New episode every Monday!

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New episode every Monday!

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New episode every Monday!

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Is that all?

Laser Inertial Fusion Energy (LIFE) – is a fusion energy concept for delivering energy to the electrical grid.

Dude, where’s my science?

Would it ever really be possible to create an artificial star? In a previous episode, we talked about a Dyson Sphere, an artificial structure built around a star. A civilization capable of building a Dyson Sphere would definitely be able to build an artificial star. In our current understanding of the Universe, however, these objects are still relegated to the realms of science fiction.

While we cannot create a whole star, it is possible to create star like conditions here on Earth. For example, scientists are working to create fusion reactors that will emulate the center of a star by fusing light elements to create energy. Check out the Laser Inertial Fusion Energy link above to find out more.

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What does it mean?

Hydrostatic equilibrium – Hydrostatic equilibrium in stars is the state in which the inward force of gravity is balanced by the outward force of the pressure inside the star, thus making the star stable against collapse or expansion. The pressure is due to the hot gas and the radiation, and each contributes different amounts to the total pressure depending on both the type of star and on what region of the star is being considered.

In human speak please!

A star is a ball of gas held together by the mutual gravitational attraction of all its constituent parts, and the more mass a star has, the higher its gravity. So why don’t stars just collapse into a tiny volume under the influence of all that gravity?

As Epo and Alkina discuss in this episode, the energy released during the fusion of light elements in the centers of stars creates pressure, and the pressure balances the inward force of the star’s gravity. This process is somewhat similar to a balloon, where the tension in the rubber of the balloon balances the pressure of the gas inside. If the pressure is increased or tension decreased, the balloon expands. If the pressure is decreased or tension increased, the balloon contracts. In stars, the role of tension is played by gravity.

Heated air inside a hot air balloon keeps the envelope from collapsing in. Photo credit: Julian Colton, Wikimedia

An interesting fact discussed in this week’s episode is that it takes a photon (light particle) emitted at the center of a star more than a million years to reach the star’s surface and escape into space. Taking the Sun as an example, it has a radius of about 700,000 km, or about twice the distance from the Earth to the Moon. If you turn on a powerful flashlight pointed at the Moon, it would take the light just over 1 second to reach the Moon. How is it possible that it takes light in the Sun more than a million years to travel only twice that distance? We expect it to take only about two seconds.

The reason light takes so long to escape from a star is that the core of a star is opaque. A photon emitted at the center has only a very short distance to travel before it gets scattered by one of the many electrons zipping around in the core. Scattering sends the photon off in some random direction, even back towards the center of the star instead of outward. This process happens millions of times a second. Sometimes the photon moves outwards, sometimes it moves inwards, with the photons going nowhere fast. Since the photons are being created in the center of the star, there are more photons there than in the outer points of the star. This means that, on average, there are more photons scattered outward than inward, and the light slowly diffuses out toward the surface of the star – which is really not a surface at all, at least not in the way we generally think of a surface. Finally, a photon will be scattered outward to a place where the star becomes transparent. At that point the photon is free to travel outward, and it escapes the star. This surface, called the photosphere, is really just a region with very thin gas high in the star’s atmosphere; there is nearly no “there” there, but the values of temperature and density in the gas above the photosphere cause the star to be transparent.

Is that all?

Stars – Lots of information about lives of of stars.

Interior Structure of Stars – Find out more about how a star keeps itself from blowing up and collapsing in on itself.

From the center to the surface – How does energy get from the center of a star to the surface?

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What does it mean?

MeV – Mega Electron Volts. The electron volt (eV) is a measurement of energy; One eV is the energy gained by an electron (or proton) dropping through an electrical potential of one volt. This is a tiny amount of energy, appropriate for describing the energies of atomic and subatomic processes. To give an example, the energy of visible light photons is about 1 eV, as is the typical energy of outer electrons in atoms. Adding the prefix “Mega” means we are talking about one million electron volts (MeV).

Binding energy – the energy it takes to break apart the nucleus of an atom into its constituent parts. Since energy is conserved, this amount of energy is also released when those parts fuse together to form a heavy nucleus. The graph below is called the curve of binding energy. It is a plot of the binding energy of all atomic nuclei (vertical axis) vs. their mass (horizontal axis). Notice that the curve rises steeply to a peak value, then drops slowly as mass increases. The peak of the curve is at Fe56 (Iron with mass 56). Stars produce energy by combining light elements, to the left of the iron peak, into heavier elements. This process is called nuclear fusion. The difference in binding energy between the initial and final nuclei is released as heat. Fusion beyond the iron peak does not release energy. Instead it requires that energy be input. That is why stars are not able to continue stable nuclear fusion of elements heavier than iron. Those elements are produced by different processes, typically accompanying supernova explosions.

Supernova (plural: supernovae) – The explosion of a star at the end of its lifetime, when it runs out of nuclear fuel in its core. Supernovae come in two types. One is called core collapse, and only massive stars can become core collapse supernovae. The other type of supernova is due to an exploding white dwarf. White dwarfs are low-mass stars, but if they acquire mass from a binary companion, they too can explode. Supernova explosions are so bright they can be easily observed in other galaxies.

In human speak please!

Each element is determined by the number of protons in its nucleus. This must match the number of electrons the atoms has, and it is the number of electrons that determines the chemistry, which in turn is how we identify atoms. The neutrons that appear in elements heavier than hydrogen help to offset the repulsive electrical forces between the positive charges of protons, but they do not affect the basic chemistry of the atom. Thus, while an element may have different number of neutrons in its nucleus, it has to have a specific number of protons to have the properties of that element. Given these two bits of information, you can make your own “recipes” for creating different types of elements. For example, below is the recipe for making an isotope of helium.

