Sun

Our Sun

The Sun is a variable star, i.e. a big ball of glowing gas that evolves over time. This yellow dwarf is a hellish ball, constantly shaken by huge explosions that send in space massive amounts of matter.
The photon flux into the space sends it, varies over time, randomly and periodically also in cycles of 11 years, 90 years and 200 years even. In addition, the temperature at the center of the Sun slowly grows in time. It is located at 2/3 the center of our galaxy, to the edge to a distance of approximately 25,000 light years from the center. The Sun moves at a speed of 220 km/s around the galactic center during the revolution that it makes 226 million years since its birth, it has 18 times around the Milky Way. Revolve around the solar system planets, asteroids, comets and the residual dust.
Our thermonuclear power plant gets its energy from nuclear fusion reactions that transform, in its core, at a temperature of 15 million degrees, hydrogen into helium, from 4.57 billion years. Consumption i.e. the loss of mass of the Sun is 4 million tons of hydrogen per second, in fact it turns 564 million tons of hydrogen into 560 million tons of helium. This thermonuclear power plant is really monstrous, it produces a huge source of heat and energy of 380 billion billion megawatts. This energy in its heart, at a temperature of 15 million degrees, is isolated from interplanetary space by 700,000 km of matter (radius of the Sun).

Within a second the sun releases more energy than produced the human civilization since birth and it lasts for 4 billion years. Sun ejects not only photons but also extremely energetic protons and electrons that make up the solar wind. This wind emerges from the surface layers, and propagates in space. Subject to these storms, comets are decorated with a tail showing the direction of the solar wind.
Earth is not completely sheltered by its magnetic screen, the solar wind speed of 400 km/s, seeping through polar slots to show us beautiful aurora borealis and australis, with white lights, green and red.
In the solar system, the Sun has captured 99.86% of the total mass of dust and gas from the original nebula. Jupiter, the largest planet in the system, has captured 71% of the remainder. The other planets are shared the residue of the gravitational evolution. The Sun is mainly composed of hydrogen (74%) and helium (24), the residual material is formed of oxygen (0.77%), carbon (0.29%), iron (0.16%), neon (0.12%), N (0.09), silicon (0.07%), magnesium (0.05%) and sulfur (0.04). Its matter is so hot it remains in state of plasma, electrons are separated from the nuclei.

Distance Earth Sun

Because of the ellipticity of the orbit of the Earth, the Earth - the Sun ranges from 3.3%. The perception of change in the apparent diameter of the sun between perihelion and aphelion did not comment to the naked eye. It is often believed that it was during the winter in the northern hemisphere, the sun is farthest from Earth. In reality the Earth is closest to the Sun (perihelion: 147 095 271 km), January 4 and below (aphelion: 152 091 174 km) on 4 July. Seasonal temperatures are mainly influenced by the height of the Sun in the sky. The seasons are indeed due to the inclination of the axis of rotation of the Earth, not to the variation of its distance from the Sun. During the winter of the northern hemisphere, the inclination of the axis of rotation of the Earth that the sun never rises very high. The semi-major axis of the Earth's orbit around the Sun is 149 597 870 km is the original definition of the astronomical unit (AU).

It takes 8 minutes and twenty seconds for sunlight reaches the Earth. The speed of movement of the Sun (217 km/s), it can go through a light year every 1400 years and an astronomical unit every 8 days. This image allows you to compare the relative size of the Sun when it is closest to Earth in January (left) than when it is at this later, in July (right). The angular size of the Sun is significantly lower in July, when it is below. If the orbit of our planet around the sun was perfectly circular, our star seems to have always the same size. These two images of the Sun were taken from Spain in 2006.

Image: On 4 January the sun (left) is at perihelion, 147 095 271 km above the Earth. On 4 July the Sun (right) is at aphelion 152 091 174 km above the Earth.
source image Superspace.

Solar protuberances

The protuberances characterize the activity of the Sun.
This activity appears to vary within a solar cycle during which the activity of the Sun varies from a maximum to another.
In absolute terms, solar activity is regulated by a cycle of an average of 11.2 years but the duration can vary between 8 and 15 years.
Eruptive prominences of the sun are enormous solar geysers matter that takes place on the chromosphere and climbing to hundreds of thousands of kilometers in space.
Soho Space Satellite Launched in 1995, detected circulating gas complex in the solar surface, but also shock waves and explosions standing in the solar atmosphere. With this success, the 1996 mission, for an initial period of 2 years, was extended until 2007 to allow the observatory to study an entire solar cycle.
Since 30 January 2009, the most beautiful pictures of solar prominences, are coming from the Russian probe Koronas-Photon and its TESIS telescope.

