor star for Procyon B was about and it came to the

neg: Main sequence local interstellar medium, the initial composition is homogeneous throughout, consisting of about 70% hydrogen, 28% helium and trace amounts of other elements, by mass. The initial mass of the star depends on the local conditions within the cloud. (The mass distribution of newly formed stars is described empirically by the initial mass function.) During the initial collapse, this pre-main-sequence star generates energy through gravitational contraction. Upon reaching a suitable density, energy generation is begun at the core using an exothermic nuclear fusion process that converts hydrogen into helium. When nuclear fusion of hydrogen becomes the dominant energy production process and

neg: Asymptotic giant branch Asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6–10 solar masses) late in their lives. Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity thousands of times greater than the Sun. Its interior structure is characterized by a central and largely inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning), another shell where hydrogen is undergoing fusion

neg: White dwarf are thought to be the product of mass loss in binary systems or mass loss due to a large planetary companion. If the mass of a main-sequence star is between 0.5 and 8 like our sun, its core will become sufficiently hot to fuse helium into carbon and oxygen via the triple-alpha process, but it will never become sufficiently hot to fuse carbon into neon. Near the end of the period in which it undergoes fusion reactions, such a star will have a carbon–oxygen core which does not undergo fusion reactions, surrounded by an inner helium-burning shell and an outer

neg: Future of an expanding universe combined mass of more than the Chandrasekhar limit of about 1.4 solar masses happen to merge. The resulting object will then undergo runaway thermonuclear fusion, producing a Type Ia supernova and dispelling the darkness of the Degenerate Era for a few weeks. If the combined mass is not above the Chandrasekhar limit but is larger than the minimum mass to fuse carbon (about ), a carbon star could be produced, with a lifetime of around 10 (1 million) years. Also, if two helium white dwarfs with a combined mass of at least collide, a helium star may be produced, with

neg: Helium flash Helium flash A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses () and 2.0 ) during their red giant phase (the Sun is predicted to experience a flash 1.2 billion years after it leaves the main sequence). A much rarer runaway helium fusion process can also occur on the surface of accreting white dwarf stars. Low mass stars do not produce enough gravitational pressure to initiate normal helium fusion. As the hydrogen in the core is

neg: Sigma Orionis than the helium-rich primary, about magnitude 10-11 at K band infrared wavelengths. It is presumed to be a low mass star . The infrared source IRS1 is close to σ Ori A. It has been resolved to a pair of low mass objects, a proplyd, and a possible third object. The brighter object has an M1 spectral class, a mass around , and appears to be a relatively normal low mass star. The fainter object is very unusual, showing a diluted M7 or M8 absorption spectrum with emission lines of hydrogen and helium. The interpretation is that it is a

neg: Carbon-burning process massive stars, the reduction in lifetime is dominated by the neutrino losses. For example, a star of 25 solar masses burns hydrogen in the core for 10 years, helium for 10 years and carbon for only 10 years. During helium fusion, stars build up an inert core rich in carbon and oxygen. The inert core eventually reaches sufficient mass to collapse due to gravitation, whilst the helium burning moves gradually outward. This decrease in the inert core volume raises the temperature to the carbon ignition temperature. This will raise the temperature around the core and allow helium to burn in

neg: Helium flash cool down and a stable burning of helium will continue. A star with mass greater than about 2.25 starts to burn helium without its core becoming degenerate, and so does not exhibit this type of helium flash. In a very low-mass star (less than about 0.5 ), the core is never hot enough to ignite helium. The degenerate helium core will keep on contracting, and finally becomes a helium white dwarf. The helium flash is not directly observable on the surface by electromagnetic radiation. The flash occurs in the core deep inside the star, and the net effect will be

neg: SDSS J1240+6710 SDSS J1240+6710 SDSSJ1240+6710 or SDSS J124043.01+671034.68, nicknamed Dox, is a white dwarf with an atmosphere of almost pure oxygen discovered by Kepler de Souza Oliveira, Detlev Koester and Gustavo Ourique. The atmosphere also has a detectable amount of magnesium, neon (under 4%) and silicon, but no hydrogen, helium or carbon. A possible explanation for the unusual composition would be if its mass were close to the limit for collapsing to a neutron star. But its mass is only 0.56 solar masses, below the mass expected for a star that could convert carbon to oxygen, neon and magnesium. The star was

neg: Red giant causing helium in a shell to begin fusing. At the same time hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch, a second red-giant phase. The helium fusion results in the build up of a carbon–oxygen core. A star below about will never start fusion in its degenerate carbon–oxygen core. Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf. The ejection of the outer mass

neg: Georgeanne R. Caughlan the 'bible' of nuclear astrophysics. During Caughlan's stint at Caltech, the first computations of the structure and evolution of stars became possible as computers developed to sufficient power. A computer program developed by Robert V. Wagoner to simulate the synthesis of 41 nuclei in stars required astrophysical rates from Caughlan's work. The "carbon-nitrogen-oxygen (CNO) cycle" is a process in stellar fusion that converts hydrogen to helium. In 1965, Caughlan computed that if a star exceeds thirty solar masses, the CNO cycle would result in nitrogen atoms outnumbering carbon and oxygen by a factor of 100. The star eta Carinae was

