Sunlight

From Wikipedia, the free encyclopedia

 

Sunlight shining through clouds, giving rise to crepuscular rays

Photograph called Sunlight (1930s)

Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon. When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by clouds or reflects off other objects, it is experienced as diffused light. The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter.^[1] Other sources indicate an "Average over the entire earth" of "164 Watts per square meter over a 24 hour day".^[2]

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D₃ and a mutagen.

Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun. A photon starting at the center of the Sun and changing direction every time it encounters a charged particle would take between 10,000 and 170,000 years to get to the surface.^[3]

Sunlight is a key factor in photosynthesis, the process used by plants and other autotrophic organisms to convert light energy, normally from the Sun, into chemical energy that can be used to fuel the organisms' activities.

Measurement

Researchers can measure the intensity of sunlight using a sunshine recorder, pyranometer, or pyrheliometer. To calculate the amount of sunlight reaching the ground, both the eccentricity of Earth's elliptic orbit and the attenuation by Earth's atmosphere have to be taken into account. The extraterrestrial solar illuminance (E_ext), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by^[4]

${\displaystyle E_{\rm {ext}}=E_{\rm {sc}}\cdot \left(1+0.033412\cdot \cos \left(2\pi {\frac {{\rm {dn}}-3}{365}}\right)\right),}$

where dn=1 on January 1st; dn=32 on February 1st; dn=59 on March 1 (except on leap years, where dn=60), etc. In this formula dn–3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and, therefore, the maximum E_ext occurs around January 3 each year. The value of 0.033412 is determined knowing that the ratio between the perihelion (0.98328989 AU) squared and the aphelion (1.01671033 AU) squared should be approximately 0.935338.

The solar illuminance constant (E_sc), is equal to 128×10³ lx. The direct normal illuminance (E_dn), corrected for the attenuating effects of the atmosphere is given by:

${\displaystyle E_{\rm {dn}}=E_{\rm {ext}}\,e^{-cm},}$

where c is the atmospheric extinction and m is the relative optical airmass. The atmospheric extinction brings the number of lux down to around 100 000.

The total amount of energy received at ground level from the Sun at the zenith depends on the distance to the Sun and thus on the time of year. It is about 3.3% higher than average in January and 3.3% lower in July (see below). If the extraterrestrial solar radiation is 1367 watts per square meter (the value when the Earth–Sun distance is 1 astronomical unit), then the direct sunlight at Earth's surface when the Sun is at the zenith is about 1050 W/m², but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1120 W/m².^[5] In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared (above 700 nm), 42 to 43 percent visible (400 to 700 nm), and 3 to 5 percent ultraviolet (below 400 nm).^[6] At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet (UV),^[7] with most of the extra UV consisting of biologically damaging short-wave ultraviolet.^[8]

Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux. This is higher than the efficacy (of source) of most artificial lighting (including fluorescent), which means using sunlight for illumination heats up a room less than using most forms of artificial lighting.

Multiplying the figure of 1050 watts per square metre by 93 lumens per watt indicates that bright sunlight provides an illuminance of approximately 98 000 lux (lumens per square meter) on a perpendicular surface at sea level. The illumination of a horizontal surface will be considerably less than this if the Sun is not very high in the sky. Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the South Pole (see insolation).

Dividing the irradiance of 1050 W/m² by the size of the sun's disk in steradians gives an average radiance of 15.4 MW per square metre per steradian. (However, the radiance at the centre of the sun's disk is somewhat higher than the average over the whole disk due to limb darkening.) Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m².

Composition and power

Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at left of wavelength range shown) but comprise very small amounts of the Sun's total output power.

The spectrum of the Sun's solar radiation is close to that of a black body^[9][10] with a temperature of about 5,800 K.^[11] The Sun emits EM radiation across most of the electromagnetic spectrum. Although the Sun produces gamma rays as a result of the nuclear-fusion process, internal absorption and thermalization convert these super-high-energy photons to lower-energy photons before they reach the Sun's surface and are emitted out into space. As a result, the Sun does not emit gamma rays from this process, but it does emit gamma rays from solar flares.^[12] The Sun also emits X-rays, ultraviolet, visible light, infrared, and even radio waves;^[13] the only direct signature of the nuclear process is the emission of neutrinos.

