The Sun
The Sun is a yellow dwarf and is the star at the centre of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets, and dust) orbit the Sun which by itself accounts for about 98.6% of the Solar System's mass. The mean distance of the Sun from the Earth is approximately 149,600,000 kilometres, or 92,960,000 miles, and its light travels this distance in 8 minutes and 19 seconds. Energy from the Sun, in the form of sunlight, supports almost all life on Earth via photosynthesis and drives the Earth's climate and weather.
The surface of the Sun consists of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 24% of mass, 7% of volume), and trace quantities of other elements, including iron, nickel, oxygen, silicon, sulphur, magnesium, carbon, neon, calcium, and chromium. The Sun has a spectral class of G2V. G2 means that it has a surface temperature of approximately 5,780 K (5,500 °C) giving it a white colour that often, because of atmospheric scattering, appears yellow when seen from the surface of the Earth. This is a subtractive effect, as the preferential scattering of shorter wavelength light removes enough violet and blue light, leaving a range of frequencies that is perceived by the human eye as yellow. It is this scattering of light at the blue end of the spectrum that gives the surrounding sky its colour. When the Sun is low in the sky, even more light is scattered so that the Sun appears orange or even red.
Venus passes in front of the solar disc
This animation was prepared by Roland Boninsegna, physics teacher in Belgium. He explains: " This animation is composed of 30 views, beginning at 4h 45m UT in the morning of June 8, 2004, and. separated by 15 minutes. It was constructed as this event will be seen from Belgium, but can be used everywhere for general purposes. The greatest differences will be between northern and southern countries. For example, there will be around 3 to 4 min differences between the contacts, as seen in Finland and Portugal, respectively. This is not so important for the general understanding of the observation. On this animation, you will see the movement of Venus and the Sun along the ecliptic (blue line) as seen the from Earth. The North and East directions are also indicated".
This animation was prepared by Roland Boninsegna, physics teacher in Belgium. He explains: " This animation is composed of 30 views, beginning at 4h 45m UT in the morning of June 8, 2004, and. separated by 15 minutes. It was constructed as this event will be seen from Belgium, but can be used everywhere for general purposes. The greatest differences will be between northern and southern countries. For example, there will be around 3 to 4 min differences between the contacts, as seen in Finland and Portugal, respectively. This is not so important for the general understanding of the observation. On this animation, you will see the movement of Venus and the Sun along the ecliptic (blue line) as seen the from Earth. The North and East directions are also indicated".
The Sun's spectrum...
contains lines of ionized and neutral metals as well as very weak hydrogen lines. The V (Roman five) in the spectral class indicates that the Sun, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium. There are more than 100 million G2 class stars in our galaxy. Once regarded as a small and relatively insignificant star, the Sun is now known to be brighter than 85% of the stars in the galaxy, most of which are red dwarfs. The Sun orbits the centre of the Milky Way galaxy at a distance of approximately 24,000 to 26,000 light years from the galactic centre, moving generally in the direction of Cygnus and completing one revolution in about 225–250 million years (one Galactic year). It's orbital speed was thought to be 220±20 km/s, but a new estimate gives 251 km/s. This is equivalent to about one light-year every 1,190 years, and about one AU every 7 days. These measurements of galactic distance and speed are as accurate as can be, given current knowledge, but this may change as more is learned. Since our galaxy is moving with respect to the cosmic microwave background radiation (CMB) in the direction of Hydra with a speed of 550 km/s, the sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo. The Sun is currently travelling through the Local Interstellar Cloud in the low-density Local Bubble zone of diffuse high-temperature gas, in the inner rim of the Orion Arm of the Milky Way Galaxy, between the larger Perseus and Sagittarius arms of the galaxy. Of the 50 nearest stellar systems within 17 light-years (1.6×1014 km) from the Earth, the Sun ranks 4th in absolute magnitude as a fourth magnitude star (M=4.83).
