## Astronomy C

Ttonyxx
Member
Posts: 23
Joined: February 27th, 2019, 5:32 pm
Division: C
State: CA
Pronouns: He/Him/His
Has thanked: 1 time
Been thanked: 2 times
Contact:

### Re: Astronomy C

Here's a relatively short question:
A blazar has a redshift of 0.95 and has a fluctuation in brightness that lasts 140 hours as seen from Earth.
1) How fast is this blazar moving away from up in km/s?
2) If an astronaut was observing this blazar within the blazar's host galaxy, how long would he measure the fluctuation to be?
RasmitDevkota
Member
Posts: 10
Joined: October 23rd, 2020, 6:27 pm
Division: C
State: GA
Pronouns: He/Him/His
Has thanked: 6 times
Been thanked: 3 times
Contact:

### Re: Astronomy C

Ttonyxx wrote: April 16th, 2021, 6:03 pm Here's a relatively short question:
A blazar has a redshift of 0.95 and has a fluctuation in brightness that lasts 140 hours as seen from Earth.
1) How fast is this blazar moving away from up in km/s?
2) If an astronaut was observing this blazar within the blazar's host galaxy, how long would he measure the fluctuation to be?
1. $v = cz = 3 \times 10^5\ \text{km/s} \cdot 0.95 = 2.85 \times 10^5\ \text{km/s}$
2. Applying time dilation, the total elapsed time for a single fluctuation as seen from the blazar's host galaxy would be $\Delta t_{\text{host\ galaxy}} = \gamma \Delta t_{\text{Earth}} = \frac{\Delta t_{\text{Earth}}}{\sqrt{1-\frac{v^2}{c^2}}} = \frac{140\ \text{hours}}{\sqrt{1-\frac{\left(2.85 \times 10^5\ \text{km/s}\right)^2}{\left(3 \times 10^5\ \text{km/s}\right)^2}}} = 448.36\ \text{hours}$

I feel like the second one is supposed to be done differently but oh well.
Ttonyxx
Member
Posts: 23
Joined: February 27th, 2019, 5:32 pm
Division: C
State: CA
Pronouns: He/Him/His
Has thanked: 1 time
Been thanked: 2 times
Contact:

### Re: Astronomy C

RasmitDevkota wrote: April 26th, 2021, 7:54 pm
Ttonyxx wrote: April 16th, 2021, 6:03 pm Here's a relatively short question:
A blazar has a redshift of 0.95 and has a fluctuation in brightness that lasts 140 hours as seen from Earth.
1) How fast is this blazar moving away from up in km/s?
2) If an astronaut was observing this blazar within the blazar's host galaxy, how long would he measure the fluctuation to be?
1. $v = cz = 3 \times 10^5\ \text{km/s} \cdot 0.95 = 2.85 \times 10^5\ \text{km/s}$
2. Applying time dilation, the total elapsed time for a single fluctuation as seen from the blazar's host galaxy would be $\Delta t_{\text{host\ galaxy}} = \gamma \Delta t_{\text{Earth}} = \frac{\Delta t_{\text{Earth}}}{\sqrt{1-\frac{v^2}{c^2}}} = \frac{140\ \text{hours}}{\sqrt{1-\frac{\left(2.85 \times 10^5\ \text{km/s}\right)^2}{\left(3 \times 10^5\ \text{km/s}\right)^2}}} = 448.36\ \text{hours}$

I feel like the second one is supposed to be done differently but oh well.
Great job! You have basically everything right, just a few minor things.

For the first problem:
$\frac{V_r}{c}=\frac{(z+1)^2-1}{(z+1)^2+1}$
$V_r=175,065km/s$

The second problem is correct, you just have to use the correct velocity from problem 1 and you'll get an answer of around
113.7 hours
These users thanked the author Ttonyxx for the post:
RasmitDevkota (May 1st, 2021, 5:11 am)
RasmitDevkota
Member
Posts: 10
Joined: October 23rd, 2020, 6:27 pm
Division: C
State: GA
Pronouns: He/Him/His
Has thanked: 6 times
Been thanked: 3 times
Contact:

