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syo_astro, right now I don't have any general questions, but then I keep looking at the AAVSO test and then I have more questions

I'm having trouble with 28:

Star G is a M2V star at a distance of 50 parsec. Planet H orbits Star G at a distance of 0.01
AU, and has a radius equal to that of Jupiter.
(a) What is the apparent visual magnitude of Star G?
(b) Assuming that Planet H has 0 albedo, how many times brighter is Star G than Planet
H?


I got a different answer for part a (13.6 vs. their answer of 13.9). I got the absolute magnitude from this chart: http://www.uni.edu/morgans/astro/course ... temps.html and then used distance modulus to find the apparent magnitude. What did I do wrong?

And I'm not sure how to get b.

Yeah, I hand it to them (the nats event supervisors, I checked it over before it went out, and I basically had no corrections), and the test is pretty good as a kind of "base line" for what you should know. There's of course much more to learn than is just on that test. Remember the heavy use of diagrams and understand as many as you can because they really are quite important in astronomy. Also, east made a nice catch...was wondering if they'd ever bring up the webinar. The AAVSO test actually wasn't as hard as they could make it (maybe saving that for the nats test ). One important thing in general with studying is being able to know what you don't know (need work on). Basics are also important. You could save up all the notes you want on exoplanet trivia "firsts", and that may come up, but you should definitely invest some time in understanding really where these different formulas come from and their associated concepts (if you've ever taken a physics class, and it was good, then you'd know to go beyond simply just using formulas). Not saying these are all your problems, but stuff to think about if you're trying to come up with what's wrong with you're studying.

Onto answering questions. They often tend to use some glossary from Introduction to Modern Astrophysics by Carroll and Ostlie as the reference of choice for getting luminosity or apparent magnitude for stars based on luminosity and spectral class. If you don't own a copy, then fret not as your answer is within error! You can submit a FAQ for this, but quite normally 10% error is good enough. In case you're wondering, 0.1 * 13.9 = 1.39, and 13.9 - 1.39 obviously puts you're answer within range . The glossary they use may be online. Another hint as an alternative is that you can get it off an HR diagram if you're in a real pinch, but the tables are of course far more precise than guesstimating.

Part b is interesting. I already said:

"As a thinking exercise take the ratio of the flux in to flux out for the planet and you can try that for deriving the planetary equilibrium temperature (which will REALLY teach you where it comes from). This should also help you to get the answer just from that step alone.

If you want it even "more formulaic", here's another way to think about it. If you use the Stefan-Boltzmann law to find the luminosity of the planet and for the star, make the ratio for the star to the planet, and then plug it in for the planet. To get it to work, you need to plug in the equilibrium temperature (hah, here we have it again) of the planet."

since you (as I remember it) asked this questions before! Did you try that, and you don't understand it either way? If so I'll try to type up a derivation (I was going to, but I kept making it too long...on paper I do it faster, oh well).

syo_astro wrote:Yeah, I hand it to them (the nats event supervisors, I checked it over before it went out, and I basically had no corrections), and the test is pretty good as a kind of "base line" for what you should know. There's of course much more to learn than is just on that test. Remember the heavy use of diagrams and understand as many as you can because they really are quite important in astronomy. Also, east made a nice catch...was wondering if they'd ever bring up the webinar. The AAVSO test actually wasn't as hard as they could make it (maybe saving that for the nats test ). One important thing in general with studying is being able to know what you don't know (need work on). Basics are also important. You could save up all the notes you want on exoplanet trivia "firsts", and that may come up, but you should definitely invest some time in understanding really where these different formulas come from and their associated concepts (if you've ever taken a physics class, and it was good, then you'd know to go beyond simply just using formulas). Not saying these are all your problems, but stuff to think about if you're trying to come up with what's wrong with you're studying.

Onto answering questions. They often tend to use some glossary from Introduction to Modern Astrophysics by Carroll and Ostlie as the reference of choice for getting luminosity or apparent magnitude for stars based on luminosity and spectral class. If you don't own a copy, then fret not as your answer is within error! You can submit a FAQ for this, but quite normally 10% error is good enough. In case you're wondering, 0.1 * 13.9 = 1.39, and 13.9 - 1.39 obviously puts you're answer within range . The glossary they use may be online. Another hint as an alternative is that you can get it off an HR diagram if you're in a real pinch, but the tables are of course far more precise than guesstimating.