Although the solar corona is a source of extreme ultraviolet and X-ray radiation, these rays make up only a very small amount of the power output of the Sun (see spectrum at right). The spectrum of nearly all solar electromagnetic radiation striking the Earth's atmosphere spans a range of 100 nm to about 1 mm (1,000,000 nm).^{[citation needed]} This band of significant radiation power can be divided into five regions in increasing order of wavelengths:^[14]

• Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to the human eye). Due to absorption by the atmosphere very little reaches Earth's surface. This spectrum of radiation has germicidal properties, as used in germicidal lamps.
• Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the Earth's atmosphere, and along with UVC causes the photochemical reaction leading to the production of the ozone layer. It directly damages DNA and causes sunburn, but is also required for vitamin D synthesis in the skin and fur of mammals.^[15]
• Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once^[when?] held to be less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths and tanning beds) and PUVA therapy for psoriasis. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of free radicals and reactive oxygen species), and can cause cancer.^[16]
• Visible range or light spans 380 to 780 nm. As the name suggests, this range is visible to the naked eye. It is also the strongest output range of the Sun's total irradiance spectrum.
• Infrared range that spans 700 nm to 1,000,000 nm (1 mm). It comprises an important part of the electromagnetic radiation that reaches Earth. Scientists divide the infrared range into three types on the basis of wavelength:
  • Infrared-A: 700 nm to 1,400 nm
  • Infrared-B: 1,400 nm to 3,000 nm
  • Infrared-C: 3,000 nm to 1 mm.

Published tables

Tables of direct solar radiation on various slopes from 0 to 60 degrees north latitude, in calories per square centimetre, issued in 1972 and published by Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA, appear on the web.^[17]

Solar constant

Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against wavenumber

The solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit (AU) (roughly the mean distance from the Sun to Earth). The "solar constant" includes all types of solar radiation, not just the visible light. Its average value was thought to be approximately 1366 W/m²,^[18] varying slightly with solar activity, but recent recalibrations of the relevant satellite observations indicate a value closer to 1361 W/m² is more realistic.^[19]

Total solar irradiance (TSI) and spectral solar irradiance (SSI) upon Earth

Total solar irradiance (TSI) – the amount of solar radiation received at the top of Earth's atmosphere – has been measured since 1978 by a series of overlapping NASA and ESA satellite experiments to be 1.361 kilo⁠watts per square meter (kW/m²).^{[18][20][21][22]} TSI observations are continuing today with the ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM satellite experiments.^[23] Variation of TSI has been discovered on many timescales including the solar magnetic cycle ^[24] and many shorter periodic cycles.^[25] TSI provides the energy that drives Earth's climate, so continuation of the TSI time series database is critical to understanding the role of solar variability in climate change.
Spectral solar irradiance (SSI) – the spectral distribution of the TSI – has been monitored since 2003 by the SORCE Spectral Irradiance Monitor (SIM). It has been found that SSI at UV (ultraviolet) wavelength corresponds in a less clear, and probably more complicated fashion, with Earth's climate responses than earlier assumed, fueling broad avenues of new research in “the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth’s climate”.^[26]

Intensity in the Solar System

Sunlight on Mars is dimmer than on Earth. This photo of a Martian sunset was imaged by Mars Pathfinder.

Different bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from Sun. A rough table comparing the amount of solar radiation received by each planet in the Solar System follows (from data in [1]):

Planet or dwarf planet	distance (AU)		Solar radiation (W/m²)
	Perihelion	Aphelion	maximum	minimum
Mercury	0.3075	0.4667	14,446	6,272
Venus	0.7184	0.7282	2,647	2,576
Earth	0.9833	1.017	1,413	1,321
Mars	1.382	1.666	715	492
Jupiter	4.950	5.458	55.8	45.9
Saturn	9.048	10.12	16.7	13.4
Uranus	18.38	20.08	4.04	3.39
Neptune	29.77	30.44	1.54	1.47
Pluto	29.66	48.87	1.55	0.57

The actual brightness of sunlight that would be observed at the surface depends also on the presence and composition of an atmosphere. For example, Venus's thick atmosphere reflects more than 60% of the solar light it receives. The actual illumination of the surface is about 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds".^[27]

Sunlight on Mars would be more or less like daylight on Earth during a slightly overcast day, and, as can be seen in the pictures taken by the rovers, there is enough diffuse sky radiation that shadows would not seem particularly dark. Thus, it would give perceptions and "feel" very much like Earth daylight. The spectrum on the surface is slightly redder than that on Earth, due to scattering by reddish dust in the Martian atmosphere.

