Variable Stars
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Variable Stars

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Variable Stars Variable Stars Presentation Transcript

  • Variable Stars
  • Binary Systems Mizar and Alcor (in Ursa Major) – optical double SAO 75471 - a real binary
  • Binary Systems Stars orbit each other around a common centre of mass . This is closer to the star of greater mass.
  • Stars resolved by telescope – optical binary Binary Systems Stars too close to be seen - spectroscopic binary A B A A A A B B B B A B No Doppler shift No Doppler shift 1 2 3 4
  • Eclipsing binaries show periodic variability in light. This can plotted as a light curve . E.g. Algol (in Perseus) Eclipsing Binary Systems http://csep10.phys.utk.edu/astr162/lect/binaries/algol.html
  • Orbital period 2.5 days . Fainter star passes in front of brighter star reducing its magnitude from 2.2 to 3.4 over 4 hrs, staying at 3.4 for only 20 mins (primary minimum). A secondary minimum occurs when the fainter star is eclipsed by the brighter star. Eclipsing Binary Systems Algol
  • Orbital period ~ 13 days Stars (components) in Beta ( β) Lyrae are more similar in magnitude so primary and secondary minima are closer in depth. Eclipsing Binary Systems β Lyrae
  • Cepheids: stars with intrinsic variability. The most famous is Delta ( δ ) Cephei. The star varies from magnitude 3.5 to 4.4. Period is 5.3 days . Cepheid Variables δ Cephei
  • Cepheid Variables Pulsating old giant stars (spectral type F and G). The period is related to the luminosity - the longer the period, the greater the luminosity. E.g. A Cepheid with a measured period of 5.3 days will have the same luminosity as δ Cephei. Cepheids are referred to as ‘Standard Candles’ (distance indicators) and are seen in other galaxies so are used to determine distances of galaxies. Period Luminosity Apparent Magnitude Distance
  • Long-Period Variables Long-period variables such as Mira are red giants of type M. Mira had a period of 332 days . There is no direct link between the period and the luminosity as with the Cepheids. Mira
  • Semi-Regular Variables Semi-regular variables pulsate, however the periods and amplitudes vary over time. Betelgeuse: sometimes it is similar in magnitude to Rigel, othertimes it is similar to Aldebaran. Period is approx 5 years .
  • R Coronae Irregular Variables Irregular variables – R Coronae class. R Coronae is a 6 th mag star until it suddenly falls to 15 th mag. The period of variation is not consistent. Carbon accumulates in the stellar atmosphere over time blocking some of the starlight.
  • Short transient light curve Novae Novae increase in brightness for few days/weeks/months. A main sequence star and a white dwarf in a binary system. The white dwarf accretes material from the primary until there is a nuclear explosion - Nova . Some stars such as T Coronae show more than one outburst – these are recurrent novae.
  • Supernovae are explosions of stars more massive than the Sun. Type II SNe are an explosion of a very massive star. Type I SNe occur in binary systems where the primary star outburst engulfs the secondary star (a white dwarf) destroying it. The release of energy can be as much as 1000 million Suns . Supernovae Short transient light curve
  • Supernovae SN 1987A : 170 000 lyr Supernova on 24th Feb 1987. Crab Nebula : 6000 lyr Supernova on 4th July 1054 AD.
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