Stellar classifications

Stellar classifications (So far)

A-type main-sequence star

(A V) is a main-sequence (hydrogen-burning) star of spectral type A and luminosity class V.

Examples include Altair, Sirius .

These stars have spectra which are defined by strong hydrogen Balmer absorption lines.

They have masses from 1.4 to 2.1 times the mass of the Sun and surface temperatures between 7,600 and 10,000 K.

Am star

Anomalous X-ray pulsar

Anomalous X-ray Pulsars (AXPs) are now widely believed to be magnetars—young, isolated, highly magnetized neutron stars. These energetic X-ray pulsars are characterized by slow rotation periods of ~2–12 seconds and large magnetic fields of ~1013–1015 gauss (1 to 100 gigateslas). There are currently (as of 2009) 9 known and 1 candidate AXPs.

Ap and Bp stars

B-type main-sequence star

Barium star

Barium stars are G to K class giants, whose spectra indicate an overabundance of s-process elements by the presence of singly ionized barium, Ba II, at λ 455.4nm. Barium stars also show enhanced spectral features of carbon, the bands of the molecules CH, CN and C2.

Observational studies of their radial velocity suggested that all barium stars are binary stars[2][3][4] Observations in the ultraviolet using International Ultraviolet Explorer detected white dwarfs in some barium star systems.

Barium stars are believed to be the result of mass transfer in a binary star system. The mass transfer occurred when the presently-observed giant star was on the main sequence. Its companion, the donor star, was a carbon star on the asymptotic giant branch (AGB), and had produced carbon and s-process elements in its interior. These nuclear fusion products were mixed by convection to its surface. Some of that matter “polluted” the surface layers of the main sequence star as the donor star lost mass at the end of its AGB evolution, and it subsequently evolved to become a white dwarf. We are observing these systems an indeterminate amount of time after the mass transfer event, when the donor star has long been a white dwarf, and the “polluted” recipient star has evolved to become a red giant.[5] [6].

During its evolution, the barium star will at times be larger and cooler than the limits of the spectral types G or K. When this happens, ordinarily such a star is spectral type M, but the s-process excesses may cause it to show its altered composition as another spectral peculiarity. While the star’s surface temperature is in the M-type regime, the star may show molecular features of the s-process element zirconium, zirconium oxide (ZrO) bands. When this happens, the star will appear as an “extrinsic” S star.

Historically, barium stars posed a puzzle, because in standard stellar evolution theory G and K giants are not far enough along in their evolution to have synthesized carbon and s-process elements and mix them to their surfaces. The discovery of the stars’ binary nature resolved the puzzle, putting the source of their spectral peculiarities into a companion star which should have produced such material. The mass transfer episode is believed to be quite brief on an astronomical timescale. The mass transfer hypothesis predicts that there should be main sequence stars with barium star spectral peculiarities.

Be star

Binary pulsar

A binary pulsar is a pulsar with a binary companion, often a white dwarf or neutron star. (In at least one case, the double pulsar PSR J0737-3039, the companion star is another pulsar as well.) Binary pulsars are one of the few objects which allow physicists to test general relativity in the case of a strong gravitational field.

. Binary pulsar timing has thus indirectly confirmed the existence of gravitational radiation and verified Einstein’s general theory of relativity in a previously unknown regime.

Binary pulsars are one of the few tools scientists have to detect evidence of gravitational waves;

Binary pulsars are one of the few tools scientists have to detect evidence of gravitational waves; Einstein’s theory of general relativity predicts that two neutron stars would emit gravitational waves as they orbit a common center of mass, which would carry away orbital energy and cause the two stars to draw closer together. As the two stellar bodies draw closer to one another, often a pulsar will absorb matter from the other causing a violent accretion process. This interaction can heat the gas being exchanged between the bodies and produce X-ray light which can appear to pulsate, causing binary pulsars to occasionally be referred to as X-ray binaries. This flow of matter from one stellar body to another is known as an accretion disk. Millisecond pulsars (or MSP’s) create a sort of “wind”, which in the case of binary pulsars can blow away the magnetosphere of the neutron stars and have a dramatic effect on the pulse emission.

When the two bodies are in close proximity, the gravitational field is stronger, the passage of time is slowed – and the time between pulses (or ticks) is lengthened. As the pulsar clock travels more slowly through the weakest part of the field it regains time. This relativistic time delay is the difference between what one would expect to see if the pulsar were moving at a constant distance and speed around it companion in a circular orbit, and what is actually observed (Haynes 2007).

