The Sun

THE SUN

Image of the Sun in H alpha (National Solar Observatory
/Sacramento Peak)

The Sun, the central body of the Solar System, is a sphere of incandescent gas, mostly hydrogen and helium, whose mass is 2 10³³ g (2 billion of billion of billion tons), that is 99.9 % of the total mass of the Solar System itself. The diameter of the Sun is as large as 1,392,000 km, 109 times that of the Earth, and corresponds, as seen from the Earth, to an angular diameter of about 32 arc minutes, almost as large as the lunar one. This originates the eclipses phenomenon, that is the apparent overlap of the lunar and the solar disks. The mean density of the Sun is 1.4.

The motion of the Sun

The Sun participates in the rotation motion of the Galaxy, moving, with respect to the nearby stars, at a speed of 19.7 km/s towards a point on the celestial sphere, called the solar motion apex.
Furthermore it has, just like the planets, a rotational motion about its axis, which is inclined by 7^o 15' on the plane of the Ecliptic. The angular speed varies versus the latitude; in fact, since it is a gaseous sphere, it does not rotate rigidly but presents a differential rotation, which is slower at the poles and faster at the equator.
At the equator, the rotational period is about 25 days.

Energy emission of the Sun

The Sun is classified as a dwarf star, whose spectral type is G2; its surface temperature is about 5,700 degrees and it emits electromagnetic radiation mostly in the optical region and near infrared, between 2,000 Angstrom and 3 micron, with a power of 400,000 billion of billion KW (4 10³³ erg/sec).

The origin of this emission, which in the past century was believed to be due to the gravitational contraction. and the following heating of its inner layers, is instead the nuclear fusion that works in its center. Due to its large mass, the inner regions of the Sun are compressed until they reach very high temperatures (15 million degrees) and to light up the fusion, which requires high pressures and temperatures.

The nuclear fusion consists in the transformation of four hydrogen nuclei (the main component of the Sun) into an helium nucleus: the mass of the latter is slightly less than the sum of the masses of the hydrogen nuclei; the difference is converted into energy.

Sketch of the nuclear fusion inside the stars (Michiel Berger)

Each second, 594 million tons of hydrogen are transformed into 590 million tons of helium. The difference, 4 million tons, corresponds to the energy that the Sun radiates in a second, according to the law E=mc², where E is the energy output, m the mass transformed into energy and c is the speed of light.

The nuclear fusion is self regulated so that the emission of energy is stable in time; the hydrogen reservoir inside the nucleus are, however, not endless, and the total duration of this process is about 10 billion years.
Since the age of the Sun is estimated 5 billion years, 5 billion years from now the fusion will cease and it will start changing, becoming cooler and brighter, that is a red giant ; its external layers will expand swallowing the closer planets, among which the Earth, and later it will end its life as a white dwarf, that is it will become a very hot star, but very faint, and it will slowly dim.

X ray image of the Sun. The lighter regions are brighter X emission sources (Calvin J. Hamilton e Yohkoh)

The structure of the Sun

The very high temperatures inside the Sun cause an almost complete ionization of the gas, that is the electrons are stripped from the atoms and freely move in the gas. The temperature decreases from 15 million degrees in the center down to about 5,700 degrees at the surface.

Also the density of the gas decreases outwards, from about 158 g/cm³ at the center down to a 10^-7 at the surface; actually the Sun hasn't a well defined physical surface : what we see is just a surface called photosphere: a very thin gas layer (about 200 km), which surrounds the inner star and that emits radiation into the optical band.

The inner structure of the Sun (NASA/ESA)

The inner region is composed by a core, where the nuclear fusion reactions are at work, surrounded by a layer of gas called radiative zone, which in turn is surrounded by a layer called convective zone, as thick as 150,000 km.
In the radiative zone, the energy produced by the nuclear fusion is transported outside by means of photons which are transferred from one ion to the other, in a very slow process that takes some million year; moving outwards, the temperature of the gas decreases and the atoms of the heavier elements start recombining with the electrons.
When the electrons are recombined, they can absorb a photon and be stripped again from the atom; this causes a slow down in the outward path of the radiation.
Convective motions thus develop, that is hot gas bubbles go up towards the surface, where they cool, thus being a carrier for the energy, which otherwise would be trapped inside. These motions, similar to those produced in a boiling water pan, make gas bubble reach the surface, where they give birth to the photosphere granulation, that is an irregular appearance similar to a collection of rice grains very luminous and visible in the optical band of the spectrum.

