http://www.AstrophysicsSpectator.com/ Research, background, news, and commentary on Astronomy and Astrophysics. en-us http://blogs.law.harvard.edu/tech/rss © 2007 The Astrophysics Spectator editor@AstrophysicsSpectator.com http://www.AstrophysicsSpectator.com/images/GizmoIcon32.gif http://www.AstrophysicsSpectator.com/ Wed, 28 Apr 2010 00:00:00 GMT Wed, 28 Apr 2010 13:15:00 GMT http://www.AstrophysicsSpectator.com/topics/supernovae/CoreCollapse1987a.html Supernovae In February of 1987, astronomers saw the closest supernova of modern times; it was in the Large Magellanic Cloud, a neighboring dwarf galaxy.  This supernova, named SN 1987A, is incontrovertible proof that the collapse of the core of a massive star can produce a supernova.  Not only were neutrinos detected from this explosion, as one expects in the birth of a neutron star from the collapse of a stellar core, but also the star that exploded was observed many times before the supernova and found to be massive.  The surprise is that the star was a blue supergiant rather than the expected red supergiant.  Other striking features of this supernova are its unusual chemical composition, its high expansion velocity, its low luminosity, and the unusual shape of its nebula.  Some of these features are tied to the star being a blue supergiant, while others are clues to why the star was in a blue supergiant state when it exploded. Wed, 28 Apr 2010 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/supernovae/CoreCollapseNeutrinos1987a.html Supernovae A core-collapse supernova releases most of its energy as neutrinos.  This theoretical conclusion is confirmed by a single event, the supernova seen in the Large Magellanic Cloud in 1987.  Large neutrino detectors buried deep underground detect cosmic neutrinos by looking for neutrino collisions with electrons.  Three neutrino detectors saw a handful of these collisions by neutrinos traveling from the direction of the Large Magellanic Cloud just before a blue supergiant star in the nearby dwarf galaxy exploded.  The energy carried by these neutrinos is consistent with the energy generated in the core-collapse of a massive star. Wed, 07 Oct 2009 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/supernovae/SupernovaeCoreCollapse.html Supernovae The most energetic supernovae are powered by gravitational potential energy. Once a massive star consumes all of its thermonuclear fuel, it is unable to support itself against its own gravity.  The core of such a star collapses to a neutron star. The birth of a neutron star is heralded by a burst of neutrinos that blows apart the remainder of the star.  We see this expanding debris as a supernova. Thu, 10 Sep 2009 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/supernovae/ThermonuclearBurning.html Supernovae Carbon and oxygen are converted into nickel in a white dwarf through a complex network of reactions. The incremental changes tend to follow the series of atomic nuclei that are multiples in composition of the helium nucleus.  For this reason, large amounts of neon-20, magnesium-24, silicon-28, and other elements with equal and even numbers of protons and neutrons are created.  But the reactions also tear down nuclei, creating many free protons, neutrons, and helium nuclei that combine with other atomic nuclei to produce elements and isotopes that do not have equal numbers of protons and neutrons or do not have an even number of protons or of neutrons.  Because thermonuclear fusion disrupts a white dwarf, the thermonuclear reactions in a white dwarf  contribute to the rich variety of chemical elements and isotopes we find throughout the universe. Wed, 03 Jun 2009 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/supernovae/SupernovaeThermonuclear.html Supernovae Most type Ia supernovae are attributed to the thermonuclear explosion of white dwarfs.  A star becomes a white dwarf before it has completely consumed its thermonuclear fuel.  The amount of thermonuclear energy locked within a white dwarf is of order 0.1% of the white dwarf's rest mass energy. Theorists have three theories that explain what triggers the release of this energy, with each theory relying on the white dwarf being a member of a binary system.  The preferred theory is that the white dwarf grows in mass by pulling gas from its companion onto itself until it becomes gravitationally unstable; when the white dwarf collapses, its internal pressure and temperature rise until thermonuclear reactions cause it to explode. Sat, 02 May 2009 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/supernovae/ThermonuclearEnergetics.html Supernovae The thermonuclear energy locked inside a white dwarf is sufficient to blow the star apart.  