Starship Asterisk*

Posted: **Fri Jan 10, 2014 5:07 am**

NGC 7293: The Helix Nebula

Explanation: A mere seven hundred light years from Earth, in the constellation Aquarius, a sun-like star is dying. Its last few thousand years have produced the Helix Nebula (NGC 7293), a well studied and nearby example of a Planetary Nebula, typical of this final phase of stellar evolution. A total of 28.5 hours of exposure time have gone in to creating this deep view of the nebula. Combining narrow band image data from emission lines of hydrogen atoms in red and oxygen atoms in blue-green hues, it shows remarkable details of the Helix's brighter inner region, about 3 light-years across, but also follows fainter outer halo features that give the nebula a span of well over six light-years. The white dot at the Helix's center is this Planetary Nebula's hot, central star. A simple looking nebula at first glance, the Helix is now understood to have a surprisingly complex geometry.

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Posted: **Fri Jan 10, 2014 10:01 am**

This is a fine picture, and I'm glad to see Don Goldman getting an APOD. He really deserves is.

Ann

Posted: **Fri Jan 10, 2014 12:12 pm**

Wow, that's deep! The link "this deep view of the nebula" is full of interesting details, as is the rest of the astrodonimaging.com website. Thank you.

Posted: **Fri Jan 10, 2014 2:27 pm**

Wow, just amazing that this picture is really over 700 years old. Thanks APOD.

Posted: **Fri Jan 10, 2014 2:36 pm**

I'm very happy to see this image on APOD, the detail is superb! There is actually a little mistake in the text description as well as the original webpage, the total exposure time is 29 hours, not 28.5 hours.

Posted: **Fri Jan 10, 2014 2:55 pm**

Some questions occurs to me. If one lived on a planet of a star that had the expanding nebula 'engulf' it (we are only 4.4 ly from Alpha Centauri), what effects would one experience? Would the background radiation become fatal? Would there be an effective temperature rise? How much would the presence of the nebula affect local viewing of space?

Would one perceive the advancing shock wave as it entered and passed through their system?

Just asking.

Posted: **Fri Jan 10, 2014 3:27 pm**

Nitpicker wrote:Wow, that's deep! The link "this deep view of the nebula" is full of interesting details, as is the rest of the astrodonimaging.com website. Thank you.

If you enjoy Don's images, then join his mailing list! He releases one or two images a month!

Posted: **Fri Jan 10, 2014 3:48 pm**

greenewr wrote:Some questions occurs to me. If one lived on a planet of a star that had the expanding nebula 'engulf' it (we are only 4.4 ly from Alpha Centauri), what effects would one experience? Would the background radiation become fatal? Would there be an effective temperature rise? How much would the presence of the nebula affect local viewing of space?

Would one perceive the advancing shock wave as it entered and passed through their system?

The radiation levels are quite low. The peak is in the ultraviolet, and would be largely blocked by the atmosphere. Even though the gases are hot, they are also very thin, similar to the best vacuum you could create in a lab. So there would be no temperature rise. (The region of space that the ISS orbits in has a gas temperature of thousands of degrees, and those gases are denser than what we have in a nebula.) Without some good technology, people living on a planet that was engulfed by this nebula might not even be aware it existed. It might slightly attenuate the night sky, but that would seem normal. And within the stellar system itself, there probably wouldn't be enough nebular material to collect and study with a Stardust like probe, because the local solar wind and radiation pressure would clear a bubble in that region of the nebula.

Posted: **Fri Jan 10, 2014 4:08 pm**

starsurfer wrote:the total exposure time is 29 hours

I assume that for a field like this that roughly 35 arcminutes on a side, and since all exposure was in spectra viewable through our atmosphere, this could be done with an amateur telescope and camera?

What would exposure time be to get the same detail and data from one of the largest telescopes?

Posted: **Fri Jan 10, 2014 4:22 pm**

dbooksta wrote:I assume that for a field like this that roughly 35 arcminutes on a side, and since all exposure was in spectra viewable through our atmosphere, this could be done with an amateur telescope and camera?

This was done with an amateur telescope and camera (more precisely, with equipment marketed primarily to amateur astronomers, although this particular telescope costs about the same as a car).

What would exposure time be to get the same detail and data from one of the largest telescopes?