For comparison, sunlight on Saturn is slightly brighter than Earth sunlight at the average sunset or sunrise (see daylight for comparison table). Even on Pluto, the sunlight would still be bright enough to almost match the average living room. To see sunlight as dim as full moonlight on Earth, a distance of about 500 AU (~69 light-hours) is needed; there are only a handful of objects in the Solar System known to orbit farther than such a distance, among them 90377 Sedna and (87269) 2000 OO67.

Surface illumination

The spectrum of surface illumination depends upon solar elevation due to atmospheric effects, with the blue spectral component dominating during twilight before and after sunrise and sunset, respectively, and red dominating during sunrise and sunset. These effects are apparent in natural light photography where the principal source of illumination is sunlight as mediated by the atmosphere.
While the color of the sky is usually determined by Rayleigh scattering, an exception occurs at sunset and twilight. "Preferential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".^[28]

Spectral composition of sunlight at Earth's surface

The Sun's electromagnetic radiation which is received at the Earth's surface is predominantly light that falls within the range of wavelengths to which the visual systems of the animals that inhabit Earth's surface are sensitive. The Sun may therefore be said to illuminate, which is a measure of the light within a specific sensitivity range. Many animals (including humans) have a sensitivity range of approximately 400–700 nm,^[29] and given optimal conditions the absorption and scattering by Earth's atmosphere produces illumination that approximates an equal-energy illuminant for most of this range.^[30] The useful range for color vision in humans, for example, is approximately 450–650 nm. Aside from effects that arise at sunset and sunrise, the spectral composition changes primarily in respect to how directly sunlight is able to illuminate. When illumination is indirect, Rayleigh scattering in the upper atmosphere will lead blue wavelengths to dominate. Water vapour in the lower atmosphere produces further scattering and ozone, dust and water particles will also absorb selective wavelengths.^[31][32]

Spectrum of the visible wavelengths at approximately sea level; illumination by direct sunlight compared with direct sunlight scattered by cloud cover and with indirect sunlight by varying degrees of cloud cover. The yellow line shows the spectrum of direct illumination under optimal conditions. The other illumination conditions are scaled to show their relation to direct illumination. The units of spectral power are simply raw sensor values (with a linear response at specific wavelengths).

Variations in solar irradiance

Seasonal and orbital variation

On Earth, the solar radiation varies with the angle of the sun above the horizon, with longer sunlight duration at high latitudes during summer, varying to no sunlight at all in winter near the pertinent pole. When the direct radiation is not blocked by clouds, it is experienced as sunshine. The warming of the ground (and other objects) depends on the absorption of the electromagnetic radiation in the form of heat.
The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). As the orbital eccentricity changes, the average distance from the sun (the semimajor axis does not significantly vary, and so the total insolation over a year remains almost constant due to Kepler's second law,

${\displaystyle {\tfrac {2A}{r^{2}}}dt=d\theta ,}$

where ${\displaystyle A}$ is the "areal velocity" invariant. That is, the integration over the orbital period (also invariant) is a constant.

${\displaystyle \int _{0}^{T}{\tfrac {2A}{r^{2}}}dt=\int _{0}^{2\pi }d\theta =\mathrm {constant} .}$

If we assume the solar radiation power P as a constant over time and the solar irradiation given by the inverse-square law, we obtain also the average insolation as a constant.

But the seasonal and latitudinal distribution and intensity of solar radiation received at Earth's surface does vary.^[33] The effect of sun angle on climate results in the change in solar energy in summer and winter. For example, at latitudes of 65 degrees, this can vary by more than 25% as a result of Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages (see: Milankovitch cycles).

Solar intensity variation

Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies on many time scales, including the 11-year sunspot solar cycle.^[24] When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400 years or cosmogenic radionuclides for going back 10,000 years. Such reconstructions have been done.^{[34][35][36][37]} These studies show that in addition to the solar irradiance variation with the solar cycle (the (Schwabe) cycle), the solar activitiy varies with longer cycles, such as the proposed 88 year (Gleisberg cycle), 208 year (DeVries cycle) and 1,000 year (Eddy cycle).

Life on Earth

The existence of nearly all life on Earth is fueled by light from the Sun. Most autotrophs, such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known as photosynthesis. These sugars are then used as building-blocks and in other synthetic pathways that allow the organism to grow.

