## Astronomy C

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### Astronomy C

Let's get started with some simpler cosmology!

Questions:
1. What epoch did the CMB originate from? What happened that allowed the creation of the CMB?
2. The CMB currently follows a black-body radiation curve that peaks at 2.725 degrees. Determine the wavelength of one photon of the CMB.
3. (Now let's start making up values for the sake of the question). Let's say the CMB originated with a wavelength of 5E-5 m. Determine the redshift.
4. With the redshift find how fast the point that area the CMB originated from is receding from us.
5. Now given that the original area the CMB originated from is 3E11 light-years away, find the Hubble's constant.
(hopefully I didn't screw up writing anything)
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Giantpants
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### Re: Astronomy C

Name wrote:
September 4th, 2019, 7:57 pm
Questions:
1. What epoch did the CMB originate from? What happened that allowed the creation of the CMB?
2. The CMB currently follows a black-body radiation curve that peaks at 2.725 degrees. Determine the wavelength of one photon of the CMB.
3. (Now let's start making up values for the sake of the question). Let's say the CMB originated with a wavelength of 5E-5 m. Determine the redshift.
4. With the redshift find how fast the point that area the CMB originated from is receding from us.
5. Now given that the original area the CMB originated from is 3E11 light-years away, find the Hubble's constant.
(hopefully I didn't screw up writing anything)

Okay so my attempt:
1. Epoch of recombination, is the second part just formation of hydrogen atoms?
2. Using Wiens Law I got 1063 μm
3. Ngl this one confused me a bit. I found a temperature ratio formula which can give redshift, so using Wiens Law with the wavelength of 5E-5 m, I got a temperature of 57.96 K, and doing the ratio of the temperature you gave and the one for the wavelength given, I got a redshift of 21.27.
4. Just used the standard z=v/c formula, which gave me 6.38E6 km/s
5. Again also unsure, because using V=Hd, if 3E11 ly is d, and V is 6.38E6, then H is a really minimal 2.25E-18... which doesn’t seem right bc I prob did something wrong especially since if 3 and 4 are wrong then 5 wont be right either
Anyway yea figured I’d give it a shot and if I’m wrong I can learn why!
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### Re: Astronomy C

Giantpants wrote:
September 5th, 2019, 12:45 pm
Name wrote:
September 4th, 2019, 7:57 pm
Questions:
1. What epoch did the CMB originate from? What happened that allowed the creation of the CMB?
2. The CMB currently follows a black-body radiation curve that peaks at 2.725 degrees. Determine the wavelength of one photon of the CMB.
3. (Now let's start making up values for the sake of the question). Let's say the CMB originated with a wavelength of 5E-5 m. Determine the redshift.
4. With the redshift find how fast the point that area the CMB originated from is receding from us.
5. Now given that the original area the CMB originated from is 3E11 light-years away, find the Hubble's constant.
(hopefully I didn't screw up writing anything)

Okay so my attempt:
1. Epoch of recombination, is the second part just formation of hydrogen atoms?
2. Using Wiens Law I got 1063 μm
3. Ngl this one confused me a bit. I found a temperature ratio formula which can give redshift, so using Wiens Law with the wavelength of 5E-5 m, I got a temperature of 57.96 K, and doing the ratio of the temperature you gave and the one for the wavelength given, I got a redshift of 21.27.
4. Just used the standard z=v/c formula, which gave me 6.38E6 km/s
5. Again also unsure, because using V=Hd, if 3E11 ly is d, and V is 6.38E6, then H is a really minimal 2.25E-18... which doesn’t seem right bc I prob did something wrong especially since if 3 and 4 are wrong then 5 wont be right either
Anyway yea figured I’d give it a shot and if I’m wrong I can learn why!
1. Yup. More specifically after the protons and electrons cooled enough to join together, they could no longer scatter photons, thus turning the universe transparent when it was previously opaque.
2. Yup
3. Yup. Weird way to do it, more commonly the equation is z+1=Observed wavelength/Emitted Wavelength
4. Yup
5. You didn't convert units. Your answer would be kms/s/ly, while Hubble's constant is generally in kms/sec/MPC. Converting 3E11 lys to MPC, you get 91980.42 MPC. Then using V=Hd you get 69.36 as hubbles constant.
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Code: 9/1/2/1/1/1/1
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Giantpants
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### Re: Astronomy C

Cool! The units thing was a weird mistake, oops. Anyway...

1. A galaxy is observed to have an angular diameter of 6.4 degrees from Earth, and it is 1.71E8 light years away. What is the actual diameter of this galaxy?
2. A Cepheid variable is found to have a period of 4.4 days. How much brighter is it than the Sun?
3. Why was 3C 273 important in helping us better understand quasars?
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### Re: Astronomy C

Giantpants wrote:
September 6th, 2019, 2:50 pm
1. A galaxy is observed to have an angular diameter of 6.4 degrees from Earth, and it is 1.71E8 light years away. What is the actual diameter of this galaxy?
2. A Cepheid variable is found to have a period of 4.4 days. How much brighter is it than the Sun?
3. Why was 3C 273 important in helping us better understand quasars?
1. Angular Diameter (arcsec) = diameter (AU)/distance (parsecs), 6.4 degrees in arcsec is 23040. 1.71E8 light years is 5.24E8 parsecs. 23040*5.24E8= 1.2E13. Convert to light years the actual diameter is 5.85E7 light years.
2. Idk what formula you're supposed to use, but I usually use -2.43(log(days)-1)-4.05 = abs magnitude. This equals -3.13359. Convert to solar luminosities, I get 116302.754 solar luminosities.
3. 3C 273 was the first quasar ever identified and have its spectrum measured.
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Code: 9/1/2/1/1/1/1
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Giantpants
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### Re: Astronomy C

