13. Particle Physics and the Sun

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 returned months later to find that the tube contained helium gas and that the chemical properties of the radioactive substance had changed. Here was proof both that the atomic nucleus can emit energy and that chemicals can change fundamentally due to radioactive processes. The atomic nucleus could be transformed, and it could emit energy.

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 time, but by the mid-nineteenth century the debate about the age of the Sun began to collide with the debate about the age of the Earth. Most people assumed they formed at the same time. Charles Darwin’s theory of natural selection seemed to require many millions of years for the diversity of species to be achieved from simple origins. In the nineteenth century in England it was common to have scientific debates carried out in public for a public audience and scientists as well. Darwin had debated Wilberforce on the subject of natural selection and by general acclaim had won the debate. In 1871 Lord Kelvin debated Thomas Huxley who was standing in for Darwin on the issue of the age of the Sun and the age of the Earth. Darwin had estimated that the age of the Earth needed to be many hundreds of millions of years, perhaps billions of years to explain the diversity of species, but Kelvin said the Sun could be no older than half a billion years based on gravitational contraction. Darwin died without knowing whether the Earth could be old enough for his mechanism to work.

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 hot? Think for example of a fluorescent tube. In this case a very diffuse gas in the tube is cool to the touch, yet it must have a thermal temperature of thousands of degrees Kelvin because it emits visible light. The reason is that it is given high energy by electrical fields that are pumped into it from electricity running through the tube. In the case of the solar corona the energy source is magnetic energy from the Sun’s surface and its magnetic field plus convection to carry the energy outward. The physics of the energy source in the solar corona is complex, but it’s clear that magnetic fields fuel the very high temperature of the solar corona.

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 volts or greater. When the reach the upper atmosphere they have enormous energy and they can excite molecules of oxygen, ozone, and nitrogen producing strong emission. That’s why the gas in the upper atmosphere glows, and it leads to the delicate patterns and curtains of light that are familiar for the aurora. Because the excitation of the atmosphere happens near the poles, the auroras are normally only seen above the Arctic Circle. There is always several days warning of a spectacular aurora because of the time it takes the solar wind to reach the Earth. During extreme periods of solar activity these interactions can spread such that the aurora can be seen even from continental United States.

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 particles that come from the Sun. Many particles are deflected by this interaction. Others are accelerated to high energies. Some penetrate the magnetosphere sufficiently to accumulate in two donut-type regions above the Earth’s surface. These are the Van Allen radiation belts. Others stream down onto the polar regions of the Earth and create auroral displays. In general the interactions of highly charged particles with magnetic fields leads to strong radio interference, so telecommunications on the Earth are severely disrupted at times of sunspot maximum or when the solar wind is particularly strong.

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 transmitted to the Earth because the solar wind is so diffuse. It extends past the orbit of Saturn. The solar wind creates substantial effects on the Earth when it interacts with the Earth’s magnetosphere.

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 when the moon blocks out the Sun’s light, and the delicate radiation of the chromosphere and its color can clearly be seen. Beyond the chromosphere the Sun fades out into deep space.

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 with electronic detectors relatively recently. Most of the first hundred years of sunspot observations were made through telescopes but transcribed by drawings by eye. However, it is almost certain that in the mid-sixteenth century for a span of about fifty years the sunspot levels were enormously lower than the present day. Even at their maximum the number of sunspots was nearly an order of magnitude lower than we see presently. This period was called the Maunder minimum, and intriguingly it correlates with substantial climate change in Europe. Europe went through a mini ice age during exactly the same span. The correlation between low sunspot numbers and climate change was first seen for the Maunder minimum but has now been seen in other aspects of sunspot behavior as well.