The sun was formed about 4.57 billion years ago when the rapid collapse of a hydrogen molecular cloud led to the formation of a third generation T Tauri Population I star, the Sun. The nascent star assumed a nearly circular orbit about 26,000 light-years from the centre of the Milky Way Galaxy. Solar formation is dated in two ways: the Sun's current main sequence age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years. This is in close accord with the radiometric date of the oldest solar system material, at 4.567 billion years ago.
The sun was formed about 4.57 billion years ago when the rapid collapse of a hydrogen molecular cloud led to the formation of a third generation T Tauri Population I star, the Sun. The nascent star assumed a nearly circular orbit about 26,000 light-years from the centre of the Milky Way Galaxy. Solar formation is dated in two ways: the Sun's current main sequence age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years. This is in close accord with the radiometric date of the oldest solar system material, at 4.567 billion years ago.
The Sun is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation; at this rate, the Sun will have so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10 billion years as a main sequence star.The Sun does not have enough mass to explode as a supernova. Instead, in about 5 billion years, it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches around 100 million kelvins and will produce carbon, entering the asymptotic giant branch phase.
Earth's fate is precarious. As a red giant, the Sun will have a maximum radius beyond the Earth's current orbit, 1 AU (1.5×1011 m), 250 times the present radius of the Sun. However, by the time it is an asymptotic giant branch star, the Sun will have lost roughly 30% of its present mass due to a stellar wind, so the orbits of the planets will move outward. If it were only for this, Earth would probably be spared, but new research suggests that Earth will be swallowed by the Sun owing to tidal interactions. Even if Earth would escape incineration in the Sun, still all its water will be boiled away and most of its atmosphere would escape into space. In fact, even during its current life in the main sequence, the Sun is gradually becoming more luminous (about 10% every 1 billion years), and its surface temperature is slowly rising. The Sun used to be fainter in the past, which is possibly the reason why life on Earth has only existed for about 1 billion years on land. The increase in solar temperatures is such that already in about a billion years, the surface of the Earth will become too hot for liquid water to exist, ending all terrestrial life. Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The only object that will remain after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a white dwarf over many billions of years. This stellar evolution scenario is typical of low- to medium-mass stars.
Earth's fate is precarious. As a red giant, the Sun will have a maximum radius beyond the Earth's current orbit, 1 AU (1.5×1011 m), 250 times the present radius of the Sun. However, by the time it is an asymptotic giant branch star, the Sun will have lost roughly 30% of its present mass due to a stellar wind, so the orbits of the planets will move outward. If it were only for this, Earth would probably be spared, but new research suggests that Earth will be swallowed by the Sun owing to tidal interactions. Even if Earth would escape incineration in the Sun, still all its water will be boiled away and most of its atmosphere would escape into space. In fact, even during its current life in the main sequence, the Sun is gradually becoming more luminous (about 10% every 1 billion years), and its surface temperature is slowly rising. The Sun used to be fainter in the past, which is possibly the reason why life on Earth has only existed for about 1 billion years on land. The increase in solar temperatures is such that already in about a billion years, the surface of the Earth will become too hot for liquid water to exist, ending all terrestrial life. Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The only object that will remain after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a white dwarf over many billions of years. This stellar evolution scenario is typical of low- to medium-mass stars.
The Sun is a yellow main sequence star
An illustration of the structure of the Sun:
1. Core 2. Radiative zone 3. Convective zone 4. Photosphere 5. Chromosphere 6. Corona 7. Sunspot 8. Granules 9. Prominence |
The Sun is a yellow main sequence star comprising about 99% of the total mass of the Solar System. It is a near-perfect sphere, with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 km (6 mi). As the Sun exists in a plasmatic state and is not solid, it rotates faster at its equator than at its poles. This behaviour is known as differential rotation. The period of this actual rotation is approximately 25 days at the equator and 35 days at the poles. However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the apparent rotation of the star at its equator is about 28 days. The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. The tidal effect of the planets is even weaker, and does not significantly affect the shape of the Sun. The Sun does not have a definite boundary as rocky planets do, and in its outer parts the density of its gases drops approximately exponentially with increasing distance from its centre. Nevertheless, it has a well-defined interior structure, described below. The Sun's radius is measured from its centre to the edge of the photosphere. This is simply the layer above which the gases are too cool or too thin to radiate a significant amount of light, and is therefore the surface most readily visible to the naked eye. The solar core comprises 10 percent of its total volume, but 40 percent of its total mass. The solar interior is not directly observable, and the Sun itself is opaque to electromagnetic radiation. However, just as seismology uses waves generated by earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology makes use of pressure waves (infrasound) traversing the Sun's interior to measure and visualize the star's inner structure. Computer modelling of the Sun is also used as a theoretical tool to investigate its deeper layers.
