Transcript: The laws of microscopic physics are invariant with respect to time which is to say that microscopic interactions can move backwards or forwards equally well. The laws of physics really say nothing about the arrow of time, and yet when a movie is run backwards it’s obviously silly and nonsensical. Smashed wine glasses do not leap whole up onto the table. So where does the strong perception of the arrow of time come from? It comes from the second law of thermodynamics and the...

Transcript: Disorder or entropy tends to increase because disordered states are usually the most common outcome in an experiment or any situation. Take the artificial but simple example of four coins, each of which has two outcomes when it’s tossed and is independent of the other. There are two to the power four or sixteen possible states. The most orderly is either four heads or four tails which occurs two out of sixteen times or thirteen percent of the time. The most disorderly or mixed...

Transcript: The third law of thermodynamics states that it’s impossible to remove all the heat from a physical system. At the temperature absolute zero there is no heat, and there is no atomic motion. So there would be no friction, and perfect machines with perpetual motion would be possible. This can never be achieved in the real universe. Imagine taking a cold gas. If you expanded it its temperature would cool further, and its molecular or atomic motions would reduce. But no matter...

Transcript: The second law of thermodynamics states that as energy changes form the amount of heat energy will tend to increase. Another way of saying this is that the entropy or disorder of a system will tend to increase. We are familiar with this systematic change from order to disordered energy with the simple example of an automobile. Gasoline is a place where the ordered energy in chemical bonds in hydrocarbons are converted into ordered mechanical energy in the motion of an engine. ...

Transcript: The first law of thermodynamics states that energy can change forms, but the total amount of energy in a system is always conserved. This is the familiar statement of the conservation of energy familiar for example in gravitational physics with the situation in an orbit where gravitational potential energy is continually trading off between kinetic energy, but the sum or the total is always constant. It seems as if we get starlight for free from nowhere, but in fact starlight...

Transcript: Thermodynamics is the study of heat and the way it flows. In this field of physics a strong connection is made between large scale or macroscopic behavior and small scale or microscopic behavior. On the large scales we can study behavior of a perfect gas and the well known relationships between the temperature, density, and pressure of the gas when it undergoes changes. These laws extend simply to the behavior of stars. On the small scale the idea of temperature is related to...

Transcript: The most spectacular type of mass loss occurs in post-main sequence stars undergoing their planetary nebula phase. The name comes because the pale bubbles of gas looked like planets as seen through early small telescopes, but it’s a misnomer. Planetary nebulae have nothing to do with planets. They are the evolved states of stars where the gas is glowing in sheets or spheres around the central stellar core. The gas glows for two reasons: first the large amount of ultraviolet...

Transcript: In both the early and late phases of stellar evolution a star can lose mass. The mass in the outer envelope can be ejected into interstellar space where it can form material to take part in the creation of a new generation of stars. Mass loss is most spectacular among evolved stars where the behavior of the outer atmosphere becomes decoupled from the behavior of the hot, dense stellar core. For example in blue supergiants the mass loss rate is enormous. Roughly one solar mass...

Transcript: In the late eighteenth century the young English amateur astronomer John Goodricke discovered brightness variations in the star Algol while he was only seventeen years old. Soon afterwards he observed Delta Cephei carefully enough to find regular brightness variations with a timescale of five days and eight hours. The Royal Society awarded him a medal for his work, and he quickly became noted in the scientific community. Goodricke was born deaf and unable to speak in an age...

Transcript: Just like a bell or any mechanical object, stars have a particular frequency or timescale when they tend to vibrate in response to an external disturbance. If the time that it takes for energy to dam up in the atmosphere of a giant star corresponds to the natural frequency the star will oscillate or pulsate. These are regular variables. In stars where the timescale of the damming of radiation in the atmosphere and the oscillation timescale are not synchronized the variations...

Transcript: In 1595 the amateur astronomer and Lutheran pastor David Fabricius noticed the bright star in a constellation Cetus fading until it became invisible and was amazed several months later to see it reappear. The star Mira, called wonderful, has a period of eleven months and is a classic example of a long period variable star. Most stable stars are in hydrostatic equilibrium. They act like a thermostat, but giants have atmospheres that trap some fraction of the energy as it’s...

Transcript: When Shakespeare talked about love that was as constant as the pole star he was taking artistic license. Polaris is in fact a variable star along with many other bright stars in the night sky. Chinese astronomers were the first to note systematic variations in stars visible to the naked eye. Modern astronomers using digital surveys and high quality detectors have cataloged hundreds of thousands of variable stars in at least twenty eight different types. There are stars that...

Transcript: After the main sequence the core and the envelope of a star often follow utterly different evolutionary paths. For example consider the fact that in a red giant the outer envelop becomes cooler than the Sun, or redder, while the core is actually more than ten times hotter. Remember also that when we observe stars in the sky we are only every seeing the photospheric temperature reflecting the color and the energy in the outer diffuse envelope. Often the conditions in the deep...

