2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
We continue to acquaint you with the books included in the long list of the 2017 Enlightener Popular Science Literature Prize. Today it is “Cosmos. From the Solar System deep into the Universe "M. Ya. Marov. It consistently and in detail describes both the objects of the solar system and other space objects and phenomena lying outside of it. We invite you to familiarize yourself with the first chapter, which naturally deals with the Sun. Fragments of other award winning books published on the N + 1 website can be found here.
The sun is like a star. General properties
The sun is the central luminary around which all the planets and small bodies of the solar system revolve. It is not only the center of gravity, but also a source of energy that provides heat balance and natural conditions on the planets, including life on Earth. The movement of the Sun in relation to the stars (and the horizon) has been studied since ancient times to create calendars that humans used primarily for agricultural purposes. The Gregorian calendar, currently used almost everywhere in the world, is essentially a solar calendar based on the Earth's cyclic rotation around the Sun. The Sun has a visual magnitude of 26.74m and is the brightest object in our sky.
The Sun is an ordinary star located in our galaxy, simply called the Galaxy or the Milky Way, at a distance of 2/3 from its center, which is 26,000 light years, or ~ 10 kpc, and at a distance of ~ 25pc from the plane of the Galaxy. It revolves around its center at a speed of ~ 220 km / s and a period of 225–250 million years (galactic year) clockwise as viewed from the north galactic pole. The orbit is believed to be approximately elliptical and is perturbed by the galactic spiral arms due to inhomogeneous stellar mass distributions. In addition, the Sun makes periodic movements up and down relative to the plane of the Galaxy from two to three times per revolution. This leads to a change in gravitational perturbations and, in particular, has a strong effect on the stability of the position of objects at the edge of the solar system. This is the reason for the invasion of comets from the Oort Cloud into the inner solar system, which leads to an increase in impact events. In general, from the point of view of various kinds of disturbances, we are in a rather favorable zone in one of the spiral arms of our Galaxy at a distance of ~ 2/3 from its center.
In the modern era, the Sun is located near the inner side of the Orion Arm, moving inside the Local Interstellar Cloud (IMO), filled with a rarefied hot gas, possibly a remnant of a supernova explosion. As we will see in Ch. 10, this area is called the galactic habitable zone. The Sun moves in the Milky Way (relative to other nearby stars) towards the star Vega in the constellation Lyra at an angle of approximately 60 ° from the direction to the galactic center; it is called the apex movement. Interestingly, since our Galaxy is also moving relative to the Cosmic Microvawe Background (CMB - Cosmic Microvawe Background, see Chapter 11) at a speed of 550 km / s in the direction of the constellation Hydra, the resulting (residual) speed of the Sun relative to the CMB is about 370 km / s. with and directed towards the constellation Leo. Note that the Sun in its motion experiences slight disturbances from the planets, primarily Jupiter, forming with it a common gravitational center of the Solar System - the barycenter located within the radius of the Sun. Every few hundred years, the barycentric movement switches from direct (prograde) to reverse (retrograde).
The sun was formed about 4.5 billion years ago, when the rapid compression of a cloud of molecular hydrogen under the action of gravitational forces led to the formation of a variable star of the first type of stellar population in our region of the Galaxy - a star of type T Tauri (T Tauri). After the beginning of thermonuclear fusion reactions (conversion of hydrogen into helium) in the solar core, the Sun switched to the main sequence of the Hertzsprung – Russell (HR) diagram (see Chapter 6). The Sun is classified as a yellow G2V dwarf star that appears yellow when viewed from Earth due to the slight excess of yellow light in its spectrum caused by blue rays scattering in the atmosphere. The Roman numeral V in the notation G2V means that the Sun belongs to the main sequence of the HR diagram. It is assumed that in the earliest period of evolution, before the transition to the main sequence, it was on the so-called Hayashi track, where it was compressed and, accordingly, decreased luminosity while maintaining approximately the same temperature. Following an evolutionary scenario typical of low and medium mass stars located on the main sequence, the Sun has passed about half of the active stage of its life cycle (the conversion of hydrogen into helium in thermonuclear fusion reactions), which is a total of about 10 billion years, and will preserve this activity over the next approximately 5 billion years. The sun annually loses 10-14 of its mass, and the total losses throughout its life will amount to 0.01%.
By its nature, the Sun is a plasma ball with a diameter of approximately 1.5 million km. The exact values of its equatorial radius and average diameter are 695,500 km and 1,392,000 km, respectively. This is two orders of magnitude larger than the size of the Earth and an order of magnitude larger than the size of Jupiter. The average angular size of the Sun when observed from the Earth is 31 59 and varies from 31ʹ 27ʹʹ to 32ʹ 31ʹʹ, and the inclination of the axis of rotation to the ecliptic is 7, 25 °. The sun rotates around its axis counterclockwise (as viewed from the North Pole of the world), the rotation speed of the outer visible layers is 7,284 km / h. The sidereal period of rotation at the equator is 25.38 days, while the period at the poles is much longer - 33.5 days, i.e. the atmosphere at the poles rotates more slowly than at the equator. This difference arises from differential rotation caused by convection and uneven mass transfer from the core to the outside, and is associated with a redistribution of angular momentum. When viewed from Earth, the apparent rotation period is approximately 28 days.
