When a supernova occurs, is the rate of expansion of the remaining ‘star stuff’ constant and dependent on the force of the initial explosion?
In other words, in the supernova pictured above, was the width of the debris equal to only 75 light years 50,000 years ago? And, will the width of the debris equal 300 light years, 100,000 years from now?
It would seem that if there is no gravitational pull (other stars) to slow it down, the debris will expand at a constant rate. No?
Have a good holiday.
Actually, the debris will sweep up gas and dust as it expands through nearby space and the energy that is released by collision and friction (think inertia) causes the "shock wave" effect that makes the above image glow in the light of ionized hydrogen. Thus, the supernova debris is slowed down slightly every time it contacts a gas molecule or dust mote.
The rate of deceleration will depend on how dense the gas and dust are in its path. As the debris passes near to other stars, it will be greatly affected and its appearance greatly altered. And when a bow shock front gathers enough mass, it can begin to affect its own shape as its gravity pulls it in on itself.
Now for the worm in the apple: the supernova left a pulsar behind. It produces a strong "pulsar wind" which it deposits into the surrounding medium, accelerating the debris outward as the pulsar loses angular momentum ("spins down"). So supernova shells can be difficult to "date" unless recent enough to have been recorded in human culture.