Starship Asterisk*

NGC 7635: The Bubble Nebula

Explanation: Blown by the wind from a massive star, this interstellar apparition has a surprisingly familiar shape. Cataloged as NGC 7635, it is also known simply as The Bubble Nebula. Although it looks delicate, the 7 light-year diameter bubble offers evidence of violent processes at work. Above and left of the Bubble's center is a hot, O-type star, several hundred thousand times more luminous and some 45 times more massive than the Sun. A fierce stellar wind and intense radiation from that star has blasted out the structure of glowing gas against denser material in a surrounding molecular cloud. The intriguing Bubble Nebula and associated cloud complex lie a mere 7,100 light-years away toward the boastful constellation Cassiopeia. This sharp, tantalizing view of the cosmic bubble is a composite of Hubble Space Telescope image data from 2016, reprocessed to present the nebula's intense narrowband emission in an approximate true color scheme.

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Why is the star often not centered within the bubble it is creating. It seems from any viewing perspective it 'should' be centered. And if the star has migrated over the millennia, would not the bubble also change shape?

APOD Robot wrote:

This sharp, tantalizing view of the cosmic bubble is a composite of Hubble Space Telescope image data from 2016, reprocessed to present the nebula's intense narrowband emission in an approximate true color scheme.

"Approximate" is right. The ionizing O-type star is definitely not pink.

Ann

Anon wrote: ↑Thu Apr 19, 2018 4:36 am Why is the star often not centered within the bubble it is creating. It seems from any viewing perspective it 'should' be centered. And if the star has migrated over the millennia, would not the bubble also change shape?

Because the medium in which the void is being created isn't homogeneous. The expanding material slows down faster in directions where it encounters denser gas.

Ann wrote: ↑Thu Apr 19, 2018 4:44 am

APOD Robot wrote:

This sharp, tantalizing view of the cosmic bubble is a composite of Hubble Space Telescope image data from 2016, reprocessed to present the nebula's intense narrowband emission in an approximate true color scheme.

"Approximate" is right. The ionizing O-type star is definitely not pink.

It is possible, using narrow band filters, to approximate the color of emission sources. Generally not of continuum sources, however, which is why stars are usually oddly colored in such images. (You can, however, measure the temperature of the stars from such data more accurately than is possible using conventional RGB filters.)

WOW!!! COOL....close up of the Bubble!!!!
We are going to have to name all the interesting details...there is one at 4 o'clock...the "Lady Godiva Nebula"... looks like a lady on a horse, an Alien Horse....but a horse is a horse, of course, of course....right?

waiting for better weather... My Bubble...
:---[===] *

Well done, Boomer!!

Ann

Ann wrote: ↑Thu Apr 19, 2018 4:44 am

APOD Robot wrote:

This sharp, tantalizing view of the cosmic bubble is a composite of Hubble Space Telescope image data from 2016, reprocessed to present the nebula's intense narrowband emission in an approximate true color scheme.

"Approximate" is right. The ionizing O-type star is definitely not pink.

Ann

There's only so much one can do with reshuffling narrowband channels to their approx true-colour values.

I made another version of this image, this time using true-colour information from an observatory image: http://www.flickr.com/photos/wildespace/39512823160



Source image for colours: https://upload.wikimedia.org/wikipedia/ ... _Block.jpg

Thanks, I really appreciate the blue star!

Ann

I don't fully understand the reprocessing... process. I was also wondering about the "approximate true color scheme". Does that mean that if we were to actually go to that nebula in person, these are the colors we'd see? ...minus the pink O star, of course.

Chris Peterson wrote: ↑Thu Apr 19, 2018 5:00 am

Anon wrote: ↑Thu Apr 19, 2018 4:36 am Why is the star often not centered within the bubble it is creating. It seems from any viewing perspective it 'should' be centered. And if the star has migrated over the millennia, would not the bubble also change shape?

Because the medium in which the void is being created isn't homogeneous. The expanding material slows down faster in directions where it encounters denser gas.

