Is my answer to this question correct? Are there mistakes in the details? Q: Name three spiral tracers and explain why they have to be short-lived. Ans: All spiral tracers share the characteristics of being young and bright. Three spiral tracers are clouds of ionized hydrogen (emission nebulae), O and B stars, and open clusters. New hot stars are formed in the spiral arms, and those stars heat and ionize clouds of hydrogen gas into emission nebulae. Areas of ionized hydrogen gas in the spiral arms appear bright red (because the predominant emission line of hydrogen here is red). The youngest stars formed in those spiral arms are O and B stars. O and B stars are big, hot and bright blue. They give the spiral arms of a galaxy a bright blue color. Open clusters of stars are relatively young (compared to globular clusters) and are confined to the disk of a galaxy. The youngest open clusters are closest to the spiral arms. Young open clusters contain young (often O and B) stars and are bright. The brightness of ionized hydrogen clouds, O and B stars, and open clusters together help map spiral arms. Emission nebulae are heated and ionized by O and B stars. The youngest bright stars in young open clusters are O and B stars. Thus, those two first spiral tracers are dependent on O and B stars. If O and B stars are short-lived, the role of emission nebulae and young open clusters as spiral tracers are short-lived too. O and B star are indeed short-lived. O stars live for only a few million years, while B stars can live up to only about 25 million years. O stars have masses of around 20-100 solar masses and B stars have masses of around 3-18 solar masses. Their tremendously high masses makes those stars burn all of their nuclear fuels very quickly.
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