To a first order, S/N scales with the area of the aperture. This was made with a 0.5 meter aperture. Big professional scopes are in the 10 meter aperture range, so they collect 400 times more light in a given exposure interval. Such a telescope might therefore produce an image like this with four or five minutes of exposure time. That answer is somewhat complicated, however, by the fact that the larger scope has a focal length about five times greater, resulting in a significantly different pixel scale, meaning the pixel S/N characteristics might have to be considered, as well. Still, the larger scope is going to need a much, much smaller exposure time.

Posted: **Fri Jan 10, 2014 4:25 pm**

I was surprised by the complex geometry. No, really! This image shows those long fingers around the outer edge of the inner disc. It's as if new stars are being born before the source star finishes fusing the remaining elements.

I'd like to encourage more long-exposure images like this one. Australia is one of the few remaining places where you can still see by starlight. All of these lights left on after sunset don't really put a dent in the crime rate. The police helicopters still need night-vision goggles to find miscreants hiding in the bushes. But I digress. Long exposures such as this bring out the background galaxies, adding a sense of depth to the 2D image.

[Exposure: 29 hrs Total: 12.5 hrs OIII, 13.5 hrs H-a, 3 hrs RGB]

Posted: **Fri Jan 10, 2014 4:31 pm**

Psnarf wrote:Australia is one of the few remaining places where you can still see by starlight.

You might be surprised. There are regions all over the world where the skies still approach their theoretically darkest levels. Most people can reach such areas with no more than a few hours drive.

Also, reading by starlight (or the light of the Milky Way) under the darkest conditions requires that the text be approximately 2 inches high.

Posted: **Fri Jan 10, 2014 5:20 pm**

Chris Peterson wrote:
greenewr wrote:Some questions occurs to me. If one lived on a planet of a star that had the expanding nebula 'engulf' it (we are only 4.4 ly from Alpha Centauri), what effects would one experience? Would the background radiation become fatal? Would there be an effective temperature rise? How much would the presence of the nebula affect local viewing of space?

Would one perceive the advancing shock wave as it entered and passed through their system?
The radiation levels are quite low. The peak is in the ultraviolet, and would be largely blocked by the atmosphere. Even though the gases are hot, they are also very thin, similar to the best vacuum you could create in a lab. So there would be no temperature rise. (The region of space that the ISS orbits in has a gas temperature of thousands of degrees, and those gases are denser than what we have in a nebula.) Without some good technology, people living on a planet that was engulfed by this nebula might not even be aware it existed. It might slightly attenuate the night sky, but that would seem normal. And within the stellar system itself, there probably wouldn't be enough nebular material to collect and study with a Stardust like probe, because the local solar wind and radiation pressure would clear a bubble in that region of the nebula.

Up to a point, Lord Copper! But you were answering the question directly.
Don't planetary nebulae occur when the star is running out of hydrogen, cools and expands towards red giant status? They'd notice that, even if the star didn't engulf them, as we are told the Sun will do one day! Then, as the outer layers are ejected to form the nebula, the hotter inner layers of the star are exposed, before fusion slows and it cools towards being a white dwarf. That would take ?10,000 years, but surely even protoplasmic blobs would notice?
John

Posted: **Fri Jan 10, 2014 5:29 pm**

JohnD wrote:Up to a point, Lord Copper! But you were answering the question directly.
Don't planetary nebulae occur when the star is running out of hydrogen, cools and expands towards red giant status? They'd notice that, even if the star didn't engulf them, as we are told the Sun will do one day! Then, as the outer layers are ejected to form the nebula, the hotter inner layers of the star are exposed, before fusion slows and it cools towards being a white dwarf. That would take ?10,000 years, but surely even protoplasmic blobs would notice?

My understanding of the question involved another star and planetary system engulfed by the nebula, not planets around the star that created the nebula.

Posted: **Fri Jan 10, 2014 5:49 pm**

The diagram of the complex features makes me think of the planets that may have surrounded the sun. The planets (if they existed) would have all had angular momentum on their various orbital planes. If the sun had planets such as gas giants or rocky bodies could their remnants have contributed to the aspects of the diagram? It’s always been interesting to me to see the varieties of shapes that planetary nebulas take and it makes me wonder what caused all those geometries.