Something does not jibe: if it is encountering denser gas, then it should be oblong in shape! Or even more distorted

sunson wrote: ↑Thu Apr 19, 2018 1:52 pm

Chris Peterson wrote: ↑Thu Apr 19, 2018 5:00 am

Anon wrote: ↑Thu Apr 19, 2018 4:36 am Why is the star often not centered within the bubble it is creating. It seems from any viewing perspective it 'should' be centered. And if the star has migrated over the millennia, would not the bubble also change shape?

Because the medium in which the void is being created isn't homogeneous. The expanding material slows down faster in directions where it encounters denser gas.

Something does not jibe: if it is encountering denser gas, then it should be oblong in shape! Or even more distorted

It's hard to determine what the actual shape is.

Certainly, Boomer at 2 o'clock there is the image of a secretary, an African bird and on the main star the face of Freud (to the best of my knowledge and understanding)

Chris Peterson wrote: ↑Thu Apr 19, 2018 2:00 pm

sunson wrote: ↑Thu Apr 19, 2018 1:52 pm

Chris Peterson wrote: ↑Thu Apr 19, 2018 5:00 am Because the medium in which the void is being created isn't homogeneous. The expanding material slows down faster in directions where it encounters denser gas.

Something does not jibe: if it is encountering denser gas, then it should be oblong in shape! Or even more distorted

It's hard to determine what the actual shape is.

Note that it is oblong. Perhaps more than unaided perception would note without a reference circle. I may be imagining it, but it seems to me the far right edge of the bubble is really more distant from our view point.

I enjoy looking at the high-res version in the APOD. It shows such fine features! One can see multiple layers of the bubble. On the upper left, I believe I see 3 separate layers (or shells).

Beware of the Blob.

Chris Peterson wrote: ↑Thu Apr 19, 2018 2:00 pm

sunson wrote: ↑Thu Apr 19, 2018 1:52 pm

Chris Peterson wrote: ↑Thu Apr 19, 2018 5:00 am

Because the medium in which the void is being created isn't homogeneous. The expanding material slows down faster in directions where it encounters denser gas.

Something does not jibe: if it is encountering denser gas, then it should be oblong in shape! Or even more distorted

It's hard to determine what the actual shape is.

The nebula is currently thought to have 3 shells.
The sketch is from Kinematics of an ionized shell, NGC7635 in the S162 Complex (1995) and the wider-angle image is from NOAO Image Gallery
The simple symmetrical, single-shell spherical model rarely applies to exploding stars, and the shape in 3 dimensions cannot be accurately determined a 2D image.
 

Thanks, alter-ego, how interesting!

It would seem that all the shells expand more easily to the lower right than to the upper left in the images you provided. That suggests that the interstellar medium is a lot thicker at upper left than at lower right, and it helps explain the off-center position of SAO 20575 (the ionizing star) in the inner bubble.

Ann

E Fish wrote: ↑Thu Apr 19, 2018 1:14 pm Does that mean that if we were to actually go to that nebula in person, these are the colors we'd see? ...minus the pink O star, of course.

The true color of the nebula is gray. That's the only color our eyes can see for something as dim as this, and it wouldn't get any brighter if we were closer. Stars, of course, are bright enough that we can detect color in them.

E Fish wrote: ↑Thu Apr 19, 2018 1:14 pm I don't fully understand the reprocessing... process.

Digital colour images are composed of the red, green, and blue channels. Hubble's narrowband images are created by assigning specific ionised gasses (like NII, Ha, and OIII) to the red, green, and blue channels respectively, resulting in a false-colour image. But in true colours, NII is actually red, so is Ha, and OIII is greenish-blue. What I did was split the original narrowband image into RGB components, then reassign NII and Ha to the red channel, and OIII to the green and blue channels (with some colour balance tweaking to give it a more pink hue like it appears in astrophotos).

I was also wondering about the "approximate true color scheme". Does that mean that if we were to actually go to that nebula in person, these are the colors we'd see? ...minus the pink O star, of course.

The bubble is 7 light years across. The light given off by those ionised gasses is extremely faint; you wouldn't be able to see it like that with the naked eye. At best, you might see some very faint grey glow, barely visible.

alter-ego wrote: ↑Fri Apr 20, 2018 4:59 am

---

The nebula is currently thought to have 3 shells.

