The purpose of this lab assignment is to introduce you to isochrones and evolutionary tracks, and to help you understand stellar evolution.
| Isochrone diagram for three star clusters | 4 points |
| Evolutionary tracks diagram for three stars | 2 points |
| Answers to 4 follow-up questions | 1 point each |
| Answers to 2 Bonus questions | 1 point each |
Remember, lab points are worth twice as much as quiz points (labs get double-counted to make each lab worth as much as a quiz), so attempting the bonus questions is probably worthwhile.
You may turn in your lab at section or office hours. We will be available during the week of finals (although there will be no sections) if you need help in completing your lab. Of course, if you want to turn your lab in before the due date you are free to do so. No projects will be accepted without a valid excuse after 2 PM on June 8. Turning in your project after June 8 with a valid excuse may result in your not receiving your class grade on time.
Points will not be deducted for people with valid excuses for handing in labs late. Valid excuses include: severe illness, family emergency, catastrophic earthquakes, large comets or asteroids hitting the Earth and killing everybody, etc. We reserve the right to demand proof of any of these (particularly the killer asteroids). If you know in advance that you will not be able to turn in your project on the due date for whatever reason, talk to us and make arrangements.
First, you should have received in class two similar-looking diagrams. One says "Isochrones" at the bottom, and the other says "Evolutionary Tracks." If you are missing one of these, we will have extras (get them in class, or on the table in Kerr Hall where old quizzes and labs are).
Look at the "Isochrones" diagram. The word "isochrone" comes from Greek, and means "same age." An isochrone is a line that shows the positions of stars, all of which are the same age but have different masses, in the H-R diagram (If you don't understand this, come talk to the professor or one of the TA's before or after class, in section, in office hours, or make an appointment). Each isochrone is labelled with its age in millions of years- so the isochrone marked "13" shows the positions of stars that are 13 million years old in the H-R diagram. You should recognize the isochrone marked "0"- it's the Main Sequence that we discussed in class.
Use the age list with the isochrone diagram. This list contains three lists of star positions- Age Lists 1, 2, and 3. Plot each of these lists on the isochrone diagram. For each list, you should get something that looks a lot like one of the isochrones, but is between two of them (you may connect the points in each list if you like). Each of those lists represents a cluster of stars which are the same age, but have different masses. Estimate the age for each cluster- this is what astronomers do to determine the age of a star cluster.
Now look at the "Evolutionary Tracks" diagram. These tracks look a little like the isochrones. However, an isochrone shows stars of the same age but different mass, and an evolutionary track shows stars of different age but the same mass. You can also think of an evolutionary track as the path that a star follows through the H-R diagram as it ages (assuming its mass does not change as it ages). The evolutionary tracks are labelled with the mass, in solar masses, of the star that follows that evolutionary track.
Use the tracks list with the tracks diagram. This list contains data for three stars at different points in their lives. Plot the data points on the evolutionary tracks diagram.
You should give us the following, with your name on each page:
You MUST turn in a hardcopy of everything. The TA's will not look at charts or answers to questions posted on the Web, emailed to us, or in any form except a paper copy.
All parts of your project must be submitted at the same time. If you turn in an incomplete lab, you may not turn the rest of it in later, even if you submit all of the parts before the due date.
Whenever you turn in your lab, make sure we actually get it. Don't slip it under our doors, leave it on our desks, put it in our mailboxes, or give it to our office-mates. This is especially important now, at the end of the quarter when papers tend to accumulate on desks and people come in and leave at irregular hours.
How do you know if your charts looks right? One good hint is that your plots on your isochrone chart should stay on one isochrone or between two isochrones. They should not cross any isochrones. If they do, you're doing something wrong. On your evolutionary tracks chart, all the points should line up on one track. If they don't, you're doing something wrong. Make sure you're not mixing up temperature (horizontal axis) and luminosity (vertical axis).
For question 1, think about the definition of a star cluster.
For question 3, the star that reaches the end of its evolutionary track in the fewest number of years is the one that evolves the fastest.
For question 4, the speed of evolution is determined by how far along the track a star goes in a certain amount of time. For example, if a star moved 1 cm along the track in 1 million years, then in the next million years it moved half a centimeter, it would be slowing down in its evolution. If a star moved 1 cm along the track in 1 million years, then in the next million years it moved 2 cm, it would be speeding up in its evolution. (For this question, it doesn't matter how much the star slows down or speeds up- just tell us whether it's slowing down or speeding up)
Also for question 4: you may want to write the ages of the star at that point beside each point on the evolutionary tracks diagram, because the points are not evenly separated in time.