Overview of Contemporary Astronomy
The Third Evening


Quoting from Alan Lightman's, "A Modern Day Yankee In A 
Connecticut Court and other essays on Science".
Conversations with Papa Joe

The Third Evening
  Read:   Conversations_with_Papa_Joe_III.pdf
  Listen: Conversations_with_Papa_Joe_III.mp3

  Key Words and Phrases:
    Stars held together by Gravity
    Stellar creation
    Hydrogen Fusion
    Solar Neutrinos 
    Helium Fusion
    White Dwarf
    Neutron Stars (Pulsars)
    Black Holes
    Space Telescopes (EM)
    Accretion Disks
    Equations (General Relative and the Quantum Mechanics)

Choose Something Like a Star - Randall Thompson
Choose Something Like a Star - Robert Frost

  O star (the fairest one in sight),
  We grant your loftiness the right
  To some obscurity of cloud-
  It will not do to say of night,
  Since dark is what brings out your

  Some mystery becomes the proud.
  But to be wholly taciturn
  In your reserve is not allowed.
  Say something to us we can learn
  By heart and when alone repeat.
  Say something! And it says "I burn."
  But say with what degree of heat.
  Talk Fahrenheit, talk Centigrade.
  Use language we can comprehend.
  Tell us what elements you blend.
  It gives us strangely little aid,
  But does tell something in the end.

  And steadfast as Keats Eremite,
  Not even stooping from its sphere,
  It asks a little of us here.
  It asks of us a certain height,
  So when at times the mob is swayed
  To carry praise or blame too far,
  We may choose something like a star
  To stay our minds on and be staid.

WHAT ARE STARS? Wikipedia - Star https://en.wikipedia.org/wiki/Star A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable Universe contains an estimated 10^24 stars, but most are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way. For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung-Russell diagram (H-R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.

FORMATION AND EVOLUTION OF STARS Wikipedia - Formation and evolution https://en.wikipedia.org/wiki/Star#Formation_and_evolution Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions-known as molecular clouds-consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula. Most stars form in groups of dozens to hundreds of thousands of stars. Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation. All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density-often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy). When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force. As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core. These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years. Breathtaking Hubble Image Captures a Star That's Still Being Born https://www.sciencealert.com/this-stellar-nursery-snapped-by-hubble-shows-a-star-still-being-born Wikipedia - Sun https://en.wikipedia.org/wiki/Sun The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as light and infrared radiation. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers (864,000 miles), or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron. The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally and not completely accurately referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process. The Sun currently fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result. This energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. When hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand, eventually transforming the Sun into a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable - but not for about five billion years. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, and no longer produce energy by fusion, but still glow and give off heat from its previous fusion.   STELLAR FUSION PROCESES In the past decade neutrino detectors have become much for efficient. Modern detectors are also able to detect not just the energy of a neutrino, but also its flavor. We now know that the solar neutrinos detected from early experiments come not from the common pp-chain neutrinos, but from secondary reactions such as boron decay, which create higher energy neutrinos that are easier to detect. Then in 2014, a team detected low-energy neutrinos directly produced by the pp-chain. Their observations confirmed that 99 percent of the Sun's energy is generated by proton-proton fusion. Proton-proton chain CNO cycle Neutrinos Prove Our Sun Is Undergoing a Second Type of Fusion in Its Core https://www.sciencealert.com/neutrinos-prove-the-sun-is-doing-a-second-kind-of-fusion-in-its-core While the pp-chain dominates fusion in the Sun, our star is large enough that the CNO cycle should occur at a low level. It should be what accounts for that extra 1 percent of the energy produced by the Sun. But because CNO neutrinos are rare, they are difficult to detect. But recently a team successfully observed them. One of the biggest challenges with detecting CNO neutrinos is that their signal tends to be buried within terrestrial neutrino noise. Nuclear fusion doesn't occur naturally on Earth, but low levels of radioactive decay from terrestrial rocks can trigger events in a neutrino detector that are similar to CNO neutrino detections. So the team created a sophisticated analysis process that filters the neutrino signal from false positives. Their study confirms that CNO fusion occurs within our Sun at predicted levels. Triple-alpha Process (Helium-Burning) https://en.wikipedia.org/wiki/Triple-alpha_process Carbon-Burning Process https://en.wikipedia.org/wiki/Carbon-burning_process Neon-Burning Process https://en.wikipedia.org/wiki/Neon-burning_process Oxygen-Burning Process (0.01-5 years) https://en.wikipedia.org/wiki/Oxygen-burning_process Silicon-Burning Process (1 day) https://en.wikipedia.org/wiki/Silicon-burning_process r-process https://en.wikipedia.org/wiki/R-process What Happens If A Star Explodes Near The Earth? https://www.youtube.com/watch?v=evUfG3lrk5U