Heterotrophs, such as animals, use light from the Sun indirectly by consuming the products of autotrophs, either by consuming autotrophs, by consuming their products, or by consuming other heterotrophs. The sugars and other molecular components produced by the autotrophs are then broken down, releasing stored solar energy, and giving the heterotroph the energy required for survival. This process is known as cellular respiration.

In prehistory, humans began to further extend this process by putting plant and animal materials to other uses. They used animal skins for warmth, for example, or wooden weapons to hunt. These skills allowed humans to harvest more of the sunlight than was possible through glycolysis alone, and human population began to grow.

During the Neolithic Revolution, the domestication of plants and animals further increased human access to solar energy. Fields devoted to crops were enriched by inedible plant matter, providing sugars and nutrients for future harvests. Animals that had previously provided humans with only meat and tools once they were killed were now used for labour throughout their lives, fueled by grasses inedible to humans.

The more recent discoveries of coal, petroleum and natural gas are modern extensions of this trend. These fossil fuels are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within Earth for millions of years. Because the stored energy in these fossil fuels has accumulated over many millions of years, they have allowed modern humans to massively increase the production and consumption of primary energy. As the amount of fossil fuel is large but finite, this cannot continue indefinitely, and various theories exist as to what will follow this stage of human civilization (e.g., alternative fuels, Malthusian catastrophe, new urbanism, peak oil).

Cultural aspects

Claude Monet: Le déjeuner sur l'herbe

The effect of sunlight is relevant to painting, evidenced for instance in works of Claude Monet on outdoor scenes and landscapes.

Téli verőfény ("Winter Sunshine") by László Mednyánszky

Many people find direct sunlight to be too bright for comfort, especially when reading from white paper upon which the sun is directly shining. Indeed, looking directly at the sun can cause long-term vision damage. To compensate for the brightness of sunlight, many people wear sunglasses. Cars, many helmets and caps are equipped with visors to block the sun from direct vision when the sun is at a low angle. Sunshine is often blocked from entering buildings through the use of walls, window blinds, awnings, shutters, curtains, or nearby shade trees.

In colder countries, many people prefer sunnier days and often avoid the shade. In hotter countries, the converse is true; during the midday hours, many people prefer to stay inside to remain cool. If they do go outside, they seek shade that may be provided by trees, parasols, and so on.

In Hinduism, the sun is considered to be a god, as it is the source of life and energy on earth.

Sunbathing

Sunbathing is a popular leisure activity in which a person sits or lies in direct sunshine. People often sunbathe in comfortable places where there is ample sunlight. Some common places for sunbathing include beaches, open air swimming pools, parks, gardens, and sidewalk cafes. Sunbathers typically wear limited amounts of clothing or some simply go nude. For some, an alternative to sunbathing is the use of a sunbed that generates ultraviolet light and can be used indoors regardless of weather conditions. Tanning beds have been banned in a number of states in the world.

For many people with light skin, one purpose for sunbathing is to darken one's skin color (get a sun tan), as this is considered in some cultures to be attractive, associated with outdoor activity, vacations/holidays, and health. Some people prefer naked sunbathing so that an "all-over" or "even" tan can be obtained, sometimes as part of a specific lifestyle.

For people suffering from psoriasis, sunbathing is an effective way of healing the symptoms.

Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes, and is an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.

Effects on human health

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D₃ and a mutagen.^[38] A dietary supplement can supply vitamin D without this mutagenic effect,^[39] but bypasses natural mechanisms that would prevent overdoses of vitamin D generated internally from sunlight. Vitamin D has a wide range of positive health effects, which include strengthening bones^[40] and possibly inhibiting the growth of some cancers.^[41][42] Sun exposure has also been associated with the timing of melatonin synthesis, maintenance of normal circadian rhythms, and reduced risk of seasonal affective disorder.^[43]
Long-term sunlight exposure is known to be associated with the development of skin cancer, skin aging, immune suppression, and eye diseases such as cataracts and macular degeneration.^[44] Short-term overexposure is the cause of sunburn, snow blindness, and solar retinopathy.

UV rays, and therefore sunlight and sunlamps, are the only listed carcinogens that are known to have health benefits,^[45] and a number of public health organizations state that there needs to be a balance between the risks of having too much sunlight or too little.^[46] There is a general consensus that sunburn should always be avoided.