neg: Stellar mass with masses below 0.5 do not enter the asymptotic giant branch (AGB) but evolve directly into white dwarfs. (At least in theory; the lifetimes of such stars are long enough—longer than the age of the universe to date—that none has yet had time to evolve to this point and be observed.) "Low mass stars" with a mass below about 1.8–2.2 (depending on composition) do enter the AGB, where they develop a degenerate helium core. "Intermediate-mass stars" undergo helium fusion and develop a degenerate carbon-oxygen core. "Massive stars" have a minimum mass of 7–10 , but this may be as low

neg: Y Lyncis duration 1,300 days. The long secondary period variations are possibly caused by long-lived convection cells. Y Lyncis has a mass around and a luminosity around . It is a thermally pulsing asymptotic giant branch star, an evolved star with a carbon-oxygen core that is fusing helium in a shell and hydrogen in a separate shell. It is also an S-type star, where third dredge-ups have brought some carbon to the surface, but not enough to create a carbon star. Y Lyncis Y Lyncis is a semiregular variable star in the constellation Lynx. It is an asymptotic giant branch star of

neg: Variable star RPHS. They are p-mode pulsators. Stars in this class are type Bp supergiants with a period of 0.1–1 day and an amplitude of 0.1 magnitude on average. Their spectra are peculiar by having weak hydrogen while on the other hand carbon and helium lines are extra strong, a type of Extreme helium star. These are yellow supergiant stars (actually low mass post-AGB stars at the most luminous stage of their lives) which have alternating deep and shallow minima. This double-peaked variation typically has periods of 30–100 days and amplitudes of 3–4 magnitudes. Superimposed on this variation, there may be long-term

neg: Exoplanetology or a mini-Neptune with a hydrogen–helium atmosphere. Another possibility for a low-mass low-density planet is that it has a large atmosphere made up chiefly of carbon monoxide, carbon dioxide, methane, or nitrogen. In 2014, new measurements of Kepler-10c found it to be a Neptune-mass planet (17 Earth masses) with a density higher than Earth's, indicating that Kepler-10c is composed mostly of rock with possibly up to 20% high-pressure water ice but without a hydrogen-dominated envelope. Because this is well above the 10-Earth-mass upper limit that is commonly used for the term 'super-Earth', the term mega-Earth has been coined. A similarly

neg: R136a1 they lie on the Zero Age Helium Main Sequence (He-ZAMS), analogous to and parallel to the hydrogen-burning main sequence but at hotter temperatures. During helium burning, carbon and oxygen will accumulate in the core and heavy mass loss continues. This eventually leads to the development of a WC spectrum, although at LMC metallicity a star is expected to spend most of the helium burning phase with a WN spectrum. Towards the end of helium burning, core temperature increase and mass loss cause an increase in both luminosity and temperature, with the spectral type becoming WO. Several hundred thousand years will

neg: Red supergiant star to individual objects, but it may become useful for analysis of galactic structure and discovery of otherwise obscured red supergiant stars. Surface abundances of red supergiants are dominated by hydrogen even though hydrogen at the core has been completely consumed. In the latest stages of mass loss before a star explodes, surface helium may become enriched to levels comparable with hydrogen. In theoretical extreme mass loss models, sufficient hydrogen may be lost that helium becomes the most abundant element at the surface. When pre-red supergiant stars leave the main sequence, oxygen is more abundant than carbon at the surface, and

neg: Stellar evolution half the mass of the Sun will be unable to ignite helium fusion (as noted earlier), and will produce a white dwarf composed chiefly of helium. In the end, all that remains is a cold dark mass sometimes called a black dwarf. However, the universe is not old enough for any black dwarfs to exist yet. If the white dwarf's mass increases above the Chandrasekhar limit, which is for a white dwarf composed chiefly of carbon, oxygen, neon, and/or magnesium, then electron degeneracy pressure fails due to electron capture and the star collapses. Depending upon the chemical composition and pre-collapse

neg: Carbon planet being only the remnant inner core. According to some definitions of planet, this would not qualify because it formed as a star. Planets around brown dwarfs are likely to be carbon planets depleted of water. Carbon planet A carbon planet is a theoretical type of planet that contains more carbon ("Z" = 6) than oxygen ("Z" = 8). Carbon is the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. Marc Kuchner and Sara Seager coined the term "carbon planet" in 2005 and investigated such planets following the suggestion of Katharina Lodders that Jupiter formed

neg: Stellar evolution and asymptotic-giant-branch stars, with inert cores made of carbon and helium-burning shells inside the hydrogen-burning shells. Between these two phases, stars spend a period on the horizontal branch with a helium-fusing core. Many of these helium-fusing stars cluster towards the cool end of the horizontal branch as K-type giants and are referred to as red clump giants. When a star exhausts the hydrogen in its core, it leaves the main sequence and begins to fuse hydrogen in a shell outside the core. The core increases in mass as the shell produces more helium. Depending on the mass of the helium

