Transcript: Physicists in the nineteenth century made various estimates of the age of the Sun, but they were fundamentally unaware of the most efficient energy source known. Early in the twentieth century physicists Rutherford and Becquerel began a systematic study of the phenomenon of radioactivity, a situation where atoms spontaneously emit both particles and radiation. Rutherford for example sealed a small amount of a radioactive substance in a tube that contained a pure vacuum. He...

Transcript: Chemical energy cannot power the Sun, so what is the energy source? Inspired by an idea by the German physicist Hermann von Helmholtz the English physicist Lord Kelvin explored the idea of gravitational contraction. In this mechanism the Sun is slowly shrinking and gravitational potential energy is being converted into heat energy which then radiates out into space. In his estimate the Sun might last a couple of hundred million years with this mechanism. It sounds like a long...

Transcript: Above the solar chromosphere is the corona, a diffuse outer layer of gas at the amazing temperature of two million degrees Kelvin. Both the chromosphere and the corona have higher temperatures than the photosphere. How can this be? One way for gas to become hot is pressure. Higher pressure and density will lead to higher temperature. This is what happens in the interior of the Sun, but the corona is a diffuse outer layer far from the Sun’s energy source. How can it be so...

Transcript: Auroras are caused when high energy particles from the solar wind crash into the atmosphere of the Earth near its poles. They’re called the northern and southern lights respectively or the Aurora Borealis and the Aurora Australis. The solar wind takes several days to reach us from the Sun. When those particles reach the edge of the magnetosphere they are channeled along magnetic field lines and accelerated. These interactions can build up voltages up to a hundred thousand...

Transcript: Even though the solar wind is diffuse and invisible to the eye it has substantial consequences for the Earth. The particles streaming out from the Sun are ions, positively charged energetic particles with their electrons removed. When they hit the Earth they first hit the magnetosphere, the magnetic field of the Earth where the charged particles can spiral in magnetic field lines. Essentially the bow wave of the Earth is the bow wave of the magnetosphere hitting the charged...

Transcript: The diffuse solar corona expands rapidly into deep space, and charged particles are blasted out from the site of sunspots and flares on the solar surface. The result of these two effects is the solar wind. The solar wind is a diffuse stream of charged and energetic particles traveling out into space. At the distance of the Earth the speed is four hundred to a thousand kilometers per second, and the temperature is two hundred thousand degrees Kelvin although little heat is...

Transcript: The visible surface and edge of the Sun and the region where sunspots lie is the Sun’s photosphere. Just above the photosphere lies the chromosphere or color layer. This is a slender region of pink gas at a temperature of about ten thousand degrees Kelvin. The pink color comes from emission from hydrogen alpha, the single spectral transition of an excited hydrogen that comes out in the red part of the visible spectrum. The solar chromosphere is best seen during a total eclipse...

Transcript: Sunspots have been carefully observed for over four hundred years since the invention of the telescope. This long span of observations reveals intriguing patterns beyond the basic eleven year sunspot cycle. There are longer variations over centuries as well. The last few sunspot cycles have been historically high compared to those a hundred or two hundred years ago. We have to be careful about long scale variations because sunspots have only been recorded photographically or...

The Sun is a smooth and continuously varying ball of gas which reduces in density moving outward until it gradually fades away into space. We see an edge which is called the photosphere, but why do we see an edge at all if the Sun is smoothly changing in temperature density and pressure? The interior of the Sun is opaque. The opacity is high. Photons are always colliding with atoms and particles and cannot travel freely, but moving outward in the Sun at a point where the temperature...

Transcript: In 1830 the German astronomer Heinrich Schwabe started observing sunspots as a hobby. He was an amateur astronomer who spent much of his time observing the Sun. After a decade or so he noticed a regular pattern reappearing in the number and placement of sunspots and he proposed the solar cycle. The solar cycle lasts twenty-two years, and it’s composed of two complete eleven year cycles of sunspots. In a cycle of sunspots at the minimum few sunspots appear, and most of them are...

Transcript: The Sun’s photosphere is a plasma, an ionized gas made up of charged particles. The Sun also has a magnetic field. The field is tethered deep within the Sun, but field lines loop out into space. Occasionally eruptions of gas from the surface travel along the field lines. Excitation of the gas creates emission at visible wavelengths and even x-rays and also creates spectacular prominences. These transient features leaping out from the Sun’s photosphere can be huge, several...

Transcript: Galileo was the first to show that sunspots are surface features on the Sun carried around by its rotation. Be very careful ever observing sunspots with the naked eye. Galileo spent the last twenty years of his life blind from careless observations of the Sun. The best procedure is to magnify the Sun’s image with a small telescope and project it into a viewing chamber shielded from outside light. Sunspots are magnetic disturbed regions cooler than the surrounding areas. They...