In particular, white dwarfs composed of carbon and oxygen, which are more common and contain more thermonuclear energy than those composed of oxygen, neon, and magnesium, can release up to 0.1% of the star's rest mass energy as the carbon and oxygen are converted into an unstable isotope of nickel.  The energy released in the explosion goes into expanding the debris from the white dwarf to velocities approaching 10% of the speed of light.  The power we see radiated from a thermonuclear supernovae comes from the decay of radioactive nickel to iron.  The light we see from a thermonuclear supernovae is about 10% of the energy released in the explosion, or 0.01% of the white dwarf's rest mass energy. Sat, 02 May 2009 00:00:02 GMT http://www.AstrophysicsSpectator.com/topics/supernovae/ Supernovae Theorists divide supernovae into two types: core-collapse supernovae and thermonuclear detonation supernovae.  The first type occurs when a massive star exhausts its thermonuclear fuel.  The second type occurs when a white dwarf experiences a thermonuclear runaway after becoming gravitationally unstable.  These rare but intense explosions can be seen across the universe.  They are responsible for all of the heavy elements in the universe, and are therefore necessary for human life. Wed, 25 Mar 2009 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/degeneracy/NeutronStarNeutrinoCooling.html Degenerate Objects Neutron stars are strong neutrinos emitters.  The power radiated by a neutron star as neutrinos far outstrips the power radiated as x-rays from the photosphere.  Three processes are responsible for generating the neutrinos: the direct Urca process, the modified Urca process, and the neutrino bremsstrahlung process.  The first process is rapid; it  operates at the cores of the most massive neutron stars.  The remaining-two processes, which operate throughout a neutrons star, cool the neutron star more slowly.  The neutrino emission cools a neutron star in only 100,000 years. Fri, 06 Mar 2009 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/degeneracy/DegenerateDwarfNeutrinoCooling.html Degenerate Objects The principal way that a degenerate dwarf cools is through the emission of neutrinos.  Unlike the main-sequence stars, which generate neutrinos as part of their thermonuclear generation of power, degenerate dwarfs generate neutrinos from photons.  This process allows degenerate stars to radiate away all of the energy in their cores, giving them an inverted temperature structure of a cold core surrounded by a hot outer layer. Fri, 13 Feb 2009 00:00:01 GMT http://www.AstrophysicsSpectator.com/commentary/2009/Commentary20090130.html Commentary The American Physical Society is pleased with the bit of pork congress is giving the National Science Foundation and the National Aeronautic and Space Administration in the American Recovery and Reinvestment Act of 2009 that just passed the U.S. House of Representatives.  I explain why I believe their joy is misplaced, and why astronomy and astrophysics may see a long-term decline in funding because of increased government spending. Fri, 30 Jan 2009 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/degeneracy/BlackHoleInevitability.html Degenerate Objects Astrophysicists generally assume that the compact objects at the centers of galaxies are black holes.  Why couldn't these objects be massive neutron stars or some other type of degenerate body?  The reason is that under general relativity and our current understanding of particle physics, no stable degenerate object can exist with more than about 5 solar masses.  Gravity would need to deviate from general relativity for million-solar-mass degenerate objects to exist. Fri, 30 Jan 2009 00:00:02 GMT http://www.AstrophysicsSpectator.com/topics/degeneracy/DegeneracyPressureRadius.html Degenerate Objects The radii of degenerate dwarfs and of neutron stars are fundamentally linked to the fundamental constants of physics.  The neutron star is about the size of a black hole of comparable mass.  The degenerate dwarf, on the other hand, has a radius that is of order 2,000 times larger.  This difference in radius is a direct consequence of the proton being more massive than the electron by this factor.  The mass of the proton sets the absolute scale for these objects.  