APOD of June 15, 2005, Cassiopeia A

[jnichols]

What a great picture! I think it raises three questions though:

If the supernova occurred 325 years ago, why doesn't the size of Cassiopeia A as measured by the glow caused by the main shock span 650 light years instead of 125?

What phenomena are represented by which false colors?

Could the main point of the picture and writeup be better illustrated by A/B toggling between the photos taken 1 yr apart?

Can anyone shed any, uh, light on these puzzlements?

Jim N

[Recycled Electrons]

If the supernova occurred 325 years ago, why doesn't the size of Cassiopeia A as measured by the glow caused by the main shock span 650 light years instead of 125?

Even supernova ejecta do not move at the speed of light, hence there would be no reason to think the remnant should span anything near 650lyr.

The point made in the description of the picture was that it appears to be expanding faster than it actually is.

What phenomena are represented by which false colors?

As the description says, it's a composite of x-ray, optical (which most likely means all of HST's optical bands), and IR. Which wavelength corresponds to which color in the image is not documented anywhere I could find, but that shouldn't be surprising. Very little information is learned from looking at composite, false-color images. Their main purpose is to impress the public and make for pretty posters on the office walls.

Could the main point of the picture and writeup be better illustrated by A/B toggling between the photos taken 1 yr apart?

You could see the apparent expansion, but you wouldn't want people to not read the description carefully enough to realize that the rapid expansion is due to ambient dust becoming visible, and that this dust is not part of the SN remnant.

I hope this bounced some photons off your 'puzzlements.'

[jnichols]

But doesn't the glow occur when the ambient dust, i.e. dust that is already there, is heated by some sort of energy itself presumably traveling at the speed of light? This energy started 262.5 (325 - 125/2) yrs after the supernova? Wonder what caused that.

Even supernova ejecta do not move at the speed of light, hence there would be no reason to think the remnant should span anything near 650lyr.

The point made in the description of the picture was that it appears to be expanding faster than it actually is.

[pamcse]

Their main purpose is to impress the public and make for pretty posters on the office walls.

If that is the case, it worked!

I, too, had questions about the explanation... Granted, the ejecta does not move at light speed -- but the SN light eruption does. So maybe the point of the explanation is that the glow makes it appear that the ejecta has reached out this far!! The glow induced by the light-heated dust could be misconstrued as ejecta -- much further dispersed than one would expect.

If this is the case, one would expect the 'glow shell' to span about 650 LYs... at least I would.

Cheers... I have to go hang my newest wall poster: Cassiopeia A.

[Recycled Electrons]

I think what you both might be forgetting is that light echoes of this nature are one of those precious few three-dimensional phenomena that we can observe from Earth.

The dust beginning to glow that gives the appearance of rapid expansion is distributed all around the remnant in every direction. So, clouds of dust behind the star may only now be reflecting light from the initial SN explosion. Or dust to either side of the star in the plane of the sky has refracted the light from the initial explosion into our line of sight. In any of these cases, the travel time for the light from the initial explosion becomes much longer than the travel time for light that went directly from the explosion to Earth along a straight line of sight.

In this way, the light is still from the initial explosion, and thus the dust is still 325 lyr from the remnant pulsar, but that 325 lyr line is not in the plane of the sky.

Here is a diagram of a light echo put out by NASA to describe V838 Mons, the best example of these objects. It very much applies here, as well.



In the case of Cass A, the percieved expansion from the light echo is more rapid than the actual expansion of the SN ejecta.

[Recycled Electrons]

Cheers... I have to go hang my newest wall poster: Cassiopeia A.

[JohnD]

All,
The other similar star, V838 Mon has appeared again on APOD - 26th November 2005.
In 2003, the Hubble site published a 'video' of this star, http://hubblesite.org/newscenter/newsde ... 10/video/a, in fact a short series of stills that dissolved, one into the next. Has anyone produced an extension of that using newer pictures? That would be worth an APOD!

JOhn