Astronomy

Astronomy 162

1. With for example a prism you can break light up into its separate colors, in a so-called spectrum. When astronomers did this to the light of stars, they found that the stars weren't all the same. Some had more blue light, and others had more red light. Also, thin dark lines (spectral lines) could be seen in the spectra and those weren't the same for all stars, but there were groups of stars that had roughly the same spectra. The astronomers started dividing the stars into classes based on their spectra. They began with class A and worked their way down the alphabet. Some of these classes were later combined so eventually only classes A, B, F, G, K, M, N, O, R, and S remained. The Sun is a star of class G. Many years later it was discovered that most spectral classes depend mainly on the surface temperature of the stars. When you put the classes in descending order of surface temperature, then you get O B A F G K M and that is the order in which they're usually mentioned nowadays. Stars of class O are hottest, and those of class F are coolest so that's why they got different spectrum. The classes R, N, and S are rare and don't rely on just the temperature (though they contain only cool stars) and are usually placed in that order after M.

2. The B-type super giant's stars appear extreme or abnormal in terms of its photospheres nitrogen abundance. The C/N and N/O ratios are compared to surface abundances predicted by stellar evolutionary calculations which assume the star has gone through a red super giant phase and convective dredge-up. In particular we find that the N/O abundance is incompatible with the star having a previous red super giant phase, and that the nebula is likely to have been ejected while the star was a blue super giant. But the K type stars got less temperature and very young. Because of the surface temperature K stars look orange and are less mass. The results are compatible with some degree of rotationally induced mixing having occurred while the star was on or near the main-sequence. This is similar to what has recently been found for nebulae surrounding LBVs. In addition our wind analysis suggests the star currently has a relatively normal mass-loss rate in comparison with other Galactic B-type super giants and sits comfortably within the wind momentum-luminosity relationship. In light of the evidence regarding massive evolved early-type stars in the Galaxy we suggest there is no object which shows any evidence of having had a previous red super giant phase and hence of undergoing blue loops in the HR diagram

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