The radius of the neutron star is of order 15 km, and the radius of the degenerate dwarf is comparable to Earth's. Mon, 19 Jan 2009 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/degeneracy/ Degenerate Objects Degeneracy pressure—the pressure caused by the Pauli exclusion principle of quantum mechanics—is manifested by four types of astronomical object: the giant gaseous planet, the brown dwarf, the degenerate dwarf, and the neutron star. The first-three objects constitute the subclass of degenerate objects that are supported by electron degeneracy pressure. The neutron star is the subclass of degenerate objects supported by neutron and proton degeneracy pressure.  The degenerate dwarf and the neutron star are two of the three endpoints of stellar evolution (the third endpoint is the black hole). Binary star systems containing a degenerate object are the most brilliant systems in the Galaxy. Wed, 19 Nov 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/degeneracy/DegeneracyPressure.html Degenerate Objects Jupiter and Saturn have a fundamental link to the degenerate (white) dwarfs and neutron stars: all of these objects are supported against gravitational collapse by a pressure generated through the Pauli exclusion principle of quantum mechanics.  This pressure is called degeneracy pressure, and it acts through electrons in planets, brown dwarfs, and degenerate dwarfs, and through neutrons and protons in neutron stars.  It's existence is directly linked to existence of chemical elements with distinctive properties.  Wed, 29 Oct 2008 00:00:02 GMT http://www.AstrophysicsSpectator.com/topics/degeneracy/DegeneracyPressureMassLimits.html Degenerate Objects Objects supported by electron degeneracy pressure span a broad range of masses.  The low-mass end of this range, which is near the mass of Saturn, is set by the transition from pressure exerted by atoms to pressure exerted by degenerate electrons.  The high end of this range, which is 1.4 solar masses, is set by the gravitational instability that arises when the degenerate electrons have kinetic energies equal to the electron rest-mass energy. These limits are given by several fundamental constants of physics.  Despite the neutron stars being supported by neutron and proton degeneracy pressure rather than electron degeneracy pressure, they have an upper mass similar to that of the degenerate dwarf. Fri, 29 Oct 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/commentary/2008/Commentary20081001.html Commentary The Chinese government is investing in a rocket engine called the Emdrive that generates thrust with microwaves.  There appears to be a slight problem, however, with this engine: it violates conservation of momentum. Fri, 10 Oct 2008 00:00:02 GMT http://www.AstrophysicsSpectator.com/topics/overview/SizeStarsPlanets.html The Structure of the Universe The stars and planets have radii that are set by the balance of internal pressure against self-gravity. Because internal pressure has several sources, the stars and planets fall into several classes, each characterized by a specific source of pressure.  The consequence is that the objects of each class obey a unique relationship between radius and mass. Fri, 10 Oct 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/commentary/2008/Commentary20080917.html Commentary The Large Hadron Collider at CERN, a machine that accelerates protons to very high energies and then bangs them together, began operating on September 10, 2008. Some believe this machine threatens Earth. They need not worry, because the particle collisions created in this machine occur daily when cosmic rays strike Earth's atmosphere. Man can't yet rival nature's extremes. Wed, 17 Sep 2008 00:00:02 GMT http://www.AstrophysicsSpectator.com/topics/degeneracy/BrownDwarfStructure.html Degenerate Objects The structure of a brown dwarf is set by degeneracy pressure.  Unlike a star, where the mass sets both the radius and the photospheric temperature, a brown dwarf has a radius and temperature that is nearly independent of its mass.  All brown dwarfs are about the same size as Jupiter.  The photospheric temperature of a brown dwarf is set by its age, although the lifetime of a brown dwarf is set by the mass.  Because the low-mass brown dwarfs cool much faster than the high-mass brown dwarfs, infrared surveys preferentially find the more-massive brown dwarfs. Wed, 17 Sep 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/commentary/2008/Commentary20080903.html Commentary Recently some people have claimed that the Sun is entering a new Maunder Minimum—a decades-long period of few sunspots—and that this will cause the Earth's atmosphere to cool.  