Stellar Properties/Classification --------------------------------- The hottest stars are members of this spectral class. -> O The longest-lived stars are members of this spectral class. -> M (or L). If two stars have the same spectral type but significantly different luminosities, they must differ in this intrinsic property. -> Radius (or size or surface area) The *approximate* temperature IN KELVINS of a star with a peak wavelength half that of the Sun's. -> 11600 K --- By Wien's law, this is just twice the sun's temperature: 2 x 5800K = 11600 K. For stars of this type, the mass of a star can be estimated by taking the fourth-root of its luminosity. -> Main-sequence (or "dwarf") --- The mass-luminosity relation works ONLY with main-sequence stars. The Sun ------- The name of the visible outer layer of the Sun. -> Photosphere Nuclear fusion of this element provides the power source for the Sun. -> Hydrogen This property of the Sun can be determined by measurements of the motion of sunspots. -> Rotation (rate) The magnetic poles of the Sun flip once every this many years. -> Eleven --- The "full" cycle takes 22 years, since it takes another 11 years for the poles to flip back to their original orientation. By mass, the approximate percentage of the Sun's core composed of helium. -> ~50-60% --- The Sun is halfway through its main-sequence lifetime, and so has converted about half of its core hydrogen to helium. (Not exactly half, due to the fact that the Sun had some helium to start with, does not burn at an exactly constant rate, and the core can mix slightly with outer layers.) Black Holes ----------- The warping of space-time around massive objects is a manifestation of this. -> Gravity (or general relativity) This is the "point of no return" for a black hole. -> Event horizon (or Schwarzschild radius) This is the region around a rotating black hole from which some of the black holes' energy can be extracted. -> Ergosphere. The method by which black hole masses are estimated. -> Doppler shift These three properties completely describe all aspects of a black hole. -> Mass, charge, spin (or angular momentum) Neutron Stars and White Dwarfs ------------------------------ This is the final endstate of the vast majority of the stars in the Galaxy. -> White dwarf --- Only the most massive stars (a small fraction of a percent) end up as neutron stars or black holes. When nuclear reactions cease, the remnants of all but the most massive stars are held up by this quantum-mechanical pressure. -> Degeneracy --- White dwarfs are held up by electron degeneracy; neutron stars by neutron degeneracy. Pulsars are composed of a rapidly rotating post-stellar object known as this. -> Neutron star This property of the parent star is the primary determinant of the composition and structure of the stellar remnant it eventually leaves behind. -> (Initial) mass The 'lighthouse' motion of a pulsar's 'beam' is caused by a misalignment between the rotational axis of a neutron star and this. -> Magnetic field (lines, poles, etc.) Stellar Measurements -------------------- A star that is too small for normal hydrogen fusion is referred to by this name. -> Brown dwarf --- Note that brown dwarfs can still fuse deuterium (an isotope of hydrogen) for a short while. This phase of stellar evolution is the longest in a star's lifecycle, with the exception of its end-state. -> Main sequence The distance of a star showing a parallax of 10 arcsec *as measured from Saturn*. Saturn orbits the Sun at a distance of 10 AU. -> 1 parsec --- The relation [p = 1 / d] holds only for the Earth. For longer baselines we see larger parallaxes, by a factor of the increase in baseline. This is the approximate age of a cluster that contains only G, K, and M stars. -> 10 billion years --- If all stars hotter than the Sun (a G star) have evolved off the main sequence, then Sun-like stars in the cluster must be at or near the end of their lifespans (they're "next in line"). Since the sun's lifetime is 10 billion years, that must be the approximate age of the cluster. The distance from Earth in AU of a main-sequence star with a mass equal to that of the Sun, but an apparent brightness about 1 trillion times less than that of the Sun. -> 1 million AU (5 parsecs) --- Since it's a main-sequence star with the same mass as the Sun, it must also have the same approximate luminosity. So if it's a trillion times fainter, by the inverse square law, we know that it's the square root of a trillion (that is, a million) times further away. The spectral and evolutionary classification of a star with T=3500K and L=1500*Lsun. --- The star is significantly cooler than the Sun at its surface but also far, far, more luminous. From the Stefan-Boltzmann law, then, we know that the star has to be much much larger than the Sun. This means that it is a red giant (or possibly a supergiant, but the luminosity is not quite high enough to be classified as a supergiant.) Supernovae ---------- This results from the core collapse and explosion of a massive star. -> Type II Supernova The presence or absence of this element in the spectrum is used to distinguish between Type I and II Supernovae. -> Hydrogen The evolutionary classes of the 2 stars involved in a nova. -> White dwarf, red giant --- In principle a very late-stage main-sequence star or supergiant could take the place of the red giant. Most of the energy of a supernova explosion is released in this form. -> Neutrinos This is the exact mass of the progenitor star of a Type IA Supernova. -> 1.4 solar masses (Chandrasekhar limit) --- A type Ia supernova occurs when a white dwarf accretes matter to the point where it becomes unstable, and detonates in an uncontrolled series of nuclear reactions. The mass limit for a stable white dwarf is 1.4 solar masses, so all Type Ia supernovae come from white dwarfs of this mass. Nuclear Reactions ----------------- This process describes the combination of lighter elements into heavier ones. -> Nuclear fusion This extremely light, noninteracting particle is often generated in nuclear reactions. -> Neutrino A runaway chain of nuclear fusion reactions completely destroys the parent star in this explosive phenomenon. -> Supernova Ia --- Knowing the exact numerical designation is not important, but be sure to understand the physical situation that gives rise to this type of supernova versus the other type (II) Gamma rays from supernovae are generated primarily by this process. -> Radioactive decay This is the most stable element in the periodic table, from which no energy can be gained either by fusion into heavier elements or fission back into lighter ones. -> Iron Asteroids and Comets -------------------- Any difference between asteroids and comets. -> Some possibilities: composition (asteroids rocky or metallic; comets icy); location (asteroids between Mars/Jupiter; comets in outer solar system); orbits (asteroids circular; comets elliptical). When a comet is moving away from the sun, its tail points in this direction. -> Away from the sun. A spherical region in the far outer Solar System harboring huge numbers of comet-like objects. -> Oort cloud --- Kuiper belt is also a source of comets, but it is a disc. The approximate size of the object that probably killed off the dinosaurs. -> 10 km The semimajor axis of the comet responsible for the Leonid meteor shower, which peaks every 33 years. -> (~10 AU) --- Meteor showers are due to a swarm of debris orbiting the Sun. If the swarm passes through Earth's orbit every 33 years, that is the period of the swarm and hence the comet that broke apart to form it. We can then use Kepler's law to calculate its semimajor axis (which is the distance from the narrow end of the ellipse of its orbit to the center of the ellipse.