Education Activities To Accompany
Chandra Data Analysis Software
Cas-A
Activity 2: What is the distance to Cas-A and when did it
explode?
Since this remnant has been
expanding for over 300 years at incredibly high speed, by now
it is quite large.
Distances to Supernova Remnants are very difficult to determine.
The best estimates come from the fast moving knots of material
that we can actual SEE moving outward through the sky over a
period of years. (Click here to see a fantastic black and white
photo of the Crab
Nebula expanding through space over a ten year period). The
bright spots in Cas-A are examples of some of these knots. If
we know how fast the knots are moving through space, and we know
how far they move in angle across the sky, we can compute the
distance to them....
Suppose a fast moving knot
is observed to be moving (again, via the Doppler effect) at 5000
km/sec. , how far does it travel in 10 years?
L= 5000km/sec x 3 x 108 seconds = 1.5 x 1012
km.
During that time, suppose
the knot is observed to move 3 arc-seconds. What is the distance
to object?
L= theta(arc-seconds) x distance to object / 206,265
so distance= 206,265 x L / theta
or d= 206265 x 1.5 x 1012 km / 3 = about 1 x 1017
km or 3 kpc.
Suppose this knot is now
100 arc-seconds from the center of the remnant. When did the
Supernova explode?
If it has been travelling at a constant velocity, and it moves
3 arc-sec in ten years, it moved 100 arc-sec in about t= 100
arc-sec / 3 arc-sec per 10 years = 330 years ago or around 1670
AD.
(or, 3 arc-seconds per 10 years is 30 arc-seconds per 100 years
or 100 arc-seconds per 330 years). Drawing a simple picture will
help visualize this idea...