The Sun spots

A group of Sun spots.
The granulation comes from
turbulent eruptions of energy on the surface. (National Solar
Observatory/
Sacramento Peak)
On the photosphere, also dark regions can be seen, called Sun spots. They are of variable shape and dimensions.
They were observed by Galileo Galilei in 1610 with his telescope, but they were already known in ancient China. The Sun spots appear moving on the solar disk surface, as a consequence of the Sun's rotation motion, and their properties change according to cycles of about 11 years. Their dimensions are comprised between a few thousand and more than 200 thousand km, and they are surrounded by penumbra regions.,

The Sun spots activity cycle within the last 250 years (Michiel Berger)

Their dark appearance is due to the fact that they are cooler (about 4,500K) and hence fainter, than the photosphere. They often gather into groups of tens, small and large. The development of a group of spots starts as a number of small spots appear, which later expand and merge together; this process can take from one week to a few months. The origin of the Sun spots seems due to the solar magnetic field , just as most of the photospheric activity: they indeed have an intense magnetic field. Furthermore they appear the location of whirling convective motions, during which the gas coming from inside cools down reaching the surface.
Even the 11 year cycle could be explained in terms of the solar magnetic activity, and in particular it should be due to the differential rotation of the Sun, which deforms the magnetic field lines.

The solar magnetic field. The darker regions are the location of positive magnetic polarity, the lighter one of negative polarity. (GSFC NASA)

Close to the Sun spots, bright areas called faculae can be seen, visible in white light. They are produced by the gas which is channeled from the inner parts along the magnetic field lines. Finally, close to the Sun spots, flares can be noticed, that is very short lasting explosions, during which, from the solar surface jets of gas and radiation are emitted; the frequency of this phenomenon is related to the solar activity, in particular the magnetic one.

A solar flare observed in H alpha (National Solar Observatory/ Sacramento Peak)

The atmosphere and the cromosphere

Above the photosphere stands the solar atmosphere, whose lower part is called cromosphere, a hot gas layer (10-20,000 degrees) 2,000 km thick, which is revealed by the emission of a hydrogen spectral line at 6563 Angstrom, in the red zone of the visible spectrum. When observed with a red filter, the cromosphere appears very irregular due to phenomena that concern the most external gas layers. In particular, prominences can be seen , that is hot gas jets that appear as giant fire flames emitted by the surface and disappearing within a few days or weeks; and spiculae, small hydrogen flames, as large as a few hundred km, which originate in the lower and medium cromosphere and disappear after a few minutes.

One of the most spectacular
prominences ever observed,
as large as 588,000 km.
It has been imaged by the Skylab
in December, 1973 (NASA)

Ultraviolet image
of a solar eruption.
The image has been taken
by the SOHO satellite (SOlar
and Heliospheric Observatory)
in 1996. (ESA/NASA)

Beyond the cromosphere, a vast region of very hot and ionized gas is present. It is also extremely rarefied, and it is called solar corona; it has a much lower luminosity than that of the photosphere, so it is not usually visible, but during the solar eclipses which obscure the brighter part. The solar corona strongly emits in the radio band; its spectrum indicates the presence of calcium atoms lacking even 14 electrons, and iron atoms lacking 13 electrons: this indicates a gas temperature of more than a million degrees.

The coronal gas at the temperature of one and a half million degrees, observed by the Extreme UltraViolet Imaging Telescope on the SOHO (SOlar Heliospheric Observatory) probe. The solar magnetic field structures can be observed. (ESA/NASA)

The origin of this very high temperature is not yet well known. The extension of the corona cannot be easily determined, since its luminosity gradually decreases out to many million kilometers from the Sun. The Sun, moreover, keeps emitting a jet of ionized gas, called solar wind, at a speed varying between 250 and 850 km/s.

In this image of a total eclipse in 1977, the solar corona can be well seen. (Calvin J. Hamilton)

Solar total eclipse image, July 1991. Picture by Steve Albers, California.

This ionic flux, which can be considered as the extension of the corona, is pushed to very large distances from the Sun, and interacts with the magnetosphere and the ionosphere of the planets, perturbing it and producing phenomena as the polar auroras.

Hot gas plumes coming out from the Sun, perhaps sources of solar wind and charged particles. From top to bottom: the magnetic field near the South solar pole; ultraviolet image of a plume at the temperature of one million degrees, in the same region; ultraviolet image of a solar atmosphere region, a more quiet and closer to the surface zone. (ESA/NASA)

ANIMATIONS

Convection inside the Sun, MPEG, 1.2 Mb (NASA/STScI)

The solar eclipse of 1994, MPEG, 3.1 Mb (NASA/STScI)

Inner region, photosphere and solar corona, AVI, 6 Mb (NASA)

The magnetic field, prominences and the solar wind, AVI, 5.6 Mb (NASA)