Astronomy

What is this web on the surface of the Sun?

What is this web on the surface of the Sun?


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I was going through my Social Media Feed and found the attached post too frequent. The caption reads this is the best image of our Sun. Just as an example, the Universe Today's This is the Highest Resolution Image Ever Taken of the Surface of the Sun

Why? What exactly are the black lines that appear to be like kind of a web, and will such patterns be observed if in case the star was not the Sun but some other star? Are they thought to be common?


The dark lines are colder areas at the edge of the convection cells, where the cooled down plasma sinks towards the inside of the Sun. Now "colder" for the surface of the Sun, is still pretty hot, as explained here.

The yellow parts are where the plasma rises to the surface. Each yellow spot (which is actually the size of a country) is called a granule, and this web-like appearance is called granulation.

In the outer part of the Sun (the convection zone in the image below), there is convection, that is hotter plasma floats towards the top, cools down at the surface, and sinks back down, like in a lavalamp.

The existence of a convective zone in the outer part of the star is determined by the mass of the star, and all stars with a convective zone in their upper layer are thought to have such granulation patterns. So stars like our sun, or smaller have these patterns.

For larger stars, though, the convective zone is in the inner part of the star, and the outer part of the star is the radiative zone, so there might not be the same patterns on the surface.


I'll add to @usernumber's answer some graphics. Unfortunately we can't yet "has YouTubes" for some reason so I'll just add the links.

There are two videos of the Sun linked in Phil Plait's Bad Astronomy article

  • DKIST first light high-resolution video of solar granules
  • DKIST First light video of solar granulation (wide angle).

Here are the same kind of convection cells shown in more familiar settings:


Usernumber's explanation of the light and dark regions is correct, but there is more detail to be added about granulation on other stars.

Granulation is expected on other stars with surface convection zones, but the properties and timescales of the granulation can be quite different.

On the Sun, the granules appear and disappear in timescales of 10-30 minutes and the granules have a characteristic diameter of around 1500 km. There are thus about 4 million of these visible on the solar photosphere.

The size of the granules is expected to vary as the gravitational scale height in the photosphere, which is proportional to $T_{ m eff}/g$. Thus stars with lower temperatures (K- and M-stars) are expected to have smaller granules, but stars with lower surface gravities (subgiants and giants) are expected to have much bigger granulation patterns (Cranmer et al. 2014).

In fact, given that gravity scales as $R^{-2}$, the ratio of the radius of the star to the size of a granule gets smaller as gravity decreases. Thus giants are expected have far fewer, but bigger granules.

The timescales are also different. The frequency of granulation appears to scale with the peak frequency of p-mode oscillations, which in turn scales as $g/sqrt{T_{ m eff}}$, and so cooler stars have higher frequency granulation, but giants, with 1-2 orders of magnitude lower surface gravity have much more slowly changing granulation patterns (Kallinger et al. 2014).

The truth of the above has been basically confirmed using disk-integrated variability seen in stars monitored by the Kepler satellite.

Of course, the granulation pattern cannot be imaged in distant stars, except in those stars with the largest radii and largest granulation patterns. There have been claims that surface brightness variations on Betelgeuse are due to granulation, but the first really believable images are of the close hypergiant $pi^1$ Gruis (Paladini et al. 2017). This star is half the temperature of the Sun and it's gravity is about $10^5$ times lower. According to the ideas above, the granules should be 50,000 times bigger than on the Sun, i.e. a diameter of 75 million km.

The radius of $pi^1$ Gru is about 250 million km, so its surface will be covered by only around 100 granules, roughly in agreement with what is observed (see below).

VLT near infrared image of $pi^1$ Gru (ESO).


