These are most likely groups of stars that

These are most likely groups of stars that share a common point of origin in giant molecular clouds.

The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, generating magnetic fields that extend throughout the star. HP 605344-001 Laptop Keyboard

The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. FUJITSU CP270342-02 Laptop Keyboard

 Coronal loops are arching magnetic fields that reach out into the corona from active regions.Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.^[103]

Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation as the star grows older. GATEWAY NV-54 Laptop Keyboard

Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods.^[104]During the Maunder minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity. HP G42-362LA Laptop Keyboard

One of the most massive stars known is Eta Carinae,^[105] which, with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. A study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe.^[106] The reason for this limit is not precisely known,  SONY KFRSBA019A Laptop Keyboard

but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space. However, a star named R136a1 in the RMC 136a star cluster has been measured at 265 solar masses, which puts this limit into question.^[107] Lenovo 63Y0047 Laptop Keyboard

A study determined that stars larger than 150 solar masses in R136 were created through the collision and merger of massive stars in close binary systems, providing a way to sidestep the 150 solar mass limit.

The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,^[109] SONY KFRMBA152B Laptop Keyboard

due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.

With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.^[110] Compaq Presario CQ42-228LA Laptop Keyboard

For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of Jupiter.^[111][112]When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter.^[112][113] ACER Aspire 7745G Laptop Keyboard

Smaller bodies are called brown dwarfs, which occupy a poorly defined grey area between stars and gas giants.

The combination of the radius and the mass of a star determines the surface gravity. Giant stars have a much lower surface gravity than main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs.  HP Probook 4515S Laptop Keyboard

The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.

The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking the rotation rate of starspots. SONY VAIO VGN-N31S/W Laptop Keyboard

Young stars can have a rapid rate of rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial rotation velocity of about 225 km/s or greater, giving it an equatorial diameter that is more than 50% larger than the distance between the poles. This rate of rotation is just below the critical velocity of 300 km/s where the star would break apart.^[114] SONY VAIO VGN-CS14G/B Laptop Keyboard

By contrast, the Sun only rotates once every 25 – 35 days, with an equatorial velocity of 1.994 km/s. The star's magnetic field and the stellar wind serve to slow down a main sequence star's rate of rotation by a significant amount as it evolves on the main sequence.^[115]

Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. ASUS F3Sv laptop keyboard

However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.^[116]

 IBM Thinkpad R40 laptop keyboard

 In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.^[117] The rotation rate of the pulsar will gradually slow due to the emission of radiation.

The surface temperature of a main sequence star is determined by the rate of energy production at the core and its radius, and is often estimated from the star's color index.^[118]  HP Mini 210-1124TU laptop keyboard

 It is normally given as the effective temperature, which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative value, as the temperature increases toward the core.^[119] The temperature in the core region of a star is several million kelvins.^[120] SONY VAIO VGN-NW23GF laptop keyboard

The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).^[33] SONY VAIO VGN-NW320F/T laptop keyboard

Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area. SONY VAIO VPC-F12 laptop keyboard

The energy produced by stars, as a product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind,^[122] which streams from the outer layers as free protons, and electrically charged alpha, and beta particles. SONY VAIO PCG-FR55E/B laptop keyboard

Although almost massless there also exists a steady stream of neutrinos emanating from the star's core.

The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleusof a new heavier element, gamma ray photons are released from the nuclear fusion product.  HP G62-b21SL laptop keyboard

This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.

The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including itsphotosphere.^[123]  COMPAQ Presario V6405CA laptop keyboard

Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. HP Pavilion G7-1077NR laptop keyboard

From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.

Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star.  FUJITSU Lifebook S7111 laptop keyboard

If the distance of the star is known, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. SONY VAIO VGN-CR410E laptop keyboard

Gravitational microlensing has been used to measure the mass of a single star.^[124]) With these parameters, astronomers can also estimate the age of the star.

The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. HP Mini 110-3538tu laptop keyboard

The luminosity of a star is determined by the radius and the surface temperature. However, many stars do not radiate a uniform flux (the amount of energy radiated per unit area) across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux at its poles than along its equator.^[126] ASUS X85S laptop keyboard

Surface patches with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots. Larger, giant stars have much larger, more obvious starspots,^[127] and they also exhibit strong stellar limb darkening. DELL PK1303Q0100 Laptop Keyboard

That is, the brightness decreases towards the edge of the stellar disk.^[128] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.

The apparent brightness of a star is expressed in terms of its apparent magnitude, which is the brightness of a star and is a function of the star's luminosity, distance from Earth, and the altering of the star's light as it passes through Earth's atmosphere.  ASUS X53S Laptop Keyboard

Intrinsic or absolute magnitude is directly related to a star's luminosity and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).

Both the apparent and absolute magnitude scales are logarithmic units: SAMSUNG R522 Laptop Keyboard

one whole number difference in magnitude is equal to a brightness variation of about 2.5 times^[131] (the 5th root of 100 or approximately 2.512). This means that a first magnitude (+1.00) star is about 2.5 times brighter than a second magnitude (+2.00) star, and approximately 100 times brighter than a sixth magnitude (+6.00) star. SONY VAIO PCG-FR55E Laptop Keyboard

The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.

On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter. The brightest stars, on either scale, have negative magnitude numbers. DELL M711P Laptop Keyboard

The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (m_b) from the magnitude number of the fainter star (m_f), then using the difference as an exponent for the base number 2.512.

Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent;^[131]  HP AEAT5U00010 Laptop Keyboard

for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.

The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun,  HP 540 Laptop Keyboard

while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. DELL PVDG3 Laptop Keyboard

This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.

As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.^[132]  HP G62-361TX Laptop Keyboard

 The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.  DELL Vostro 1014 Laptop Keyboard