- #1
JoAstro
- 14
- 0
Homework Statement
Hi,
I am trying to use the angular velocity and angular position to find the age of the Crab Nebula but my calculations seem to be erroneous. I am supposed to get a number around 800 but I keep getting 5000+ years of age. I'm going to try to be very specific in what I'm doing to see if there is something in my procedure that is wrong and I'm not noticing.
1) I am given two images from different years with 27 years of difference - 1973 and 2000. I have two reference stars in each image (A and B) which both are 385 arc seconds apart. I have to calculate the 'plate scale' for these two stars by measuring the distance of these stars in mm. and multiplying that distance by the arc seconds given.
2) Measure the distance in mm. to the pulsar from each image.
3) Measure the distance in mm. of at least 10 knots from the pulsar in the both images (1973 and 2000)
4) Convert the distance of the knots in mm. to arc seconds. This is done by multiplying the knot's distance by the plate scale (for both images too).
5) Calculate difference of arc seconds each knot is moving between the two images to get the change in velocity between the 2 years. This is the proper motion.
6) Divide the value from 'step 5' by 27 years which is the time between the two images. This should give me the arc seconds per year in which each knot is moving which is called the angular velocity or "μ".
7) Now, I need the angular position from the image 2000 from the pulsar (this is what I am struggling with).
8) (extra)... I asked if the angular position was measured in mm. and I was told it should be in arc seconds. But trying to calculate the age with mm first and then with arc seconds, it still gives me a wrong result.
Homework Equations
⋅Plate scale[/B] = 385 arc seconds × Distance in mm.
⋅Arc seconds conversion = knot distance in mm. × plate scale
⋅Change in velocity or Proper motion = knot arc second from 1973 × knot arc second from 2000
⋅Angular velocity = μ=Δx / Δt
⋅Age of the Nebula = angular position to the pulsar in 2000 image / angular velocity
The Attempt at a Solution
(plate scale)
385 arc seconds / 220.3mm = 1.74 "/mm
(distance of one of the knots in mm and conversion to arc seconds)
Knot 1 (from 1973 image)
105.4mm × 1.74 = 183.39 arc seconds
Knot 1 (from 2000 image)
105.9mm × 1.74 = 184.26 arc seconds
(proper motion or the difference of arc seconds of the knot between 1973 and 2000)
183.39 arc seconds - 184.26 arc seconds = 0.87
(angular velocity "μ" for the knot)
0.87 / 27 = 0.032 arc seconds per year
-Here 27 years corresponds to Δt which is the time between the two images.-
(age of Nebula) - The mm. value is the distance to the pulsar.
1. Attempt with angular position in mm:
153.2mm / 0.032 = 4787.5 years
2. Attempt with angular position in arc seconds:
153.2mm × 1.74 = 266.56 arc seconds
266.56 / 0.032 = 8330 years
Both results are wrong but I can't spot where my mistake is. I can't understand in full the last step for the angular procedure but it might also be something else.
I hope my explanation makes sense.
Thank you in advance