Posted: **Fri Jan 10, 2014 5:51 pm**

Chris Peterson wrote:
JohnD wrote:Up to a point, Lord Copper! But you were answering the question directly.
Don't planetary nebulae occur when the star is running out of hydrogen, cools and expands towards red giant status? They'd notice that, even if the star didn't engulf them, as we are told the Sun will do one day! Then, as the outer layers are ejected to form the nebula, the hotter inner layers of the star are exposed, before fusion slows and it cools towards being a white dwarf. That would take ?10,000 years, but surely even protoplasmic blobs would notice?
My understanding of the question involved another star and planetary system engulfed by the nebula, not planets around the star that created the nebula.

Ah! You WERE answering the question, which I misread!
JOhn

Posted: **Fri Jan 10, 2014 6:35 pm**

Always Awe Inspiring!!!!

P.N.s are one of my favorite objects.

I like the wider angle shots which show just how far reaching the outer shell has reached. But this shows some awesome detail, which I like...

:---[===] *

Posted: **Fri Jan 10, 2014 7:02 pm**

wow, are we a puny lot. Takes what, 16 light hours to cross the solar system and this thing is 6 light years across ,

Posted: **Fri Jan 10, 2014 8:09 pm**

Chris Peterson wrote: (The region of space that the ISS orbits in has a gas temperature of thousands of degrees, and those gases are denser than what we have in a nebula.)

Chris,
Can you expand on this statement? I thought space was cold. Perhaps my understanding of 'gas temperature' is limited.

Thanks.

Posted: **Fri Jan 10, 2014 9:08 pm**

Chris Peterson wrote:
Psnarf wrote:Australia is one of the few remaining places where you can still see by starlight.
You might be surprised. There are regions all over the world where the skies still approach their theoretically darkest levels. Most people can reach such areas with no more than a few hours drive.

And these days more and more people, including Don Goldman, are operating dark site observatories remotely, so as to enjoy the best of both worlds.

Posted: **Fri Jan 10, 2014 9:54 pm**

Wadsworth wrote:
Chris Peterson wrote: (The region of space that the ISS orbits in has a gas temperature of thousands of degrees, and those gases are denser than what we have in a nebula.)
Chris,
Can you expand on this statement? I thought space was cold. Perhaps my understanding of 'gas temperature' is limited.

Space itself has no temperature in a meaningful sense. Temperature is a measure of the kinetic energy of particles, whether solid, liquid, or gas. Of course, no region of space is entirely free of particles, so we can measure the temperature by looking at the kinetic energy of those particles. The outer reaches of Earth's atmosphere is called the thermosphere, starting at 85 km and extending to 600 km. The temperature of this region is over 2000°C. The reason the ISS doesn't burn up is because the atmospheric density is so low. It doesn't get hit by particles often enough to transfer their kinetic energy in comparison with radiative effects. Basically, the ISS gets hot in the Sun, and cold in the shade, because radiative heat transfer is many times more efficient in the thin thermosphere than convective heat transfer.

The same is true in many nebulas. The gas molecules may be very energetic, with temperatures of thousands of degrees. But the nebular density is so low that almost no heat is transferred convectively.

Posted: **Sat Jan 11, 2014 12:32 pm**

"Here's looking at you, kid."
Did anyone else think this looks like a giant eyeball?
jambo

Posted: **Sat Jan 11, 2014 1:50 pm**

jambo@hughes.net wrote:
"Here's looking at you, kid."

Did anyone else think this looks like a giant eyeball?

Jambie, I think this is the beginning of a beautiful friendship.

http://asterisk.apod.com/viewtopic.php? ... 98#p168398
http://apod.nasa.gov/apod/ap120131.html

Posted: **Sat Jan 11, 2014 3:09 pm**

If astronomers had a category for "Things Which Look Like Eyes" it would be pretty full.

Posted: **Sat Jan 11, 2014 3:24 pm**

geckzilla wrote:
If astronomers had a category for "Things Which Look Like Eyes" it would be pretty full.

http://arovingiwillgo.wordpress.com/2011/04/07/reptiles-of-all-kinds-at-fernbank/ wrote:
[b][color=#0000FF]Gecko eye patterns[/color][/b]

Starship Asterisk*

APOD: NGC 7293: The Helix Nebula (2014 Jan 10)

APOD: NGC 7293: The Helix Nebula (2014 Jan 10)

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