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, 07 May 2008 00:00:00 GMT Mon, 05 May 2008 20:00:00 GMT http://www.AstrophysicsSpectator.com/topics/stars/Protostars.html Stars A star begins its life as a brilliant protostar, powered only by its self-gravity, and outshining main-sequence stars of similar mass.  It is physically-large and luminous. A one-solar-mass protostar can be 1,000 times as luminous and 70 times as large in radius as the Sun.  A protostar shrinks rapidly in size, causing its luminosity to fall rapidly, but throughout this evolution, its temperature changes very little. A solar-mass star lives in the protostar stage for only about 10 million years, which is a brief prologue to its 9 billion year life on the main-sequence. Wed, 07 May 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/stars/BinaryStarBirth.html Stars Theories of stellar birth must explain why most stars are in orbit with nearby companion stars.  Four theories have received considerable attention: the capture of a star by another star, the breakup of a rapidly-spinning star into two stars, the creation of a companion star from an accretion disk orbiting a star, and the fragmentation of a collapsed molecular cloud into two stars.  The last two are currently the most favored theories. Fri, 18 Apr 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/stars/BinaryStars.html Stars Of the stars in the Galactic disk, only about one-third are without a companion star.  Most stars are in binary star systems, and many are in systems containing three or four stars. Binary star systems are surprisingly small, with most smaller than the Solar System.  Usually the two stars in a system are not dramatically different in mass.  Binary stars are interesting for three reasons: they enable us to weight stars, they give us a clue about how stars form, and they give rise to the compact binary systems, which are the x-ray-bright systems containing a degenerate dwarf, a neutron star, or a black hole. Fri, 04 Apr 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/stars/BinaryStarMasses.html Stars Binary star systems provide us with our only opportunity to measure the masses of stars, the Sun excepted.  Astronomers measure the positions of the stars of a binary on the sky and the Doppler shift of the emission lines in each star's spectrum to derive the characteristics of each star's orbit.  Under Newtonian mechanics, the masses these stars are directly related to the sizes and period of the orbits.  With the latest instrumentation, astronomers are able to derive the stellar masses of angularly-resolved binary stars with errors of less than 5%, and in some cases of less than 1%.  Binary stars that cannot be resolved—the spectroscopic binaries—can have the masses of their individual stars determined to better than 1% if the stars eclipse each-other. Wed, 19 Mar 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/milkyway/MilkyWayLocalDensity.html Milky Way Galaxy The local mass density of the local Galactic disk has been estimated in two ways: from the dynamics of the stars in the disk, and from the number of stars seen near the Sun. In the past, these two measures gave very different results, with the star counts giving a much lower density than the estimate from dynamics.  More recent estimates have closed this gap so that the two estimated are in agreement. Fri, 29 Feb 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/milkyway/MolecularCloudCollapse.html Milky Way Galaxy The gas pressure inside a molecular cloud is too weak to prevent the collapse of the cloud into numerous stars. However, other sources of pressure—magnetic fields and supersonic turbulence within the molecular cloud—are capable of slowing this collapse.  The debate over how these sources of pressure act on a cloud has lead the theoretical community to two very different theories for the collapse of molecular clouds.  Under one theory, static regions in a turbulent cloud collapse rapidly into stars.  Under the second theory, turbulence and magnetic fields slow, but do not stop, the collapse of the whole cloud. Mon, 18 Feb 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/milkyway/MolecularClouds.html Milky Way Galaxy The coldest regions of the Galaxy are the dark nebulae that block our view of the Galactic disk.  These clouds of dust and hydrogen molecules shield their interiors from starlight, which allows their interiors to cool to temperatures only several degrees higher than the 2.7° Kelvin of the microwave background. These clouds can become unstable to gravitational collapse, which leads to the formation of stars. Wed, 30 Jan 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/gravity/GravitationalStability.html Newtonian Gravity The gravitational stability of groups of stars and of clouds of gas is measured by a length scale called the Jeans length.  A system that is larger than the Jeans length will break up into clumps that are a Jeans length in size. These clumps will collapse into gravitationally-stable systems in a timescale called the Jeans time.  All one needs to know to calculate these scales is the average mass density and the average random velocity of the stars or gas in space. Wed, 30 Jan 2008 00:00:02 GMT http://www.AstrophysicsSpectator.com/topics/milkyway/InterstellarMedium.html Milky Way Galaxy The space between the stars is filled with gas, dust, magnetic fields, and cosmic rays.  Together they form within the Galactic disk a tenuous, pressure-balanced atmosphere of surprising variety.  The cool, dense clouds we see in the distance against the stellar background are the most obvious manifestation of this material.  Surrounding these clouds is a warm, tenuous gas that is interspersed with bubbles of hot, thin gas. Wed, 16 Jan 2008 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/milkyway/InterstellarMediumTwoPhase.html Milky Way Galaxy The gas in the interstellar medium is segregated into cool, dense regions surrounded by warm, tenuous regions. Despite the differences in density and temperature, each of these regions is thermally stable and in pressure equilibrium with the other.  The immediate reason these regions exist simultaneously is tied to how different chemicals within the interstellar gas emit radiation, but the deeper reason is that the temperature and density of the interstellar gas depend on the rate at which young, massive stars are born. Wed, 16 Jan 2008 00:00:02 GMT http://www.AstrophysicsSpectator.com/topics/milkyway/OpenClustersDistanceMethods.html Milky Way Galaxy Deriving the distance to the stars has always been a challenging problem.  Stars bound together in a cluster give astronomers more techniques for determining distance than are available for finding the distance to an individual star within the Galactic plane.  In addition to the annual parallaxes of stars within a cluster, the proper motions and spectral red shifts of individual stars and of stars in binary star systems within the cluster provide the means of deriving the distance to an open cluster. Distance can also be estimated by comparing the brightness and color of cluster stars to stellar models. Wed, 19 Dec 2007 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/milkyway/OpenClusters.html Milky Way Galaxy Open clusters are scattered throughout the Galactic plane. They contain hundreds to thousands of stars within a radius of order 10 parsecs.  They are young, living only 1 billion years before they are fully disrupted by encounters with giant molecular clouds. Ten open clusters lie within 300 parsecs of the Sun, with the closest being the Hyades, and the best-known being the Pleiades, both in the constellation Taurus.  These nearby clusters have been studied in detail, providing theorists with the best collections of stars for testing theories of stellar evolution. Wed, 5 Dec 2007 00:00:01 GMT http://www.AstrophysicsSpectator.com/topics/milkyway/SagittariusAStarAdvectiveAccretion.html Milky Way Galaxy The central Galactic black hole should be orbited by an accretion disk.  If it exists, it is unusually dim, unlike the accretion disks seen orbiting other black hole candidates.  Many theorists are pursing a theory that the accretion disk carries energy into the black hole before the energy is radiated away. Other theorists believe the energy is converted into the kinetic energy of a wind.  In either theory, the accretion disk radiates only a tiny fraction of the power that it generates. Wed, 21 Nov 2007 00:00:01 GMT