GRAVITY EVENTUALLY WINS Star are born and stars die... just like us. The big massive stars have but short lives, a few millions of years. Stars like our sun last for a good 10 billions of years, and the little red stars like Barnard's Star might last for 100s of billions of years. How long stars live, is determined by their mass (which must be at least 80 Jupiter masses to sustain thermonuclear fusion of hydrogen). There are four (4) fates for the end of stars depending on their masses and the masses of their cores: Red/Brown Dwarf - less than 0.076 Ms <== Main Sequence 0.076-0.8 Ms Stars less than about 0.6 solar masses, when nuclear fuel is used up, gravitational collapse shrinks the star, but no more than the gas temperature-pressure-volume laws of classical physics allow. We have not found any white dwarf less massive than 0.6 solar masses. Part of the answer is that the universe may not be old enough for lower mass stars to have evolved off the main sequence. White Dwarf - 0.08 and 1.44 Ms <== Main Sequence 0.8-8 Ms Stars with core masses between 0.08 and 1.44 solar masses are destined to become white dwarfs. White dwarfs are degenerate matter. Further collapse is halted by electron degeneracy pressure. The vast majority of stars are in this mass range and are destined to become white dwarfs. Neutron Star - 1.44 and 2.35 Ms <== Main Sequence 8-30 Ms Core masses between 1.44 and 2.35 solar masses overcome electron degeneracy pressure and collapse to form neutron stars, a star that is essentially one gigantic nucleus. Further collapse is halted by neutron degeneracy pressure. Black Hole - ~2.4 or more Ms <== Main Sequence > 30 Ms But for cores with mass of ~2.4 or more solar masses, neutron degeneracy pressure does not stop the collapse and the star becomes a black hole with zero physical size, but with all the mass. Gravity really wins! In each case, gravity eventually wins. But, to what extent is determined by the mass and the relative pressures of the quantum mechanical forces, electron and neutron degeneracy pressure. What Happens If A Star Explodes Near The Earth? https://www.youtube.com/watch?v=evUfG3lrk5U Degenerate matter https://en.wikipedia.org/wiki/Degenerate_matter Degenerate matter is a highly dense state of fermionic matter in which particles must occupy high states of kinetic energy to satisfy the Pauli exclusion principle. The description applies to matter composed of electrons, protons, neutrons or other fermions. The term is mainly used in astrophysics to refer to dense stellar objects where gravitational pressure is so extreme that quantum mechanical effects are significant. This type of matter is naturally found in stars in their final evolutionary states, such as white dwarfs and neutron stars, where thermal pressure alone is not enough to avoid gravitational collapse. Degenerate matter is usually modelled as an ideal Fermi gas, an ensemble of non-interacting fermions. In a quantum mechanical description, particles limited to a finite volume may take only a discrete set of energies, called quantum states. The Pauli exclusion principle prevents identical fermions from occupying the same quantum state. At lowest total energy (when the thermal energy of the particles is negligible), all the lowest energy quantum states are filled. This state is referred to as full degeneracy. This degeneracy pressure remains non-zero even at absolute zero temperature. Adding particles or reducing the volume forces the particles into higher-energy quantum states. In this situation, a compression force is required, and is made manifest as a resisting pressure. The key feature is that this degeneracy pressure does not depend on the temperature but only on the density of the fermions. Degeneracy pressure keeps dense stars in equilibrium, independent of the thermal structure of the star. A degenerate mass whose fermions have velocities close to the speed of light (particle energy larger than its rest mass energy) is called relativistic degenerate matter. The concept of degenerate stars, stellar objects composed of degenerate matter, was originally developed in a joint effort between Arthur Eddington, Ralph Fowler and Arthur Milne. Eddington had suggested that the atoms in Sirius B were almost completely ionized and closely packed. Fowler described white dwarfs as composed of a gas of particles that became degenerate at low temperature. Milne proposed that degenerate matter is found in most of the nuclei of stars, not only in compact stars.