neg: Helium star Helium star A helium star or helium strong star is a class O or B star (blue), which has extraordinarily strong helium lines and weaker than normal hydrogen lines, indicating strong stellar winds and a mass loss of the outer envelope. "Extreme helium stars" (EHe) entirely lack hydrogen in their spectra. Previously, a "helium star" was a synonym for a B class star, but this usage is considered obsolete. A "helium star" is also a term for a hypothetical star that could occur if two helium white dwarfs with a combined mass of at least 0.5 solar masses merge and

neg: PV Telescopii PV Telescopii PV Telescopii (also known as HD 168476) is a class B-type (blue) supergiant extreme helium star in the constellation Telescopium. It is also the prototype of variable stars called PV Telescopii variables. PV Telescopii shows radial velocity changes thought to be due to radial pulsations caused by a strange mode instability It appears as a star of magnitude 9.3. Despite a mass thought to be less than the sun, it is actually around 25,000 more luminous. The spectrum shows a strong hydrogen deficiency and strongly enhanced helium and carbon lines. PV Telescopii variables are helium supergiants which vary

neg: Thermal runaway degenerate core reaches a critical mass of about 0.45 solar masses, helium fusion is ignited and takes off in a runaway fashion, called the helium flash, briefly increasing the star's energy production to a rate 100 billion times normal. About 6% of the core is quickly converted into carbon. While the release is sufficient to convert the core back into normal plasma after a few seconds, it does not disrupt the star, nor immediately change its luminosity. The star then contracts, leaving the red giant phase and continuing its evolution into a stable helium-burning phase. A nova results from runaway

neg: Main sequence occurring in regions with steeper temperature gradients, higher opacity or both. The main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. Stars below about 1.5 times the mass of the Sun () primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the proton–proton chain. Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of carbon, nitrogen and oxygen as intermediaries in the CNO cycle that produces helium from hydrogen atoms. Main-sequence stars with more

neg: Helium planet over time this will lead to an overall loss of methane. With time, the methane in the atmospheres of hot ice giants will also get depleted. A helium-rich planetary object may also form from a low-mass white dwarf, which gets depleted of hydrogen via mass transfer in a close binary system with a second, massive object like a neutron star. One scenario involves an AM CVn type of symbiotic binary star composed of two helium-core white dwarfs surrounded by a circumbinary helium accretion disk formed during mass transfer from the less massive to the more massive white dwarf. After it

neg: Carbon This carbyne is of considerable interest to nanotechnology as its Young's modulus is 40 times that of the hardest known material – diamond. In 2015, a team at the North Carolina State University announced the development of another allotrope they have dubbed Q-carbon, created by a high energy low duration laser pulse on amorphous carbon dust. Q-carbon is reported to exhibit ferromagetism, fluorescence, and a hardness superior to diamonds. Carbon is the fourth most abundant chemical element in the observable universe by mass after hydrogen, helium, and oxygen. Carbon is abundant in the Sun, stars, comets, and in the atmospheres

neg: Sun approximately 120 million years of active life left, but much happens. First, the core, full of degenerate helium ignites violently in the helium flash, where it is estimated that 6% of the core, itself 40% of the Sun's mass, will be converted into carbon within a matter of minutes through the triple-alpha process. The Sun then shrinks to around 10 times its current size and 50 times the luminosity, with a temperature a little lower than today. It will then have reached the red clump or horizontal branch, but a star of the Sun's mass does not evolve blueward along

neg: Helium flash energy is absorbed into the star's upper layers. Thus the helium flash is mostly undetectable to observation, and is described solely by astrophysical models. After the core's expansion and cooling, the star's surface rapidly cools and contracts in as little as 10,000 years until it is roughly 2% of its former radius and luminosity. It is estimated that the electron-degenerate helium core weighs about 40% of the star mass and that 6% of the core is converted into carbon. During the red giant phase of stellar evolution in stars with less than 2.0 the nuclear fusion of hydrogen ceases in

neg: Carbon-burning process a shell around the core. Outside this is another shell burning hydrogen. The resulting carbon burning provides energy from the core to restore the star's mechanical equilibrium. However, the balance is only short-lived; in a star of 25 solar masses, the process will use up most of the carbon in the core in only 600 years. The duration of this process varies significantly depending on the mass of the star. Stars with below 8–9 Solar masses never reach high enough core temperature to burn carbon, instead ending their lives as carbon-oxygen white dwarfs after shell helium flashes gently expel the

neg: VY Canis Majoris a radius of . The star has evolved rapidly because of its high mass. The time spent to the red hypergiant phase is estimated to be between 100,000 and 500,000 years, and thus VY CMa had left its main sequence more than a million years ago. The future evolution of VY CMa is uncertain, but like the most cool supergiants, the star will certainly explode as a supernova. It has begun to fuse helium into carbon, whereas a main sequence star fuses hydrogen into helium. In 2009, Nathan Smith of the University of California, Ken Hinkle and Nils Ride of