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 reduces below six thousand degrees Kelvin there’s a thin zone where the density has reduced to the point where photons no longer interact with particles. At this point they travel freely through space, and we see the Sun as an edge. This is just like the situation in a cloud. The density, pressure, and temperature in a cloud are not much different from outside the cloud, but the pressure of water vapor is slightly higher so that water vapor can condense. When you go in and out of a cloud in an airplane there is no hard or sharp edge. In fact the density is just slightly higher, sufficient that light bounces around inside the cloud; the cloud is opaque. The edge of the cloud is the place where light travels freely. Thus the photosphere is the edge of the Sun where the opacity is reduced to the point where light travels freely. Radiation that has taken hundreds of thousands of years to reach the surface from the core travels in only eight minutes to the Earth.

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 within ten degrees of the Sun’s equator. After a couple of years the number of spots increases, and most of them are found at relatively high latitudes thirty degrees from the Sun’s equator. Near the maximum many sunspots appear, and they are at latitudes of around twenty degrees from the Sun’s equator. As eleven years from the minimum approaches the number again reduces towards the minimum, and once again the latitude is within ten degrees of the Sun’s equator.

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 times the Earth’s dimension, and they can come and go in timescales of only an hour. The largest blasts from active sunspot sites are called solar flares.

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 have temperatures of four thousand to forty-five hundred degrees Kelvin versus fifty-seven hundred degrees on average for the solar photosphere. The gas motions in a sunspot are controlled by intense magnetic fields.

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 boiling of the solar surface. It can also be measured by the Doppler Effect in the gas motions and the light emitted by cells near the surface. The Sun has highly complex behavior, and solar seismology is a rich field. Hundreds of modes of oscillation have been observed in the Sun, and recently the principles of seismology have begun to be applied to other stars as well.

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 difference is so large that energy cannot move fast enough by radiation so convection occurs. In convection, pockets of hot gas rise and then fall and carry energy outwards, much as happens in a pan of boiling water.

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 the very center of the Sun the temperature is fifteen million Kelvin. The conditions in the Sun are predicted by the same gas laws that apply to calculations of the atmospheres of planets. At the center of the Sun the pressure is two hundred and fifty billion times the pressure at the sea level of Earth. The density is a hundred and sixty times that of water, twenty times that or iron. One cubic inch of the Sun’s interior would weigh five pounds brought to Earth, yet it’s a gas, a plasma. Despite the violent reactions occurring in the center of the Sun, the Sun is stable. It’s more like a reactor, and it’s certainly not a bomb.

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. The frequent collisions make a high opacity which means the radiation cannot travel freely. It’s the difference between having a town square where you walk across it freely because no one else is there and a town square full of a crowd of people where you would have to jostle your way through colliding many times before you could cross the square. In one case the opacity is zero. In the other case the opacity is high. In the Sun the opacity is high enough that we cannot see inside the surface’s few thousandths of a kilometer.

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 difference is so large that energy cannot move fast enough by radiation so convection occurs. In convection, pockets of hot gas rise and then fall and carry energy outwards, much as happens in a pan of boiling water.

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 the core is ten to the fifth kilograms per cubic meter. This drops by a factor of two out to the edge of the fusion core and then down to about ten to the three kilograms per cubic meter halfway out the Sun. The fusion core, one quarter of the radius, one sixty-fourth of the volume, contains half the Sun’s mass and generates ninety-nine percent of its energy. Remember that these statements are based on a model of the Sun’s behavior. We cannot see directly into the Sun’s core.

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 best way is to put large tanks of extremely pure fluid deep underground to shield from cosmic rays and look for the exceptionally rare interactions between a neutrino and a particle in the fluid. Essentially a form of cleaning fluid works best. Deep underground mines to detect neutrinos experiments have been in operation for over thirty years. Ray Davis at Brookhaven Lab was the pioneer of this type of experiment. Solar neutrinos have been detected for that length of time which is a profound confirmation that fusion actually does occur in the center of the Sun. Neutrinos allow us to see into the heart of the Sun where fusion actually occurs. However, in detail the rate of occurrence of neutrino interactions was one-third the prediction of standard solar models. This remained an extreme puzzle indicating that perhaps we didn’t understand stellar fusion at all until the discovery relatively recently that neutrinos can oscillate and change their flavor. This explains the shortfall of solar neutrinos.

