The hotter the object, the shorter the wavelength, which is why the corona emits "soft x rays," whose waves are much shorter than those of visible or ultra-violet light. However, as a rule, temperatures are expected to drop the further one gets from the furnace, whereas the million-degree corona lies outside the surface layer where sunlight originates, whose temperature is less that 6000 C.Īll hot objects emit electromagnetic waves-for example, visible light is emitted by the hot filament inside a lightbulb. It is almost certain that its energy comes from the Sun's internal furnace, which also supplies the rest of the Sun's heat. The source of the corona's heat remains a puzzle. Such high levels of ionization require the atoms to be buffeted around by extremely high temperatures, around 1,000,000 C (1,800,000 F).
Unknown spectral lines emitted by the corona were similarly credited to a new element "coronium" until Edlen showed that they came from the familiar atoms of iron, nickel and calcium, after they had lost an appreciable number of electrons (e.g. Later, in 1895, William Ramsey actually discovered helium on Earth. In the 19th century, some of the spectral lines of sunlight did not match the lines of any substance on Earth, and it was proposed that they came from a new unknown chemical element, named helium (from the Greek helios-Sun). Much of that is sunlight scattered by coronal dust, but some light is also produced by the corona itself, in narrowly defined colors ("spectral lines") characteristic of its emitting atoms. XThe most remarkable aspect of the corona is its high temperature, deduced by the Swedish astronomer Bengt Edlen in 1942 after a study of the corona's light. Astronomers have named them "helmet streamers" because of their resemblance to spiked helmets worn by knights (and also to those used by some German soldiers up to 1918). From the tops of such "arches" long streamers may extend, to distances of the Sun's diameter or even more, looking like pulled taffy, as if some process was pulling material away from the tops of the arches into space, (which is of course what the solar wind actually does). Structures observed in the corona above sunspots often have horseshoe-shaped outlines, again suggesting that they follow magnetic field lines. For instance, short "plumes" rising from the polar regions of the Sun look very much like field lines coming out of the end of a bar magnet, and they therefore suggest that the Sun, in addition to the intense fields of sunspots, also has a global magnetic field like the Earth's. Structures visible in the corona at such times suggest that they are shaped by magnetic fields, and therefore, that the corona consists of plasma. The contribution from the corona is minuscule in comparison.During a total eclipse of the Sun, when for a few minutes the Moon completely covers the Sun's face, a glow appears around the darkened Sun-the solar corona, the Sun's outermost atmosphere. So although the Sun’s corona at a temperature of over a million kelvins (1,800,000 F ) is significantly hotter than the photosphere, the vast majority of the light we use to measure the effective temperature of the Sun comes from its photosphere. These different methods all show that the effective temperature of the Sun’s surface is around 5,800 kelvins (9,980 degrees Fahrenheit ). Another approach is to record which absorption lines are present in the solar spectrum and determine HTML their strengths both the elements present and their strengths are sensitive to temperature. This means that we can determine the effective temperature of the Sun by measuring the amount of light it emits at each wavelength and comparing the resulting spectrum we see to models. The color of light a star emits is related to its temperature. In fact, the photosphere is often referred to as the “surface” of the Sun, even though there is no real solid surface. This is also why the corona can be seen only when the light from the photosphere is blocked (via a total eclipse or an occulting disk in a coronagraph). Because of this, the light we receive from the Sun overwhelmingly comes from its photosphere only a tiny fraction comes from its corona.
The density of the plasma in the Sun’s atmosphere falls off precipitously as we move outward from the photosphere (its visible surface) to the corona. How is the temperature of the Sun’s surface measured through its much hotter atmosphere, the corona?
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