Sun

The Sun

The Sun, also known as Sol or Helios, is the star located at the centre of our Solar System. It is a colossal and remarkably spherical mass of scorching plasma. Its core undergoes nuclear fusion reactions, resulting in intense incandescence, while it emits energy primarily in the form of visible light and infrared radiation, with a small portion in ultraviolet wavelengths. Undoubtedly, it serves as the paramount source of energy for sustaining life on Earth. Throughout history, the Sun has been revered in numerous cultures and has remained a focal point of astronomical exploration since ancient times.

The Galactic Center is where the Sun revolves, positioned at a distance ranging from 24,000 to 28,000 light-years. In relation to Earth, it is approximately 1 AU (1.496×108 km) away, equivalent to about 8 light-minutes. The Sun's diameter measures around 1,391,400 km (864,600 mi), which is 109 times larger than that of the Earth. Its mass is approximately 330,000 times greater than the Earth's, constituting about 99.86% of the total mass of the Solar System. Hydrogen makes up roughly three-quarters of the Sun's mass (~73%), while helium accounts for most of the remaining mass (~25%), along with smaller amounts of heavier elements such as oxygen, carbon, neon, and iron.

The Sun, classified as a G-type main-sequence star (G2V), is commonly referred to as a yellow dwarf, even though its light is actually white. It originated around 4.6 billion years ago through the gravitational collapse of matter within a vast molecular cloud. The majority of this matter accumulated at the center, while the remaining material flattened into a rotating disk that eventually formed the Solar System. The central mass became incredibly hot and dense, leading to the initiation of nuclear fusion in its core. Every second, the Sun's core combines approximately 600 billion kilograms (kg) of hydrogen to form helium and converts 4 billion kg of matter into energy.

In the distant future, as hydrogen fusion in the Sun's core decreases, the Sun will lose hydrostatic equilibrium, leading to a significant increase in density and temperature in its core. This will result in the expansion of its outer layers, eventually transforming the Sun into a red giant. As a consequence, the Earth will become uninhabitable in approximately five billion years. Following this, the Sun will shed its outer layers and evolve into a white dwarf, a dense cooling star that no longer produces energy through fusion. Despite this, it will continue to glow and emit heat from its previous fusion for trillions of years. Eventually, it is predicted to become a super dense black dwarf, ceasing to emit any further energy.

General Characteristics

The Sun, a G-type main-sequence star, accounts for approximately 99.86% of the Solar System's mass. With an absolute magnitude of +4.83, it is believed to be brighter than around 85% of the stars in the Milky Way, which mainly consist of red dwarfs. Classified as a Population I star, the Sun is rich in heavy elements. Its formation, about 4.6 billion years ago, might have been triggered by shockwaves from nearby supernovae. This hypothesis is supported by the high abundance of heavy elements, like gold and uranium, in the Solar System compared to Population II stars, which are deficient in heavy elements. The heavy elements could have plausibly been produced through endothermic nuclear reactions during a supernova or through transmutation via neutron absorption within a massive second-generation star.

The Sun stands out as the most luminous entity in the sky of the Earth, boasting an apparent magnitude of −26.74. In comparison, Sirius, the next brightest star, has an apparent magnitude of −1.46, making the Sun approximately 13 billion times brighter.

The mean distance between the centers of the Sun and the Earth is defined as one astronomical unit, which is approximately 150 million kilometers or 93 million miles. Throughout the year, as the Earth moves from perihelion around January 3rd to aphelion around July 4th, the instantaneous distance varies by about ± 2.5 million km or 1.55 million miles. When considering the average distance, it takes approximately 8 minutes and 20 seconds for light to travel from the Sun's horizon to the Earth's horizon. However, light from the closest points of the Sun and the Earth takes about two seconds less. This sunlight's energy plays a crucial role in supporting almost all life on Earth through photosynthesis, as well as influencing the Earth's climate and weather.

The Sun's boundary is not clearly defined, however, its density decreases exponentially as you move higher above the photosphere. The Sun's radius is typically measured from its center to the edge of the photosphere, which is the visible surface of the Sun. This measurement indicates that the Sun is almost a perfect sphere, with a slight oblateness estimated at 9 millionths. This means that the difference between its polar diameter and equatorial diameter is only 10 kilometers. The gravitational pull of the planets has a weak tidal effect on the Sun and does not greatly impact its shape.

Rotation

The equator of the Sun experiences a faster rotation compared to its poles. This difference in rotation speed is a result of convective motion caused by heat transfer and the Coriolis force generated by the Sun's rotation. When observed from a frame of reference defined by the stars, the rotational period at the equator is approximately 25.6 days, while at the poles it is around 33.5 days. As seen from Earth during its orbit around the Sun, the apparent rotational period of the Sun at its equator is approximately 28 days. However, when viewed from a vantage point above its north pole, the Sun rotates counterclockwise around its axis of spin.

According to a study conducted on solar analogs, it has been suggested that the early Sun had a rotation speed up to ten times faster than its current rate. This increased rotation would have resulted in a much more active surface, leading to higher levels of X-ray and UV emissions. Additionally, sunspots would have covered a significant portion of the surface, ranging from 5% to 30%. Over time, the rotation rate gradually decreased due to magnetic braking, caused by the interaction between the Sun's magnetic field and the outflowing solar wind. However, traces of this rapid primordial rotation can still be observed at the Sun's core, which has been found to rotate once per week, four times faster than the average surface rotation rate. age:4.7billion years

Composition

The Sun consists mainly of the elements hydrogen and helium. At this time in the Sun's life, they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere. All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant.

The Sun's original chemical composition was inherited from the interstellar medium out of which it formed. Originally it would have been about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements. The hydrogen and most of the helium in the Sun would have been produced by Big Bang nucleosynthesis in the first 20 minutes of the universe, and the heavier elements were produced by previous generations of stars before the Sun was formed, and spread into the interstellar medium during the final stages of stellar life and by events such as supernovae.

Since the Sun formed, the main fusion process has involved fusing hydrogen into helium. Over the past 4.6 billion years, the amount of helium and its location within the Sun has gradually changed. The proportion of helium within the core has increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements have settled from the photosphere toward the center of the Sun because of gravity. The proportions of heavier elements are unchanged. Heat is transferred outward from the Sun's core by radiation rather than by convection (see Radiative zone below), so the fusion products are not lifted outward by heat; they remain in the core, and gradually an inner core of helium has begun to form that cannot be fused because presently the Sun's core is not hot or dense enough to fuse helium. In the current photosphere, the helium fraction is reduced, and the metallicity is only 84% of what it was in the protostellar phase (before nuclear fusion in the core started). In the future, helium will continue to accumulate in the core, and in about 5 billion years this gradual build-up will eventually cause the Sun to exit the main sequence and become a red giant.

The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System. Typically, the solar heavy-element abundances described above are measured both by using spectroscopy of the Sun's photosphere and by measuring abundances in meteorites that have never been heated to melting temperatures. These meteorites are thought to retain the composition of the protostellar Sun and are thus not affected by the settling of heavy elements. The two methods generally agree well.