APOD: Halo of the Cat's Eye (2014 Jun 01)

[MarkBour]

How has an estimate of the age of the outer filaments been made?

Basically he same way the age of the universe (from the Big Bang) is determined:

• 1) A Doppler measurement of the current expansion rate in combination
  
  2) with a simple physical model of the average expansion rate over time.

Run the model in reverse back to the start of the "explosion."

Thanks for the reply. So, I suppose, to figure out the expansion rate from a Doppler effect, you are looking at the difference between the largest and the smallest values? Does one have a way to sample all sorts of small locations from the region? Is there any way to make sure that the Doppler measurements are actually coming from the nebula itself? I mean, I am imagining you are looking at some emission spectra. It would be interesting to learn more about the technique in detail.

P.S. In keeping with the naming of this nebula, I suppose we could call this halo the Cat's Eye's vitreous humor

[neufer]

So, I suppose, to figure out the expansion rate from a Doppler effect, you are looking at the difference between the largest and the smallest values? Does one have a way to sample all sorts of small locations from the region? Is there any way to make sure that the Doppler measurements are actually coming from the nebula itself? I mean, I am imagining you are looking at some emission spectra.

• All your assumptions are correct. According to Wikipedia:

<<The Cat's Eye Nebula (or NGC 6543) is a well-studied planetary nebula. While the bright inner nebula is rather small—the major axis of the inner ellipse is 16.1 arcseconds, while the distance between the condensations is 24.7 arcseconds—it has an extended halo of matter that the progenitor star ejected during its red giant phase. This halo extends over a diameter of about 300 arcseconds (5 arcminutes).

In recent years observations made using the Hubble Space Telescope have allowed a new method of determining distances [to planetary nebula]. All planetary nebulae are expanding, and observations several years apart and with high enough angular resolution will reveal the growth of the nebula in the plane of the sky. This is typically very small—only a few milliarcseconds a year or less. Spectroscopic observations can reveal the velocity of expansion of the nebula along the line of sight using the Doppler effect. Then, comparing the angular expansion with the known expansion velocity, the distance to the nebula can be calculated.

Hubble Space Telescope observations of NGC 6543 several years apart have been used to calculate its distance. Its [current] angular expansion rate is 3.457 milliarcseconds per year, while its [current] expansion velocity along the line of sight has been found to be 16.4 km/s. Combining these two results implies that NGC 6543 is 1001 ± 269 parsecs, or about 3300 light-years away from Earth.

The angular expansion of the nebula can also be used to estimate its age. If it has been expanding at a constant rate of 10 milliarcseconds a year, then it would take 1000 ± 260 years to reach [an average inner core] diameter of 20 arcseconds. This may be an upper limit to the age, as ejected material will be slowed as it encounters material ejected from the star at earlier stages of its evolution, as well as the interstellar medium.>>

[Ron-Astro Pharmacist]

I think I've asked this before but should the original star had gas giant planets within close proximity (or far) from the star sloughing off matter – could their remnants be noticeable or dectected amongst the star material being ejected?

The creation of a planetary nebula is actually pretty gentle. Any planets in the system should be largely unaffected, although their orbits will necessarily change somewhat because of the lost central mass. But it's reasonable to assume that most planets would still be in orbit. They wouldn't have a visible effect on the nebula, but could potentially be detected using one of the techniques used for doing so (transits, Doppler, direct imaging, etc).

Thanks Chris – It makes sense that a rocky planet or highly gravitationally bound gas giant would escape the less violent demise of a sun-mass star. It would be interesting to use such a "demise" in ways to study its planets. In searching I ran across this bit of information.

http://www.rochester.edu/news/show.php?id=3123

[geckzilla]

http://www.rochester.edu/news/show.php?id=3123

Boo! Entomic nebulas should be the name!

[MarkBour]

... wikipedia article ...

Fascinating. So, we *have* had enough time to observe the expansion of some planetary nebula directly. That seems to provide a wonderful strengthening of confidence in the distance estimate. I suppose we don't know for certain that the nebula is spherical, but it would seem to provide strong evidence, nonetheless.

And thanks, Ron-Astro pharmacist for that other article with a number of great images and examples.

[geckzilla]

I suppose we don't know for certain that the nebula is spherical, but it would seem to provide strong evidence, nonetheless.

I can't think of any disk-shaped planetaries. There are some with rings, double rings, hourglass/double cone shaped ones, cylinders, perfect spheres, concentric spheres, complex spring-like spirals, and a lot that are just blobby which take up roughly spherical volumes. If anyone knows of a disk-shaped planetary nebula I'd love to know about it.

[starsurfer]

This image is 10 years old! Haloes are one of my favourite features of planetary nebulae and many are now known to possess them. Interestingly, there is actually HST data for part of the halo, see here.

Romano Corradi is legendary! He published an amazing paper about planetary nebula haloes 10 years ago, see here.