neg: V838 Monocerotis shells of dust surrounding the star, created by the star during previous similar outbursts. The brightening may have been a so-called helium flash, where the core of a dying low-mass star suddenly ignites helium fusion disrupting, but not destroying, the star. Such an event is known to have occurred in Sakurai's Object. However, several pieces of evidence support the argument that the dust is interstellar rather than centered on V838 Monocerotis. A dying star that has lost its outer envelopes would be appropriately hot, but the evidence points to a young star instead. According to some evidence, V838 Monocerotis may

neg: IK Pegasi When helium was exhausted in the core a helium-burning shell formed in addition to the hydrogen-burning one and the star moved to what astronomers term the asymptotic giant branch, or AGB. (This is a track leading to the upper-right corner of the H-R diagram.) If the star had sufficient mass, in time carbon fusion could begin in the core, producing oxygen, neon and magnesium. The outer envelope of a red giant or AGB star can expand to several hundred times the radius of the Sun, occupying a radius of about (3 AU) in the case of the pulsating AGB star

neg: Subgiant subgiant branch. The helium core mass of a Z=0.001 (extreme population II) star at the end of the main sequence is nearly double that of a Z=0.02 (population I) star. The low metallicity star is also over 1,000 K hotter and over twice as luminous at the start of the subgiant branch. The difference in temperature is less pronounced at the end of the subgiant branch, but the low metallicity star is larger and nearly four times as luminous. Similar differences exist in the evolution of stars with other masses, and key values such as the mass of a star

neg: IK Tauri is brighter than prominent stars such as Rigel (K-band magnitude +0.18) and comparable to Sirius (K-band magnitude −1.35). As a Mira variable, IK Tauri is an asymptotic giant branch (AGB) star, originally around . It has exhausted its core hydrogen and helium, is not massive enough to ignite its carbon-oxygen core, and is now alternately fusing in concentric hydrogen and helium shells. As the inert core grows and the hydrogen shell nears the surface, mass loss becomes very high and the star becomes highly obscured visually, an "infrared star". It will then quickly lose its entire atmosphere, creating a planetary

neg: Stellar evolution resulting in a white dwarf composed chiefly of oxygen, neon, and magnesium, provided that it can lose enough mass to get below the Chandrasekhar limit (see below), and provided that the ignition of carbon is not so violent as to blow the star apart in a supernova. A star of mass on the order of magnitude of the Sun will be unable to ignite carbon fusion, and will produce a white dwarf composed chiefly of carbon and oxygen, and of mass too low to collapse unless matter is added to it later (see below). A star of less than about

neg: Red giant the star takes as it moves along the red-giant branch, which ends with the complete collapse of the core, depends on the mass of the star. For the Sun and stars of less than about the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate, it will continue to heat until it reaches a temperature of roughly 10 K, hot enough to begin fusing helium to carbon via the triple-alpha process. Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a

neg: AM Canum Venaticorum star period systems with high accretion rates it could be as high as 5th magnitude. In most cases the accretor light output is swamped by the accretion disk. Some AM CVn variables have been detected at x-ray wavelengths. These contain extremely hot accretor stars, or possible hot spots on the accretor due to direct impact accretion. The donor star can potentially be either a helium (or possibly hybrid) white dwarf, a low-mass helium star, or an evolved main-sequence star. In some cases a donor white dwarf may have a comparable mass to the accretor although it is inevitably somewhat lower even

neg: Atomic mass (nucleon count). The amount that the ratio of atomic masses to mass number deviates from 1 is as follows: the deviation starts positive at hydrogen-1, then decreases until it reaches a local minimum at helium-4. Isotopes of lithium, beryllium, and boron are less strongly bound than helium, as shown by their increasing mass-to-mass number ratios. At carbon, the ratio of mass (in daltons) to mass number is defined as 1, and after carbon it becomes less than one until a minimum is reached at iron-56 (with only slightly higher values for iron-58 and nickel-62), then increases to positive values in

neg: La Superba helium flash, which begins helium core burning on the horizontal branch. Once even the core helium is exhausted, a degenerate carbon-oxygen core remains. Fusion continues in both hydrogen and helium shells at different depths in the star, and the star increases luminosity on the asymptotic giant branch (AGB). La Superba is currently an AGB star. In the AGB stars, fusion products are moved outwards from the core by strong deep convection known as a dredge-up, thus creating a carbon abundance in the outer atmosphere where carbon monoxide and other compounds are formed. These molecules tend to absorb radiation at shorter

neg: Helium planet loses most of its mass, the less massive white dwarf may approach planetary mass. Helium planets are expected to be distinguishable from regular hydrogen-dominated planets by strong evidence of carbon monoxide and carbon dioxide in the atmosphere. Due to hydrogen-depletion, the expected methane in the atmosphere cannot form because there is no hydrogen for the carbon to combine with, and hence carbon combines with oxygen instead, forming CO and CO. Due to the atmospheric composition, helium planets are expected to be white or grey in appearance. Such a signature can be found in Gliese 436 b, which has a predominance