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 years later it was detected. Neutrinos interact very weakly with matter and could pass through thousands of miles of iron with a small probability of interacting or being stopped. There are three flavors or types of neutrino associated with the three families in the standard model of particle physics: the electron neutrino, the muon neutrino, and the tau neutrino in increasing energy or mass. For a long time physicists thought the neutrino had no mass, but now it appears the neutrino must have a small amount of mass because the three types of neutrinos can oscillate or change from one to the other. The fact that the neutrino has mass has important consequences for astronomy and cosmology because neutrinos are routine byproducts of solar fusion.

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 magnetic field is superimposed in the particle regime such that particles move in the magnetic field. Due to the opposite charge of electrons and positrons they curved in opposite directions in the magnetic fields. In this way antimatter was discovered in the 1930s.

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 release of gamma rays or radiant energy. In the third step of the process two tritium nuclei combine to form a helium nucleus, two protons and two neutrons, with two free protons left over to participate in fusion products further. At each of the three steps in the process energy is released, and the result of all this energy release is the power from the Sun.

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 composition. For each kilogram processed 0.007 kilograms is released as pure energy. The mass-energy conversion efficiency is therefore 0.7 percent. Each helium nucleus created releases about four times ten to the minus eleven Joules. In the Sun every second four million tons of hydrogen are converted into energy and radiated into space. Over the whole Sun the energy release is four times ten to the twenty-six Joules per second. That’s four hundred trillion trillion Watts which is a lot of light bulbs.

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 the universe. On Earth fission has even occurred naturally in the ground in an underground mine in Gabon, Africa; 1.7 billion years ago the concentration of uranium was sufficient for energy to be released. Humans have tried to purify uranium and other radioactive elements to create nuclear reactors. The sustaining reaction is moderated by carbon rods which will absorb the neutrons, and in an uncontrolled version of the reaction sufficient mass of uranium or another heavy element creates an uncontrolled reaction and catastrophic energy release.

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 a fistful is sufficient to power a city. Contrast this with a chemical energy source such as coal or any fossil fuel. In this case a single atom of carbon combining with oxygen to carbon dioxide releases only six times ten to the minus nineteen Joules, fifty million times less energy.

Transcript: The only way in which humans have so far harnessed fusion is in a hydrogen bomb, the most violent product of human creation. We’re obviously interested in fusion as a power source for a simple reason. What goes in in fusion is light elements like hydrogen, and what comes out is deuterium or helium or tritium. Other light elements, if they are radioactive they have very short half-lives and decay quickly. Fusion compared to fission is therefore a very clean energy source and equally efficient. The problem is that fusion requires protons to be brought into close proximity. They are positively charged, and they repel each other with the electrical force. Temperatures of millions of degrees are required to force hydrogen to fuse into heavier elements and release energy. We have so far managed to have this happen in the lab fleetingly and not in a sustained reaction. Since most material substances melt at temperatures of thousands of degrees Kelvin a temperature of millions of degrees cannot be held in a conventional container. Physicists therefore used magnetic bottles to hold hydrogen to make fusion occur. There is still some hope that fusion will be a feasible energy source, but that day may be decades away.

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 a fistful is sufficient to power a city. Contrast this with a chemical energy source such as coal or any fossil fuel. In this case a single atom of carbon combining with oxygen to carbon dioxide releases only six times ten to the minus nineteen Joules, fifty million times less energy.

Transcript: The mass of any atomic nucleus is less than the separate masses of its protons and neutrons. The difference is the binding energy. In other words sticking protons and neutrons together somehow causes some of their mass to vanish. The answer is connected with Einstein’s equation E = mc2. Mass and energy are really two different forms of the same thing so the vanishing mass of the protons and neutrons is simply converted into energy, and that’s the idea behind fusion. The binding energy of a particular isotope is the amount of energy released when it’s created. You can calculate its amount by using Einstein’s equation. The binding energy is also the amount of energy you need to add to a nucleus to break it up again into protons and neutrons. The binding energy per nucleon, that is proton or neutron, differs for different elements. It peaks at iron; that’s the most stable element. So for elements heavier than iron a decay by fission into smaller pieces releases energy. The binding energy is less of the residual pieces and the excess is released as energy.