Differential rotation affects the structure of the magnetic field and, in particular, leads to twisting of the magnetic field lines. Magnetic field loops projected towards the surface of the Sun cause sunspots and prominences. According to existing concepts, a kind of magnetic hydrodynamic dynamo combining the interaction of poloidal and toroidal fields in the inner convective zone of the Sun is responsible for the generation of the solar magnetic field. The dynamo mechanism is associated with an 11-year cycle of solar activity and a change in the polarity of the solar magnetic field every 11 years.
The figure of the Sun is almost spherical, its oblateness is insignificant, only 9 ppm. This means that its polar radius is less than the equatorial one by only ~ 10 km. The mass of the Sun is 1.99x1033 g (~ 330,000 Earth masses), and the average density is 1.41 g / cm3 (almost 4 times less than the Earth's density). The sun contains 99.86% of the mass of the entire solar system. The acceleration of gravity (at the equator) g = 274.0 m / s2 (27.94gE), the second cosmic velocity Ve = 617.7 km / s (55 times greater than for the Earth).
The effective temperature of the solar "surface" (Teff = 5777 K) refers to the visible layer - the photosphere, while the temperature in the center of the core is ~ 1, 57x107 K, and the temperature of the outer atmosphere (corona) is ~ 5x106 K. At such high temperatures, gases are in a plasma state. The photosphere is mainly responsible for all the radiation emitted, as the gas above the photosphere is too cold and too thin to emit significant amounts of light. The brightness of the Sun is enormous, it is 3.85x1033 erg / s and roughly corresponds to the Planck radiation of a black body at a temperature of ~ 6000 K.
About 1 billion years after entering the Main Sequence (estimated between 3, 8 and 2.5 billion years ago), the Sun's brightness has increased by about 30%. It is quite obvious that the problems of the climatic evolution of the planets are directly related to the change in the luminosity of the Sun. This is especially true of the Earth, the surface temperature of which, necessary to preserve liquid water (and, probably, the origin of life), could only be achieved due to the higher content of greenhouse gases in the atmosphere to compensate for low insolation. This problem is called the “paradox of the young sun”. In the subsequent period, the brightness of the Sun (as well as its radius) continued to grow slowly. It is estimated that the Sun gets about 10% brighter every one billion years. Accordingly, the surface temperatures of the planets (including the temperature on Earth) are slowly increasing. In about 3.5 billion years from now, the brightness of the Sun will increase by 40%, and by that time conditions on Earth will be similar to conditions on today's Venus.
Currently, the amount of energy per unit area of the Earth's surface (the solar constant referring to the upper boundary of the atmosphere) is 1,368 W x m2, or ~ 2 cal x cm-2 x min-1. This is approximately one billionth of the solar power. During the 11-year solar cycle (see below), the solar constant changes insignificantly, within ~ 0.2%, although the spectral composition of radiation changes significantly, primarily in the UV and X-ray wavelength ranges. These energetically small ranges have a decisive effect on the state of the upper atmosphere and near-planetary space. The atmosphere and clouds attenuate sunlight almost exponentially, and the amount of energy reaching the earth's surface is almost 30% less (~ 1,000 W / m2 than in clear weather and when the Sun is near its zenith.
By the end of its life, the Sun will go into the state of a red giant. The hydrogen fuel in the core will be depleted, its outer layers will expand greatly, and the core will shrink and heat up. Hydrogen fusion will continue along the shell surrounding the helium core, and the shell itself will constantly expand. More and more helium will be formed and the core temperature will rise. When the core temperature reaches ~ 100 million degrees, helium will start burning with the formation of carbon. This is probably the final phase of the Sun's activity, since its mass is insufficient for the beginning of the later stages of nuclear fusion with the participation of heavier elements - nitrogen and oxygen (see Chapter 6). Due to its relatively small mass, the life of the Sun will not end in a supernova explosion. Instead, intense thermal pulsations will occur, which will cause the Sun to shed its outer shells and form a planetary nebula. In the course of further evolution, a very hot degenerate core is formed - a white dwarf, devoid of its own sources of thermonuclear energy, with a very high density of matter, which will slowly cool and, as the theory predicts, in tens of billions of years will turn into an invisible black dwarf.
Marov M. Ya. Cosmos. From the solar system deep into the universe. - M.: Fizmatlit, 2016.