One of the standard candle methodologies

[ehabmozart]

In one of the standard candle methodologies, it is given in my book that by looking at the EM spectrum , we can know the temp. of the star by wien's law... To this point, everything's fine... now, the next sentence is FROM THE WIDTH OF THE SPECTRAL LINES, YOU CAN DETERMINE WHETHER OR NOT IT IS A MAIN SEQUENCE STAR... This part is completely confusing.. I understand nothing and don't know what do they mean by this. What is the width of spectral line. DO they mean the absobtion lines and how would that determine whether it is a main sequence star or NOT? I really need help, so whoever can contribute with a fine answer, i am so thankful to him/her. Thanks!

 

[Drakkith]

The spectral line can be either an absorption or emission line depending of what you are looking for. In this case it is an absorption line. As for how the width tells you if it's a main sequence star or not, I don't know.

 

[Bobbywhy]

“In the current star classification system, the Morgan-Keenan system, the spectrum letter is enhanced by a number from 0 to 9 indicating tenths of the range between two star classes, so that A5 is five tenths between A0 and F0, but A2 is two tenths of the full range from A0 to F0. Lower numbered stars in the same class are hotter. Another dimension that is included in the Morgan-Keenan system is the luminosity class expressed by the Roman numbers I, II, III, IV and V, expressing the width of certain absorption lines in the star's spectrum. It has been shown that this feature is a general measure of the size of the star, and thus of the total luminosity output from the star. Class I are generally called supergiants, class III simply giants and class V either dwarfs or more properly main-sequence stars. For example, our Sun has the spectral type G2V, which might be interpreted as "a 'yellow' two tenths towards 'orange' main-sequence star". The apparently brightest star Sirius has type A1V.”:
http://en.wikipedia.org/wiki/Stellar_classification

Hertzsprung-Russell Diagrams and Structure of Spectral Lines:
http://web.njit.edu/~gary/321/Lecture6.html

Classification of stars using Spectral line widths:
http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml

 

[Chronos]

See http://web.njit.edu/~gary/321/Lecture6.html for discussion.