The sketch is from Kinematics of an ionized shell, NGC7635 in the S162 Complex (1995) and the wider-angle image is from NOAO Image Gallery

The simple symmetrical, single-shell spherical model rarely applies to exploding stars, and the shape in 3 dimensions cannot be accurately determined a 2D image.

Ann wrote: ↑Fri Apr 20, 2018 5:17 am
It would seem that all the shells expand more easily to the lower right than to the upper left in the images you provided. That suggests that the interstellar medium is a lot thicker at upper left than at lower right, and it helps explain the off-center position of SAO 20575 (the ionizing star) in the inner bubble.

It is also suggestive that the 3 shells might have been generated at 3 specific past times
when a moving SAO 20575 might have been more centrally located to each in turn.

neufer wrote: ↑Fri Apr 20, 2018 1:41 pm

alter-ego wrote: ↑Fri Apr 20, 2018 4:59 am

---

The nebula is currently thought to have 3 shells.

The sketch is from Kinematics of an ionized shell, NGC7635 in the S162 Complex (1995) and the wider-angle image is from NOAO Image Gallery

The simple symmetrical, single-shell spherical model rarely applies to exploding stars, and the shape in 3 dimensions cannot be accurately determined a 2D image.

Ann wrote: ↑Fri Apr 20, 2018 5:17 am
It would seem that all the shells expand more easily to the lower right than to the upper left in the images you provided. That suggests that the interstellar medium is a lot thicker at upper left than at lower right, and it helps explain the off-center position of SAO 20575 (the ionizing star) in the inner bubble.

It is also suggestive that the 3 shells might have been generated at 3 specific past times
when a moving SAO 20575 might have been more centrally located to each in turn.

Yeah, I was wondering the same thing.
Thinking there might be proper motion data for BD+60 2522 (aka SAO 20575), I found dRA/dt = +3.1mas/yr, and dDEC/dt = +4.6mas/yr. The star and the two circular shells align in direction, and quite well in position angle (PA from velocity = 214° compared to PA = 205° from estimates for shell center locations).
The radial shell center locations from the star ~51 arcsec (shell 1), and ~190 arcsec (shell 2). Given radial velocity = 0.00555 masarcsec/yr, the past elapsed times are easily calculated: The star was nominally centered in shell1 ~ 9200 years ago and centered in shell 2 ~ 34300 years ago. (A cruder estimate for shell 3 ~ 80,000 years ago) The link states the estimated age of the surrounding nebula ~40,000 years old. I'd say the things fall in line pretty well to support stellar motion wrt apparent shell centers, i.e. the times reasonably correspond to when stellar burps occurred.

You can tell the expanding bubble encountered strong resistance at around 10-11 o'clock, hence the thick shells and strands of interstellar material. Around 2 to 6 o'clock there was little resistance and that part bubble expanded considerably further. Plus, of course, there's the proper motion of the star.

The photos of the Bubble Nebula makes me wonder what the Solar System looks like from a few light years away; maybe there's a bubble as well? After all, we are located in the so-called Local Bubble. http://en.wikipedia.org/wiki/Local_Bubble

Thanks to Art and alter-ego. The idea that SAO 20575 is moving and leaving shells behind it (which are remnants of past outbursts of the star) is intriguing and very plausible.

Many high-mass stars are runaways, so it makes good sense that SAO 20575 would be at least a "slow runaway" which is asymmetrically located inside its own shells, due to the fact that it is moving inside them.

Ann

wildespace wrote: ↑Sat Apr 21, 2018 10:05 am The photos of the Bubble Nebula makes me wonder what the Solar System looks like from a few light years away; maybe there's a bubble as well? After all, we are located in the so-called Local Bubble. http://en.wikipedia.org/wiki/Local_Bubble

The bubble around us is in the interstellar medium, which is many orders of magnitude less dense than the region around NGC 7635. It would be difficult to detect even instrumentally, and would not be visible at all.