Image: the coronal mass ejections (CME). Eruptive prominences of the Sun are huge geysers solar matter that take place on the chromosphere. Crédit photo : NASA

Image: Extremely hot gases with a solar spicule moving speed of 50 000 km / h in a tube of the magnetic field. They are particularly evident around the visible spot at the bottom left of the image. The spicules have a shelf life of about 5 minutes, beginning in the form of long tubes of gas quickly raising and falling towards the Sun.
Credit: K. Reardon (Osservatorio Astrofisico di Arcetri, INAF) IBIS, DST, NSO

Solar activity

The observatory TESIS was designed by the Laboratory of X-ray astronomy of the Sun of the Lebedev Institute.
It is designed to study solar activity and space weather, the heating of the corona, the mechanism of eruptions and the solar cycle.
TESIS should take a million pictures of the Sun.
The probe-Photon Koronas weighing 1920 kg (600 kg payload) was launched January 30, 2009 from the Baikonur Russian Plesetsk in the Arkhangelsk region.
This is the third type probe Korona (Russian initials for near-Earth orbital space observations of solar activity). Two missions, Solar Orbiter, the European Space Agency, and Solar Probe Plus, the U.S. space agency should be closer to the Sun, respectively about 35 and 7 million kilometers by 2015 to 2017.
We can see much more closely, filaments and coronal mass ejections (CME), called, solar prominences.
BeBetween 2015 and 2017 solar activity correspond to the middle of the solar cycle 24.

NB: The filaments are elongated solar material gas clouds cooled and suspended above the solar surface by magnetic forces.

Video: A very long filament wound in the solar corona has finally exploded, December 6, 2010. The SDO (Solar Dynamics Observatory) NASA filmed the explosion in ultraviolet light helium. This filament measured nearly a million miles long or about half a solar radius.
The SDO has had time to film this event before the rotation of the sun does hide the view.
Credit: NASA's GSFC, SDO AIA Team

Image: This photo shows the huge bulge taken by TESIS, imagine the size of the bulge compared to the size of the Earth represented by the little blue dot in the upper right of the picture.

Cycle of the Sun

The easy observation of sunspots allows to notice not only that the rotation of the sun on itself is made in 27 days but also that the activity of the warm and cold zones of the Sun respect a cycle. The solar cycle is the period during which the activity of the Sun varies from a maximum to the other one. Theoretically, the solar activity is adjusted by a cycle of an average period of 11,2 years but the duration can vary between 8 and 15 years. The cycle of 11 years was determined for the first time by the German astronomer Heinrich Schwabe by 1843. In 1849, the Swiss astronomer Johann Rudolf Wolf ( 1816-1893 ) establishes a method of calculation of the solar activity based on the number of spots. The cycles of Schwabe are numbered from the maximum of 1761. In 2003, the cycle n°23 is on the decline, the cycle n°24 will begin in 2012. The variations of the solar activity are translated on Earth, by fluctuations in the distribution of the waves radio. The most got range of frequencies covers the said diametric waves or the short waves which propagate at long distance. During these magnetic thunderstorms, the very strong ionization of the high layers of the atmosphere can perturb the communications with satellites with the consequences that we can imagine for telecommunications. Sunspots appear group in the warm photosphere (5800 K) there as a colder, dark zone (4500 K) surrounded with a clearer region (4500 K in 5800 K) and are due to a local increase of the magnetic field.

