(*There is no actual house party.)
In this presentation we will model how stars of different masses evolve, in order to determine the age of a star cluster, using a "house party" model--basically the dumbest story you will ever hear in introductory astronomy, but the one you will remember for the rest of your lives...because it's so true.
First, how stars of different masses evolve differently on an H-R diagram.
Here a massive protostar will move rapidly from right-to-left, increasing in temperature (and decreasing in size) until it is hot enough to become a main-sequence star, fusing hydrogen in its core. After it has depleted all the hydrogen in its core, it begins to die, moving from left-to-right, cooling down (and increasing in size), becoming a supergiant. All of these processes for a massive star take an extremely short amount of time.
In contrast, a medium-mass star will take longer to progress from a protostar, stay on the main-sequence line longer, and take longer to move off the main-sequence line as it dies, becoming a giant.
A low-mass star will take even longer to progress from a protostar to a main-sequence star. Recall that a low-mass "red dwarf" star will have a main-sequence lifetime of 56+ billion years, which is longer than the current age of the universe. Yes, these stars will eventually die, but none have ever died yet, so effectively every low-mass star that has made it to the main-sequence stage is still fusing hydrogen in its core.
Second, the "house party" model.
Let's imagine who will show up first to my house party, at 4:00 PM or so--my mom, and my aunties. "Oh hi! Are we too early? We brought a jello salad, and a casserole--we'll just stick this in the oven to finish up. Do you need any help cleaning up your place? Do you have enough food in the refrigerator? Can we help wash your dog? Vacuum the carpet? Oh, look at the time--I know your friends haven't shown up yet, but we have a long day tomorrow, so good luck with your party!" Come early, stay for a brief time, and leave early--these people are like: massive stars!
Then when would most people show up to my house party? Some time later, at some reasonable hour, and stay for a decent amount of time, and take off not too late in the evening. So these people are most like: medium-mass stars.
And then there are these people. Knocking on my door at 3:00 AM. "Uh, heyyyyy. Is the party still going on? We, uh, maded you some brownies...but then we ated them. Whoa, hey! Is that an Xbox? You have Halo 4? Duuude! Duuude." So they sit on my couch and frag each other until their buzz wears off... And then the next morning at 10:00 AM, they're still there. Come late, still here. These people are like: low-mass stars...
So, what time did you say you were going to show up my house party?
Third, applying the house party model to determining the age of a star cluster.
Here are the labels for the different mass stars on the main-sequence line. Just remember where they are, as we're going to remove these labels, and more importantly, think about what kind of house party guests these labels correspond to.
This is an H-R diagram of an actual star cluster. Who is at the house party? (This is called the turn-off point.) Who are the people to the left of them? Are they coming to, or leaving from the house party? Who are the people to the right of them? Are they coming to, or leaving from the house party? Now, about what time of the evening is this H-R diagram snapshot taken?
Another H-R diagram of another star cluster. Who is at the house party? Who are the people to the left of (and above) them? Are they coming to, or leaving from the house party? Who are the people to the right of them? Are they coming to, or leaving from the house party? Now, about what time of the evening is this H-R diagram snapshot taken?
The idea of the turn-off point works much like browsing Facebook albums of photos taken from my house party. None of the photos have time stamp information, but it is possible to determine approximately when that evening the photos were taken just from seeing who is in the each of the photos--mom and aunties, college students, or stoners? The house party model is a dumb story, but it works. And if you discuss the house party model on an exam essay question in a consistent and conscientious manner...you are probably going to do great. This is how many models in science work.
In the subsequent in-class activity you will be working in groups to compare evolution rates of different-mass stars, and rank relative star cluster ages given their H-R diagrams.