### Question --- Sketch the HR diagrams for a typical globular cluster and open cluster. Identify the various observed populations and interpret them on the basis of stellar evolution theory. How would you go about constructing lines of constant stellar radius on the diagram? ### Answer --- ##### Sketch the HR diagrams for a typical globular cluster and open cluster. Identify the various observed populations and interpret them on the basis of stellar evolution theory. **Globular Clusters:** ![[HR_globularCluster_merged.png|align:center]] The [[Hertzsprung-Russell Diagram|HR-Diagram]] for a [[Stellar Clusters#Globular Cluster|globular cluster]] is composed of only older, evolved stars, as represented above. This is because they formed long ago when there was still enough gas in the [[Interstellar Medium|ISM]] for large molecular clouds to collapse and create such stellar-dense environments. The younger cluster environments are the [[Stellar Clusters#Open Cluster|open clusters]]. - See few/no hot [[Spectral Classes#O]][[Spectral Classes#B]][[Spectral Classes#A]][[Spectral Classes#F]] stars since they have short lifetimes - See many more [[Hertzsprung-Russell Diagram#Post-Main Sequence]] stars ([[Hertzsprung-Russell Diagram#Red Giant Branch (RGB)|RGB]], [[Hertzsprung-Russell Diagram#Asymptotic Giant Branch (AGB)|AGB]], [[Hertzsprung-Russell Diagram#Horizontal Branch]]) The observed turn-off point of the main sequence can be used to estimate the cluster's age, since the turn off-point migrates down the [[Hertzsprung-Russell Diagram|HR-Diagram]] as a cluster ages. *(see this [video](https://www.youtube.com/watch?v=wbvgjzW3Xz0&ab_channel=Astronomy1101%3AFromPlanetstotheCosmosOnline) for visualization)* If the turning point is at a very high temperature and luminosity, this means the cluster is very young and the stars haven't had the chance to become red giants yet. However, over time the population migrates to red giants and the turn-off point moves down along the [[Hertzsprung-Russell Diagram#Main Sequence (MS)|main sequence]]. > [!cian] > The [[Stellar classes#Blue straggler|blue stragglers]] at the end of the [[Hertzsprung-Russell Diagram#Main Sequence (MS)|main sequence]] and the instability gap due to [[Stellar Classes#RR Lyrae]] stars. > > A star labelled the blue straggler area will only be able to survive on the main sequence for a certain amount of time before turning into a red giant. > - A star just past the turnoff could stay on the main sequence for about 3 billion years. > - A star past the [[Hertzsprung-Russell Diagram#Main Sequence (MS)|main sequence]] can fuse helium for about 50 million years before becoming a red giant. **Open Cluster:** ![[HR_openCluster_merged.png|align:center]] The [[Hertzsprung-Russell Diagram|HR-Diagram]] for an [[Stellar Clusters#Open Cluster|open cluster]] is composed of only younger, un-evolved stars, as represented above. Therefore, it is mostly composted of [[Hertzsprung-Russell Diagram#Main Sequence (MS)|main sequence]] stars since they haven't had enough time to transition to their next step in evolution. ##### How would you go about constructing lines of constant stellar radius on the diagram? ![[Hertzsprung-Russell Diagram#Lines of Constant Radius]]