Astronomy C

[Jehosaphat]

I just went back through my work and realized I forgot to account for some kiloparsecs and stuff. I fixed it and got your answers, so that's good.

Suppose we receive mysterious transmissions from deep space, and in order to decide if we should look deeper into them, we attempt to locate them. We find their source around a star, and we take a hydrogen spectra and find a line’s wavelength to be 413.3 nm, above it’s typical wavelength of 410.2 nm.

1. What is the redshift of this star?
2. In km/s, how fast is the star moving away from us?
3. How long ago was this light transmitted?
4. What is the absolute magnitude of the star, if the apparent magnitude is 28.5?
5. This star is pretty far out there, but it is nothing compared to the farthest known Type 1a Supernova. What is its name and what is its redshift?
6. To cap off all my redshift talk, what redshifts is the CANDELS survey studying?

Click to Show Answer

1. 
2. 
3. 
4. 
5. SN UDS10Wil, 1.914
6. 8 to 1.5

Looks good to me! Your turn

[Ttonyxx]

A distant star has a temperature of , a radius of , a mass of and has a redshift of
1) How far is this star in kilometers? Assume .
2) If this star has a proper motion of milliarcseconds per year, what is its true space velocity in ?
3) What is the luminosity of this star in solar luminosities? If this was a perfect system in which all its mass was turned into energy, how long could the star shine for?
4) Before the formation of this star, it was a cloud of the same mass but with a 10x larger radius. If we assume this cloud to be a sphere, how long did it take this cloud to collapse (ignoring rotation and magnetic fields)?

[EKT26]

A distant star has a temperature of , a radius of , a mass of and has a redshift of
1) How far is this star in kilometers? Assume .
2) If this star has a proper motion of milliarcseconds per year, what is its true space velocity in ?
3) What is the luminosity of this star in solar luminosities? If this was a perfect system in which all its mass was turned into energy, how long could the star shine for?
4) Before the formation of this star, it was a cloud of the same mass but with a 10x larger radius. If we assume this cloud to be a sphere, how long did it take this cloud to collapse (ignoring rotation and magnetic fields)?

ok so dont clown on me too hard, im kinda rusty but uh

Click to Show Answer

1. 9.57 E20 km (i hope i didnt scuff units like at regs  )
2. 6.2 km/s ? (im v bad with proper motion)
3. 109 L_sun, 4.01 E11 years (last one seems kinda long and that was also assuming that the only thing stopping the star was running out of mass)
4. 1.375 E5 seconds (uhhhh, way too short, I tried calculating acceleration due to gravity of a particle at the farthest distance and then yossing that into one of the kinematic equations/free-fall equation but with the new acceleration, put i have no idea how to account for the pressure of the gas if thats even a concern)

they're pretty scuffed and i havent done astro in a minute so this will be good to brush off the rust 2 days before states lol

[Ttonyxx]

A distant star has a temperature of , a radius of , a mass of and has a redshift of
1) How far is this star in kilometers? Assume .
2) If this star has a proper motion of milliarcseconds per year, what is its true space velocity in ?
3) What is the luminosity of this star in solar luminosities? If this was a perfect system in which all its mass was turned into energy, how long could the star shine for?
4) Before the formation of this star, it was a cloud of the same mass but with a 10x larger radius. If we assume this cloud to be a sphere, how long did it take this cloud to collapse (ignoring rotation and magnetic fields)?

ok so dont clown on me too hard, im kinda rusty but uh

Click to Show Answer

1. 9.57 E20 km (i hope i didnt scuff units like at regs  )
2. 6.2 km/s ? (im v bad with proper motion)
3. 109 L_sun, 4.01 E11 years (last one seems kinda long and that was also assuming that the only thing stopping the star was running out of mass)
4. 1.375 E5 seconds (uhhhh, way too short, I tried calculating acceleration due to gravity of a particle at the farthest distance and then yossing that into one of the kinematic equations/free-fall equation but with the new acceleration, put i have no idea how to account for the pressure of the gas if thats even a concern)

they're pretty scuffed and i havent done astro in a minute so this will be good to brush off the rust 2 days before states lol

1. Good!
2. Not quite, here's the solution. I made the proper motion too large so the answer doesn't make sense whoops (probably should've checked it beforehand my bad).