The Sun is certainly quiet in 2008, but this is the normal quiet of a minimum in the 11 year sunspot cycle.  Clearly the tendency to interpret normal variations as fundamental changes is not confined to the global warming alarmists. Fri, 05 Sep 2008 00:00:02 GMT http://www.AstrophysicsSpectator.com/topics/degeneracy/BrownDwarf.html Degenerate Objects A class of object, long predicted by astrophysicists, sits in the mass range between the giant gaseous planets and the M dwarf stars.  These objects are called brown dwarfs.  They are massive enough to burn deuterium, but they are too light to burn hydrogen.  The first brown dwarf was observed orbiting an M dwarf star in 1988, and since that time, hundreds of additional brown dwarfs have been found. They are cool, so they are primarily emitters of infrared radiation.  In the early stages of their lives, they are powered by deuterium fusion and gravitational potential energy, but when they consume their deuterium, and when the electron degeneracy pressure stops their shrinkage, they grow cold and dark. Fri, 05 Sep 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/observation/StellarTypes.html Observational Astronomy In the nineteenth century, astronomers recognized that stars could be classified by their spectra into a handful of types.  Over time, this system was refined to characterize a star in terms of prototypical stars with similar spectra. This is the meaning of the jargon that the Sun is a G2 V star: the G2 refers to the pattern of lines in the Sun's spectrum, which is directly dependent on temperature, and the V refers to the widths of these lines, which are dependent on luminosity.  The advantage of this system is that astronomers can determine what stars are like the Sun in temperature and luminosity simply by looking at the patterns of lines in the stars' spectra. Fri, 15 Aug 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/observation/MagnitudesAndColors.html Observational Astronomy The magnitude system used by astronomers ranks stars by brightness, with the brightest stars having the lowest values of magnitude.  A star's magnitude is generally measured after the starlight has passed through a colored filter, which gives a measure of a star's color. Literally dozens of filter systems are used by astronomers.  The most common system for measuring color over the infrared, visible, and ultraviolet wavelengths is the Johnson-Cousins UBVRI system. Fri, 01 Aug 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/stars/HertzsprungRussellLocalStars.html Stars The HR diagram of the stars within 10 parsecs is presented on this page.  The diagram reveals that we are surrounded largely by two types of star: dark main-sequence stars and degenerate dwarfs.  Stars like the Sun are the exception rather than the rule, and the more luminous A stars and red giants are rather rare.  The brilliant and massive supergiant O and B stars, of which Rigel in the constellation Orion is an example, are completely absent from the local stellar neighborhood, despite their prominence in the night sky.  Most stars in the Galactic disk are much less luminous than the Sun, and most of the stellar mass of the Galactic disk is in these stars. Wed, 16 Jul 2008 00:00:02 GMT http://www.AstrophysicsSpectator.com/tables/ClosestBrightStars.html Stars Little more than 350 stars are known to be within 10 parsecs of the Sun.  Most of these are too dim to see with the unaided eye. Several, however, are among the brightest stars in the night sky.  The 10 brightest are listed in a table on this page, along with their distances, apparent visual magnitudes, absolute visual magnitudes, color indices, and stellar types. Wed, 16 Jul 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/stars/HertzsprungRussellClusters.html Stars The nearby stars are of all ages, which gives them a broad variety of luminosities and colors.  To see stars of the same age, to see the effects of mass and composition alone on a star's color and luminosity, one must examine star clusters. All of the stars in a star cluster are born at about same time. The open clusters scattered in the Galactic disk provide us with collections of young stars.  The ancient globular clusters that swarm around the Galactic center provide us with collections of old stars.  By creating Herzsprung-Russell diagrams for both types of star cluster—plots of the colors and luminosities of stars—astrophysicists gain insight into how stars, especially stars more massive than the Sun, change over billions of years. Fri, 06 Jun 2008 00:00:01 GMT