Name wrote:
September 6th, 2019, 3:51 pm
1. Angular Diameter (arcsec) = diameter (AU)/distance (parsecs), 6.4 degrees in arcsec is 23040. 1.71E8 light years is 5.24E8 parsecs. 23040*5.24E8= 1.2E13. Convert to light years the actual diameter is 5.85E7 light years.
2. Idk what formula you're supposed to use, but I usually use -2.43(log(days)-1)-4.05 = abs magnitude. This equals -3.13359. Convert to solar luminosities, I get 116302.754 solar luminosities.
3. 3C 273 was the first quasar ever identified and have its spectrum measured.

Bit of discrepancy,
1. I used a different formula than you and got a different answer. I initially used Diameter = (2*pi*distance*angular diameter in degrees)/360, which works so that diameter and distance are in the same unit, and I ended up getting 1.91E7 lightyears. To confirm, I also used Diameter = (anglular diameter in arcseconds*distance)/206265, which gave me the same answer. I still may be overlooking something though?
2. I used a bit of a different variation of Period Luminosity Relationship, but we got roughly the same magnitude (I got -3.17), so that's cool. The discrepancy comes with the solar luminosities. Using the formula Magnitude = 4.83 - 2.5log(L/Lsol), which gives me a luminosity of 6.06E29 W, so like 1584 times brighter than the sun. (your answer gave me 1528 times brighter). Same idea here, Im prob just missing something silly so lmk if you think I did something wrong lol
3. yea this is right ofc, this and it's one of the optically brightest and closest.
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### Re: Astronomy C

Giantpants wrote:
September 7th, 2019, 10:18 am
Name wrote:
September 6th, 2019, 3:51 pm
1. Angular Diameter (arcsec) = diameter (AU)/distance (parsecs), 6.4 degrees in arcsec is 23040. 1.71E8 light years is 5.24E8 parsecs. 23040*5.24E8= 1.2E13. Convert to light years the actual diameter is 5.85E7 light years.
2. Idk what formula you're supposed to use, but I usually use -2.43(log(days)-1)-4.05 = abs magnitude. This equals -3.13359. Convert to solar luminosities, I get 116302.754 solar luminosities.
3. 3C 273 was the first quasar ever identified and have its spectrum measured.

Bit of discrepancy,
1. I used a different formula than you and got a different answer. I initially used Diameter = (2*pi*distance*angular diameter in degrees)/360, which works so that diameter and distance are in the same unit, and I ended up getting 1.91E7 lightyears. To confirm, I also used Diameter = (anglular diameter in arcseconds*distance)/206265, which gave me the same answer. I still may be overlooking something though?
2. I used a bit of a different variation of Period Luminosity Relationship, but we got roughly the same magnitude (I got -3.17), so that's cool. The discrepancy comes with the solar luminosities. Using the formula Magnitude = 4.83 - 2.5log(L/Lsol), which gives me a luminosity of 6.06E29 W, so like 1584 times brighter than the sun. (your answer gave me 1528 times brighter). Same idea here, Im prob just missing something silly so lmk if you think I did something wrong lol
3. yea this is right ofc, this and it's one of the optically brightest and closest.
1. lmao so 1. I screwed up by counting 0s wrong when I converted ly to parsec and 2. I screwed up again when I used the AU to parsec conversion when converting AU to ly, so your answer is correct
2. you're right I realized I plugged it in wrong lol
Maybe I should stick with just doing non-math stuff like before...
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Fossils: 16/13/6/9/2/3/8

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Giantpants
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### Re: Astronomy C

That's alright, happens to the best of us lol. Your turn!
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### Re: Astronomy C

We do not know what dark matter is. An alternative theory of dark matter is that our current understanding of how gravity works is incorrect over large distances.
1. How does gravitational lensing support the dark matter theory over an alternative gravity theory?
2. What happened to the galaxies as the two galaxy cluster collided in the Bullet Cluster? What about gas clouds? The theorized dark matter?
3. How does the Bullet Cluster also support the fact that dark matter exists?
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nirvaan (May 28th, 2020, 7:26 am)
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Code: 9/1/2/1/1/1/1
Fossils: 16/13/6/9/2/3/8

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Steuben42
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### Re: Astronomy C

1. Gravitational lensing results from large amounts of mass, more than we observe directly in galaxies, implying the existence of dark matter; the alternative gravity theory fails to explain lensing.
2. They passed through each other slowed but relatively undisturbed. The gas was separated from the galaxies due to strong interaction. Dark matter, however, stuck with the galaxies, as it's theorized to be weakly interacting.
3. Since without dark matter, gravitational lensing would occur around the greatest concentrations of baryonic matter - the gas - but calculations have shown the matter to be concentrated with the galaxies, not the gas.
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