Transcript: The Sun oscillates or vibrates at many frequencies like a bell. Solar oscillations can be used to study the interior of the Sun just as geologists use seismic waves to study the Earth’s interior. In fact, apart from neutrinos this is the only way to reliably map out conditions inside the Sun. The best known solar oscillation has a five minute period and corresponds to cells near the surface moving up and down by distances of about ten kilometers. This represents the kind of...

Transcript: Energy produced in the core of the Sun travels out using two of the three basic modes of heat transport. Helium exists in the core, but some is found further out because it preexisted in the solar nebula and it has had time to diffuse out from the core where it’s generated by fusion. Throughout most of its volume energy travels outward in the Sun by radiation, electromagnetic waves of different frequencies and wavelengths. In the outer quarter of its radius the temperature...

Transition: The part of the Sun we see is its surface layer at only fifty-seven hundred degrees Kelvin. Gas that hot has the electrons stripped off from the protons, but it’s far too cool for fusion to occur. However the temperature, pressure, and density all increase as you move towards the center of the Sun. At a region where the temperature exceeds ten million degrees fusion can occur of hydrogen with protons fusing to form helium nuclei in three stages in the proton-proton chain. At...

Transcript: We can’t see into the Sun. The Sun is opaque like a frosted pane of glass. Opacity or optical depth is the degree to which a material transmits light. If a material transmits all of the light incident on it, it is transparent, and its opacity or optical depth is zero. If it transmits none of the light it’s opaque, and its opacity or optical depth is high. In the Sun, radiation suffers collisions with atoms or ions that exist there at high temperature and very high density. ...

Transcript: Energy produced in the core of the Sun travels out using two of the three basic modes of heat transport. Helium exists in the core, but some is found further out because it preexisted in the solar nebula and it has had time to diffuse out from the core where it’s generated by fusion. Throughout most of its volume energy travels outward in the Sun by radiation, electromagnetic waves of different frequencies and wavelengths. In the outer quarter of its radius the temperature...

Transcript: Knowing the size, composition, and energy source of the Sun, astronomers can calculate the physical conditions at any point within its volume. This is called the standard solar model. Fusion occurs within the core, the inner quarter of the Sun’s radius. The temperature at the very center is fifteen million Kelvin. One quarter of the radius out has dropped to ten million, the edge of the fusion zone, and at half the radius is down to about five million Kelvin. The density at...

Transcript: At the first step in the proton-proton chain in the Sun and other low mass stars neutrinos are produced. Since neutrinos interact so weakly with ordinary matter they flee the Sun almost instantly. Ten to the fourteen neutrinos pass through every square meter of the Earth’s surface every second. Ten trillion pass through your body every second, and you don’t feel a thing. Getting neutrinos to interact is difficult, so detecting them is an extreme experimental challenge. The...

Transcript: Neutrinos were predicted as a consequence of the conservation of energy. This fundamental principle applies to most interactions in the universe. In the 1930s particle reactions were observed where when all the energies and momenta were added up some energy and momentum was missing. The experimenters predicted the existence of a weakly interacting neutral particle to account for the missing energy and momentum. Wolfgang Pauli named it the neutrino, little neutral one. Twenty...

Transcript: The positron is the antiparticle of the electron. Every particle in the zoo of nature has an antiparticle. The positron is positively charged whereas the electron is negatively charged. It has the same mass however. Electrons and positrons if they combine annihilate to form pure energy or gamma rays. The converse process is also possible. Gamma rays can spontaneously form electron-positron pairs. Positrons were first detected in film and bubble chamber experiments where a...

Transcript: The energy source of the Sun is the conversion by fusion of hydrogen into helium in a three step process called the proton-proton chain. In the first step protons fuse to form deuterium, a nucleus with a proton and a neutron. The release products are a positron, the antiparticle of the electron, and a neutrino, a tiny nearly massless particle. In the second step of the process deuterium has another proton added to form tritium, two protons and a neutron bound together with the...

Transcript: The Sun is not burning in a conventional sense. In 1871 Hermann von Helmholtz calculated that if the Sun were burning by chemical reactions it would be the equivalent of seven thousand kilograms of coal for each square meter of its surface. No chemical reaction can produce energy with the efficiency that the Sun does. The Sun is powered by the fusion of hydrogen into helium with two consequences: the release of huge amounts of energy and a gradual change of the chemical...

Transcript: Fission is such an efficient energy source that humans have long tried to harness it. A massive atomic nucleus can be split by a neutron. Since the decay of a massive nucleus can also release a neutron, this raises the possibility of a chain reaction where there’s sufficient density or purity of radioactive atoms that the neutrons released by the decay of one atom always trigger the decay of another atom and so on in a sustaining reaction. Fission occurs in a number of ways in...

Transcript: The decay of a massive atomic nucleus with the release of particles or energy or the splitting of a massive nucleus into two or more pieces is called fission. In fission the sum of the fragments is less than the mass of the original nucleus. The excess is released as energy according to E = mc2. Fission is a highly efficient energy source. When a single atom of uranium 235 decays it releases three times ten to the minus eleven Joules of energy. Not much for a single atom, but...