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Sun Spots and Cycles
When observing the Sun with appropriate filtration, the most immediately visible features are usually its sunspots, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures. Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field gives rise to strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. The largest sunspots can be tens of thousands of kilometres across. The number of sunspots visible on the Sun is not constant, but varies over an 11-year cycle known as the solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by Spörer's law.
Sunspots usually exist as pairs with opposite magnetic polarity. The magnetic polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.
The solar cycle has a great influence on space weather, and is a significant influence on the Earth's climate since luminosity has a direct relationship with magnetic activity. Solar activity minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures.
The solar cycle has a great influence on space weather, and is a significant influence on the Earth's climate since luminosity has a direct relationship with magnetic activity. Solar activity minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures.
Early Understanding Of The Sun
Worshipping Ra, Sun God
Humanity's most fundamental understanding of the Sun is as the luminous disk in the sky, whose presence above the horizon creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural phenomenon. Worship of the Sun was central to civilizations such as the Inca of South America and the Aztecs of what is now Mexico. Many ancient monuments were constructed with solar phenomena in mind; for example, stone megaliths accurately mark the summer or winter solstice (some of the most prominent megaliths are located in Nabta Playa, Egypt, Mnajdra, Malta and at Stonehenge, England); Newgrange, a prehistoric human-built mount in Ireland, was designed to detect the winter solstice; the pyramid of El Castillo at Chichén Itzá in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumn equinoxes.
During the Roman era the winter solstice was a holiday celebrated as Sol Invictus literally "unconquered sun". With respect to the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac, and so Greek astronomers considered it to be one of the seven planets after which the seven days of the week are named in some languages.
In Paganism the sun is associated with life. At the summer solstice Pagans have the festival of Midsummer, sometimes called Litha. The God in his light aspect is at the height of his power and is crowned Lord of Light. It is a time of plenty and celebration.
Yule is the time of the winter solstice in the Northern Hemisphere, the time of greatest darkness and the longest night of the year. When the sun child representing the Male Divinity in many Pagan traditions is reborn of the Goddess, an image of the return of all new life born through the love of the Gods. The Norse had a God Ullr, and within the Northern Tradition Yule is regarded as the New Year.
In Paganism the sun is associated with life. At the summer solstice Pagans have the festival of Midsummer, sometimes called Litha. The God in his light aspect is at the height of his power and is crowned Lord of Light. It is a time of plenty and celebration.
Yule is the time of the winter solstice in the Northern Hemisphere, the time of greatest darkness and the longest night of the year. When the sun child representing the Male Divinity in many Pagan traditions is reborn of the Goddess, an image of the return of all new life born through the love of the Gods. The Norse had a God Ullr, and within the Northern Tradition Yule is regarded as the New Year.
Safe Sun Watching
Never view the Sun directly with the naked eye or with any unfiltered optical device, such as binoculars or a telescope
If you're thinking of viewing the Sun, your first concern should always be eye safety. Serious eye damage can result from even a brief glimpse of our closest star. See the Stargazing Live Guide below for advice on safe sun viewing...and look out for our Adrian's photograph of the Venus transit.
If you're thinking of viewing the Sun, your first concern should always be eye safety. Serious eye damage can result from even a brief glimpse of our closest star. See the Stargazing Live Guide below for advice on safe sun viewing...and look out for our Adrian's photograph of the Venus transit.
