Main sequence - Wikipedia
In astrophysics, the mass–luminosity relation is an equation giving the relationship between a star's mass and its luminosity, first noted by Jakob Karl Ernst Halm. Note: Exoplanet mass estimated from mass-radius. relationship when not available. A whole new world: .. More on This Topic. Starstruck. 6 days ago Smaller stars, up to eight times the mass of our own sun, typically evolve into white dwarves. A star If a white dwarf takes on enough mass it reaches a level called the Chandrasekhar Limit. Science & InnovationStarstruck.
Not all of these model stars will be dynamically stable. This matter radiates roughly as a black body.
This enables the composition and structure of their atmospheres to be studied by soft X-ray and extreme ultraviolet observations. As was explained by Leon Mestel inunless the white dwarf accretes matter from a companion star or other source, its radiation comes from its stored heat, which is not replenished. White dwarfs have an extremely small surface area to radiate this heat from, so they cool gradually, remaining hot for a long time.
Since the white dwarf has no energy sink other than radiation, it follows that its cooling slows with time. The rate of cooling has been estimated for a carbon white dwarf of 0. After initially taking approximately 1. Once we adjust for the selection effect that hotter, more luminous white dwarfs are easier to observe, we do find that decreasing the temperature range examined results in finding more white dwarfs.
The white dwarf luminosity function can therefore be used to find the time when stars started to form in a region; an estimate for the age of our Galactic disk found in this way is 8 billion years. Stellar structure This diagram shows a cross-section of a Sun-like star, showing the internal structure. Because there is a temperature difference between the core and the surface, or photosphereenergy is transported outward.
White dwarf - WikiVisually
The two modes for transporting this energy are radiation and convection. A radiation zonewhere energy is transported by radiation, is stable against convection and there is very little mixing of the plasma. By contrast, in a convection zone the energy is transported by bulk movement of plasma, with hotter material rising and cooler material descending.
Convection is a more efficient mode for carrying energy than radiation, but it will only occur under conditions that create a steep temperature gradient. Consequently, there is a high temperature gradient in the core region, which results in a convection zone for more efficient energy transport. The outer regions of a massive star transport energy by radiation, with little or no convection. This results in a steady buildup of a helium-rich core, surrounded by a hydrogen-rich outer region.
By contrast, cool, very low-mass stars below 0. Since it is the outflow of fusion-supplied energy that supports the higher layers of the star, the core is compressed, producing higher temperatures and pressures. Both factors increase the rate of fusion thus moving the equilibrium towards a smaller, denser, hotter core producing more energy whose increased outflow pushes the higher layers further out.
Thus there is a steady increase in the luminosity and radius of the star over time. This effect results in a broadening of the main sequence band because stars are observed at random stages in their lifetime. Selecting the picture below of Gliese and its companion, Gliese B, will take you to the caption for the picture at the Space Telescope Institute.
With the discovery of several hundred brown dwarfs in recent infrared surveys, astronomers have now extended the spectral type sequence to include these non-planets. Just beyond the M-stars are the L dwarfs with surface temperatures of about K to K with strong absorption lines of metal hydrides and alkali metals. Cooler than the L dwarfs are the T dwarfs. At their cooler temperatures, methane lines become prominent.
Stars with too much mass have so much radiation pressure inside pushing outward on the upper layers, that the star is unstable.
It blows off the excess mass. The limit is roughly about to perhaps solar masses. The picture of Eta Carinae below shows two dumbbell-shaped lobes of ejected material from the star in an earlier episode of mass ejection.
Selecting the image will take you to more information about the image at the Space Telescope Institute will display in another window.
The picture below from the Hubble Space Telescope shows the violet Pistol Star surrounded by hydrogen gas fluorescing from the copious ultraviolet light coming from the star. Selecting the image will bring up the press release from the Space Telescope Institute in another window.