Binary star

A binary star is a star system consisting of two stars orbiting around their common center of mass

If components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these close binary systems can exchange mass, which may bring their evolution to stages that single stars cannot attain

When a binary system contains a compact object such as a white dwarf, neutron star or black hole, gas from the other (donor) star can accrete onto the compact object. This releases gravitational potential energy,

Black dwarf

Black star (semiclassical gravity)

Blue dwarf (red-dwarf stage)

Blue giant

A blue giant is a massive star that has exhausted the hydrogen fuel in its core and left the main sequence.

As they grow older they expand and cool, eventually becoming red giants, or continuing fusion into a more luminous or massive star

These stars, the massive and middle-aged blue giants, represent a transitory phase where the star is either to become a bright giant (and eventually a planetary nebula and massive white dwarf) or a supergiant (and eventually a supernova or rare oxygen-neon white dwarf) and no star remains as this kind of blue giant for very long

Blue straggler

Blue supergiant

Bright giant

Brown dwarf

Brown dwarfs, were originally called black dwarfs, a classification for dark substellar objects floating freely in space which were too low in mass to sustain stable hydrogen fusion (the term black dwarf currently refers to a white dwarf that has cooled down so that it no longer emits significant heat or visible light)

The standard mechanism for star birth is through the gravitational collapse of a cold interstellar cloud of gas and dust. As the cloud contracts it heats up from the release of gravitational potential energy. Early in the process the contracting gas quickly radiates away much of the energy, allowing the collapse to continue. Eventually, the central region becomes sufficiently dense to trap radiation. Consequently, the central temperature and density of the collapsed cloud increases dramatically with time, slowing the contraction, until the conditions are hot and dense enough for thermonuclear reactions to occur in the core of the protostar. For most stars, gas and radiation pressure generated by the thermonuclear fusion reactions within the core of the star will support it against any further gravitational contraction. Hydrostatic equilibrium is reached and the star will spend most of its lifetime fusing hydrogen into helium as a main-sequence star.

If, however, the mass of the protostar is less than about 0.08 solar mass, normal hydrogen thermonuclear fusion reactions will not ignite in the core. Gravitational contraction does not heat the small protostar very effectively, and before the temperature in the core can increase enough to trigger fusion, the density reaches the point where electrons become closely packed enough to create quantum electron degeneracy pressure.

Lithium: Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature 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 line in a candidate brown dwarf’s spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test

Carbon detonation

Carbon star

A carbon star is a late-type star similar to a red giant (or occasionally to a red dwarf) 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.

In normal stars (such as the Sun), the atmosphere is richer in oxygen than carbon. Ordinary stars not exhibiting the characteristics of carbon stars, but that are cool enough to form carbon monoxide, are therefore called oxygen stars.

CH star

CH stars are particular type of carbon stars which are characterized by the presence of exceedingly strong CH absorption bands in their spectra. They belong to the star population II, meaning they’re metal poor and generally pretty middle-aged stars, and are underluminous compared to the classical C-N carbon stars. Many CH stars are known to be binaries, and it’s reasonable to believe this is the case for all CH stars. Like Barium stars, they are probably the result of a mass transfer from a former classical carbon star, now a white dwarf, to the current CH-classed star.

The CH stars are Population II stars with similar evolutionary state, spectral peculiarities, and orbital statistics, and are believed to be the older, metal-poor analogs of the barium stars.

Compact star

The term compact star (sometimes compact object) is used to refer collectively to white dwarfs, neutron stars, other exotic dense stars, and black holes.

These objects are all small for their mass. The term compact star is often used when the exact nature of the star is not known, but evidence suggests that it is very massive and has a small radius, thus implying one of the above-mentioned possibilities. A compact star which is not a black hole may be called a degenerate star.

Compact stars form the endpoint of stellar evolution. A star shines and thus loses energy. The loss from the radiating surface is compensated by the production of energy from nuclear fusion in the interior of the star. When a star has exhausted all its energy and undergoes stellar death, the gas pressure of the hot interior can no longer support the weight of the star and the star collapses to a denser state: a compact star.

Although compact stars may radiate, and thus cool off and lose energy, they do not depend on high temperatures to maintain their pressure. Barring external perturbation or baryon decay, they will persist virtually forever, although black holes are generally believed to finally evaporate from Hawking radiation. Eventually, given enough time (when we enter the so-called degenerate era of the universe), All stars will have evolved into dark, compact stars.

Dark-energy star

Dark star (dark matter)

Dark star (Newtonian mechanics)

Double star

Electroweak star

Exotic star

F-type main-sequence star

Flare star

FU Orionis star

Fusor (astronomy)

Fuzzball (string theory)

G-type main-sequence star

Giant star


Helium star

Herbig Ae/Be star



Intergalactic star

An intergalactic star is a star which does not belong to a galaxy. These stars were a source of much discussion in the scientific community during the late 1990s and are generally thought to be the result of colliding galaxies.