The number on the top left of the chemical symbol signifies the total number of nucleons (protons and neutrons) in the nucleus of an atom. The number of the bottom left is known as the atomic number, which is also the number of protons in the nucleus. Here is another example in which nitrogen (N) and hydrogen (H) creates two different types of elements, carbon (C) and helium (He):

To create heavier elements requires higher temperatures and pressures than are present when a star is young. Therefore, heavier elements are created later in the evolution of the star, when its center is hotter. This goes on until the star can no longer sustain fusing the heavy elements at its core.

Is that all?

Binding Energy – What does Einstein’s E = mc² have to do with binding energy.

Nuclear binding energy – A more technical discussion of binding energy.

Periodic Table of Elements – An interactive periodic table for students.

Multimedia? Yep, we’ve got it right here!

The Periodic Table of Videos – A series of videos about all the elements in the periodic table.

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What does it mean?

Isotope – Isotopes of a given atom differ in the number of neutrons in their nucleus. Using carbon as an example, the most common isotope has six protons and six neutrons in the nucleus. But there is a different isotope that has six protons (that’s what makes it carbon) and seven neutrons. Both isotopes behave the same in chemical reactions. Other elements can also have different numbers of neutrons, and thus will have different isotopes. The chemical behavior of different isotopes of a given atom is always the same.

Deuterium – is an isotope, or a variation, of the hydrogen atom with a neutron in its nucleus. Most hydrogen nuclei contain only a proton. Deuterium, which makes up just a fraction of naturally occurring hydrogen, has a proton and a neutron.

Positron – is the anti-particle of the electron. It has all the same characteristics as an electron except for its charge, which is positive.

Lepton – One of the 3 most fundamental building blocks of the Universe, along with quarks and bosons. The most common leptons are the electron and neutrino.

In human speak please!

The law of charge conservation states that electrical charge cannot be created or destroyed. This means that for any physical process, the electric charge is conserved, or in other words, the net sum of positive and negative charges before a reaction must be the same as after the reaction.

In the Sun, two protons (left side of the equation above), each with a positive charge, fuse together to make deuterium, an isotope of hydrogen with a proton (positively charged) and a neutron. The neutron has no electrical charge, and so the deuterium nucleus is missing one of the original positive charges that started the reaction. Where did the charge go? The extra positive charge is carried away by a positron, an electron with a positive charge, often represented as an e with a superscript plus sign.

A positron is the antiparticle of the electron, and both belong to the lepton family of elementary particles. Another conservation law states that the lepton number, which is the number of leptons in a reaction, must be conserved. But at the beginning of the reaction there are only two protons, neither of which is a lepton. That means the reaction starts out with a lepton number of zero. However, the complete reaction produces not only the deuterium and positron, but also an electron neutrino (represented by the Greek letter nu with a subscript e), another kind of lepton. Because the positron is an antiparticle it has lepton number -1. The electron neutrino has lepton number +1. When added together the total lepton number of the products is zero, just as it is for the protons that start the reaction.

As you can see, a seemingly simple nuclear reaction can have a lot of things going on. Yet this process is happening in our Sun hundreds of millions of times every second, which is a good thing, as without these reactions there would be no energy produced in the Sun, and no life as we know it on Earth.

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What does it mean?

Neutrino – A fundamental particle that has no charge, almost zero mass and that travels at nearly the speed of light. Neutrinos can pass through most matter without interacting with it.

Sandbox – In computer science, sandbox means to isolate a computer program such that it is unable to affect any other part of the computer, software or hardware.

In human speak please!

Neutrinos are created in stars as lighter elements get fused into heavier ones. They were first predicted in 1930 by physicist Wolfgang Pauli, who was trying to explain why some nuclear reactions appeared to violate certain conservation laws of physics. He hypothesized that there was a neutral particle that was escaping detection and carrying away some of the energy and momentum in the nuclear reactions that scientists were studying.

It wasn’t until 26 years later in 1956 that neutrinos were finally observed by scientists. A few years after that it was discovered that there are three different types of neutrinos. The first neutrinos from the Sun were detected in 1968. The Sun produces a huge number of neutrinos every second. Just to give you an idea of how huge a number, consider this; every second about 100,000,000,000 neutrinos created in the Sun pass through every 6.5 square centimeters (about a square inch) of your body.

Is that all?

The neutrino and its friends – A short description of neutrinos along with brief historical notes.

Hunting Solar Neutrinos – An interview with astrophysicist John Bahcall, who helped determine the production rate of neutrinos from our Sun.

Wolfgang Pauli – A brief biography of Wolfgang Pauli.

Multimedia? Yep, we’ve got it right here!

The Search for Neutrinos – An AstronomyCast podcast about neturinos.

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Dude, where’s my science?

As solar systems form, there is material that is left over that does not go into making stars, planets or moons. This material can end up making comets and asteroid that orbit a central star, approximately in the plane of the solar system. The debris field can obscure observations of objects if we happen to be looking at the solar system edge on. Epo and Alkina will have to visit the solar system if they want to solve the mystery of the artifact for which they were supposedly looking. What will they discover? Keep reading to find out.

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In human speak please!

The students address many of the primary forms of evidence for dark matter and the scientists responsible for discovering that evidence. However, the Universe is a big place and as such there is more to dark matter than just what has been covered in this year’s Special Episodes. For further inquiry, here is one more piece of evidence for dark matter:

Detailed maps of the Cosmic Microwave Background imply that dark matter must exist. If the Universe was composed of only “normal” matter then the various fluctuations of the CMB would be different than what is observed. What is observed indicates that there is dark matter.

Is that all?

Evidence for dark matter – Chandra X-ray Telescope’s website about dark matter and the evidence for it.

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