Epidemiological data shows that people who have more exposure to the sun have less high blood pressure and cardiovascular-related mortality. While sunlight (and its UV rays) are a risk factor for skin cancer, "sun avoidance may carry more of a cost than benefit for over-all good health."^[47] A study found that there is no evidence that UV reduces lifespan in contrast to other risk factors like smoking, alcohol and high blood pressure.^[47]

Effect on plant genomes

Elevated solar UV-B doses increase the frequency of DNA recombination in Arabidopsis thaliana and tobacco (Nicotiana tabacum) plants.^[48] These increases are accompanied by strong induction of an enzyme with a key role in recombinational repair of DNA damage. Thus the level of terrestrial solar UV-B radiation likely affects genome stability in plants.

Stellar population

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In 1944, Walter Baade categorized groups of stars within the Milky Way from their spectra. Two main divisions were defined as Population I and II, with another division known as Population III added in 1978^[1]. Often now simply abbreviated as either Pop I, II or III, these differences were later shown to be highly significant, dividing stars into classes by their chemical composition or metallicity, whose small proportion of stellar matter consists of the "heavier chemical elements" beyond the more common elements of hydrogen and helium.^[2][3] By coincidence, each population group definition has decreasing metal content and increasing age. Hence, the first stars in the universe (low metal content) were deemed Population III, and recent stars (high metallicity) are Population I.

Stellar populations

Observation of the spectra of stars has revealed that the metallicity of older stars have fewer heavy elements compared to the Sun. This immediately suggests that metallicity has evolved through the generations of stars by the process of stellar evolution. In current cosmological models, the matter created in the Big Bang was mostly hydrogen and helium, with only a very tiny fraction of light elements like lithium and beryllium. After this, when the universe cooled sufficiently, the first stars were born as extremely metal-poor Population III stars. Without metals, it is postulated that their stellar masses were hundreds of times that of the Sun. In turn, these massive stars evolved very quickly, and their nucleosynthetic processes quickly created the first 26 elements (up to iron in the periodic table).^[4]

Current theoretical stellar models show that most high-mass Population III stars quickly exhausted their fuel and exploded in extremely energetic pair-instability supernovae. Those explosions would have thoroughly dispersed their material, ejecting metals into the interstellar medium (ISM), to be incorporated into the later generations of stars. Their destruction suggests that no galactic high-mass Population III stars should be observable. However, some Population III stars might be seen in high-redshift galaxies whose light originated during the earlier history of the universe.^{[citation needed]} None have been discovered. Stars too massive to produce pair-instability supernovae would have collapsed into black holes through a process known as photodisintegration, but some matter may have escaped during this process in the form of relativistic jets, and this could have "sprayed" the first metals into the universe.^[5][6]

A rendering of Mu Arae, a metal-rich population I star.

It has been proposed that recent supernovae SN 2006gy and SN 2007bi may have been pair-instability supernovae in which such super-massive Population III stars exploded. It has been speculated that these stars could have formed relatively recently in dwarf galaxies containing primordial metal-free interstellar matter; past supernovae in these galaxies could have ejected their metal-rich contents at speeds high enough for them to escape the galaxy, keeping the metal content of the galaxy very low.^[7]

The oldest observed stars, known as Population II, have very low metallicities;^[8][9] as subsequent generations of stars were born they became more metal-enriched, as the gaseous clouds from which they formed received the metal-rich dust manufactured by previous generations. As those stars died, they returned metal-enriched material to the interstellar medium via planetary nebulae and supernovae, enriching further the nebulae out of which the newer stars formed. These youngest stars, including the Sun, therefore have the highest metal content, and are known as Population I stars.

Population I stars

Population I, or metal-rich stars, are young stars with the highest metallicity out of all three populations. The Earth's Sun is an example of a metal-rich star. These are common in the spiral arms of the Milky Way galaxy. Generally, the youngest stars, the extreme Population I, are found farther toward the center of a galaxy, and intermediate Population I stars are farther out. The Sun is an intermediate Population I star. Population I stars have regular elliptical orbits of the galactic centre, with a low relative velocity. It was hypothesized that the high metallicity of Population I stars makes them more likely to possess planetary systems than the other two populations, because planets, particularly terrestrial planets, are thought to be formed by the accretion of metals.^[10] However, observations of the Kepler data-set have found smaller planets around stars with a range of metallicities, while only larger, potential gas giant planets are concentrated around stars with relatively higher metallicity — a finding that has implications for theories of gas giant formation.^[11] Between the intermediate Population I and the Population II stars comes the intermediary disc population.

