Distance from Earth
150 million km
Density
1410 kg/m3
Luminosity
3.9 × 1026 J/s
Surface temperature
5500 °C
Interior temperature
15 million degrees Celsius
Equatorial rotation period
25 days
Composition
92 % hydrogen, 7.8 % helium
Surface gravity
290 N/kg (29 × Earth gravity)
Escape velocity
618 km/s
Photosphere thickness
400 km
Chromosphere thickness
2500 km
Core pressure
250 billion atmospheres
Sunspot cycle
11 years
Age
4.5 billion years
The main elements present in the Sun are hydrogen (92 %), followed by helium (7.8 %), and less than 1 % of heavier elements like oxygen, carbon, nitrogen and neon. The Sun is entirely gaseous with an average density 1.4 times that of water. Because the pressure in the core is much greater than at the surface, the core density is eight times that of gold, and the pressure is 250 billion times that on Earth’s surface (Table 3.3).Table 3.3Composition of the Sun
Element
Abundance (percentage of total number of atoms)
Abundance (percentage of total mass)
Hydrogen
91.2
71.0
Helium
8.7
27.1
Oxygen
0.078
0.097
Carbon
0.043
0.40
Nitrogen
0.0088
0.096
Silicon
0.0045
0.099
Magnesium
0.0038
0.076
Neon
0.0035
0.058
Iron
0.0030
0.14
Sulfur
0.0015
0.040
Energy and Luminosity
The Sun produces a 100 million times more energy than all the planets combined. Just over half this energy is in the form of visible light, with the rest being infrared (heat) radiation. Only about a billionth of the Sun’s energy reaches us here on Earth.
The Sun’s energy comes from the burning of its hydrogen gas via the process of nuclear fusion. In this process four hydrogen atoms combine to make one helium nuclei. During this process some mass is lost and it is this mass that is converted into energy. Every second the Sun converts over 600 million tonnes of hydrogen into helium, and this results in 4.5 million tonnes of matter being converted into energy every second.
Energy generated in the core is carried outward to the surface by radiation and convection processes. Core temperature is about 15 million degrees Celsius, while at the surface the temperature is around 5500 °C. The surface and interior temperature are too hot to have any liquid or solid material.
The luminosity of a star is an indication of the total amount of energy it produces every second. This rate depends on the core temperature and pressure of the star, which in turn depends on its mass. The Sun’s luminosity is 3.9 × 1026 J/s.
Throughout its life the Sun has increased its luminosity by about 40 % and it will continue to increase from some time.
Zones of the Sun
The Sun has several different layers or zones of activity. At the centre is the core, which is where energy is produced via nuclear fusion reactions. Above this is the radiative zone, where energy travels very slowly upwards. Closer to the surface is the convective zone where heat is transported much faster to the surface, or photosphere. Surrounding the photosphere is the solar atmosphere that contains two zones—the chromosphere and corona.
The Core of the Sun
The core of the Sun is the central region where nuclear reactions convert hydrogen into helium. These reactions release the energy that ultimately leaves the Sun as visible light. For these reactions to take place a very high temperature is needed. The temperature close to the centre is about 15 million degrees Celsius and the density is about 160 g/cm3 (i.e. 160 times that of water). Both the temperature and density decrease outwards from the centre of the Sun. The core occupies the innermost 25 % of the Sun’s radius. At about 175,000 km from the centre the temperature is only half its central value and the density drops to 20 g/cm3.
The Radiative Zone
Surrounding the core of the Sun is the radiative zone. This zone occupies 45 % of the solar radius and is the region where energy, in the form of gamma ray photons, is transported outward by the flow of radiation generated in the core. The high-energy gamma ray photons are knocked about continually as they pass through the radiative zone, some are absorbed, some re-emitted and some are returned to the core. It may take the photons a hundred thousand years to find their way through the radiative zone. At the outermost boundary of the radiative zone, the temperature is about 1.5 million degrees, and the density is about 0.2 g/cm3. This boundary is called the interface layer or tachocline. It is believed that the Sun’s magnetic field is generated in this layer. The changes in fluid flow velocities across the layer can stretch magnetic field lines of force and make them stronger. There also appears to be sudden changes in chemical composition across this layer.
The Convective Zone
The outermost zone is called the convective zone, because energy is carried to the surface by a process of convection. It extends from a depth of about 210,000 km up to the visible surface and occupies about 30 % of the Sun’s radius. In this zone, plasma gas, heated by the radiative zone beneath, rises in giant convection currents to the surface, spreading out, cooling, and then shrinking—similar to the boiling of water in a pot. Rising cells of gas are visible on the surface as a granular pattern. The granules are around 1000 km in diameter. The convection cells release energy into the Sun’s atmosphere. At the surface the temperature is around 5600° and density is practically zero.
Once the plasma gas reaches the surface of the Sun, it cools and settles back into the Sun to the base of the convection zone, where it receives more heat from the top of the radiative zone. The process then repeats itself. The photons escaping from the Sun, have lost energy on their way up from the core and changed their wavelength so most emission is in the visible region of the electromagnetic spectrum.