Transcript: The universe is full of things that are held together by forces: atoms, molecules, solar systems, stars, and galaxies. The binding energy of a system is the energy required to take it apart. For example, a speed of eleven kilometers per second corresponds to the kinetic energy needed to liberate anything from the gravitational binding energy of the Earth, or consider an atom. The binding energy of a hydrogen atom is a tiny quantity, 2.2 x 10-16 Joules. This means that any incoming photon with a frequency greater than 3.3 x 1015 Hertz or a wavelength less than a hundred nanometers gives sufficient energy to the system to liberate an electron.

Transcript: The equivalence between mass and energy denoted by the equation E = mc2 has profound consequences for the way we look at the physical world. We know that energy comes in many forms, and mass is just one of these. In a sense mass is potential energy since normally it is frozen in the form of stable particles. In the equation E = mc2 the arrows go both ways. Just as mass is a form of energy, so energy is a form of mass. A rapidly moving car has slightly more mass than a stationary car. A spent battery has slightly less mass than a fully charged battery. These effects are tiny but real and measurable, and so instead of the conservation of energy we should really talk about the conservation of mass-energy.

Transcript: The nucleus of the atom contains a prodigious potential energy source. Mass and energy are related by Einstein’s famous equation E = mc2. Since c is a very large number, three hundred thousand kilometers per second, c2 is an even larger number. So a tiny amount of mass is equivalent to a huge amount of energy. When you plug the numbers in you can see how dramatic this is. The mass-energy equivalent to the tip of a pencil lead is sufficient to run a family home for a day. The mass-energy equivalent to an adult person is sufficient to provide ten to the nineteen Joules which is the energy requirement of the United States for a year. The inefficiency of chemical energy is also clear. The Saturn V rocket that took astronauts to the moon had ninety percent of its mass in the form of fuel that had to be burned up to launch the projectile. Ten grams of mass energy would have provided the same energy source. For a final every day example, consider a quarter pounder hamburger. This provides the average person who eats it two hundred and fifty calories or a million Joules, but the mass-energy in that amount of material is ten to the sixteen Joules. Thus we only extract one part in ten to ten of the total energy of the hamburger in the form of chemical energy.

Transcript: Working early in the twentieth century physicist Marie Curie was able to show that radioactive processes released millions of times more energy per atom than any chemical process known. Marie Curie was a pioneer. With her husband she was the first to isolate a radioactive element. She was the first female professor in the six hundred year history of the Sorbonne. She was the first person ever to win two Nobel Prizes; however Marie and many others who worked on radioactivity paid heavily for being pioneers. Unaware of the damaging effects of radiation on human skin and tissue they died from radioactive poisoning.

Transcript: The three basic types of radioactive decay are called alpha, beta, and gamma decay. In the alpha process an atom spontaneously emits a helium nucleus. Helium nucleus contains two protons and two neutrons so alpha decay reduces the atomic number by two. In beta decay a neutron decays into a proton, an electron, and a neutrino. A neutron is only stable when bound in an atomic nucleus. Free neutrons will decay radioactively. The final type of emission is called gamma radiation. Gamma rays are high energy photons released spontaneously by radioactive atoms.

Transcript: Radioactive decay is a phenomenon of the atomic nucleus. In these processes an element changes its chemical properties, that is its atomic number, by the emission of particles and or radiation. Radioactivity is a random process. It’s impossible to predict exactly when a particular radioactive decay will occur. However, in a collection of atoms there is a well defined half-life or time that it takes one starting point, that is the parent isotope, to turn into the decayed product or daughter isotope. Physicists early on did not understand the fundamental nature of the radioactive process, and they categorized the three types of decay as alpha decay, beta decay, and gamma decay.