neg: Main sequence hydrogen in the star to be consumed during the main-sequence lifetime. The outer regions of a massive star transport energy by radiation, with little or no convection. Intermediate-mass stars such as Sirius may transport energy primarily by radiation, with a small core convection region. Medium-sized, low-mass stars like the Sun have a core region that is stable against convection, with a convection zone near the surface that mixes the outer layers. This results in a steady buildup of a helium-rich core, surrounded by a hydrogen-rich outer region. By contrast, cool, very low-mass stars (below 0.4 ) are convective throughout. Thus

neg: Helium star transform into a red giant after losing its hydrogen envelope in the future. Helium star A helium star or helium strong star is a class O or B star (blue), which has extraordinarily strong helium lines and weaker than normal hydrogen lines, indicating strong stellar winds and a mass loss of the outer envelope. "Extreme helium stars" (EHe) entirely lack hydrogen in their spectra. Previously, a "helium star" was a synonym for a B class star, but this usage is considered obsolete. A "helium star" is also a term for a hypothetical star that could occur if two helium white

neg: Star Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss. When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a

neg: Planet Jupiter masses and a sub-brown dwarf of 7 Jupiter masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter masses, making them brown dwarfs according to the IAU working definitions. In close binary star systems one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438. These shrunken white dwarfs may become a helium planet or carbon planet. Some large satellites (moons) are of similar size or larger than the

neg: Wolf–Rayet star dust. Usually this takes place on those belonging to binary systems as a product of the collision of the stellar winds forming the pair, as is the case of the famous binary WR 104; however this process occurs on single ones too. A few (roughly 10%) of the central stars of planetary nebulae are, despite their much lower (typically ~0.6 solar) masses, also observationally of the WR-type; i.e. they show emission line spectra with broad lines from helium, carbon and oxygen. Denoted [WR], they are much older objects descended from evolved low-mass stars and are closely related to white dwarfs,

neg: BX Circini and the average surface temperature has been measured at 23,390 ±90 K using optical spectra, but 1750 K cooler if analysing it in both the visual and ultraviolet. The temperature appears to vary by 3450 K. It has an extremely low proportion of hydrogen, which was first noticed in 1980. In fact, over 99% of its composition appears to be helium, qualifying it as an extreme helium star. Its origin is unclear, but thought to be a result of the merger of a helium white dwarf with a carbon/oxygen one. The two merge violently, with material from the lighter helium

neg: NGC 3603-B brightest single star. HD 97950B is a Wolf-Rayet (WR) star, with spectra dominated by strong broadened emission lines. Type WN6 indicates that ionised nitrogen lines are strong in comparison to ionised carbon lines, and the suffix "h" indicates that hydrogen is also seen in the spectrum. This type of WR star is not the classical stripped helium-burning aged star, but a young highly luminous object with CNO cycle fusion products showing at the surface due to strong conventional and rotational mixing, and high mass loss rates from the atmosphere. The emission lines are generated in the stellar wind and the

neg: Supernova whenever the mass of the star at the time of core collapse is low enough not to cause complete fallback to a black hole, any massive star may result in a supernova if it loses enough mass before core collapse occurs. Type Ib supernovae are the more common and result from Wolf–Rayet stars of Type WC which still have helium in their atmospheres. For a narrow range of masses, stars evolve further before reaching core collapse to become WO stars with very little helium remaining and these are the progenitors of Type Ic supernovae. A few percent of the Type

neg: V605 Aquilae nebulosity. Although the star could not be detected directly, scattered light showed a [WC4] spectral type, quite different from the spectrum at peak brightness. In 2013, the central star was detected at magnitude 20.2, with an estimated four magnitudes of extinction. The spectral type is now [WC4], a hydrogen-deficient, helium and carbon-rich object with strong emission lines. In 1921, the surface has been estimated to consist of 98% helium and 1% carbon, typical of an RCB star. By 2006, the abundances were measured as 55% helium, 45% carbon, and 5% oxygen, typical of a WC star. Both are very unusual,

neg: Brown dwarf densities and pressures needed. Further gravitational contraction is prevented and the result is a "failed star", or brown dwarf that simply cools off by radiating away its internal thermal energy. Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which reach the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. Fusion of lithium-7 and a proton occurs producing two helium-4 nuclei. The temperature necessary for this reaction is just below that necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is eventually depleted. Therefore, the

neg: Helium-4 nucleon (carbon). However, due to the lack of intermediate elements, this process needs three helium nuclei striking each other nearly simultaneously (see triple alpha process). There was thus no time for significant carbon to be formed in the few minutes after the Big Bang, before the early expanding universe cooled to the temperature and pressure where helium fusion to carbon was no longer possible. This left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4.