Part b is interesting. I already said:

"As a thinking exercise take the ratio of the flux in to flux out for the planet and you can try that for deriving the planetary equilibrium temperature (which will REALLY teach you where it comes from). This should also help you to get the answer just from that step alone.

If you want it even "more formulaic", here's another way to think about it. If you use the Stefan-Boltzmann law to find the luminosity of the planet and for the star, make the ratio for the star to the planet, and then plug it in for the planet. To get it to work, you need to plug in the equilibrium temperature (hah, here we have it again) of the planet."

since you (as I remember it) asked this questions before! Did you try that, and you don't understand it either way? If so I'll try to type up a derivation (I was going to, but I kept making it too long...on paper I do it faster, oh well).

I completely forgot I asked this before. I saw the response you made, but couldn't think about it at the time since I was working on an English essay. Sorry about that!

I do have a copy of Introduction to Modern Astrophysics! I'll have to check it out. Thanks again!

syo_astro wrote:Yeah, I hand it to them (the nats event supervisors, I checked it over before it went out, and I basically had no corrections), and the test is pretty good as a kind of "base line" for what you should know. There's of course much more to learn than is just on that test. Remember the heavy use of diagrams and understand as many as you can because they really are quite important in astronomy. Also, east made a nice catch...was wondering if they'd ever bring up the webinar. The AAVSO test actually wasn't as hard as they could make it (maybe saving that for the nats test ). One important thing in general with studying is being able to know what you don't know (need work on). Basics are also important. You could save up all the notes you want on exoplanet trivia "firsts", and that may come up, but you should definitely invest some time in understanding really where these different formulas come from and their associated concepts (if you've ever taken a physics class, and it was good, then you'd know to go beyond simply just using formulas). Not saying these are all your problems, but stuff to think about if you're trying to come up with what's wrong with you're studying.

Onto answering questions. They often tend to use some glossary from Introduction to Modern Astrophysics by Carroll and Ostlie as the reference of choice for getting luminosity or apparent magnitude for stars based on luminosity and spectral class. If you don't own a copy, then fret not as your answer is within error! You can submit a FAQ for this, but quite normally 10% error is good enough. In case you're wondering, 0.1 * 13.9 = 1.39, and 13.9 - 1.39 obviously puts you're answer within range . The glossary they use may be online. Another hint as an alternative is that you can get it off an HR diagram if you're in a real pinch, but the tables are of course far more precise than guesstimating.

Part b is interesting. I already said:

"As a thinking exercise take the ratio of the flux in to flux out for the planet and you can try that for deriving the planetary equilibrium temperature (which will REALLY teach you where it comes from). This should also help you to get the answer just from that step alone.

If you want it even "more formulaic", here's another way to think about it. If you use the Stefan-Boltzmann law to find the luminosity of the planet and for the star, make the ratio for the star to the planet, and then plug it in for the planet. To get it to work, you need to plug in the equilibrium temperature (hah, here we have it again) of the planet."

since you (as I remember it) asked this questions before! Did you try that, and you don't understand it either way? If so I'll try to type up a derivation (I was going to, but I kept making it too long...on paper I do it faster, oh well).

syo_astro, I attempted to solve it, but I got an answer that was way too big . Would you mind posting a derivation? You can do it on paper if you want.

Thanks so much!

Heh, ok...let's see if I do this wrong.