The lower temperatures in the convective zone allow heavier ions (such as carbon, nitrogen, oxygen, calcium, and iron), to hold onto some of their electrons. This makes the material more opaque so that it is harder for radiation to get through. This traps heat that ultimately makes the fluid unstable and it starts to ‘boil’ or convect (Fig. 3.3).
Fig. 3.3Interior structure of the Sun. Energy is transferred by radiation in the inner regions, and by convection in the outer region.
The Photosphere
The photosphere is a thin shell of gases about 200 km thick and forms the visible surface of the Sun. Most of the energy radiated by the Sun passes through this layer. It has a temperature of about 5500 °C. From Earth the surface looks smooth, but it is actually turbulent and granular because of convection currents. Material boiled off from the surface of the Sun is carried outward by the solar wind.
The surface of the Sun also contains dark areas called sunspots. Sunspots appear dark because they are cooler than the surrounding photosphere—about 3500 °C compared to 5500 °C. They radiate only about one fifth as much energy as the rest of the photosphere (see Fig. 3.4).
Fig. 3.4Sunspots on the surface of the Sun. Large sunspots contain dark umbral centres, grey penumbral haloes, many large and small single and overlapping spots, and surrounding whitish plages (Credit: J. Wilkinson).
Sunspots vary in size from 1000 km to over 40,000 km. As they move across the surface of the Sun, sunspots usually change shape—some disappear and new ones appear.
Their lifetime seems to depend on their size, with small spots lasting only several hours, while larger spots may persist for weeks or months. The rate of movement of sunspots can be used to estimate the rotational period of the Sun. At the equator, sunspots take about 25 days to move once around the Sun. At the poles sunspots take about 36 days to go around the Sun. This odd behaviour is due to the fact that the Sun is not a solid body like the Earth. Sometimes sunspots appear in isolation, but often they arise in groups.
Sunspots and sunspot groups are directly linked to the Sun’s intense magnetic fields. Such spots are areas where concentrated magnetic fields break through the hot gases of the photosphere. These magnetic fields are so strong that convective motion beneath the spots is greatly reduced. This in turn reduces the amount of heat brought to the surface as compared to the surrounding area, so the spot becomes cooler. Data obtained from space probes like SOHO have shown that the strength of the magnetic fields around sunspots is thousands of times stronger than the Earth’s magnetic field.
A typical sunspot is about 10,000 km across. Each has two parts: a black central region called the umbra, which in turn is surrounded by a grey region, the penumbra. The darker the area, the lower the temperature. It is possible to view sunspots from Earth by projecting the image of the Sun from a telescope onto a white screen or by using a telescope fitted with a Herschel wedge. Observations of the Sun in this way need to be made carefully so as not to damage the viewer’s eyes—NEVER look at the Sun through an unprotected telescope!
Like many other features of the Sun, the number and location of sunspots vary in a cycle of about 11 years. Heinrich Schwabe, a German astronomer, first noted this cycle in 1843. Sunspot maximums occurred in 1968, 1979, 1990 and 2001. Sunspot minimums occurred in 1965, 1976, 1986, 1997 and 2008. The average latitude of sunspots also varies throughout the sunspot cycle. At the beginning of a sunspot cycle, most sunspots are at moderate latitudes, around 28° north or south. Sunspots arising much later in each cycle typically form closer to the Sun’s equator. The variation in the number of sunspots is now known to be the most visible aspect of a profound oscillation of the Sun’s magnetic field that affects other aspects of both the surface and interior (see Figs. 3.5 and 3.6).
Fig. 3.5Variations in sunspot numbers tend to go through a maximum/minimum cycle every 11 years. The figure shows sunspot numbers since 1960 with predicted numbers for solar cycles 24 and 25.
Fig. 3.6Variations in the average latitude of sunspots.
The SOHO probe has been able to monitor the Sun for the entire 11-year sunspot cycle (number 23) and the rise of the current cycle (number 24). At the same time, this probe has also monitored the total solar irradiance and variations in the extreme ultraviolet flux, both of which are important to our understanding of the impact of solar variability on Earth’s climate (see Fig. 3.5).
Ejection of material from the surface of the Sun often follows solar flares or other solar phenomena. Sometimes this material reaches Earth and gets trapped in the magnetic field around Earth’s polar regions. This material consists mostly of charged particles (ions and electrons), which interfere with communication systems and produce magnetic and ionospheric disturbances such as auroras. An aurora is a bright display of coloured lights in the night sky. Auroras are produced when charged particles (from the Sun) get trapped in the Earth’s magnetic field and collide with atoms in our upper atmosphere.
Another visible phenomenon of the Sun’s photosphere associated with its magnetic field is the faculae. These are irregular patches or streaks brighter than the surrounding surface. They are clouds of incandescent gas in the upper regions of the photosphere. Such clouds often precede the appearance of sunspots.