WHERE DO THE ELEMENTS COME FROM? R-process https://en.wikipedia.org/wiki/R-process In nuclear astrophysics, the rapid neutron-capture process, also known as the r-process, is a set of nuclear reactions that is responsible for the creation of approximately half of the atomic nuclei heavier than iron; the "heavy elements", with the other half produced by the p-process and s-process. The r-process usually synthesizes the most neutron-rich stable isotopes of each heavy element. The r-process can typically synthesize the heaviest four isotopes of every heavy element, and the two heaviest isotopes, which are referred to as r-only nuclei, can only be created via the r-process. P-process https://en.wikipedia.org/wiki/P-process The term p-process (p for proton) is used in two ways in the scientific literature concerning the astrophysical origin of the elements (nucleosynthesis). Originally it referred to a proton capture process which is the source of certain, naturally occurring, neutron-deficient isotopes of the elements from selenium to mercury. These nuclides are called p-nuclei and their origin is still not completely understood. Although it was shown that the originally suggested process cannot produce the p-nuclei, later on the term p-process was sometimes used to generally refer to any nucleosynthesis process supposed to be responsible for the p-nuclei. S-process https://en.wikipedia.org/wiki/S-process The slow neutron-capture process, or s-process, is a series of reactions in nuclear astrophysics that occur in stars, particularly AGB stars. The s-process is responsible for the creation (nucleosynthesis) of approximately half the atomic nuclei heavier than iron. In the s-process, a seed nucleus undergoes neutron capture to form an isotope with one higher atomic mass. If the new isotope is stable, a series of increases in mass can occur, but if it is unstable, then beta decay will occur, producing an element of the next higher atomic number. The process is slow (hence the name) in the sense that there is sufficient time for this radioactive decay to occur before another neutron is captured. A series of these reactions produces stable isotopes by moving along the valley of beta-decay stable isobars in the table of nuclides. A range of elements and isotopes can be produced by the s-process, because of the intervention of alpha decay steps along the reaction chain. The relative abundances of elements and isotopes produced depends on the source of the neutrons and how their flux changes over time. Each branch of the s-process reaction chain eventually terminates at a cycle involving lead, bismuth, and polonium.

BOOK RECOMMENDATION Astrophysical Formulae: Radiation, Gas Processes, and High Energy Physics (Volume 1) bu Kenneth R. Lang https://www.amazon.com/Astrophysical-Formulae-Radiation-Processes-Astrophysics/dp/3540296921/ Astrophysical Formulae: Space, Time, Matter, and Cosmology (Volume 2) by Kenneth Lang https://www.amazon.com/Astrophysical-Formulae-Space-Matter-Cosmology/dp/3540646647 Astrophysical Formulae is a comprehensive, widely-used reference to the fundamental formulae employed in astronomy, astrophysics and general physics. All the basic formulae in a particular field are given, with references to both the original work and recent research papers. Where possible the formulae have been developed from basic principles. If you want to know something about a given area, or find the formula that you need or know might exist, the first step is to look for it in Astrophysical Formulae or its references, rather than searching through a library or journals. Over the past two decades, Astrophysical Formulae has become a standard reference found on numerous individual bookshelves and in all libraries that deal with astronomy, astrophysics and physics. This third, enlarged and revised edition will be similarly used by current and future generations of students and scientists in these fields. The new edition of Astrophysical Formulae has been divided into two books - Volume I. Radiation, Gas Processes and High Energy Astrophysics and Volume II. Space, Time, Matter and Cosmology. They together contain over 4000 formulae and 5000 references, more than doubling the number found in previous versions. Past editions have also been improved upon by collecting all the references together in one alphabetical bibliography, instead of listing by chapter, and numerous references have been added for papers published during the past three decades. sam.wormley@gmail.com