APOD: NGC 7293: The Helix Nebula (2012 Oct 04)

[APOD Robot]

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 58 hours of exposure time have gone in to creating this deep view of the nebula. Accumulating narrow band data from emission lines of hydrogen atoms in red and 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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[bystander]

For another view of the Helix from Spitzer and GALEX in IR and UV, http://asterisk.apod.com/viewtopic.php?t=29679

[Ann]

Fantastic! The blue blaze from the central parts of the Helix in this image had my socks knocked off!

Ann

[Boomer12k]

It is interesting to note....How much Oxygen must there be there???

As it looks like an Eye....and the top looks like a Brain.....it looks like "THE THING FROM 700 LIGHT YEARS".....a B grade Sci-Fi movie Starring Mike "Touch" Connors, and Peter Graves....with Lee Van Cleef as the bad guy of course....

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[starsurfer]

Stunning detail, goes to show it's really worth going really deep! The Helix Nebula has always been one of my favourite nebulae and it's great to see the main bright parts together with the full extent of the outer halo! Even more amazing is that this image was taken with a 4 inch telescope!!!

[neufer]

It is interesting to note....How much Oxygen must there be there???

About 1% Oxygen by mass (all of it from previous supernova).

The Usual Suspects: Ten most common elements in the Milky Way Galaxy estimated spectroscopically,

Code: Select all

Z 	Element 	Mass fraction in parts per million
--------------------------------------------------
1 	 Hydrogen 	739,000
2 	 Helium   	240,000 
.....................................
8 	 Oxygen   	 10,400 	
6 	 Carbon   	  4,600 	
10 	Neon     	  1,340 	
26 	Iron     	  1,090 	
7 	 Nitrogen 	    960 	
14 	Silicon  	    650 	
12 	Magnesium 	   580
16 	Sulfur    	   440

Note, however, that you are only observing the doubly ionized oxygen [O III] in blue green colors.

<<In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atoms undergoing nominally "forbidden" energy transitions not normally allowed by the selection rules of quantum mechanics. Forbidden lines of nitrogen ([N II] at 654.8 and 658.4 nm), sulfur ( at 671.6 and 673.1 nm), and oxygen ([O II] at 372.7 nm, and [O III] at 495.9 and 500.7 nm) are commonly observed in astrophysical plasmas. These lines are extremely important to the energy balance of such things as planetary nebulae and H II regions. Also, the forbidden 21-cm hydrogen line is of the utmost importance for radio astronomy as it allows very cold neutral hydrogen gas to be seen.>>

<<Stars more massive than 8 solar masses (M_⊙) will likely end their lives in a dramatic supernova explosion. Planetary nebula may result from the death of intermediate and low mass stars down to 0.8 M_⊙.

Stars spend most of their lifetime shining as a result of nuclear fusion reactions that convert hydrogen to helium in the star's core. Outward pressure from fusion in the core balances inward collapse due to the star's own gravity. Such stars are said to be in the main sequence.

Intermediate to low mass stars run out of hydrogen in their cores after tens of millions to billions of years in the main sequence. Gravity compresses the core and it heats up. Currently the sun's core has a temperature of approximately 15 million K, but when it runs out of hydrogen, the compression of the core will cause the temperature to rise to about 100 million K.

The outer layers of the star expand enormously and become much cooler in contrast to the very high temperature of the core; and the star becomes a red giant. The core continues to contract and heat up, and when its temperature reaches 100 million K, helium nuclei begin to fuse into carbon and oxygen. The resumption of fusion reactions stops the core's contraction. Helium burning (fusion of helium nuclei) soon forms an inert core of carbon and oxygen, with both a helium-burning shell and a hydrogen-burning shell surrounding it. In this last stage the star will observationally be a red giant again and structurally an asymptotic giant branch star.

Helium fusion reactions are extremely temperature sensitive, with reaction rates being proportional to T⁴⁰ (under relatively low temperatures). This means that just a 2% rise in temperature more than doubles the reaction rate. These conditions cause the star to become very unstable—a small rise in temperature leads to a rapid rise in reaction rates, which releases a lot of energy, increasing the temperature further. The helium-burning layer rapidly expands and therefore cools, which reduces the reaction rate again. Huge pulsations build up, which eventually become large enough to throw off the whole stellar atmosphere into space.