Although the way in which these stars form is still a mystery, the most common theory is that the collision of two or more galaxies can toss certain stars out into the vast regions of empty space

Iron star

In astronomy, an iron star is a hypothetical type of star that could occur in the universe in 101500 years. The premise behind iron stars states that cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into iron-56 nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting stellar-mass objects to cold spheres of iron.[1] The formation of these stars is only a possibility if the proton does not decay.

K-type main-sequence star

Lambda Boötis star

Late-type star

Lead star


Magnetospheric eternally collapsing object

Main sequence

Main sequence star

Mercury-manganese star

Neutron star

O-type main-sequence star

OB star

Optical pulsar

Peculiar star

PG 1159 star

Photometric-standard star

Planetar (astronomy)

Population I star

Population II star

Population III star

Pre-main-sequence star

Preon star

P cont.



Q star

Quark star


Radio star

Radio-quiet neutron star

Rapidly oscillating Ap star

Red dwarf

Red giant

Red supergiant

Relativistic star

S-type star

Shell star

Soft gamma repeater

Stellar black hole

Sub-brown dwarf

Subdwarf B star

Subdwarf star



T Tauri star

Technetium star

Thorne–Żytkow object

Variable star

User:Headbomb/White dwarf

White dwarf

A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf’s mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy.

White dwarfs are thought to be the final evolutionary state of all stars whose mass is not high enough to become a neutron star—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. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, around 1 billion K, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf.[6] Usually, therefore, white dwarfs are composed of carbon and oxygen. If the mass of the progenitor is above 8 solar masses but below 10.5 solar masses,the core temperature suffices 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.

The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported by the heat generated by fusion against gravitational collapse. It 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 Chandrasekhar limit—approximately 1.4 solar masses—beyond which it cannot be supported by electron degeneracy pressure. A carbon-oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation.

Wolf–Rayet star

Wolf-Rayet stars are evolved, massive stars (over 20 solar masses initially), which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s

While our own Sun loses approximately 10−14 solar masses every year, Wolf–Rayet stars typically lose 10−5 solar masses a year.

Wolf–Rayet stars are very hot, with surface temperatures in the range of 25,000 K to 50,000 K.

In 1867, astronomersdiscovered three stars in the constellation Cygnus that displayed broad emission bands on an otherwise continuous spectrum. Most stars display absorption bands in the spectrum, as a result of overlying elements absorbing light energy at specific frequencies. The number of stars with emission lines is quite low, so these were clearly unusual objects..

It was later shown that the the emission bands, )or pickering lines) resulted from the presence of helium;

In addition to helium, emission lines of carbon, oxygen and nitrogen were identified in the spectra of Wolf–Rayet stars.

Wolf–Rayet stars are a normal stage in the evolution of very massive stars, in which strong, broad emission lines of helium and nitrogen (“WN” sequence) or helium, carbon, and oxygen (“WC” sequence) are visible. Due to their strong emission lines they can be identified in nearby galaxies. About 300 Wolf–Rayets are catalogued in our own Milky Way Galaxy.

The characteristic emission lines are formed in the extended and dense high-velocity wind region enveloping the very hot stellar photosphere, which produces a flood of UV radiation that causes fluorescence in the line-forming wind region. This ejection process uncovers in succession, first the nitrogen-rich products of CNO cycle burning of hydrogen (WN stars), and later the carbon-rich layer due to He burning (WC and WO stars). Most of these stars are believed finally to progress to become supernovae of Type Ib or Type Ic. 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, rather than to the very young, very massive stars that comprise the bulk of the WR class..

It is possible for a Wolf–Rayet star to progress to a “collapsar” stage in its death throes: This is when the core of the star collapses to form a black hole, pulling in the surrounding material. This is thought to be the precursor of a long gamma-ray burst.

X-ray pulsar

X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity. The X-ray periods range from as little as a fraction of a second to as much as several minutes.

An X-ray pulsar consists of a magnetized neutron star in orbit with a normal stellar companion and are a type of binary star system. The magnetic field strength at the surface of the neutron star is typically about 108 Tesla, over a trillion times stronger than the strength of the magnetic field measured at the surface of the Earth (60 nT).

