The Sun ☉

November 30, 2020

The Sun ☉

Our Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma , heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as light and infrared radiation. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometres (864,000 miles), or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally and not completely accurately referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.

In its core the Sun currently fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result. This energy, which can take between 10,000 and 170,000 years to escape the core, is the source of the Sun's light and heat. When hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand, eventually transforming the Sun into a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable – but not for about five billion years. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, and no longer produce energy by fusion, but still glow and give off heat from its previous fusion.

The enormous effect of the Sun on Earth has been recognized since prehistoric times. The Sun has been regarded by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the predominant calendar in use today.

The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,368 W/m² (watts per square meter) at a distance of one astronomical unit (AU) from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is attenuated by Earth's atmosphere, so that less power arrives at the surface (closer to 1,000 W/m²) in clear conditions when the Sun is near the zenith. Sunlight at the top of Earth's atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light. The atmosphere in particular filters out over 70% of solar ultraviolet, especially at the shorter wavelengths. Solar ultraviolet radiation ionizes Earth's dayside upper atmosphere, creating the electrically conducting ionosphere.

The Sun's color is white, with a CIE color-space index near (0.3, 0.3), when viewed from space or when the Sun is high in the sky, and the Solar radiance per wavelength peaks in the green portion of the spectrum. When the Sun is low in the sky, atmospheric scattering renders the Sun yellow, red, orange, or magenta. Despite its typical whiteness, most people mentally picture the Sun as yellow; the reasons for this are the subject of debate. The Sun is a G2V star, with G2 indicating its surface temperature of approximately 5,778 K (5,505 °C, 9,941 °F), and V that it, like most stars, is a main-sequence star. The average luminance of the Sun is about 1.88 giga candela per square metre, but as viewed through Earth's atmosphere, this is lowered to about 1.44 Gcd/m².However, the luminance is not constant across the disk of the Sun (limb darkening).

Radius, diameter & circumference

The sun is nearly a perfect sphere. Its equatorial diameter and its polar diameter differ by only 6.2 miles (10 km). The mean radius of the sun is 432,450 miles (696,000 kilometers), which makes its diameter about 864,938 miles (1.392 million km). You could line up 109 Earths across the face of the sun. The sun's circumference is about 2,713,406 miles (4,366,813 km).

It may be the biggest thing in this neighborhood, but the sun is just average compared to other stars. Betelgeuse, a red giant, is about 700 times bigger than the sun and about 14,000 times brighter.

"We have found stars that are 100 times bigger in diameter than our sun. Truly those stars are enormous," NASA says on its SpacePlace website. "We have also seen stars that are just a tenth the size of our sun."

According to NASA's solar scientist C. Alex Young, if the sun were hollow, it would take about one million Earths to fill it.

It's possible that the sun is even larger than previously thought. Xavier Jubier, an engineer and solar eclipse researcher, creates detailed models of solar and lunar eclipses to determine precisely where the moon's shadow would fall during the solar eclipse. But when he matched actual photos and historical observations with the models, he found precise eclipse shapes only made sense if he scaled up the sun's radius by a few hundred kilometers.

Even missions like NASA's Solar Dynamics Observatory (SDO) and measurements of the inner planets across the face of the sun don't refine the star's radius as precisely as desired "It's harder than you think just to put a ruler on these images and figure out how big the sun is — [SDO] doesn't have enough precision to nail this down," NASA researcher Ernie Wright told Space.com. "Similarly, with the Mercury and Venus transits, it turns out [a measurement based on those is] not quite as precise as you'd like it to be."Wright said different papers using a variety of methods have produced results that differ by as much as 930 miles (1,500 km).That could be a problem if you are planning to skirt the edges of the next solar eclipse."For most people, yes, it doesn't really matter; it won't change everything," Jubier said. "But the closer you get to the edge of the [eclipse] path, the more risk you take."

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Note: This is a brief summary of the sun. i will keep writing about the entire solar system and the i will move on to something bigger. Next will be Mercury .