Population II stars

Population II, or metal-poor stars, are those with relatively little metal. The idea of a relatively small amount must be kept in perspective as even metal-rich astronomical objects contain low percentages of any element other than hydrogen or helium; metals constitute only a tiny percentage of the overall chemical makeup of the universe, even 13.8 billion years after the Big Bang. However, metal-poor objects are even more primitive. These objects are formed during an earlier time of the universe. Intermediate Population I stars are common in the bulge near the centre of our galaxy, whereas Population II stars found in the galactic halo are older and thus more metal-poor. Globular clusters also contain high numbers of population II stars.^[12] It is thought that population II stars created all the other elements in the periodic table, except the more unstable ones. An interesting characteristic of Population II stars is that despite their lower overall metallicity, they often have a higher ratio of alpha elements (O, Si, Ne, etc.) relative to Fe as compared to Population I stars; current theory suggests this is the result of Type II supernovae being more important contributors to the interstellar medium at the time of their formation, whereas Type Ia supernovae metal enrichment came later in the universe's evolution.^[13]

Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of Timothy C. Beers et al. and the Hamburg-ESO survey of Norbert Christlieb et al., originally started for faint quasars. Thus far, they have uncovered and studied in detail about ten very metal-poor stars (such as Sneden's Star, Cayrel's Star, BD +17° 3248) and three of the oldest stars known to date: HE0107-5240, HE1327-2326 and HE 1523-0901. Caffau's star was identified as the most metal-poor star yet when it was found in 2012 using Sloan Digital Sky Survey data. However, in February 2014 the discovery of an even lower metallicity star was announced, SMSS J031300.36-670839.3 located with the aid of SkyMapper astronomical survey data. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are HD 122563 (a red giant) and HD 140283 (a subgiant).

Population III stars

Possible glow of Population III stars imaged by NASA's Spitzer Space Telescope

Population III stars^[14] are a hypothetical population of extremely massive and hot stars with virtually no metals, except possibly for intermixing ejecta from other nearby Pop III supernovas. Their existence is inferred from physical cosmology, but they have not yet been observed directly. Indirect evidence for their existence has been found in a gravitationally lensed galaxy in a very distant part of the universe.^[15] Their existence may account for the fact that heavy elements—which could not have been created in the Big Bang—are observed in quasar emission spectra.^[4] They are also thought to be components of faint blue galaxies. These stars likely triggered the universe's period of reionization, a major phase transition of gases leading to the opacity observed today. Observations of the galaxy UDFy-38135539 suggest it may have played a role in this reionization process. Some theories hold that there were two generations of Pop III stars.^[16]

Artist's impression of the first stars, 400 million years after the Big Bang.

Current theory is divided on whether the first stars were very massive or not—theories proposed in 2009 and 2011 suggest the first star groups might have consisted of a massive star surrounded by several smaller stars.^[17][18][19] One theory, developed by computer models of star formation, is that with no heavy elements and a much warmer interstellar medium from the Big Bang, it was easy to form stars with much greater total mass than the ones visible today.^{[citation needed]} Typical masses for Pop  III stars are expected to be about several hundred solar masses, which is much larger than that of current stars. Analysis of data of extremely low-metallicity Population II stars such as HE0107-5240, which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 20 to 130 solar masses.^[20] On the other hand, analysis of globular clusters associated with elliptical galaxies suggests pair-instability supernovae, which are typically associated with very massive stars, were responsible for their metallic composition.^[21] This also explains why there have been no low-mass stars with zero metallicity observed, although models have been constructed for smaller Pop  III stars.^[22][23] Clusters containing zero-metallicity red dwarfs or brown dwarfs (possibly created by pair-instability supernovae^[9]) have been proposed as dark matter candidates,^[24][25] but searches for these and other MACHOs through gravitational microlensing have produced negative results.

Detection of Population III stars is a goal of NASA's James Webb Space Telescope.^[26] New spectroscopic surveys, such as SEGUE or SDSS-II, may also locate Pop III stars.^{[citation needed]} Stars observed in the Cosmos Redshift 7 galaxy at z = 6.60 may be Population III stars. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it.^[27][28]

Faint young Sun paradox

From Wikipedia, the free encyclopedia

 

Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right.

