Astronomy C

[syo_astro]

For part e of that question, it asks us to use the phase diagram for water along with the temperature to determine if the planet is habitable. How can I use the phase diagram for water to help determine the planet's habitability? I'm not exactly sure how I can use the diagram to my advantage.

It is given at the end of this part that the pressure of this planet is the same for that of Earth (which if you don't know you can look up now as good preparation!). From the previous part with the equilibrium temperature it is presumed you would have calculated the temperature for the planet. The temperature in K is on the top of the graph, and the pressure in bars (I believe it is 1 bar, you should check really) is on the right side of the graph. If you match that to a point just as you would read a graph then you should see this falls around the line that divides the portion of the graph that is solid and liquid with a black line. This implies that water on this planet can exist likely either in solid OR liquid phases. The question only requires liquid water, therefore we can answer now. If you look up about phase diagrams you should see what the lines separate and also features such as the triple or critical point, which are all important. Hope that helps, are you still confused?

[cifutielu]

For part e of that question, it asks us to use the phase diagram for water along with the temperature to determine if the planet is habitable. How can I use the phase diagram for water to help determine the planet's habitability? I'm not exactly sure how I can use the diagram to my advantage.

It is given at the end of this part that the pressure of this planet is the same for that of Earth (which if you don't know you can look up now as good preparation!). From the previous part with the equilibrium temperature it is presumed you would have calculated the temperature for the planet. The temperature in K is on the top of the graph, and the pressure in bars (I believe it is 1 bar, you should check really) is on the right side of the graph. If you match that to a point just as you would read a graph then you should see this falls around the line that divides the portion of the graph that is solid and liquid with a black line. This implies that water on this planet can exist likely either in solid OR liquid phases. The question only requires liquid water, therefore we can answer now. If you look up about phase diagrams you should see what the lines separate and also features such as the triple or critical point, which are all important. Hope that helps, are you still confused?

I'm not confused any more, thanks!

However, I'm not exactly sure how to get Question 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 know for part a, it's just distance modulus. But where do you get the absolute magnitude from? The HR diagram?

I have no idea how to get part 2

[syo_astro]

An HR diagram wouldn't make much sense. It's relative brightness, and you should understand HR diagrams work for stars or stellar objects (so from the tip top hypergiants to the low low mass brown dwarfs). Even then that would be for ABSOLUTE brightness, NOT some kind of relative flux or something of the sort.

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.

[cifutielu]

How can I solve 24 on the practice test?

Image B1 shows the radial velocity curve of host Star A, around which Planet B orbits. Star A has the same mass, radius, and luminosity as the sun. Assume that the system has no inclination and Planet B has 0 eccentricity (a circular orbit).

(a) What is the distance from Star A to Planet B, in AU, assuming Planet B has a mass much less than that of Star A?

(b) What is the velocity of Planet B in its orbit around Star A, in km/s?

(c) What is the mass of Planet B, in Jupiter masses?

(d) Planet B has a radius of 0.8 Jupiter radii. What is the density of Planet B, in g/cm3?

[syo_astro]

I believe I recall enough on radial velocity stuff to figure this out. Let's see (mind you I'm doing this off memory for what the graphs looked like).
a) You're given the velocity graph showing basically the star moving away and toward you, which you can derive period from. You are also given that the star has the same mass as the Sun. Noting that, you can use Kepler's third law to derive the semi-major axis (also known as distance between the star and planet).
b) The orbit is a circle, and let's rather call this speed. Speed is distance over time. Distance around a circle is circumference, or 2*pi*radius. You have radius and period from part a, so use those to find circular velocity for this part.
c) For this you need to use the binary star/system mass ratio (or at least I think that's the easiest way). You should derive it for practice, it also helps you understand Kepler's third law! To summarize, it states that m_a*v_a = m_b*v_b. Neatly, this also works for radius, but we use velocity here because you can get it from the graph (and it's more trouble than it's worth trying to find barycentric distances when you can just do something simpler given your data).
d) This is simply density. For part c and this one look up what a Jupiter mass and radius is. Then dimensional analysis away at it.

Sorry this was brief, but did that help? If you have problems on a specific part please ask. Be very careful on converting units (since that's heavily tested on this question). Lastly, look some of this up maybe to help.

[sciolymom]

In the Astronomy wiki, there is a very nice formula sheet at the bottom under resources. A question though:

Under "Hubble's Law" it lists the value of H as H = 20km/s/Mpc. Further down under "Age of the Universe" it lists the value of H as 70km/s/Mpc, which is the number we are familiar with. Is that just a typo?

[syo_astro]

In the Astronomy wiki, there is a very nice formula sheet at the bottom under resources. A question though:

Under "Hubble's Law" it lists the value of H as H = 20km/s/Mpc. Further down under "Age of the Universe" it lists the value of H as 70km/s/Mpc, which is the number we are familiar with. Is that just a typo?

Ah, that formula sheet isn't perfectly up to date really. Don't worry too much about Hubble's Law/constant as it doesn't relate to this year's topic (star/planet formation, etc). On the wiki itself, it just lists it as "70" (no units, gasp?!). Also, while that sheet may give some formulas, I guarantee the topic this year is new and will force you to look up some math that hasn't been looked up before. This is exactly exemplified by the posts explaining that AAVSO test. Minus some classic stuff about blackbodies, spectroscopic parallax, trigonometric parallax, etc (all of which are still extremely important), quite a bit is new for the event here.

This all said, if you are really curious you can look up about the Planck satellite. I never bothered to look it up, but I'm pretty sure they updated the Hubble constant value from previous observations using it. I believe recently it's stayed pretty much around 70 (but has deviated slight bits here and there).

[cifutielu]

In the Astronomy wiki, there is a very nice formula sheet at the bottom under resources. A question though:

Under "Hubble's Law" it lists the value of H as H = 20km/s/Mpc. Further down under "Age of the Universe" it lists the value of H as 70km/s/Mpc, which is the number we are familiar with. Is that just a typo?

Ah, that formula sheet isn't perfectly up to date really. Don't worry too much about Hubble's Law/constant as it doesn't relate to this year's topic (star/planet formation, etc). On the wiki itself, it just lists it as "70" (no units, gasp?!). Also, while that sheet may give some formulas, I guarantee the topic this year is new and will force you to look up some math that hasn't been looked up before. This is exactly exemplified by the posts explaining that AAVSO test. Minus some classic stuff about blackbodies, spectroscopic parallax, trigonometric parallax, etc (all of which are still extremely important), quite a bit is new for the event here.

This all said, if you are really curious you can look up about the Planck satellite. I never bothered to look it up, but I'm pretty sure they updated the Hubble constant value from previous observations using it. I believe recently it's stayed pretty much around 70 (but has deviated slight bits here and there).

Hey syo_astro, are there any big resources that you recommend for the astro test?

[syo_astro]

Supposedly my tests are impossible, so good you didn't specify *my* astro test . I'm trying to rack up ideas for questions myself. Even then, I don't know of any really all-encompassing sources right now for the planets/brown dwarfs aspects. Star formation I think there's plenty (though, even there some parts can get super complicated). I'd like to ask first do you have trouble with anything in particular?

[EastStroudsburg13]

The Chandra National Science Olympiad series has been uploaded to YouTube! They're a series of videos designed to give a brief overview of the event. The first one can be found here.