Chapter 16Our Star, The SunThe material of Chapter 16 introduces the Sun, the closest example of a star and the only one that we can view in sufficient detail to resolve features of its surface (and interior !).

1. Examine the characteristic features of the Sun photosphere, chromosphere, corona and how they change with time.

2. Study the activity cycle of the Sun and learn about its deep interior, where sunlight is generated.

A typical view of the Sun: a spherical star that exhibits limb-darkening (brightness drops towards the edge) and a nearly featureless disk.

The Sun has actually changed in size and temperature since it first formed 4 billion years ago.

Like all stars, the Sun is in hydrostatic equilibrium (balance), otherwise its size would change very quickly ! The weight of each layer in the Sun is balanced by the buoyant force exerted by the pressure exerted by gas below it. The pressure comes from radiation (light) and the high temperature of the gas. The ultimate power source is nuclear fusion in the Suns core.

The core is where hydrogen is converted to helium via the proton-proton chain, Tcritical = 10 million K.

H1 + H1 D2 + positronD2 + H1 He3He3 + He3 He4 + 2H1

The proton-proton chain in the 15 million K Suns core simplified.

The source of energy in the Suns core is the same as that which powers an H bomb (hydrogen bomb).

The short-wavelength gamma rays produced in the core of the Sun gradually diffuse outwards, scattered (absorbed and re-emitted at slightly lower energies) to longer wavelengths by interactions with gas atoms and ions until they eventually emerge at the Suns surface 10,000-200,000 years later as visible light photons.

Neutrinos, on the other hand, do not interact with gas atoms and ions at all, and reach Earth 8 minutes later, detected only through the occasional interactions they make with atoms of some substances.

The Solar Neutrino Problem

The observed flux of neutrinos at Earth is on the order of to the amount predicted by the Standard Solar Model. Current thinking in subatomic physics suggests that the electron neutrinos generated in the Suns core are oscillating between the states of electron and muon neutrinos on their way to Earth, so that detectors on Earth, tuned to detect electron neutrinos, are missing half of neutrinos actually generated by the Sun.

The heat of the Sun diffuses outwards by simple radiative transfer in the deep interior, but is helped by convective eddies over the last 28.6%.

Solar granulation: an indication of the outer convective region of the Sun. Shock waves from the convection zone also heat the gas in solar chromosphere and corona to very high temperatures.

The solar granulation and model (lower).

Models of the Suns interior make use of mathematical relationships describing the conditions of hydrostatic equilibrium, thermal equilibrium, radiative equilibrium, and the general continuity of mass in the Sun in order to calculate the mass, density, temperature, and pressure at all points in the solar interior.

The standard solar model is well verified by observation, including helioseismology, which matches the oscillations seen at the Suns surface with the parameters of seismic waves passing through the Suns outer regions. Neutrinos detected from the Suns core also provide information about the nuclear reactions occurring there.

Helioseismology.

The Sun, Earth to scale, and sunspots.

Bright = Hot

Dark = Cool

Sun: March 9, 2011 from Atrium

White light images of the Sun showing sunspots, limb darkening, and the solar rotation. The orange surface is the solar photosphere = light sphere.

The motion of sunspots across the Suns disk indicates that the Sun rotates differentially, that is the equatorial regions rotate faster than regions near the poles. Observations indicate a rotational period of 24 days at the solar equator and 30 days 60 away from the equator, corresponding to observed rates from Earth (synodic rates) of ~27 days and ~32 days, respectively.

Differential rotation also occurs in the gaseous planets Jupiter and Saturn. On the Sun it is responsible for twisting up the subsurface toroidal magnetic field.

Nomenclature for sunspot features.

Sunspots are cool regions (T ~ 4500K) of the photosphere, where strong magnetic fields have restricted normal turbulence. Because they are cooler than the surrounding photosphere (T ~ 5800K) they are less radiative, and appear dark through contrast with the brighter photosphere.

A complex sunspot group.

The limb darkening of the Sun has a similar explanation. Our line of sight at the edge of the solar disk penetrates to shallower, cooler depths than does the line of sight at disk centre.

The optical depth to which we can see into the Suns photosphere (photon sphere) is about the same no matter where we look, but we can see to deeper, hotter regions near disk centre.

Sunspots, and the sunspot cycle of ~11 years. The number of spots increases rapidly and declines slowly during the 11-year sunspot cycle. (Sunspot number is a curious function that counts 10 for each spot group, and 1 for each individual spot.)

