Star
The effective temperature of a star is the temperature of a black body with the same luminosity per surface area as the star and is defined according to the Stefan–Boltzmann law . Notice that the total (bolometric) luminosity of a star is then, where is the stellar radius. The definition of the stellar radius is obviously not straightforward. More rigorously the effective temperature corresponds to the temperature at the radius that is defined by the Rosseland optical depth. The effective temperature and the bolometric luminosity are the two fundamental physical parameters needed to place a star on the Hertzsprung–Russell diagram. Both effective temperature and bolometric luminosity actually depend on the chemical composition of a star.
The effective temperature of our Sun is around 5780 kelvin (K). Stars actually have a temperature gradient, going from their central core up to the atmosphere. The "core temperature" of the sun—the temperature at the centre of the sun where nuclear reactions take place—is estimated to be 15 000 000 K.
The color index of a star indicates its temperature from the very cool—by stellar standards, that is—red M stars that radiate heavily in the infrared to the very blue O stars that radiate largely in the ultraviolet. The effective temperature of a star indicates the amount of heat that the star radiates per unit of surface area. From the warmest surfaces to the coolest is the sequence of star types known as O, B, A, F, G, K, and M.
A red star could be a tiny red dwarf, a star of feeble energy production and a small surface or a bloated giant or even supergiant star such as Antares or Betelgeuse, either of which generates far greater energy but passes it through a surface so large that the star radiates little per unit of surface area. A star near the middle of the spectrum, such as the modest Sun or the giant Capella radiates more heat per unit of surface area than the feeble red dwarf stars or the bloated supergiants, but much less than such a white or blue star as Vega or Rigel.
Read more about this topic: Effective Temperature
Famous quotes containing the word star:
“Relying on any one disciplinary approach—time-out, negotiation, tough love, the star system—puts the parenting team at risk. Why? Because children adapt to any method very quickly; today’s effective technique becomes tomorrow’s worn dance.”
—Ron Taffel (20th century)
“The obvious parallels between Star Wars and The Wizard of Oz have frequently been noted: in both there is the orphan hero who is raised on a farm by an aunt and uncle and yearns to escape to adventure. Obi-wan Kenobi resembles the Wizard; the loyal, plucky little robot R2D2 is Toto; C3PO is the Tin Man; and Chewbacca is the Cowardly Lion. Darth Vader replaces the Wicked Witch: this is a patriarchy rather than a matriarchy.”
—Andrew Gordon, U.S. educator, critic. “The Inescapable Family in American Science Fiction and Fantasy Films,” Journal of Popular Film and Television (Summer 1992)
“A brush had left a crooked stroke
Of what was either cloud or smoke
From north to south across the blue;
A piercing little star was through.”
—Robert Frost (1874–1963)