Gas is accreted from the stellar companion and is channeled by the neutron star’s magnetic field on to the magnetic poles producing two or more localized X-ray hot spots similar to the two auroral zones on the Earth but far hotter. At these hotspots the infalling gas can reach half the speed of light before it impacts the neutron star surface. So much gravitational potential energy is released by the infalling gas, that the hotspots, which are estimated to about one square kilometer in area, can be up to ten thousand times or more luminous than the Sun.

Temperatures of millions of degrees are produced so the hotspots emit mostly X-rays. As the neutron star rotates, pulses of X-rays are observed as the hotspots move in and out of view if the magnetic axis is tilted with respect to the spin axis.

The gas that supplies the X-ray pulsar can reach the neutron star by a variety of ways that depend on the size and shape of the neutron star’s orbital path and the nature of the companion star.

Some companion stars of X-ray pulsars are very massive young stars, usually OB supergiants (see stellar classification), that emit a radiation driven stellar wind from their surface. The neutron star is immersed in the wind and continuously captures gas that flows nearby. Vela X-1 is an example of this kind of system.

In other systems, the neutron star orbits so closely to its companion that its strong gravitational force can pull material from the companion’s atmosphere into an orbit around itself, a mass transfer process known as Roche lobe overflow. The captured material forms a gaseous accretion disc and spirals inwards to ultimately fall onto the neutron star as in the binary system Cen X-3.

For still other types of X-ray pulsars, the companion star is a Be star that rotates very rapidly and apparently sheds a disk of gas around its equator. The orbits of the neutron star with these companions are usually large and very elliptical in shape. When the neutron star passes nearby or through the Be circumstellar disk, it will capture material and temporarily become an X-ray pulsar. The circumstellar disk around the Be star expands and contracts for unknown reasons, so these are transient X-ray pulsars that are observed only intermittently, often with months to years between episodes of observable X-ray pulsation.

Radio pulsars (rotation-powered pulsars) and X-ray pulsars exhibit very different spin behaviors and have different mechanisms producing their characteristic pulses although it is accepted that both kinds of pulsar are manifestations of a rotating magnetized neutron star. The rotation cycle of the neutron star in both cases is identified with the pulse period.

The major differences are that radio pulsars have periods on the order of milliseconds to seconds, and all radio pulsars are losing angular momentum and slowing down. In contrast, the X-ray pulsars exhibit a variety of spin behaviors. Some X-ray pulsars are observed to be continuously spinning faster or slower (with occasional reversals in these trends) while others show either little change in pulse period or display erratic spin-down and spin-up behavior.

The explanation of this difference can be found in the physical nature of the two pulsar classes. Over 99% of radio pulsars are single objects that radiate away their rotational energy in the form of relativistic particles and magnetic dipole radiation, lighting up any nearby nebulae that surround them. In contrast, X-ray pulsars are members of binary star systems and accrete matter from either stellar winds or accretion disks. The accreted matter transfers angular momentum to (or from) the neutron star causing the spin rate to increase or decrease at rates that are often hundreds of times faster than the typical spin down rate in radio pulsars. Exactly why the X-ray pulsars show such varied spin behavior is still not clearly understood.

Yellow hypergiant

A yellow hypergiant is a massive star with an extended atmosphere,

Yellow hypergiants, have been observed to experience periodic eruptions, resulting in periodic or continuous dimming of the star, respectively.

Yellow hypergiants appear to be extremely rare in the universe. Due to their extremely rapid rate of consumption of nuclear fuel, yellow hypergiants generally only remain on the main sequence for a few million years before destroying themselves in a massive supernova or hypernova.

Yellow hypergiants are post-red supergiants, rapidly evolving toward the blue supergiant phase. They are in a so-called “Yellow Evolutionary Void,” a part of the Hertzsprung-Russell diagram where post-red supergiants exhibit atmospheric instability while evolving blueward; however, there exists strong chemical and surface gravity.

They can quickly run out of core nuclear fuel and implode to become Type II supernovae.

According to the current physical models of stars, a yellow hypergiant should possess a convective core surrounded by a radiative zone, as opposed to a sun-sized star, which consists of a radiative core surrounded by a convective zone.

Due to the extremely high pressures which exist at the core of a yellow hypergiant, portions of the core or perhaps the entire core may be composed of degenerate matter.

Due to the sheer size of these stars, in addition to powerful magnetic fields and their extreme energy output, yellow hypergiants are less effective at retaining surface material than other kinds of stars. They therefore have very large, extended atmospheres.

Young stellar object

A Young stellar object (YSO) denotes a star in its early stage of evolution.

This class consists of two groups of objects: protostars and pre–main sequence stars.


In astrophysics, stars are classified by their surface temperature, that is associated to specific spectral patterns. An early schema from the 19th century ranked stars from A to P, which is the origin of the currently used spectral classes. After several transformations, today the spectral classification includes 7 main types: O, B, A, F, G, K, M.