These spots can reach dimensions of several tens of thousand km.
At the beginning of the solar cycle, spots appear rather to high latitude in both hemispheres (the North and the South). Throughout the cycle, spots are going to get closer to the equator till the beginning of the following cycle.
The Ulysses probe, launched on October 6th, 1990 by the shuttle Discovery, is the fruit of a cooperation ESA-NASA the mission of which is the exploration of the heliosphere.
The probe glances through for the first time successively the regions of the South Poles (1994) and the North (1995) of the Sun, invisible since the Earth. Its purpose was to go out of the plan of the ecliptic (plan in which turn planets around the Sun), by using the enormous gravitational field of Jupiter, to observe the poles of the Sun.
One of the enigma not resolved by the first polar passage in 1994 and 1995 concerns the temperature of the poles of the Sun.
During its passages over the South Pole then over the North Pole, for a period of minimum solar energy, the probe had measured the temperatures of the big polar holes. Strangely, the temperature of the polar hole the North was about 7 in 8 percent lower than that of south polar hole (source: Solar Wind Ion Composition Spectrometer).

Image: The Ulysses probe, launched on 6 October 1990 by the shuttle Discovery. The mission was arrested on 1 July 2008 following the deterioration of the energy source of the probe. This vessel was the first and the only one to fly over the poles of the Sun to study the heliosphere, the vast bubble around our star. Designed for a period of 5 years, longevity has been exceptional, a record 6822 operating days (18 years 246 days).

Temperatures of north and south poles

Image: Against this, the measures taken by the Ulysses probe, the temperature of the north and south poles of the Sun thousand kelvin.
The extension of the mission will be decided because of the peak activity of the Sun, the poles being overflown again in 2000 and 2001, the period of maximum activity.
The latest overflights have occurred between November 2006 and April 2007 (South Pole) and between November 2007 and March 2008 (South Pole).

Life of a star

"The universe needed places denser than the galaxies, to reach the complexity, it invents then stars" Trinh Xuan Thuan.
Pushed by the gravity, the small clouds of hydrogen and helium of the young galaxy, collapse and the density increases gradually. The gaseous balls ignite, it is the birth of stars as the Sun. The nuclear energy loosened in its balls, stop the gravitational collapse and a balance settles down between the pressure of the radiation and that of the gravity.

The big stars live some million years, stars average as our Sun, exhaust their reserve of hydrogen only at the end of 9 billion years and the small stars will burn their fuel, 20 billion for years. When the hydrogen is consumed, the gravitational pressure gets over it, the density increases and the temperature reaches 100 million degrees. The nuclei of helium 4, produced by the combustion of the hydrogen, group together to form nuclei of carbon 12. The pressure of the radiation resumes vigor, the contraction stops, the star swells excessively, cools and becomes a red giant.

Later

300 million years later, the combustion of the helium is ended, the heart of the red giant contracts again, for lack of a sufficient radiation.
The temperature reaches then 500 million degrees, and it is now in the tour of the carbon to waste away for make the other elements always more complex, as the neon, the oxygen, the sodium, the magnesium, the aluminum, the silicon, the phosphor, the sulfur.

These sequences are going to repeat many a time by accelerating and towards the end of its life, the heart of the star contains some iron, some cobalt and some nickel, result of the combustion of the silicon.
In stars, real cosmic ovens, are going to be made, more and more heavy chemical elements necessary for the walking forward towards the complexity.

Cycle proton-proton

In the stars of solar type, a suite of reactions called "chain proton-proton" operates in several stages.
At first 2 protons merge in a nucleus of deuterium (isotope of the hydrogen or the heavy hydrogen, because formed by a proton and by a neutron) with emission of a positron (or antielectron) and of neutrinos which take 2 % of the global energy.
The deuterium merges with a proton to give a nucleus of helium 3 (2 protons and 1 only neutron) and a photon; two of these unstable nuclei merge to lead to the very unstable beryllium 6 which splits at once to give finally the stable nucleus of helium 4 with formation of 2 protons. 6 protons are thus necessary so that a stable nucleus of helium can form, with restoration of 2 protons; the balance sheet is many 4 protons for a nucleus He4. Chains protons-protons require a temperature superior to 10 million degrees. A small quantity of helium 3 forms of the beryllium 7, which, during the other chains of reactions, leads to the lithium 7 or to the boron 8 giving of the beryllium 8 (with intense release of neutrinos): all these nuclei, very unstable, quickly transmute in helium 4.

Image: The proton-proton cycle, first merging two protons in a nucleus of deuterium to create a helium nucleus, the portion of the photons are released.
The hydrogen atoms launched against each other by the enormous pressure, will be transformed into helium atoms, this process generates fusion of atoms with a mass slightly smaller and this difference is released as energy.