neg: Hydrogen-deficient star relatively few of these stars were known. Large-scale stellar surveys since then have greatly increased the number and variety of known hydrogen-deficient stars. As of 2008, about 2,000 hydrogen-deficient stars were known. Despite being relatively rare, there are many different types of hydrogen-deficient stars. They can be grouped into five general classes: massive or upper-main-sequence stars, low-mass supergiants, hot subdwarf stars, central stars of planetary nebulae, and white dwarfs. There have been other classification schemes, such as one based on carbon content. Wolf-Rayet stars show bright bands in continuous spectra that come from ionized atoms such as helium. Although there

neg: Isotope "nuclide", and is still sometimes used in contexts where "nuclide" might be more appropriate, such as nuclear technology and nuclear medicine. An isotope and/or nuclide is specified by the name of the particular element (this indicates the atomic number) followed by a hyphen and the mass number (e.g. helium-3, helium-4, carbon-12, carbon-14, uranium-235 and uranium-239). When a chemical symbol is used, e.g. "C" for carbon, standard notation (now known as "AZE notation" because "A" is the mass number, "Z" the atomic number, and E for element) is to indicate the mass number (number of nucleons) with a superscript at the

neg: Helium flash material to the compact star. Note that similar flashes occur on neutron stars. Shell helium flashes are a somewhat analogous but much less violent, nonrunaway helium ignition event, taking place in the absence of degenerate matter. They occur periodically in asymptotic giant branch stars in a shell outside the core. This is late in the life of a star in its giant phase. The star has burnt most of the helium available in the core, which is now composed of carbon and oxygen. Helium fusion continues in a thin shell around this core, but then turns off as helium becomes

neg: Planetary nebula K, the available helium nuclei fuse into carbon and oxygen, so that the star again resumes radiating energy, temporarily stopping the core's contraction. This new helium burning phase (fusion of helium nuclei) forms a growing inner core of inert carbon and oxygen. Above it is a thin helium-burning shell, surrounded in turn by a hydrogen-burning shell. However, this new phase lasts only 20,000 years or so, a short period compared to the entire lifetime of the star. In either scenario, the venting of atmosphere continues unabated into interstellar space, but when the outer surface of the exposed core reaches temperatures

neg: Subdwarf O star Subdwarf O star A subdwarf O star (sdO) is a type of hot, but low-mass star. O-type subdwarfs are much dimmer than regular O-type main-sequence stars, but with a brightness about 10 to 100 times that of the Sun, and have a mass approximately half that of the Sun. Their temperature ranges from 40,000 to 100,000 K. Ionized helium is prominent in their spectra. Gravity acceleration is expressed by log "g" between 4.0 and 6.5. Many sdO stars are moving at high velocity through the Milky Way and are found at high galactic latitudes. The structure of a subdwarf O

neg: Lambda Velorum apparent magnitudes +2.14 to +2.30. λ Velorum is an evolved star that has exhausted the hydrogen in its core region. It has about seven times the mass of the Sun. It is likely to be on or approaching the asymptotic giant branch (AGB), although its properties do not exclude it being a slightly more massive star on the red giant branch (RGB). As an AGB star it has an inert core of carbon and oxygen and is alternately fusing helium and hydrogen in two shells outside the core. The star's outer envelope has expanded to form a deep, convective, hydrogen

neg: R Coronae Borealis of the metal lines disappear although the Ca doublet remains strong. Forbidden "nebular" lines of [O], [O], and [N] can be detected at times. The spectrum at maximum indicates that hydrogen in R Coronae Borealis is strongly depleted, helium is the dominant element, and carbon is strongly enhanced. At minimum, the spectrum shows the development of carbon clouds that obscure the photosphere, leaving chromospheric lines visible at times. R Coronae Borealis is about 90% helium and less than 1% hydrogen. The majority of the remainder is carbon. The qualifies as a carbon-enhanced extreme helium star. Modelling the pulsations suggests that

neg: Star branch. After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate. During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a

neg: NGC 3603-C be a binary, but its companion has not been observed. HD 97950C is a Wolf-Rayet (WR) star, with spectra dominated by strong broadened emission lines. Type WN6 indicates that ionised nitrogen lines are strong in comparison to ionised carbon lines, and the suffix "h" indicates that hydrogen is also seen in the spectrum. This type of WR star is not the classical stripped helium-burning aged star, but a young highly luminous object with CNO cycle fusion products showing at the surface due to strong conventional and rotational mixing, and high mass loss rates from the atmosphere. The emission lines are

neg: Intergalactic star of chemical elements other than hydrogen and helium within a star) indicating an inner galactic origin, since stars outside the disks of galaxies tend to have low metallicity and are older. Some recently discovered supernovae have been confirmed to have exploded hundreds of thousands of light years from the nearest star or galaxy. Although the precise mass of the intergalactic star population cannot be known exactly, it is estimated that locally they make up 10 percent of the mass of the Virgo cluster of galaxies. This means that, most likely, intergalactic stars in the Virgo cluster would collectively have a

neg: Ross 154 the Sun where convection only occurs in the outer layers, a red dwarf with a mass this low will be entirely convective. Based on the relatively high projected rotation, this is probably a young star with an estimated age of less than a billion years. The abundance of elements heavier than helium is about half that in the Sun. No low-mass companions have been discovered in orbit around Ross 154. Nor does it display the level of excess infrared emission that would suggest the presence of circumstellar dust. Such debris disks are rare among M-type star systems older than about