A popular mnemonic for remembering this order is “Oh, Be A Fine Girl, Kiss Me”.

This is called “Morgan-Keenan spectral classification”, even though its form was already by Annie Cannon, also based on the work of other astronomers from the Harvard College Observatory. The classes, listed from hottest to coldest, are:



Star Color


30,000 – 60,000 °K



10,000 – 30,000 °K



7,500 – 10,000 °K



6,000 – 7,500 °K

White (yellowish)


5,000 – 6,000 °K

Yellow (like the Sun)


3,500 – 5,000 °K



2,000 – 3,500 °K


Notice that hottest stars are blue, while coldest stars are red. This seems unusual to most people, who associate red with hot and blue with cold. This is because we see fire as yellow, orange or red, but light produced by hotter sources is blue. However, blue sourced are hard to find on Earth because it requires a large amount of energy.

Also notice that this is true for light-emitting objects. However, the color of a common object, like a blue shirt or a piece of red paper, is not related to its temperature. Confusion also arises when one considers how artists or photographers may refer to the color of light: usually they describe reds as “warm” colors and blues as “cool”.

Kelvin Temperature

K means Kelvin degrees, that can be calculated adding 273 to Celsius degrees. Here are 4 examples of common temperatures in Fahrenheit, Celsius and Kelvin degrees:





Water boils




Room Temperature




Water Freezes




Absolute Zero




However, star temperatures are much higher, so the following table can be useful:

Conditions in different temperatures




1,808 °K

1,535 °C

Melting point of iron

2,013 °K

1,740 °C

Boiling point of lead

3,683 °K

3,410 °C

Melting point of tungsten

3,925 °K

3,652 °C

Sublimation point of carbon

5,780 °K

5,500 °C

Surface temperature of the Sun

5,828 °K

5,555 °C

Boiling point of tungsten

Spectral types

The seven spectral classes were subdivided into tenths (for example B0, B1, B2, B3, …, B9, A0, A1, A2, A3, … A9, F0, F1, F2, F3…). The Sun is a G2 star.

Class O stars are very hot and luminous, being blue in colour. Naos (in the constellation Puppis) shines with a power close to a million times solar. These stars have prominent ionized and neutral helium lines and only weak hydrogen lines. Class O stars emit most of their radiation in ultra-violet.

Class B stars are again very luminous, Rigel (in the great constellation Orion) is a prominent B class blue supergiant. Their spectra have neutral helium and moderate hydrogen lines. As O and B stars are so powerful, they live for a very short time. They do not stray far from the area in which they were formed as they don’t have the time. They therefore appear clustered together in the OB associations, which are associated with giant molecular clouds. The Orion OB association is an entire spiral arm of our Galaxy.

Class A stars are amongst the more common naked eye stars. Deneb in Cygnus is another very powerful star. Sirius, that appears the brightest star as seen from Earth, is also an A class star, but not nearly as powerful. As with all class A stars, they are white. Many white dwarfs are also A. They have strong hydrogen lines and also ionized metals.

Class F stars are still quite powerful and they are average-sized, such as Fomalhaut in Pisces Australis. Their spectra is characterized by the weaker hydrogen lines and ionized metals, their colour is white with a slight tinge of yellow.

Class G stars are probably the most well known for the reason that our Sun is of this class. They have even weaker hydrogen lines than F but along with the ionized metals, they have neutral metals.

Class K are orange stars which are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus, while others like Alpha Centauri B are smaller. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals.

Class M is the most common class by the number of stars. All red dwarfs, such Proxima Centauri, the closest star to our Solar Sysem, go in here, and they are plentiful. M is also host to most giants and some supergiants such as Antares in Scorpio and Betelgeuse in Orion, as well as Mira variable stars. These red giants are old stars. The spectrum of an M star shows lines belonging to molecules and neutral metals but hydrogen is usually absent. Titanium oxide can be strong in M stars.

M stars may be dwarf stars or supergiant stars, and A stars can be white dwarfs or white giants as well. However, not all combinations are possible. For example, F and G stars must be average-sized stars.

This can be understood through the Hertzsprung-Russell diagram, that is very important in astrophysics and relates temperature and spectral classification of stars with their luminosity and size.

While these descriptions of stellar colors are traditional in astronomy, they really describe the light as we see them from Earth, after it has been scattered by the atmosphere. The Sun is not in fact a yellow star, but has the color temperature of a body of 5780 K, that is a white with no trace of yellow which is sometimes used as a definition for standard white.

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