Layers of the Sun

Core:
The core is the zone where occurs the nuclear reactions (fusion of the atoms d' hydrogen).
Radiative zone:
The radiative zone is an ionized area of dense gases bombarded by the rays G resulting from the fusion of the protons of the core. These rays G rebound on gases, are absorbed then re-emitted in the form of x-rays and of radiation U.V.
Convective zone:
The convective zone transports energy of the heart towards surface by convection. The gases bring l' energy on the surface of the Sun and turns over towards the bottom after having lost their energy.

Photosphere: The photosphere of 160 km d' thickness only is responsible for emission of energy which bathes planets, it is mottled granules.
Chromosphere:
The chromosphere is a semitransparent layer visible during eclipses. This is that formed the projections. The spicules are these long jets proposed material.
Crown:
The Crown is the external atmosphere of the Sun. She undulates and change forms of jets of gas emissions. This is the visible part of the Sun.

Image: Own work, autor Kelvinsong

What is the size of a star?

It is through the law of Stefan-Boltzmann, that astronomers can easily calculate the radii of stars (see Note below against). In 1879, the Austrian physicist Josef Stefan, who is interested in the radiation of hot bodies, discovers that the total energy emitted by an object is proportional to the power of 4 of its absolute temperature.
The biggest stars discoveries are sagitarii kilowatts, V354 Cephei and KY Cygni, are approximately 1 500 times larger than our Sun.
Our Sun has a diameter of 1 392 000 km.
Antares, super giant red closest to us has a diameter of about ≈ 700 times that of the Sun, or nearly 1 billion miles.
Betelgeuse is a red super giant, one of the largest stars known. If Betelgeuse were at the center of our solar system, its radius, ≈ 650 times that of the Sun, would extend between the orbit of Mars and Jupiter.
Aldebaran is a red giant of magnitude 0.86 and spectral type K5 III, which means that it is orange and has largely left the main sequence after using all its hydrogen. It burns primarily of helium and reached a diameter of ≈ 45 times that of the Sun.

Rigel is a blue super giant, 55 000 times brighter than the Sun. With a diameter of nearly 116 000 000 km, ≈ 35 times that of the sun, Rigel would extend until the orbit of Venus in our solar system.
Arcturus is 20 times larger than the sun, its magnitude is -0.04 and its distance to the sun is ≈ 37 light-years.
Pollux is ≈ 8 times larger than the sun, its magnitude is 1.09 and its distance to the sun is ≈ 33.7 light-years.

Image: Sizes compared to some super giant stars like Antares, Betelgeuse, Rigel, Aldebaran and some white dwarfs as Arcturus, Pollux, Sirius and the Sun.
© astronoo.com

NB: Thanks to the law of Stefan-Boltzmann constant, astronomers can calculate the radii of the stars.
The brightness of a star L is: L = 4πσR2T4
L is the luminosity, σ is the constant of Stefan-Boltzmann constant, R the radius of the star and T its temperature.

The death of a star

The death of a star can be sweet or rapes, it depends on its mass.
Below 1,4 times the mass of the Sun, the star goes out in the serenity, it will pass of the size of a red giant (approximately 50 million Km of beam), in that of the Earth (approximately 6000 km of beam). The star becomes a white dwarf.
Between 1,4 and 5 times the mass of the Sun, its agony is much more violent. Its beam narrows until 10 Km. The final density is enormous, nuclei cannot resist and the heart of the star becomes a gigantic nucleus of neutrons. The collapse provokes a terrible explosion which is going to throw the superior layers of the star in the space and we shall see shining in the sky, a supernova.
Above 5 times the mass of the Sun, the collapse is extremely violent. This one cannot be any more arrested. The heart of the star A black hole becomes.
The violence of the collapse produces a gigantic explosion which throws the superior layers of the star in the space.

As in the previous case a supernova is going to extend, on hundreds of billion Km, sowing the interstellar middle of heavy elements, made during the life of the star and during the explosion.

Image: The violence of the collapse of a star, produces a huge explosion which projects the upper layers of the star into space, playing a key role in the history of life.
During its supernova explosion the star releases the chemical elements that it has synthesized in its existence and in the explosion itself. These chemicals travel in the interstellar medium to spread into space.