## Astronomy C

Unome
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### Re: Astronomy C

carboxylofthebells wrote:Hi guys, I'm not sure if this has been posted yet in the forum, so please forgive me for bringing this up again.

Concerning the practice test posted under 2017 (user rwayzataso), of the math question #71 (Given that Type Ia supernova BrightBurst exploded with an apparent magnitude of +14.8, calculate the distance to the remnants. Give your answer in lightyears.),

For the distance modulus equation, I have ݀ d = 10^(m-M+5)/5, but for the answer it has eliminated that 5.

Solution the user posted:
Using the distance modulus, d = 10
(14.8+19.3)/5 =6.61*10^6 pc
1 pc = 3.26 ly
d = 2.16*10^7 ly

So, is this an error in the solution itself or is it a special case for Type 1a Supernovae to eliminate the "5"?

Thanks~!
It's definitely not a special case relating to a specific type of object, since the distance modulus equation is derived solely from the inverse square law governing luminosity, and the definition of magnitude; these don't change with the type of object. I'm not too good at the math questions in Astronomy, but I got: 7.82 = log (d) in parsecs, and 2.15e8 in light-years.
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raxu
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### Re: Astronomy C

raxu wrote:On a practice test, where a question was
An exoplanet orbits a host star at an orbital radius of 2.01 AU in 3.6 years. What is its orbital velocity in km/s?
I thought of using 2pi*r/T, but it gave an answer 1/2 of the official answer... Am I missing something?
Don't worry - got it It's just a unit problem that I've fixed.
The answer is $16.6\mathrm{km/s}$, by the way, by using $\frac{2\pi r}{T}$ and converting correctly
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y_utsumi
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### Re: Astronomy C

Need help with these questions:

1) The image attached shows a Mira variable star: Chi Cygni. The first cycle shows the apparent magnitude change from ~3.0 to ~ 14.0. How many times brighter is Chi Cygni at the top of the cycle than the bottom of the cycle?

I tried using $L_A/L_B=2.512^(M_B-M_A)$ but got 25118.9. The answer is 17,000 ± 500 times.

2) The image attached shows an absorption spectrum of type Ia supernova SN UDS10Wil, focusing on the bands
created by the absorption of Fe II, Fe III, Si II, and S II. What was this supernova’s apparent magnitude at peak luminosity?

I calculated the redshift to be 1.91, then used that to find recessional velocity to be 572727 km/s, then used that and Hubble's Law to find distance to be 8067 Mpc. I used distance, absolute magnitude of Type Ia SN (-19.3), and the distance modulus and got 25.23 as the apparent magnitude. The answer is 10.35 ± 0.4.

antoine_ego
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### Re: Astronomy C

Any idea when the MIT test is coming out? I looked over our tests, and while we got 10th, we got every single DSO question wrong (literally no points). Our other team did the same. Any reason for this? The rest of the test seemed easier than I expected.
Rest in Peace Len Joeris
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[/b]
Acton-Boxborough Regional High School Captain 17-19

Unome
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### Re: Astronomy C

antoine_ego wrote:Any idea when the MIT test is coming out? I looked over our tests, and while we got 10th, we got every single DSO question wrong (literally no points). Our other team did the same. Any reason for this? The rest of the test seemed easier than I expected.
For us it was the opposite; we got 52 points on the first page (through the one where it asked for a phrase for each description), missing maybe 8 points, and 6 points on the math (3 right out of ~15). It was especially depressing because we finished in 14th, same as last year. Math seems to still be our weak point, and I have no idea what to do about that.
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antoine_ego
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### Re: Astronomy C

Unome wrote:
antoine_ego wrote:Any idea when the MIT test is coming out? I looked over our tests, and while we got 10th, we got every single DSO question wrong (literally no points). Our other team did the same. Any reason for this? The rest of the test seemed easier than I expected.
For us it was the opposite; we got 52 points on the first page (through the one where it asked for a phrase for each description), missing maybe 8 points, and 6 points on the math (3 right out of ~15). It was especially depressing because we finished in 14th, same as last year. Math seems to still be our weak point, and I have no idea what to do about that.

Oh wow, we did the exact opposite. Apparently, we managed to miss every single "pure" DSO question, so we probably messed up the identifications. The general section that followed it was rather easy, so I got 35 I think. The math was much easier than on previous years, so I got 29. I skimmed over the DSO IDs after I'd finished the math, and I though we got them right lol. Looks like I gotta prep the DSOs more.
Rest in Peace Len Joeris
[b]2016 Air Trajectory Nationals - 3rd
2018 Hovercraft Nationals - 6th
2018 Mousetrap Nationals - 6th
2018 Nationals - Team 9th Place!
2019 Astronomy Nationals - 3rd!
2019 Nationals - Team 9th Place!
[/b]
Acton-Boxborough Regional High School Captain 17-19

Unome
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### Re: Astronomy C

antoine_ego wrote:
Unome wrote:
antoine_ego wrote:Any idea when the MIT test is coming out? I looked over our tests, and while we got 10th, we got every single DSO question wrong (literally no points). Our other team did the same. Any reason for this? The rest of the test seemed easier than I expected.
For us it was the opposite; we got 52 points on the first page (through the one where it asked for a phrase for each description), missing maybe 8 points, and 6 points on the math (3 right out of ~15). It was especially depressing because we finished in 14th, same as last year. Math seems to still be our weak point, and I have no idea what to do about that.