What is this web on the surface of the Sun? - Astronomy

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Table of Contents Enchanted Learning
All About Astronomy
Site Index
Our Solar System Stars Glossary Printables, Worksheets, and Activities
The Sun The Planets The Moon Asteroids Kuiper Belt Comets Meteors Astronomers

THE SUN
Introduction to the Sun Solar Structure Size, Mass Flares, , Prominences Sun's Birth Solar Eclipses Activities,
Web Links
Solar Rotation Sunspots Sun's Death

Introduction to The Sun
Our sun is a star located at the center of our Solar System. It is a huge, spinning ball of hot gas and nuclear reactions that lights up the Earth and provides us with heat.

The sun's absolute magnitude (its intrinsic brightness) is +4.83. Its stellar type is G (a star that absorbs strong metallic lines in its spectrum).

The Greeks called the Sun "Helios" the Romans called it "Sol."

Planet-Sun Orbital Diagram
Label the aphelion (farthest point in orbit) and perihelion (closest point in orbit) of a planet in orbit.
Answers Our sun is a medium-sized yellow star that is 93,026,724 miles (149,680,000 km or 1 Astronomical Unit) from the Earth.

The Earth is closest to the Sun (this is called perihelion) around January 2 each year (91.4 million miles = 147.1 million km) it is farthest away from the Sun (this is called aphelion) around July 2 each year (94.8 million miles = 152.6 million km).

THE TEMPERATURE OF THE SUN
The Sun's core can reach 10 to 22.5 million°F. The surface temperature is approximately 9,900°F (5,500°C). The outer atmosphere of the Sun (which we can see during a solar eclipse) gets extremely hot again, up to 1.5 to 2 million degrees. At the center of big sunspots the temperature can be as low as 7300 °F (4300 K, 4000 °C). The temperature of the Sun is determined by measuring how much energy (both heat and light) it emits.

COMPOSITION OF THE SUN
The Sun is made up of about 2 x 10 30 kilograms of gas. It is composed of about 75% hydrogen and 25% helium. About 0.1% is metals (made from hydrogen via nuclear fusion). This ratio is changing over time (very slowly), as the nuclear reactions continue, converting smaller atoms into more massive ones.

Since the Sun formed 4.5 billion years ago, it has used up about half of its initial hydrogen supply.

Our Sun is a seond or third generation star. Second generation stars do not just burn hydrogen, they also burn heavier elements, like helium and metals (elements heavier than hydrogen and helium), and were formed from supernova explosions (the debris of exploded population II stars).

The element helium was named after the Sun (called "Helios" in Greek) because it was first discovered on the Sun. Helium is plentiful on the Sun but rare on Earth. The element helium was discovered by Jules Janssen during the total solar eclipse of 1868 when he detected a new line in the solar absorption spectrum Norman Lockyer suggested the name helium.

The composition of the Sun is studied using spectroscopy in which the visible light (the spectrum) of the Sun is studied.

NUCLEAR ENERGY PRODUCTION
At the Sun's core, nuclear fusion produces enormous amounts of energy, through the process of converting hydrogen nuclei into helium nuclei (nuclear fusion).

Although the nuclear output of the sun is not entirely consistent, each second the Sun converts about 600,000,000 tons of hydrogen nuclei into helium nuclei. These fusion reactions convert part of these atoms' mass (roughly 4 million tons) into energy, and release an enormous amount of this heat and light energy into the Solar System. In these fusion reactions, the Sun loses 4 million tons of mass each second. The Sun will run out of fuel in about 5 billion (5,000,000,000) years. When this happens, the Sun will explode into a planetary nebula, a giant shell of gas that will destroy the planets in the Solar System (including Earth).

AGE OF THE SUN
The Sun formed 4.5 billion years ago, as the solar system coalesced from a cloud of gas and dust.

STUDYING THE SUN
Astronomers study the Sun using special instruments. Scientists analyze how and why the amount of light from the Sun varies over time, the effect of the Sun's light on the Earth's climate, spectral lines, the Sun's magnetic field, the solar wind, and many other solar phenomena. The outer regions of the Sun (the corona) are studied during solar eclipses.

NEVER LOOK DIRECTLY AT THE SUN! Looking at the Sun can blind you or cause cataracts.