The Chromosphere
The chromosphere is the first layer of the Sun’s atmosphere. It lies just above the photosphere and is a few thousand kilometres thick. During a solar eclipse, when the Moon passes in front of the Sun, the chromosphere appears as a red shell around the Sun. The chromosphere is much hotter than the photosphere ranging from 4200 °C near the surface to 8200 °C higher up. It consists largely of hydrogen, helium and calcium.
When viewed with a hydrogen alpha filter, dark features called filaments can often be seen against the surface of the Sun. These structures are huge masses of burning plasma ejected upwards from the photosphere and suspended in the Sun’s chromosphere and corona by strong magnetic fields. When seen around the limb of the Sun, these eruptions, can be seen as gigantic ‘flame-like’ structures, and are called prominences. The prominences can reach a temperature of 50,000 °C. Some prominences last for only a few hours while others last for weeks. Prominences can only be seen during a total solar eclipse or by using a hydrogen-alpha telescope (Fig. 3.7).
Fig. 3.7Prominence eruptions on the Sun. Taken by the author through a H-alpha solar telescope on 23rd April 2015 (Credit: J. Wilkinson).
The Corona
The corona is the upper layer of the Sun’s atmosphere. During a solar eclipse, it appears as a pale white glowing area around the Sun. Temperatures in the corona reach as high as one million degrees Celsius because of interactions between gases and the photosphere’s strong magnetic fields. The corona can extend millions of kilometres into space. The corona consists mainly of ionised gas or plasma.
A coronal mass ejection or CME is an expulsion of a part of the corona and ionised particles into space. Such events can represent the loss of several billion tonnes of matter from the Sun at speeds between 10 and 1000 km/s. Some CME’s are triggered by solar flares and are associated with strong magnetic fields in the corona. Sometimes, clouds of ejected particles are carried by the solar wind towards Earth (see Fig. 3.8).
Fig. 3.8In March 2000 an erupting filament lifted off the active solar surface and blasted this enormous bubble of magnetic plasma into space (a coronal mass ejection). The Sun itself (white circle) has been blocked out in this picture of the event (Credit: NASA/ESA/SOHO).
A coronal hole is a large region in the corona that is less dense and is cooler than its surrounds. They are areas where open magnetic field lines project out from the Sun’s surface. Such holes may appear at any time during a solar cycle but they are most common during the declining phase of the cycle. Coronal holes allow denser and faster ‘gusts’ of the solar wind to escape the Sun. They are sources of many disturbances in Earth’s ionosphere and geomagnetic field (see Fig. 3.9).
Fig. 3.9Two coronal holes on the Sun developed over several days in Jan 2011. This UV image was taken by the SDO space probe (Credit: NASA/SDO).
Solar flares occur when the magnetic field of the Sun changes rapidly to create an explosion of charged particles through the Sun’s corona. Such events last from a few minutes to a few hours and can send charged particles, X-rays, ultraviolet rays and radio waves into space. Flares can release energy equivalent to more than a billion one-megaton thermonuclear explosions in a few seconds. They are sometimes so violent that they cause additional ionisation in the Earth’s ionosphere and may disrupt radio communications.
The Solar Wind
The solar wind is an erratic flow of highly ionised gas particles that are ejected into space from the Sun’s upper atmosphere. This wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft. The SOHO, Wind and ACE probes have measured the speed of the solar wind.
The Ulysses space probe provided the first-ever three-dimensional map of the heliosphere from the equator to the poles. Instruments on board the Ulysses space probe also found that the solar wind blows faster around the Sun’s poles (750 km/s) than in equatorial regions (350 km/s).
The SOHO space probe found that the solar wind originated from honeycomb-shaped magnetic fields surrounding large bubbling cells near the Sun’s poles.
Near the Earth, the particles in the solar wind move at speeds of about 400 km/s. These particles often get trapped in Earth’s magnetic field, especially around the poles, and produce auroras.
The solar
wind produces a huge bubble in space called the heliosphere. The heliosphere stretches outward from the Sun in all directions to a distance well beyond the world of planets. At its outer boundary, called the heliopause, lies the broader realm of the Oort cloud. The heliosphere is a protective zone, carved out by the solar wind and sustained by the Sun’s extended magnetic field. It “protects” in the sense that the solar plasma that flows continually outward from the Sun is strong enough to fend off most of the plasma that comes in stellar winds from other stars, and to keep out all but the most energetic cosmic rays.
Cycles in Solar Activity
While most of the Sun’s activity follows the 11-year sunspot cycle, conditions in the heliosphere are driven by a 22-year magnetic cycle. The Sun’s magnetic field is like that of a giant bar magnet with a north and South Pole. Data from the Ulysses space probe showed that at during the last solar maximum (2001), the Sun’s north and south poles changed places. Ulysses next passed over the Sun’s poles during the solar minimum period 2007/2008. At this time the Sun’s magnetic polarity was opposite to that of the previous solar minimum.
The Japanese Hinode probe (launched 2006) was the first to be able to measure small changes in the Sun’s magnetic field. The magnetic field of the Sun influences the way in which charged particles move through the heliosphere.
The Solar System in Close-Up Page 6