Oh wow, we did the exact opposite. Apparently, we managed to miss every single "pure" DSO question, so we probably messed up the identifications. The general section that followed it was rather easy, so I got 35 I think. The math was much easier than on previous years, so I got 29. I skimmed over the DSO IDs after I'd finished the math, and I though we got them right lol. Looks like I gotta prep the DSOs more.
I've been trying #16b and having some problems. So what I've been trying is to take the star's luminosity of 10^4 solar luminosities and radius of 370 solar radii, and divide the luminosity by the cross-sectional area (370^2 * pi for future readers) to get the flux in terms of solar flux. Then I've just been plugging this into Lstar / Lsun = (Tstar / Tsun)^4 but I end up around 800 K lower than the middle of the correct range (which is 3000 K +- 300 K). Any idea what I'm doing wrong? It seems to me like I'm doing it wrong entirely since #16c asks for the flux, which I would have already found in the process of computing the temperature if I'm supposed to be doing it the way I am.
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jonboyage
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### Re: Astronomy C

Unome wrote:
antoine_ego wrote:
Unome wrote: For us it was the opposite; we got 52 points on the first page (through the one where it asked for a phrase for each description), missing maybe 8 points, and 6 points on the math (3 right out of ~15). It was especially depressing because we finished in 14th, same as last year. Math seems to still be our weak point, and I have no idea what to do about that.

Oh wow, we did the exact opposite. Apparently, we managed to miss every single "pure" DSO question, so we probably messed up the identifications. The general section that followed it was rather easy, so I got 35 I think. The math was much easier than on previous years, so I got 29. I skimmed over the DSO IDs after I'd finished the math, and I though we got them right lol. Looks like I gotta prep the DSOs more.
I've been trying #16b and having some problems. So what I've been trying is to take the star's luminosity of 10^4 solar luminosities and radius of 370 solar radii, and divide the luminosity by the cross-sectional area (370^2 * pi for future readers) to get the flux in terms of solar flux. Then I've just been plugging this into Lstar / Lsun = (Tstar / Tsun)^4 but I end up around 800 K lower than the middle of the correct range (which is 3000 K +- 300 K). Any idea what I'm doing wrong? It seems to me like I'm doing it wrong entirely since #16c asks for the flux, which I would have already found in the process of computing the temperature if I'm supposed to be doing it the way I am.
Since you're using solar units, you don't need to use pi. You can just stick to L☉ = (R/R☉)^2 * (T/T☉)^4. I went backwards from the answer you said you got and took out the pi and I got right in the middle of the range of correct answers. If you weren't using solar units, you would have had to include pi and the stefan-boltzman constant, to get the formula L = 4πR^2 * σT^4
I was in a bin

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Unome
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### Re: Astronomy C

jonboyage wrote:
Unome wrote:
antoine_ego wrote:
Oh wow, we did the exact opposite. Apparently, we managed to miss every single "pure" DSO question, so we probably messed up the identifications. The general section that followed it was rather easy, so I got 35 I think. The math was much easier than on previous years, so I got 29. I skimmed over the DSO IDs after I'd finished the math, and I though we got them right lol. Looks like I gotta prep the DSOs more.
I've been trying #16b and having some problems. So what I've been trying is to take the star's luminosity of 10^4 solar luminosities and radius of 370 solar radii, and divide the luminosity by the cross-sectional area (370^2 * pi for future readers) to get the flux in terms of solar flux. Then I've just been plugging this into Lstar / Lsun = (Tstar / Tsun)^4 but I end up around 800 K lower than the middle of the correct range (which is 3000 K +- 300 K). Any idea what I'm doing wrong? It seems to me like I'm doing it wrong entirely since #16c asks for the flux, which I would have already found in the process of computing the temperature if I'm supposed to be doing it the way I am.
Since you're using solar units, you don't need to use pi. You can just stick to L☉ = (R/R☉)^2 * (T/T☉)^4. I went backwards from the answer you said you got and took out the pi and I got right in the middle of the range of correct answers. If you weren't using solar units, you would have had to include pi and the stefan-boltzman constant, to get the formula L = 4πR^2 * σT^4
Thanks. I just talked it over with my partner and figured this out.

at least I'm starting to understand the math now...
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jonboyage
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### Re: Astronomy C

Unome wrote:
jonboyage wrote:
Unome wrote: I've been trying #16b and having some problems. So what I've been trying is to take the star's luminosity of 10^4 solar luminosities and radius of 370 solar radii, and divide the luminosity by the cross-sectional area (370^2 * pi for future readers) to get the flux in terms of solar flux. Then I've just been plugging this into Lstar / Lsun = (Tstar / Tsun)^4 but I end up around 800 K lower than the middle of the correct range (which is 3000 K +- 300 K). Any idea what I'm doing wrong? It seems to me like I'm doing it wrong entirely since #16c asks for the flux, which I would have already found in the process of computing the temperature if I'm supposed to be doing it the way I am.
Since you're using solar units, you don't need to use pi. You can just stick to L☉ = (R/R☉)^2 * (T/T☉)^4. I went backwards from the answer you said you got and took out the pi and I got right in the middle of the range of correct answers. If you weren't using solar units, you would have had to include pi and the stefan-boltzman constant, to get the formula L = 4πR^2 * σT^4
Thanks. I just talked it over with my partner and figured this out.

at least I'm starting to understand the math now...
Nice
I was in a bin

Rustin '19
UPenn '23

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