Sunspot numbers.

Recent sunspot numbers

Sunspots are associated with strong magnetic regions of the photosphere, and cycle through different latitude regions during the 11-year cycle, beginning near both poles, ending near the equator (butterfly diagram).

The Butterfly Diagram.

Holland in winter during the Maunder Minimum.

The chromosphere as seen in the light of H radiation (right) and at the limb, where spicules are visible (below right).

Close-up view of spicules.

Magnetic field lines on the Sun and constraint of ions.

Active (magnetic) regions in the solar chromosphere seen in spectroheliograms taken in the light of Ca K radiation. Bright regions are called plages or faculae.

More views of the chromosphere from H spectroheliograms. A solar flare is developing at right.

Active regions on the Sun are associated with sunspot zones, which are 5 to 45 away from the Suns equator.

Prominences associated with sunspots protrude into the solar corona. Their feet rest on active sunspots.

Solar prominences seen on the limb.

Features of the solar corona can be seen in X-rays (right) or radio images (below), which isolate hot regions, even when gas density is low.

The Babcock mechanism as a qualitative explanation for the solar cycle of activity.

The solar corona. Its extent varies with the sunspot cycle (largest during sunspot maximum).

Solar corona showing coronal holes.

The magnetic Sun.

The active Sun, displaying prominences and a time sequence of a flare developing.

Solar flare debris injected into the solar wind, eventually to reach Earth a few days later.

The auroral zone on Earth lies within 20 to 30 of Earths magnetic poles.

Displays of the aurora borealis.

Noctilucent clouds, perhaps an indicator of solar activity but more likely to climate change.

Astronomical Terminology

Photosphere = light sphere. The region (disk) of the Sun from which light appears to originate. It also constitutes the solar atmosphere.Chromosphere = coloured sphere. The region lying above the solar photosphere containing spicules where the temperature increases with height.Corona = crown. The outermost region of hot gases surrounding the Sun where temperatures reach 1-2 million K, and where the solar wind originates.Sunspot cycle. The interval of ~11 years over which sunspot numbers increase and wane.Proton-proton chain. The sequence of nuclear reactions in which hydrogen nuclei (protons) interact and are fused to become helium nuclei ( particles), with the release of energy.Convection zone. The upper 29% of the Sun where energy is carried by convective bubbles of hot gas.

Astronomical Terminology (continued)

Granulation. The mottled structure of the photosphere caused by hot bubbles of gas at the Suns surface.Spicule. A spikey jet of hot gas from the solar chromosphere erupting into the solar corona.Prominence. Huge gaseous eruptions of arching clouds of ionized particles streaming between sunspots of opposite polarity through the corona.Filament. The dark projection of a prominence viewed against the Suns surface in monochromatic light.Sunspot. A region of cool gas (T 4500K) in the surrounding solar photosphere (T 5800 K) appearing as a dark penumbral region, often with a surrounding gray penumbra, that is the site of strong magnetic field.Floculli, Plages. Light and dark markings in the solar chromosphere seen in monochromatic images of the Sun.

Astronomical Terminology (continued)

Maunder Minimum. A period of ~60 years in the late 1600s and early 1700s when sunspot numbers were at an unusual low and northern hemisphere winters were unusually cold. Possibly related to Sprer minima at earlier epochs when sunspot activity may also have been low.Optical Depth. How far into the Suns photosphere we can see before the gas becomes completely opaque.Neutrino. A nearly massless nuclear particle that travels at nearly the speed of light and is produced during the production of a deuteron through a collision of two protons.Solar Wind. The stream of ionized and neutral gas particles away from the Sun.Aurora Borealis = Northern Lights. Radiation produced in Earths upper atmosphere by the streaming of charged particles towards the magnetic poles.

Sample Questions1. In the proton-proton chain process, the mass of four protons is slightly greater than the mass of a helium nucleus. Explain what happens to this difference in mass.

Answer. The difference in mass is accounted for by the energy released in the process in the form of gamma rays, neutrinos, and positrons.

2. The Sun has a radius equal to about 2.3 light seconds. Explain why a gamma ray produced in the Suns core does not emerge from the Suns surface 2.3 seconds later.

Answer. The ionized gas near the Suns core is opaque to gamma rays and absorbs the photons before they can travel very far. The light is re-emitted as photons of slightly lower energy in random directions, repeating the absorption and re-emission process a multitude of times before the light eventually emerges at the Suns surface.

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