The faint young Sun paradox describes the apparent contradiction between observations of liquid water early in Earth's history and the astrophysical expectation that the Sun's output would be only 70 percent as intense during that epoch as it is during the modern epoch. The issue was raised by astronomers Carl Sagan and George Mullen in 1972.^[1] Explanations of this paradox have taken into account greenhouse effects, astrophysical influences, or a combination of the two.

The unresolved question is how a climate suitable for life was maintained on Earth over the long timescale despite the variable solar output and wide range of terrestrial conditions.^[2]

Early solar output

Early in Earth's history, the Sun's output would have been only 70 percent as intense as it is during the modern epoch. In the environmental conditions existing at that time, this solar output would have been insufficient to maintain a liquid ocean. Astronomers Carl Sagan and George Mullen pointed out in 1972 that this is contrary to the geological and paleontological evidence.^[1]

According to the Standard Solar Model, stars similar to the Sun should gradually brighten over their main sequence lifetime due to contraction of the stellar core caused by fusion.^[3] However, with the predicted solar luminosity 4 billion (4 × 10⁹) years ago and with greenhouse gas concentrations the same as are current for the modern Earth, any liquid water exposed to the surface would freeze. However, the geological record shows a continually relatively warm surface in the full early temperature record of Earth, with the exception of a cold phase, the Huronian glaciation, about 2.4 to 2.1 billion years ago. Water-related sediments have been found dating to as early as 3.8 billion years ago.^[4] Hints of early life forms have been dated from as early as 3.5 billion years,^[5] and the basic carbon isotopy is very much in line with what is found today.^[6] A regular alternation between ice ages and warm periods is only found occurring in the period since one billion years ago.^{[citation needed]}

Greenhouse hypothesis

When it first formed, Earth's atmosphere may have contained more greenhouse gases. Carbon dioxide concentrations may have been higher, with estimated partial pressure as large as 1,000 kPa (10 mbar), because there was no bacterial photosynthesis to convert the CO₂ gas to organic carbon and gaseous oxygen. Methane, a very active greenhouse gas that reacts with oxygen to produce carbon dioxide and water vapor, may have been more prevalent as well, with a mixing ratio of 10⁻⁴ (100 parts per million by volume).^[7][8]

Based on a study of geological sulfur isotopes, in 2009 a group of scientists including Yuichiro Ueno from the University of Tokyo proposed that carbonyl sulfide (OCS) was present in the Archean atmosphere. Carbonyl sulfide is an efficient greenhouse gas and the scientists estimate that the additional greenhouse effect would have been sufficient to prevent Earth from freezing over.^[9]

Based on an "analysis of nitrogen and argon isotopes in fluid inclusions trapped in 3.0- to 3.5-billion-year-old hydrothermal quartz" a 2013 paper concludes that "dinitrogen did not play a significant role in the thermal budget of the ancient Earth and that the Archean partial pressure of CO₂ was probably lower than 0.7 bar".^[10] Burgess, one of the authors states "The amount of nitrogen in the atmosphere was too low to enhance the greenhouse effect of carbon dioxide sufficiently to warm the planet. However, our results did give a higher than expected pressure reading for carbon dioxide – at odds with the estimates based on fossil soils – which could be high enough to counteract the effects of the faint young Sun and will require further investigation."^[11] Also, in 2012-2016 the research by S.M. Som, based on the analysis of raindrop impressions and air bubbles trapped in ancient lavas, have further indicated a low atmospheric pressure below 1.1 bar and probably as low as 0.23 bar during an epoch 2.7 bn years from present.^[12]

Following the initial accretion of the continents after about 1 billion years,^[13] geo-botanist Heinrich Walter and others contend that a non-biological version of the carbon cycle provided a negative temperature feedback. The carbon dioxide in the atmosphere dissolved in liquid water and combined with metal ions derived from silicate weathering to produce carbonates. During ice age periods, this part of the cycle would shut down. Volcanic carbon emissions would then restart a warming cycle due to the greenhouse effect.^[14][15]

According to the Snowball Earth hypothesis, there may have been a number of periods when Earth's oceans froze over completely. The most recent such period may have been about 630 million years ago.^[16] Afterwards, the Cambrian explosion of new multicellular life forms started.