SOLAR EXPLORATION
The Ulysses spacecraft, a joint mission of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA), was launched from the space shuttle on October, 1990 to explore the sun. It has studied the sun's magnetism, solar prominences and coronal mass ejections (orbiting over the south pole of the Sun in 1994 and over the north pole in 1995), and will will complete a second solar orbit in December, 2001.


Thinking Ahead

For most of the twentieth century, black holes seemed the stuff of science fiction, portrayed either as monster vacuum cleaners consuming all the matter around them or as tunnels from one universe to another. But the truth about black holes is almost stranger than fiction. As we continue our voyage into the universe, we will discover that black holes are the key to explaining many mysterious and remarkable objects—including collapsed stars and the active centers of giant galaxies.

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    Einstein’s theory of general relativity predicts that the wavelength of electromagnetic radiation will lengthen as it climbs out of a gravitational well. Photons must expend energy to escape, but at the same time must always travel at the speed of light, so this energy must be lost through a change of frequency rather than a change in speed. If the energy of the photon decreases, the frequency also decreases. This corresponds to an increase in the wavelength of the photon, or a shift to the red end of the electromagnetic spectrum – hence the name: gravitational redshift. This effect was confirmed in laboratory experiments conducted in the 1960s.

    The converse is also true. The observed wavelength of a photon falling into a gravitational well will be shortened, or gravitationally ‘blueshifted’, as it gains energy.

    As an example, take the white dwarf star Sirius B, with a gravitational field

    100,000 times as strong as the Earth’s. Although it sounds extreme, this is still considered a relatively weak field, and the gravitational redshift can be approximated by:

    where z is the gravitational redshift, G is Newton’s gravitational constant, M is the mass of the object, r is the photon’s starting distance from M, and c is the speed of light. In this case, the gravitational redshift suffered by a photon emitted from the star’s surface is a tiny 3 × 10 -4 . In other words, wavelengths are shifted by less than one part in 30,000.

    For radiation emitted in a strong gravitational field, such as from the surface of a neutron star or close to the event horizon of a black hole, the gravitational redshift can be very large and is given by:

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    What is this web on the surface of the Sun? - Astronomy

    The photosphere is the visible surface of the Sun that we are most familiar with. Since the Sun is a ball of gas, this is not a solid surface but is actually a layer about 100 km thick (very, very, thin compared to the 700,000 km radius of the Sun). When we look at the center of the disk of the Sun we look straight in and see somewhat hotter and brighter regions. When we look at the limb, or edge, of the solar disk we see light that has taken a slanting path through this layer and we only see through the upper, cooler and dimmer regions. This explains the "limb darkening" that appears as a darkening of the solar disk near the limb.

    A number of features can be observed in the photosphere with a simple telescope (along with a good filter to reduce the intensity of sunlight to safely observable levels). These features include the dark sunspots, the bright faculae, and granules. We can also measure the flow of material in the photosphere using the Doppler effect. These measurements reveal additional features such as supergranules as well as large scale flows and a pattern of waves and oscillations.

    The Sun rotates on its axis once in about 27 days. This rotation was first detected by observing the motion of sunspots in the photosphere. The Sun's rotation axis is tilted by about 7.15 degrees from the axis of the Earth's orbit so we see more of the Sun's north pole in September of each year and more of its south pole in March.

    Since the Sun is a ball of gas it does not have to rotate rigidly like the solid planets and moons do. In fact, the Sun's equatorial regions rotate faster (taking about 24 days) than the polar regions (which rotate once in more than 30 days). The source of this "differential rotation" is an area of current research in solar astronomy.


