Sk -69 202

The designation Sk means that Nick Sanduleak catalogued the star. The -69 is the rough declination of the star and the 202 means that it is the 202nd star in the catalogue.

The properties of SK -69 202 are

• Luminosity ~ (3-6) x 10**38 ergs per second ~ 100,000 L(Sun)
• Spectral Class = B3 I supergiant
• T(surface) ~ 16,000 Kelvins
• Mass ~ 20 M(Sun)
• Radius ~ 3 x 10**12 cm

Note that SK -69 202 is a blue supergiant whereas the typical TYPE II supernova is thought to be produced by a red supergiant. The inference that TYPE II supernovae are produced by red supergiants is based on theoretical modeling of the light curves of supernovae and numerical simulations of massive stars.

This issue was resolved rather early on when theorists noted that blue supergiants could in fact produce TYPE II supernova if some additional effects were included in the calculations. It was found that if one considered the facts that:

• stars in the LMC seem to have fewer heavy elements than stars in our Galaxy (lower metallicity) and hence have different opacities in their interiors, and
• massive stars can have intense stellar winds (and hence mass loss) which may allow them to shed much of their hydrogen envelopes,

Evolved stars which were ready to supernova could appear as blue supergiants without having to stretch the physics or the reader's credibility!

This has some other interesting consequences for the overall evolution of the supernova luminosity:

• The upper row denotes SK -69 202 while the lower row denotes a Red SuperGiant (RSG).
• Given the small initial size of SK -69 202, R ~ 3 x 10**12 centimeters compared to 7 x 10**13 centimeters for a typical RSG, the material ejected during the outburst of SN1987A must undergo a large expansion before it can be seen in the optical portion of the spectrum. Hmmmm. Why is this so?
  • Initially, SN1987A radiates strongly in the UV (because it is hot) and it must increase in radius by roughly a factor of 10 and in order to become cool enough to be radiate strongly in the optical.
  • The large increase in its radius ===> a large increase in its volume ===> the gas will cool strongly (adiabatic losses).
  • This effect is more pronounced for SN1987A than for typical Type II SN because of its small initial size. A more typical TYPE II SN starts larger (and hence cooler) and so does not have to expand much to be seen in the optical ===> a typical TYPE II SN does not waste as much of its thermal energy in the expansion.
• This means that by the time SK -69 202 reaches the size of a RSG (where it can radiate strong optical emeission) it will have used up a large part of the energy budgeted for optical radiation.
• This predicts that the evolution of SN1987A will be quicker (as is observed) and SN1987A will be fainter (as is also observed) than a typical Type II Supernova.

  So, although the observation that a blue supergiant exploded was initially puzzling, it turned out that such an observation was easily accomodated by theory and it in fact explained some of the odd behavior of the light curve of SN1987A.