neg: Sirius of a carbon–oxygen mixture that was generated by helium fusion in the progenitor star. This is overlaid by an envelope of lighter elements, with the materials segregated by mass because of the high surface gravity. The outer atmosphere of Sirius B is now almost pure hydrogen—the element with the lowest mass—and no other elements are seen in its spectrum. Since 1894, there have been observed irregularities in the orbits of Sirius A and B with an apparent periodicity of 6–6.4 years. A 1995 study found such a companion to likely exist, with a mass of roughly 0.05 solar masses- a

neg: Lithium burning Lithium burning Lithium burning is a nucleosynthetic process in which lithium is depleted in a star. Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which by definition must achieve the high temperature (2.5 × 10 K) necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of lithium-7 and a proton producing two helium-4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium

neg: Hydrogen-deficient star a faint blue star in 1994 to a yellow supergiant in 1996. One proposed mechanism for this migration is the final helium flash scenario. He-sdB are subdwarfs with class B spectra with broader than usual H, HeI, and HeII lines. JL 87 in 1991 was the first He-sdB star to be reported. Since then this class of stars has been shown to have a wide range of hydrogen-to-helium ratios. Compact He-sdO stars have class O spectra, are typically nitrogen-rich, and may or may not be carbon-rich. Low-gravity He-sdO stars overlap with their compact cousins, but have lower surface gravity. It

neg: Extreme helium star carbon to helium is always in the range of 0.3 to 1%. This is despite wide variation of other abundance ratios in EHe stars. The known extreme helium stars are supergiants where hydrogen is underabundant by a factor of 10,000 or more. The surface temperatures of these stars range from 9,000–35,000 K. They are primarily composed of helium, with the second most abundant element, carbon, forming about one atom per 100 atoms of helium. The chemical composition of these stars implies that they have undergone both hydrogen and helium burning at some stage of their evolution. Two possible scenarios were

neg: Helium is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4. All heavier elements (including those necessary for rocky planets like the Earth, and for carbon-based or other life) have thus been created since the Big Bang in stars which were hot enough to fuse helium itself. All elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe. Helium-4, by contrast, makes up about 23% of the universe's ordinary matter—nearly all the ordinary matter that is not hydrogen.

neg: U Aquilae Cepheid variable star ranging between magnitudes 6.08 and 6.86 over a period of 7.02 days. It is an evolved star which has exhausted its core hydrogen and is now fusing helium into carbon. U Aquilae U Aquilae is a binary star system in the constellation Aquila, Located approximately away from Earth. The primary star (component A) is a yellow supergiant with a radius of and a luminosity of . The secondary (component B) is a blue main-sequence star, twice the mass of the sun and around thirty times more luminous. It is hotter than the primary star at 9,300 K,

neg: Giant star degenerate helium fusion begins explosively, but most of the energy goes into lifting the degeneracy and the core becomes convective. The energy generated by helium fusion reduces the pressure in the surrounding hydrogen-burning shell, which reduces its energy-generation rate. The overall luminosity of the star decreases, its outer envelope contracts again, and the star moves from the red-giant branch to the horizontal branch. When the core helium is exhausted, a star with up to about has a carbon–oxygen core that becomes degenerate and starts helium burning in a shell. As with the earlier collapse of the helium core, this starts

neg: Rho Cassiopeiae a solar mass or 10,000 Earth masses. The luminosity remains roughly constant during the outbursts at , but the radiation output shifts towards the infra red. Surface abundances of most heavy elements on Rho Cas are enhanced relative to the Sun, but carbon and oxygen are depleted. This is expected for a massive star where hydrogen fusion takes place predominantly via the CNO cycle. In addition to the expected helium and nitrogen convected to the surface, sodium is strongly enhanced, indicating that the star had experienced a dredge-up while in a red supergiant stage. Therefore, it is expected that Rho

neg: Carbon star the modern spectral types C-R and C-N, the abundance of carbon is thought to be a product of helium fusion, specifically the triple-alpha process within a star, which giants reach near the end of their lives in the asymptotic giant branch (AGB). These fusion products have been brought to the stellar surface by episodes of convection (the so-called third dredge-up) after the carbon and other products were made. Normally this kind of AGB carbon star fuses hydrogen in a hydrogen burning shell, but in episodes separated by 10-10 years, the star transforms to burning helium in a shell, while the

neg: V391 Pegasi the ejection of the hydrogen envelope of a red giant star at or just before the onset of helium fusion. The ejection left only a tiny amount of hydrogen on the surface - less than 1/1000 of the total stellar mass. The future for the star is to eventually cool down to make a low mass white dwarf. Most stars retain more of their hydrogen after the first red giant phase, and eventually become asymptotic giant branch stars. The reason that some stars, like V391 Pegasi, lose so much mass is not well known. In 2007, a gas giant planet

neg: Nucleosynthesis the beginning of the universe, but no rational physical scenario for this could be identified. Gradually it became clear that hydrogen and helium are much more abundant than any of the other elements. All the rest constitute less than 2% of the mass of the Solar System, and of other star systems as well. At the same time it was clear that oxygen and carbon were the next two most common elements, and also that there was a general trend toward high abundance of the light elements, especially those composed of whole numbers of helium-4 nuclei. Arthur Stanley Eddington first