    Space Weather

    The Sun is the source of the solar wind a flow of gases from the Sun that streams past the Earth at speeds of more than 500 km per second (a million miles per hour). Disturbances in the solar wind shake the Earth's magnetic field and pump energy into the radiation belts. Regions on the surface of the Sun often flare and give off ultraviolet light and x-rays that heat up the Earth's upper atmosphere. This "Space Weather" can change the orbits of satellites and shorten mission lifetimes. The excess radiation can physically damage satellites and pose a threat to astronauts. Shaking the Earth's magnetic field can also cause current surges in power lines that destroy equipment and knock out power over large areas. As we become more dependent upon satellites in space we will increasingly feel the effects of space weather and need to predict it.


    The Sun's Surface

    The Sun may look yellow and smooth in our sky, but it actually has quite a mottled "surface." Actually, the Sun doesn't have a hard surface as we know it on Earth but instead has an outer layer of an electrified gas called "plasma" that appears to be a surface. It contains sunspots, solar prominences, and sometimes gets roiled up by outbursts called flares. How often do these spots and flares happen? It depends on where the Sun is in its solar cycle. When the Sun is most active, it is in "solar maximum" and we see lots of sunspots and outbursts. When the Sun quiets down, it is in "solar minimum" and there is less activity. In fact, during such times, it can look pretty bland for long periods of time.


    Surface and Structure

    The surface of the Earth is very young – this means that the surface changed a lot from when it was first formed. Erosion and tectonic processes, like earthquakes, for example, destroy, recreate, and reshape most of Earth’s surface.

    Earth is currently the only known planet where water can exist in liquid form on the surface. Most of our planet is covered by water, around 71%. The vast oceans keep the temperatures on Earth stable, and this is crucial in the maintaining of life. Water is essential for life, at least the way we know it.

    Water is also responsible for most of the erosion and weathering of the Earth’s continents, a process which is unique in our Solar System. Our Earth has four major main layers: an inner core at the center, an outer core enveloping it, the mantle, and the crust.


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      . I introduce astronomy's place in science, and give a sense of the size and time scales involved. Also discussion of the scientific method and how astrology is not a science and what makes astronomy a science.

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    Pseudoscience vs. science article. Borrowing from Carl Sagan's "The Demon-Haunted World", I take up the subject of UFOs as alien spacecraft. This article is not part of the regular textbook. Other documents on Astronomy Notes about fake science and news: "The Seven Warning Signs of Bogus Science" and "Fake or Real? How to Self-Check the News and Get the Facts" (from NPR's All Tech Considered: original link).

    . I discuss the celestial sphere, motions of the Sun (solar and sidereal days, time zones, equation of time, and seasons), motions of the Moon (phases and eclipses, including my own pictures of some solar eclipses), and planetary motions. Update: additional diagrams and animations for describing phases of the moon.

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    . This chapter covers: The Sun, interiors of stars, and nuclear fusion, neutrinos, the solar neutrino problem, and helioseismology. The concept of hydrostatic equilibrium is used to explain the mass-luminosity relation and the reason for the mass cut-off at the high and low ends. Updates: 2017 solar eclipse pictures, fixed broken links to external websites and added additional resources.

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    . This chapter covers cosmology: the study of the nature, origin, and evolution of the universe as a whole. The distance-scale ladder topic is dealt with in the Steps to the Hubble Constant document. I discuss Olbers' Paradox, the cosmic microwave background radiation, the fate of the universe (open or closed), dark matter, dark energy, inflation, and the cosmological constant. Updates: fixed broken links to external websites and updated material about cosmic microwave background radiation from Planck mission, observations of first galaxies, dark matter, temperature power spectrum (also added graph from 2018 final Planck data release), BICEP2 discussion, dark energy, and tension with Hubble Constant measurements.

    Appendices

    Angular Momentum in Astronomy. I define angular momentum and give several examples of angular momentum in astronomy: Kepler's second law of orbital motion, Earth-Moon system, rapidly spinning neutron stars, accretion disk in a binary system, and a collapsing galactic cloud.