Greater radiogenic heat

The radiogenic heat from the decay of 4 isotopes affecting Earth's internal heat budget over time: ⁴⁰K (yellow), ²³⁵U (red), ²³⁸U (green) and ²³²Th (violet). In the past the contribution from ⁴⁰K and ²³⁵U was much higher and thus the overall radiogenic heat output was higher.

In the past, the geothermal release of decay heat, emitted from the decay of the isotopes potassium-40, uranium-235 and uranium-238 was considerably greater than it is today.^[17] The figure to the right shows that the isotope ratio between uranium-238 and uranium-235 was also considerably different than it is today, with the ratio essentially equivalent to that of modern low-enriched uranium. Therefore, natural uranium ore bodies, if present, would have been capable of supporting natural nuclear fission reactors with common light water as its moderator. Any attempts to explain the paradox must therefore factor in both radiogenic contributions, both from decay heat and from any potential natural nuclear fission reactors.

The primary mechanism for Earth warming by radiogenic heat is not the direct heating (which contribute less than 0.1% to the total heat input even of early Earth) but rather the establishment of the high geothermal gradient of the crust, resulting in greater out-gassing rate and therefore the higher concentration of greenhouse gases in early Earth atmosphere. Additionally, a hotter deep crust would limit the water absorption by crustal minerals, resulting in a smaller amount of high-albedo land protruding from the early oceans, causing more solar energy to be absorbed.

Greater tidal heating

The Moon was much closer to Earth billions of years ago,^[18] and therefore produced considerably more tidal heating.^[19]

Alternatives

Phanerozoic Climate Change

A minority view, propounded by the Israeli-American physicist Nir Shaviv, uses climatological influences of solar wind, combined with a hypothesis of Danish physicist Henrik Svensmark for a cooling effect of cosmic rays, to explain the paradox.^[20] According to Shaviv, the early Sun had emitted a stronger solar wind that produced a protective effect against cosmic rays. In that early age, a moderate greenhouse effect comparable to today's would have been sufficient to explain an ice-free Earth. Evidence for a more active early Sun has been found in meteorites.^[21]

The temperature minimum around 2.4 billion years goes along with a cosmic ray flux modulation by a variable star formation rate in the Milky Way. The reduced solar impact later results in a stronger impact of cosmic ray flux (CRF), which is hypothesized to lead to a relationship with climatological variations.

An alternative model of solar evolution may explain the faint young Sun paradox. In this model, the early Sun underwent an extended period of higher solar wind output. This caused a mass loss from the Sun on the order of 5−10 percent over its lifetime, resulting in a more consistent level of solar luminosity (as the early Sun had more mass, resulting in more energy output than was predicted). In order to explain the warm conditions in the Archean era, this mass loss must have occurred over an interval of about one billion years. However, records of ion implantation from meteorites and lunar samples show that the elevated rate of solar wind flux only lasted for a period of 0.1 billion years. Observations of the young Sun-like star π¹ Ursae Majoris matches this rate of decline in the stellar wind output, suggesting that a higher mass loss rate can not by itself resolve the paradox.^[22]

Examination of Archaean sediments appears inconsistent with the hypothesis of high greenhouse concentrations. Instead, the moderate temperature range may be explained by a lower surface albedo brought about by less continental area and the "lack of biologically induced cloud condensation nuclei". This would have led to increased absorption of solar energy, thereby compensating for the lower solar output.^[23]

On Mars

Usually, the faint young Sun paradox is framed in terms of Earth's paleoclimate. However, the issue also appears in the context of the climate on ancient Mars, where apparently liquid water was present, in significant amounts (hydrological cycle, lakes, rivers, rain, possibly seas and oceans), billions of years ago. Subsequently, significant liquid water disappeared from the surface of Mars. Presently, the surface of Mars is cold and dry. The variable solar output, assuming nothing else changed, would imply colder (and drier) conditions on Mars in the ancient past than they are today, apparently contrary to the empirical evidence from Mars exploration that suggest the wetter and milder past. An explanation of the faint young Sun paradox that could simultaneously account for the observations might be that the sun shed mass through the solar wind, though sufficient rate of mass shedding is so far unsupported by stellar observations and models.^[24]

An alternative possible explanation posits intermittent bursts of powerful greenhouse gases, like methane. Carbon dioxide alone, even at a pressure far higher than the current one, cannot explain temperatures required for presence of liquid water on early Mars.^[25]