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A star's luminosity can be determined from two stellar characteristics: size and effective temperature. [11] The former is typically represented in terms of solar radii, R ⊙, while the latter is represented in kelvins, but in most cases neither can be measured directly.
L ★ is the star's luminosity (bolometric luminosity) in watts; L 0 is the zero point luminosity 3.0128 × 10 28 W; M bol is the bolometric magnitude of the star; The new IAU absolute magnitude scale permanently disconnects the scale from the variable Sun.
A more complex definition of absolute magnitude is used for planets and small Solar System bodies, based on its brightness at one astronomical unit from the observer and the Sun. The Sun has an apparent magnitude of −27 and Sirius, the brightest visible star in the night sky, −1.46. Venus at its brightest is -5.
Combined magnitude (3rd brightest star in night sky) −0.04: star Arcturus: seen from Earth 4th brightest star to the naked eye [47] −0.01: star Alpha Centauri A: seen from Earth 4th brightest individual star visible telescopically in the night sky +0.03: star Vega: seen from Earth originally chosen as a definition of the zero point [48] +0. ...
It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is the apparent magnitude a star would be if the distance between ...
At a stellar core temperature of 18 million kelvin, the PP process and CNO cycle are equally efficient, and each type generates half of the star's net luminosity. As this is the core temperature of a star with about 1.5 M ☉ , the upper main sequence consists of stars above this mass.
Thus, from the Stefan–Boltzmann law, the luminosity is related to the surface temperature T S, and through it to the color of the star, by = where σ B is Stefan–Boltzmann constant, 5.67 × 10 −8 W m −2 K −4. The luminosity is equal to the total energy produced by the star per unit time.
One nominal solar luminosity is defined by the International Astronomical Union to be 3.828 × 10 26 W. [2] The Sun is a weakly variable star, and its actual luminosity therefore fluctuates. [3] The major fluctuation is the eleven-year solar cycle (sunspot cycle) that causes a quasi-periodic variation of about ±0.1%. Other variations over the ...