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    Diminishing solar activity may bring new Ice Age by 2030

    This image of the Sun was taken by NASA Solar Dynamics Observations mission on 15 July 2015, at a wavelength of 304 Angstroms. Image credit: NASA Solar Dynamics Observations. The arrival of intense cold similar to the one that raged during the “Little Ice Age”, which froze the world during the 17th century and in the beginning of the 18th century, is expected in the years 2030&mdash2040. These conclusions were presented by Professor V. Zharkova (Northumbria University) during the National Astronomy Meeting in Llandudno in Wales by the international group of scientists, which also includes Dr Helen Popova of the Skobeltsyn Institute of Nuclear Physics and of the Faculty of Physics of the Lomonosov Moscow State University, Professor Simon Shepherd of Bradford University and Dr Sergei Zharkov of Hull University.

    It is known that the Sun has its own magnetic field, the amplitude and spatial configuration of which vary with time. The formation and decay of strong magnetic fields in the solar atmosphere results in the changes of electromagnetic radiation from the Sun, of the intensity of plasma flows coming from the Sun, and the number of sunspots on the Sun’s surface. The study of changes in the number of sunspots on the Sun’s surface has a cyclic structure vary in every 11 years that is also imposed on the Earth environment as the analysis of carbon-14, beryllium-10 and other isotopes in glaciers and in the trees showed.

    There are several cycles with different periods and properties, while the 11-year cycle, the 90-year cycle are the best known of them. The 11-year cycle appears as a cyclical reduction in stains on the surface of the Sun every 11 years. Its 90-year variation is associated with periodic reduction in the number of spots in the 11-year cycle in the 50-25%. In 17th century, though, there was a prolonged reduction in solar activity called the Maunder minimum, which lasted roughly from 1645 to 1700. During this period, there were only about 50 sunspots instead of the usual 40-50 thousand sunspots. Analysis of solar radiation showed that its maxima and minima almost coincide with the maxima and minima in the number of spots. In this 1677 painting by Abraham Hondius, “The Frozen Thames, looking Eastwards towards Old London Bridge,” people are shown enjoying themselves on the ice. In the 17th century there was a prolonged reduction in solar activity called the Maunder minimum, which lasted roughly from 1645 to 1700. During this period, there were only about 50 sunspots recorded instead of the usual 40-50 thousand. Image credit: Museum of London. In the current study published in 3 peer-reviewed papers the researchers analysed a total background magnetic field from full disk magnetograms for three cycles of solar activity (21-23) by applying the so-called “principal component analysis”, which allows to reduce the data dimensionality and noise and to identify waves with the largest contribution to the observational data. This method can be compared with the decomposition of white light on the rainbow prism detecting the waves of different frequencies. As a result, the researchers developed a new method of analysis, which helped to uncover that the magnetic waves in the Sun are generated in pairs, with the main pair covering 40% of variance of the data (Zharkova et al, 2012, MNRAS). The principal component pair is responsible for the variations of a dipole field of the Sun, which is changing its polarity from pole to pole during 11-year solar activity.

    The magnetic waves travel from the opposite hemisphere to the Northern Hemisphere (odd cycles) or to Southern Hemisphere (even cycles), with the phase shift between the waves increasing with a cycle number. The waves interacts with each other in the hemisphere where they have maximum (Northern for odd cycles and Southern for even ones). These two components are assumed to originate in two different layers in the solar interior (inner and outer) with close, but not equal, frequencies and a variable phase shift (Popova et al, 2013, AnnGeo).

    The scientists managed to derive the analytical formula, describing the evolution of these two waves and calculated the summary curve which was linked to the variations of sunspot numbers, the original proxy of solar activity, if one used the modulus of the summary curve (Shepherd et al, 2014, ApJ). By using this formula the scientists made first the prediction of magnetic activity in the cycle 24, which gave 97% accuracy in comparison with the principal components derived from the observations.