neg: Carbon detonation A white dwarf is the remnant of a small to medium size star (our sun is an example of these). At the end of its life, the star has burned its hydrogen and helium fuel, and thermonuclear fusion processes cease. The star does not have enough mass to either burn much heavier elements, or to implode into a neutron star or type II supernova as a larger star can, from the force of its own gravity, so it gradually shrinks and becomes very dense as it cools, glowing white and then red, for a period many times longer than the

neg: White dwarf among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910. The name "white dwarf" was coined by Willem Luyten in 1922. White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star, that of about 10 solar masses. This includes over 97% of the other stars in the Milky Way. After the hydrogen-fusing period of a main-sequence star of low or medium mass ends, such a star will expand to a red giant during which it fuses helium to

neg: White dwarf the mass of a main-sequence star is lower than approximately half a solar mass, it will never become hot enough to fuse helium in its core. It is thought that, over a lifespan that considerably exceeds the age of the Universe (c. 13.8 billion years), such a star will eventually burn all its hydrogen, for a while becoming a blue dwarf, and end its evolution as a helium white dwarf composed chiefly of helium-4 nuclei. Due to the very long time this process takes, it is not thought to be the origin of the observed helium white dwarfs. Rather, they

neg: White dwarf but not neon, in which case an oxygen–neon–magnesium white dwarf may form. Stars of very low mass will not be able to fuse helium, hence, a helium white dwarf may form by mass loss in binary systems. The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy. As a result, it cannot support itself by the heat generated by fusion against gravitational collapse, but is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a non-rotating white dwarf, the

neg: V445 Puppis an enrichment of helium and carbon, and a higher level of ionization, suggested that it was the first observed instance of a helium nova. This is theorized to occur when a white dwarf star predominantly accretes helium (rather than hydrogen) from an orbiting companion. When sufficient helium has accumulated along a shell on the surface of the white dwarf, a run-away thermonuclear explosion results in a nova outburst. Hence, V445 Puppis may belong to a binary star system and be surrounded by an accretion disk of matter drawn from the companion star. At present, the system is being obscured by

neg: Red giant and helium core flash. When the ascent of the red-giant branch ends they puff off their outer layers much like a post-asymptotic-giant-branch star and then become a white dwarf. Very low mass stars are fully convective and may continue to fuse hydrogen into helium for up to a trillion years until only a small fraction of the entire star is hydrogen. Luminosity and temperature steadily increase during this time, just as for more-massive main-sequence stars, but the length of time involved means that the temperature eventually increases by about 50% and the luminosity by around 10 times. Eventually the level

neg: IK Pegasi thermonuclear fusion to produce helium. This category of super-soft sources consist of high-mass white dwarfs with very high surface temperatures ( to ). Should the white dwarf's mass approach the Chandrasekhar limit of 1.4 it will no longer be supported by electron degeneracy pressure and it will undergo a collapse. For a core primarily composed of oxygen, neon and magnesium, the collapsing white dwarf is likely to form a neutron star. In this case, only a fraction of star's mass will be ejected as a result. If the core is instead made of carbon-oxygen, however, increasing pressure and temperature will

neg: Photosphere certain wavelengths. The surface of a star is defined to have a temperature given by the effective temperature in the Stefan–Boltzmann law. Stars, except neutron stars, have no solid surface. Therefore, the photosphere is typically used to describe the Sun's or another star's visual surface . The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively. All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon

neg: Helium star subsequently start nuclear fusion of helium, with a lifetime of a few hundred million years. This may only happen if these 2 binary masses share the same type of envelope phase. It is believed this is the origin of the extreme helium stars. The helium star's great capability of transforming into other stellar objects has been observed over the years. In 2014, a helium nova named V445 Puppis exploded, along with a following explosion of the star SN2012Z, causing a high-mass transfer between the two. It is observed to have caused a growing helium star that has the potential to

neg: History of Solar System formation and evolution hypotheses to form carbon, and for three helium atoms to form carbon-12 was so unlikely as to have been impossible over the age of the Universe. However, in 1952 the physicist Ed Salpeter showed that a short enough time existed between the formation and the decay of the beryllium isotope that another helium had a small chance to form carbon, but only if their combined mass/energy amounts were equal to that of carbon-12. Hoyle, employing the anthropic principle, showed that it must be so, since he himself was made of carbon, and he existed. When the matter/energy level of carbon-12 was

neg: Helium planet of carbon monoxide. Helium planet A helium planet is a planet with a helium-dominated atmosphere. This contrasts with ordinary gas giants such as Jupiter and Saturn, whose atmospheres consist primarily of hydrogen, with helium as a secondary component only. Helium planets might form in a variety of ways. Gliese 436 b is a candidate helium planet. There are several hypotheses for how a helium planet might form. A helium planet might form via hydrogen evaporation from a gaseous planet orbiting close to a star. The star will drive off lighter gases more effectively through evaporation than heavier gasses, and over