The current stellar classification system

The current stellar classification system originated in the early 20th century, when stars were classified from A to Qbased on the strength of the hydrogen line.^[135] It was not known at the time that the major influence on the line strength was temperature; the hydrogen line strength reaches a peak at over 9000 K, and is weaker at both hotter and cooler temperatures.  DELL Vostro 3550 Laptop Keyboard

When the classifications were reordered by temperature, it more closely resembled the modern scheme.^[136]

Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, toM, which are so cool that molecules may form in their atmospheres. HP Pavilion dv5-1104tu Laptop Keyboard

The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types have special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. GATEWAY NV-59 Laptop Keyboard

However, this system breaks down at extreme high temperatures: class O0 and O1 stars may not exist.^[137]

In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by the surface gravity. ACER Aspire 5930 Laptop Keyboard

These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors addVII (white dwarfs). Most stars belong to the main sequence, which consists of ordinary hydrogen-burning stars. These fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.^[137] DELL Vostro 3450 Laptop Keyboard

The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; ACER Aspire 5610Z Laptop Keyboard

"m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.^[137]

White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature index. TOSHIBA NSK-TAJ01 Laptop Keyboard

Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.

During their stellar evolution, some stars pass through phases where they can become pulsating variables.  Lenovo Thinkpad T420 Laptop Keyboard

Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and cepheid-like stars, and long-period variables such as Mira.^[139]  HP Mini 110-1032NR Laptop Keyboard

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.^[139] This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars.

Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novaeand supernovae.HP Pavilion zv6233ea Laptop Keyboard

A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.^[4] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.^[140] SONY VAIO VGN-FS940 laptop keyboard

 Some novae are also recurrent, having periodic outbursts of moderate amplitude.^[139]

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.^[139] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days. SONY VAIO VGN-FS730F laptop keyboard

The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. HP Pavilion dv3-2157cl laptop keyboard

The temperature at the core of a main sequence or giant star is at least on the order of 10⁷ K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.^[141][142] SONY Vaio PCG-K315S laptop keyboard

As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. HP Pavilion dv3-2157cl laptop keyboard

Eventually the helium content becomes predominant and energy production ceases at the core. Instead, for stars of more than 0.4 solar masses, fusion occurs in a slowly expanding shell around the degeneratehelium core.^[143]

In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. HP Pavilion dv6-2030ev laptop keyboard

There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.

The radiation zone is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. HP G61-465SL laptop keyboard

In this region the plasma will not be perturbed and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with highopacity as in the outer envelope.^[142]  LENOVO IdeaPad S10 20015 laptop keyboard

The occurrence of convection in the outer envelope of a main sequence star depends on the mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.^[144] SONY VAIO VGN-FS315M laptop keyboard

Red dwarf stars with less than 0.4 solar masses are convective throughout, which prevents the accumulation of a helium core.^[2] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.

The portion of a star that is visible to an observer is called the photosphere. HP Mini 110-1150EV laptop keyboard

This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate out into space. It is within the photosphere that sun spots, or regions of lower than average temperature, appear.

Above the level of the photosphere is the stellar atmosphere. HP Envy 15-1090eg laptop keyboard

In a main sequence star such as the Sun, the lowest level of the atmosphere is the thin chromosphere region, where spicules appear and stellar flares begin. This is surrounded by a transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). SONY Vaio VGN-FW390 laptop keyboard

Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.^[145] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.^[144] Despite its high temperature, the corona emits very little light. The corona region of the Sun is normally only visible during a solar eclipse. TOSHIBA Satellite M105-S3004 laptop keyboard

From the corona, a stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout the bubble-shaped region of the heliosphere.

A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition, as part of stellar nucleosynthesis. GATEWAY MX8715 laptop keyboard

The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is released as electromagnetic energy, according to the mass-energy equivalence relationshipE = mc².^[1]

The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. SONY Vaio VGN-AW330J laptop keyboard

As a result the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.

In massive stars, heavier elements can also be burned in a contracting core through the neon burning process and oxygen burning process. SONY Vaio PCG-K315S Laptop Keyboard

The final stage in the stellar nucleosynthesis process is the silicon burning process that results in the production of the stable isotope iron-56. Fusion can not proceed any further except through an endothermic process, and so further energy can only be produced through gravitational collapse.^[147] HP Business Notebook nx9000 Laptop Keyboard

The example below shows the amount of time required for a star of 20 solar masses to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.

Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties"^[1]  HP pavilion DV7-1000 Laptop Keyboard