The easier, "formulaic way":
Stefan-Boltzmann Law gives us that L = sigma*4*pi*R^2*T^4 for our blackbody. Let's define something for the planet: L_p = sigma*4*pi*R_p^2*T_p^4. Now we find:
L_p / L = (sigma*4*pi*R^2*T^4) / (sigma*4*pi*R_p^2*T_p^4) = (R^2*T^4) / (R_p^2*T_p^4), and we now define T_p (the planet equilibrium temperature) = T*(1-a)^(0.25) * (R/(2D))^0.5, with 0 albedo we ignore the a term, so it becomes
T * (R/(2D))^0.5

We plug this equilibrium temperature into that ratio we had beforehand:
(R^2*T^4) / (R_p^2 * ( T * (R/(2D))^0.5 )^4) = (R/R_p)^2 * 1 / (R/(2D))^(4/2) = 4D^2 / R_p^2...which I think should be the answer. Again, remember, around 10% error and you're fine (at least for good graders, and nats has good people...hopefully your state does too).

Now that I've been through that I really ask that you take the time to look up more about the planetary equilibrium temperature to concretely understand where this all comes from. I only did it out this way because probably I'd give not as simple an answer as I could to explain how it works. I do think people on this website are smart enough to at least get a rough idea of how it works . At least observe the logic of this, you are making a ratio of luminosities. If you make a ratio of luminosities, then you are making a difference of absolute magnitudes, which may help show you that (if you think in magnitudes for some reason) we're comparing the brightness that we get for both. In addition, the inverse square law logically would show that the light from the star hitting the planet would be emitted by the planet less brightly than the original star's luminosity because the light is travelling a further distance and hitting a small surface area (thus we have lowered the intensity, which is why this at its core involves flux). If that doesn't work/help...well, I guess I could go the route of setting fluxes equal, but I feel like that might confuse more, so I hope that helped!

I saw this question:

Cepheid Variables occupy which branch of the HR Diagram?

Would it be looking for instability strip? Or something else?

JT016 wrote:I saw this question:

Cepheid Variables occupy which branch of the HR Diagram?

Would it be looking for instability strip? Or something else?

The question is probably looking for instability strip, but I don't think that's a good question for this year. Perhaps you were looking at past year tests, which I highly recommend for certain math problems, early stellar evolution, and even some variability as related to those young stars (nothing except math really for the planets aspect of the event). But main sequence, post-main sequence, etc questions should be given a critical eye for usefulness this year.

syo_astro wrote:

JT016 wrote:I saw this question:

Cepheid Variables occupy which branch of the HR Diagram?

Would it be looking for instability strip? Or something else?

The question is probably looking for instability strip, but I don't think that's a good question for this year. Perhaps you were looking at past year tests, which I highly recommend for certain math problems, early stellar evolution, and even some variability as related to those young stars (nothing except math really for the planets aspect of the event). But main sequence, post-main sequence, etc questions should be given a critical eye for usefulness this year.

That actually happened to be a question from a competition this year. And the answer they were looking for was apparently "horizontal branch."

Well, hopefully at other competitions you won't deal with questions like those! I don't like the question personally because (since I like playing devil's advocate and technicalities) to my knowledge not all cepheids are horizontal branch pulsators (delta scuti and BL Her I THINK are examples?). Besides that, many other cepheids (basically the classical and some other type II) live up around the supergiants. They ALL are in the instability strip, though, which makes asking about the instability strip a more common/logical question to ask. I kept tons of extra random info in my binder/notes, but to be honest usually it wasn't so helpful at the end of the day. If you're worried the person writing that competition may write something else for you, I guess you could shoot him/her/the tourny director an email, but I would say really don't worry otherwise. Good luck studying, keep the questions coming if people have them!

I need help with Question 25 part d...

25. Image B2 shows the light curve of Star C, displaying transits due to Planet D. Star C is a
K1 star with a mass of 0.80 Solar Masses and radius of 0.79 Solar Radii. The orbital period
of Planet D is 2.22 days. Assume that the system has no inclination and Planet D has 0
eccentricity.
(a) Which point (A-E) shows the Primary Eclipse, when Planet D blocks light from Star
C?
(b) Which point (A-E) shows the Secondary Eclipse, when Star C blocks light from Planet
D?
(c) What is the transit depth of the Primary Eclipse, in terms of the % of normal (noneclipse)
system flux?
(d) What is the radius of Planet D, in Jupiter radii?
(e) What is the total duration of the Primary Eclipse, in seconds?

I honestly don't understand how to get the radius of the planet from the transit timing graph... ... any help is appreciated!