### Re: Astronomy C

1) What are the conditions for a Einstein Ring to form? Hint: Think about the relative positions of the different objects involved.
2) When you look out in the sky through your powerful telescope one night, you notice multiple images of a certain galaxy all clustered around a single area. What type of gravitational lensing is most likely to be the cause of these multiple images?
3) Suggest at least one possible object class (e.g. stars, black holes, galaxies) that could be responsible for the lensing effect in the problem above.
4) The form of lensing from the second question has many applications in astronomy. Name and explain a useful application.
Ttonyxx
Member
Posts: 23
Joined: February 27th, 2019, 5:32 pm
Division: C
State: CA
Pronouns: He/Him/His
Has thanked: 1 time
Been thanked: 2 times
Contact:

### Re: Astronomy C

RasmitDevkota wrote: May 1st, 2021, 5:41 am 1) What are the conditions for a Einstein Ring to form? Hint: Think about the relative positions of the different objects involved.
2) When you look out in the sky through your powerful telescope one night, you notice multiple images of a certain galaxy all clustered around a single area. What type of gravitational lensing is most likely to be the cause of these multiple images?
3) Suggest at least one possible object class (e.g. stars, black holes, galaxies) that could be responsible for the lensing effect in the problem above.
4) The form of lensing from the second question has many applications in astronomy. Name and explain a useful application.
1) There has to be a perfect alignment where the object is behind the gravitational lens so that the light is bent around it and it looks like a ring.
2) This is the Strong type of gravitational lensing.
3) Galaxy
4) One useful application is to study the object that is being lensed because essentially it's giving a zoomed-in view of the object that we otherwise would not be able to see.

RasmitDevkota
Member
Posts: 10
Joined: October 23rd, 2020, 6:27 pm
Division: C
State: GA
Pronouns: He/Him/His
Has thanked: 6 times
Been thanked: 3 times
Contact:

### Re: Astronomy C

Ttonyxx wrote: May 6th, 2021, 4:09 pm
RasmitDevkota wrote: May 1st, 2021, 5:41 am 1) What are the conditions for a Einstein Ring to form? Hint: Think about the relative positions of the different objects involved.
2) When you look out in the sky through your powerful telescope one night, you notice multiple images of a certain galaxy all clustered around a single area. What type of gravitational lensing is most likely to be the cause of these multiple images?
3) Suggest at least one possible object class (e.g. stars, black holes, galaxies) that could be responsible for the lensing effect in the problem above.
4) The form of lensing from the second question has many applications in astronomy. Name and explain a useful application.
1) There has to be a perfect alignment where the object is behind the gravitational lens so that the light is bent around it and it looks like a ring.
2) This is the Strong type of gravitational lensing.
3) Galaxy
4) One useful application is to study the object that is being lensed because essentially it's giving a zoomed-in view of the object that we otherwise would not be able to see.

All correct!
Ttonyxx
Member
Posts: 23
Joined: February 27th, 2019, 5:32 pm
Division: C
State: CA
Pronouns: He/Him/His
Has thanked: 1 time
Been thanked: 2 times
Contact:

### Re: Astronomy C

For this problem assume that there is a theoretically small galaxy that has a 1kpc diameter.
1) If a star in the galaxy moves 5 milliarcseconds across the sky in 6 months, how far is the galaxy?
2) According to Hubble's law, how fast is the galaxy receding from us?
3) Is the value above accurate? Why?
4) What is the size of the galaxy in arcseconds?
64015197
Member
Posts: 2
Joined: January 7th, 2020, 9:31 am
Division: C
State: MN
Has thanked: 0
Been thanked: 0

### Re: Astronomy C

1) 1/.005 = 200 pc
2) v = Hd = 0.014 km/s
3) The value is inaccurate because the galaxy and the star's motion are affected by peculiar motion.
4) I got a really big number which probably is not right

Ttonyxx
Member
Posts: 23
Joined: February 27th, 2019, 5:32 pm
Division: C
State: CA
Pronouns: He/Him/His
Has thanked: 1 time
Been thanked: 2 times
Contact:

### Re: Astronomy C

Close! For 1,
since the parallax angle is actually half the amount it moves (diagram below) in 6 months, it would be 1/0.0025 = 400pc

3 is correct
For 4,
the number is supposed to be really big since the galaxy is large and close. Using $D = \frac{d\theta}{206265}$, you get an answer of around 5.16*10^5 arcseconds

### Who is online

Users browsing this forum: No registered users and 4 guests