Click to Show Answer

3. Yup!
4. Close! Again, the numbers I gave were a little weird so that answer doesn't really make sense. There's a useful equation you can use:

Click to Show Answer

[EKT26]

I guess I'll go (even though I got half the questions wrong :/ )

There is a main sequence star with with a temperature of 10,000K.
1. What are its spectral and luminosity classes?
2. What is the peak wavelength it emits at in nm?
3. What is the specific intensity/spectral radiance of the peak wavelength?
4. What the name of the formula that is given after integrating Planck's law over all wavelengths and solid angles?

[RiverWalker88]

I guess I'll go (even though I got half the questions wrong :/ )

There is a main sequence star with with a temperature of 10,000K.
1. What are its spectral and luminosity classes?
2. What is the peak wavelength it emits at in nm?
3. What is the specific intensity/spectral radiance of the peak wavelength?
4. What the name of the formula that is given after integrating Planck's law over all wavelengths and solid angles?

Either this felt like it went pretty well or pretty terribly, mostly depending on whether or not I actually calculated the right thing.

Click to Show Answer

1. B9V
2. Two-hundred ninety nano-meters (Wien's law)
3. Uhh... Well, I tried at least. I used the M = sigma T^4 version of the Stefan-Boltzmann law to get 567000000 W/m^2.
4. The Stefan-Boltzmann Law

[EKT26]

I guess I'll go (even though I got half the questions wrong :/ )

There is a main sequence star with with a temperature of 10,000K.
1. What are its spectral and luminosity classes?
2. What is the peak wavelength it emits at in nm?
3. What is the specific intensity/spectral radiance of the peak wavelength?
4. What the name of the formula that is given after integrating Planck's law over all wavelengths and solid angles?

Either this felt like it went pretty well or pretty terribly, mostly depending on whether or not I actually calculated the right thing.

Click to Show Answer

1. B9V
2. Two-hundred ninety nano-meters (Wien's law)
3. Uhh... Well, I tried at least. I used the M = sigma T^4 version of the Stefan-Boltzmann law to get 567000000 W/m^2.
4. The Stefan-Boltzmann Law

Click to Show Answer

Very nice! 1, 2, and 4 are good. I honestly thought 4 would give people more trouble.

For 3 I was looking for Planck's law with 10,000K and 2.898 E-7 m as the inputs for temperature and wavelength respectively. 
I used  which I got from the BoB.
With the other numbers I got 4.10 E14 W sr^-1 m^-3. 

I know the units are funky and that this is the less common use/meaning of the word intensity, but in case anyone didn't know, it can be interpreted as energy emitted per second per square radian per unit surface area and then the "specific" part gives it that extra per unit wavelength (or per unit frequency). The newer term for it according to Wikipedia is "radiance" since intensity already means W m^-2. My bad for the confusion. I'm impressed nonetheless.

It's your turn!

[RiverWalker88]

I thought that three might go something like that, but I looked at that law just slightly too long and my brain decided it was time to try another roundabout way.

So, I can't seem to muster enough creativity for a good question. I'll just leave it with something simple:
We don't observe the first generation of stars undergoing the CNO cycle. Why might this be?

[Ttonyxx]

I thought that three might go something like that, but I looked at that law just slightly too long and my brain decided it was time to try another roundabout way.

So, I can't seem to muster enough creativity for a good question. I'll just leave it with something simple:
We don't observe the first generation of stars undergoing the CNO cycle. Why might this be?

Click to Show Answer

It's because the first generation of stars didn't undergo the CNO cycle. The composition of the first generation of stars made it so that they couldn't go through the CNO cycle, but there were other processes of fusion that are used, namely the proton-proton chain reaction. 

[RiverWalker88]

I thought that three might go something like that, but I looked at that law just slightly too long and my brain decided it was time to try another roundabout way.

So, I can't seem to muster enough creativity for a good question. I'll just leave it with something simple:
We don't observe the first generation of stars undergoing the CNO cycle. Why might this be?

Click to Show Answer

It's because the first generation of stars didn't undergo the CNO cycle. The composition of the first generation of stars made it so that they couldn't go through the CNO cycle, but there were other processes of fusion that are used, namely the proton-proton chain reaction. 

Yep! To elaborate a bit, Carbon, nitrogen, and oxygen hadn't been nucleosynthesized in a high enough quantity that early on, so it just wasn't a major process. Your answer was fine, I just find that cool.