    Inspired by this success, the authors extended the prediction of these two magnetic waves to the next two cycle 25 and 26 and discovered that the waves become fully separated into the opposite hemispheres in cycle 26 and thus have little chance of interacting and producing sunspot numbers. This will lead to a sharp decline in solar activity in years 2030&mdash2040 comparable with the conditions existed previously during the Maunder minimum in the XVII century when there were only about 50-70 sunspots observed instead of the usual 40-50 thousand expected.

    The new reduction of the solar activity will lead to reduction of the solar irradiance by 3W/m 2 according to Lean (1997). This resulted in significant cooling of Earth and very severe winters and cold summers. “Several studies have shown that the Maunder Minimum coincided with the coldest phase of global cooling, which was called “the Little Ice Age”. During this period there were very cold winters in Europe and North America. In the days of the Maunder minimum the water in the river Thames and the Danube River froze, the Moscow River was covered by ice every six months, snow lay on some plains year round and Greenland was covered by glaciers” – says Dr Helen Popova, who developed a unique physical-mathematical model of the evolution of the magnetic activity of the Sun and used it to gain the patterns of occurrence of global minima of solar activity and gave them a physical interpretation.

    If the similar reduction will be observed during the upcoming Maunder minimum this can lead to the similar cooling of the Earth atmosphere. According to Dr Helen Popova, if the existing theories about the impact of solar activity on the climate are true, then this minimum will lead to a significant cooling, similar to the one occurred during the Maunder minimum.

    However, only the time will show soon enough (within the next 5-15 years) if this will happen.

    Dr. Helen Popova of the Skobeltsyn Institute of Nuclear Physics and of the Faculty of Physics of the Lomonosov Moscow State University. Image credit: Lomonosov Moscow State University. “Given that our future minimum will last for at least three solar cycles, which is about 30 years, it is possible, that the lowering of the temperature will not be as deep as during the Maunder minimum. But we will have to examine it in detail. We keep in touch with climatologists from different countries. We plan to work in this direction”, Dr Helen Popova said.

    The notion that solar activity affects the climate, appeared long ago. It is known, for example, that a change in the total quantity of the electromagnetic radiation by only 1% can result in a noticeable change in the temperature distribution and air flow all over the Earth. Ultraviolet rays cause photochemical effect, which leads to the formation of ozone at the altitude of 30-40 km. The flow of ultraviolet rays increases sharply during chromospheric flares in the Sun. Ozone, which absorbs the Sun’s rays well enough, is being heated and it affects the air currents in the lower layers of the atmosphere and, consequently, the weather. Powerful emission of corpuscles, which can reach the Earth’s surface, arise periodically during the high solar activity. They can move in complex trajectories, causing aurorae, geomagnetic storms and disturbances of radio communication.

    By increasing the flow of particles in the lower atmospheric layers air flows of meridional direction enhance: warm currents from the south with even greater energy rush in the high latitudes and cold currents, carrying arctic air, penetrate deeper into the south. In addition, the solar activity affects the intensity of fluxes of galactic cosmic rays. The minimum activity streams become more intense, which also affects the chemical processes in the Earth’s atmosphere

    The study of deuterium in the Antarctic showed that there were five global warmings and four Ice Ages for the past 400 thousand years. The increase in the volcanic activity comes after the Ice Age and it leads to the greenhouse gas emissions. The magnetic field of the Sun grows, what means that the flux of cosmic rays decreases, increasing the number of clouds and leading to the warming again. Next comes the reverse process, where the magnetic field of the Sun decreases, the intensity of cosmic ray rises, reducing the clouds and making the atmosphere cool again. This process comes with some delay.

    Dr Helen Popova responds cautiously, while speaking about the human influence on climate.

    “There is no strong evidence, that global warming is caused by human activity. The study of deuterium in the Antarctic showed that there were five global warmings and four Ice Ages for the past 400 thousand years. People first appeared on the Earth about 60 thousand years ago. However, even if human activities influence the climate, we can say, that the Sun with the new minimum gives humanity more time or a second chance to reduce their industrial emissions and to prepare, when the Sun will return to normal activity”, Dr Helen Popova summarised.



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