Transcript: The core of an evolving star like a red giant contracts until the temperature reaches roughly two hundred million Kelvin. At this point a new energy source is available from the fusion of helium nuclei by the triple alpha process. This is a two stage reaction. In the first stage two helium four nuclei combine to form a beryllium 8 nucleus with a photon, and in the second stage a beryllium 8 nucleus combines with a helium 4 nucleus to form a carbon 12 nucleus with a photon...

Transcript: Eventually all main sequence stars must exhaust their hydrogen fuel supply. This is true whether or not they are high mass and live their lives quickly or low mass and live their lives very slowly. The star must then eventually pass through either the red giant or the white dwarf stage. For a star like the Sun and more massive than the Sun it goes to a red giant stage. The Sun will spend roughly a billion years in this phase of evolution. As a star leaves the main sequence...

Transcript: Conceptually we can divide the evolution of stars into three rough stages: the early stages or pre-main sequence stages of evolution, the main sequence itself when hydrogen is converted into helium, and post pain sequence stages which vary depending on the mass of a star. To take the example of the Sun, the Sun has spent roughly thirty million years reaching the main sequence, will spend nine billion years in total on the main sequence, followed by about a billion years as a red...

Transcript: We say that the Sun is a typical star, and that’s not precisely true. The Sun is indeed intermediate in mass range between the lowest mass and highest mass stars, but it’s not typical numerically. The distribution of stellar masses is called the initial mass function. This is the relative numbers of stars of different masses that emerge average over star formation regions throughout the Milky Way. The initial mass function is a power law, and it’s quite steep which means that...

Transcript: The fusion process that dominates stars of less than about one and a half times the mass of the Sun and core temperatures less than fifteen million Kelvin is called the proton-proton chain. There are three steps in this reaction which converts hydrogen into helium. In the first step that lasts about ten million years per proton, two protons combine to form deuterium with the release of a positron and a neutrino. In the second step which occurs relatively quickly the deuterium...

Transcript: Stellar fusion cannot be understood without the quantum theory of matter. In classical physics the electrical repulsion force between two protons as they approach each other is an insurmountable barrier, but in the quantum theory there’s a finite probability that the protons will ignore the barrier and be able to fuse. This problem was worked out in the 1930s by Hans Bethe, a German physicist who immigrated to Cornell University. Bethe calculated the probability that protons...

Transcript: Imagine a set of stars with different masses, all of which are just reaching the main sequence and beginning to consume hydrogen for the first time. This is called the zero age main sequence. In theory it would be a line across the HR diagram, but in practice the properties of such stars form a band because stars of different ages have different chemical compositions which gives them slightly different observed properties.

Transcript: Star formation in molecular clouds occurs more slowly and less efficiently than the simple theory of gravitational collapse would predict. Regions of star formation do not only contain gas and dust however. They are threaded by weak magnetic fields. When the clouds begin to collapse the magnetic field lines are trapped and entrained and so the magnetic field increases as the cloud collapses. The gravity is opposed by magnetic pressure as the field lines are compressed and...

Transcript: When a new star turns on in the center of a disk shaped cocoon of gas and dust material is blown out along the rotation axes at speeds that range from twenty kilometers per second all the way up to two hundred kilometers per second. Bipolar outflows or jets are distinctive features of young stellar objects. When these jets hit the interstellar material or gas they create bright shocks and regions of emission.

Transcript: Star formation results in a young star embedded in a disk of gas and dust. In its early stages every young star has a higher luminosity than its eventual main sequence luminosity. The disk of gas and dust is equally important. This infrared emitting material extends hundreds of astronomical units, and protostellar disks form the missing link between the initial collapsing gas cloud and the formation of planets themselves. Starting in the mid-1980s astronomers used infrared and...

Transcript: The most important pre-main sequence stars are called T Tauri stars, named after the twentieth cataloged variable star in the constellation of Taurus. T Tauri stars are transitions between infrared stars in opaque cocoons or nebulae and stable stars settling for the first time onto the main sequence. The density of T Tauri stars in a rich star forming region like the Orion nebula exceeds the density of all stars in the solar neighborhood. T Tauri stars vary irregular in their...

Transcript: Stars must be hot enough in their cores for fusion to occur, a temperature of about ten million Kelvin or higher. Objects lower than this boundary which corresponds to a mass of eight percent the mass of the Sun are called brown dwarfs. Brown dwarfs have a mass range from a few times the mass of Jupiter up to eighty Jupiter masses at which point an object becomes a star. If you imagine the hypothetical experiment of adding mass to a gas giant planet the following occurs. As...