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A star is a big ball of gas, with fusion going on at its center, held together by gravity!. Massive Star. Sun-like Star. Low-mass Star. Stars!. There are variations between stars, but by and large they’re really pretty simple things. MASS!.
A star is a big ball of gas, with fusion going on at its center, held together by gravity!Massive StarSun-like StarLow-mass StarStars!There are variations between stars, but by and large they’re really pretty simple things.MASS!What is the most important thing about a star?The mass of a normal star almost completely determines itsLUMINOSITY and TEMPERATURE!
  • Note: “normal” star means a star that’s fusing Hydrogen into Helium in its center (we say “hydrogen burning”).
  • HOW and WHY is that so?The core supports the weight of the whole star!PRESSURE &TEMPERATURERATE OF FUSIONMASSThe mass of a star determines the pressure in its core:The more mass the star has, the higher the central pressure!The core pressure determines the rate of fusion…luminosity!…which in turn determines the star’sWe call this its Apparent MagnitudeRigel and Betelgeuse, stars in Orion with apparent magnitudes 0.9 and 0.3Usually, what we know is how bright the star looks to us here on Earth…The Magnitude Scale
  • Magnitudes are a way of assigning a number to a star so we know how bright it is
  • The historical magnitude scale…
  • Greeks ordered the stars in the sky from brightest to faintest…
  • …so brighter stars have smaller magnitudes.
  • Later, astronomers accepted and quantified this system.
  • Modern measurements showed it was actually a logarithmic scale
  • Every one magnitude corresponds to a factor of 2.51 change in brightness
  • 5 magnitudes change is 100 change in brightness
  • because (2.51)5 = 2.51 x 2.51 x 2.51 x 2.51 x 2.51 = 10099.626Brighter = Smaller magnitudesFainter = Bigger magnitudes
  • Magnitudes can even be negative for really bright stuff!
  • b – apparent brightnessm – apparent magnitudelog scale, no kidding?The last thing to introduce is the question of massThat question can be translating into question: What are binary stars ????????A large ball of gas that creates and emits its own radiation.startwo balls – not necessarily gas, not necessarily emitting radiationBinary Stars>60% of Stars are in Binary SystemsContains two (or sometimes more) stars which orbit around their common center of mass. Importance - only when a star is in a binary system that we have the possibility of deriving its true mass.The more unequal the masses are, the more it shifts toward the more massive star.The period – watching the system for many years. The distance between the two stars - if we know the distance to the system and their separation in the sky.→ the masses can be derived.The masses of many single stars can then be determined by extrapolations made from the observation of binaries. Visual Binary StarsVisual BinariesThe ideal case:Both stars can be seen directly, and their separation and relative motion can be followed directly.the most common case:Spectroscopic BinariesUsually, binary separation d can not be measured directly because the stars are too close to each other.information from: Doppler Shifts for Binary StarsIdealized binary star system: two stars have equal masses and are in circular orbits and each star has a single spectral line at the same frequency when the stars are at rest. Spectroscopic Binary StarThe approaching star produces blue shifted lines; the receding star produces red shifted lines in the spectrum.have patienceDoppler shift → Measurement of radial velocities→ Estimate of separation d→ Estimate of massesEclipsing Binary StarEclipsing BinariesThere is the rare case when the system is turned so that we see it directly edge-on. This is called an eclipsing binary system. In the case of an eclipsing binary, we see each star pass directly in front of the other one. In these cases, the masses can be directly determined for the stars.Algol known colloquially as the Demon Star, is a bright star in the constellationPerseus. It is one of the best known eclipsing binaries, the first such star to be discovered.Peculiar “double-dip” light curveNASAX-ray BinariesA special class of binary stars is the X-ray binaries, so-called because they emit X-rays. X-ray binaries are made up of a normal star and a collapsed star (a white dwarf, neutron star, or black hole). These pairs of stars produce X-rays if the stars are close enough together that material is pulled off the normal star by the gravity of the dense, collapsed star. The X-rays come from the area around the collapsed star where the material that is falling toward it is heated to very high temperatures (over a million degrees!). An eclipsing binary, with an indication of the variation in intensity. An animation of an eclipsing binary system undergoing mass transfer.What is the defining characteristic of aneclipsing binary system? (That at some pointin its orbit one star eclipses the other along ourline of sight.)How many eclipses occur during a completeorbital cycle? (Two for this system) Add thatmost systems have two eclipses but not allsystems.When does the large dip in the light curveoccur? (When the hot blue star is eclipsed.)
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