The ejected gases form a cloud of material around the now-exposed core of the star. As more and more of the atmosphere moves away from the star, deeper and deeper layers at higher and higher temperatures are exposed. When the exposed surface reaches a temperature of about 30,000 K, there are enough ultraviolet photons being emitted to ionize the ejected atmosphere, making it glow. The cloud has then become a planetary nebula.>>

[Anthony Barreiro]

The complex geometry illustration from the link in the caption is quite informative.

The Helix nebula is notoriously difficult to observe in a small telescope. You would think that at magnitude 7 it would be a piece of cake, but its light is spread out over such a large area in the sky that it tends to be washed out by even low levels of light pollution. If you're observing from a dark location at the right time of the year, this is definitely one to add to your observing list. Evenings when the Moon is not in the sky during the next couple of months will be your last chance until the pre-dawn hours next summer.

[Wadsworth]

Excellent APoD!

[neufer]

[url=http://hubblesite.org/newscenter/archive/releases/2004/32/image/e/format/web_print/]The complex geometry illustration[/url] [b][color=#0000FF] from the link in the caption is quite informative.[/color][/b]

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[Anthony Barreiro]

[url=http://hubblesite.org/newscenter/archive/releases/2004/32/image/e/format/web_print/]The complex geometry illustration[/url] [b][color=#0000FF] from the link in the caption is quite informative.[/color][/b]

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Art, would the geometry change significantly if we replaced the poultry with tofu, for those of us who are vegetarian? And, come to think of it, is it a malapropism to talk about the geometry of an object 700 light years from Earth?

[Boomer12k]

Thanks Art, for the info and chart....


:---[===] *

[Ann]

Anthony Barreiro wrote:

And, come to think of it, is it a malapropism to talk about the geometry of an object 700 light years from Earth?

Good point! I haven't googled it, but I think that "geometry" means "geo" + "metry" which might mean "Earth" + "measure" as in "measuring the Earth". It's kind of strange to talk about measuring the Earth when we try to work out the structure of a planetary nebula. Perhaps in this case we shouldn't talk about "geometry" but, maybe, "planetary-nebula-metry"?

Similarly, it's a bit strange to talk about the "geology" of, say Mars. But my favorite groan-inducing expression is when astronomers talk about galaxies having "global", that is, "overall" properties. But even though most galaxies do have a spherical component (as in the halo of the Milky Way, and of course the overall shape of elliptical galaxies), it still feels to me as if someone is trying to fit the shape (and size!!) of the Earth onto galaxies, when galaxies are said to have "global" properties!

Ann

[Joe Stieber]

The Helix nebula is notoriously difficult to observe in a small telescope. You would think that at magnitude 7 it would be a piece of cake, but its light is spread out over such a large area in the sky that it tends to be washed out by even low levels of light pollution.

The Helix Nebula does indeed have a reputation for being hard to see, but I find it relatively easy to spot with a small instrument. From the Pine Barrens of Southern New Jersey with naked-eye limiting magnitudes in the 5 to 6 range overhead (reasonably, but not spectacularly dark), I can spot it with my 10x42 binoculars and it’s a piece of cake with my 16x70 binoculars (but they show just a small fuzzy patch).

With my 12.5-inch Newtonian telescope, it’s not as easy to see (because it's bigger and more spread out), but add a narrowband filter (e.g., passing O-III around 500 nm) and it becomes much more prominent. On a dark night with good transparency, structural detail begins to emerge in the scope with a filter.

It’s not difficult to locate the Helix starting from first magnitude Fomalhaut in Piscis Austrinus. Swing about 10 degrees northwest to fifth magnitude Upsilon Aquarii (the brightest at the northern end of a distinctive row of three stars) then move a little more than a degree west and you’re there. Currently, the Helix is about 10 degrees south-southeast of Neptune.

[Anthony Barreiro]

The Helix nebula is notoriously difficult to observe in a small telescope. You would think that at magnitude 7 it would be a piece of cake, but its light is spread out over such a large area in the sky that it tends to be washed out by even low levels of light pollution.

The Helix Nebula does indeed have a reputation for being hard to see, but I find it relatively easy to spot with a small instrument. From the Pine Barrens of Southern New Jersey with naked-eye limiting magnitudes in the 5 to 6 range overhead (reasonably, but not spectacularly dark), I can spot it with my 10x42 binoculars and it’s a piece of cake with my 16x70 binoculars (but they show just a small fuzzy patch).

With my 12.5-inch Newtonian telescope, it’s not as easy to see (because it's bigger and more spread out), but add a narrowband filter (e.g., passing O-III around 500 nm) and it becomes much more prominent. On a dark night with good transparency, structural detail begins to emerge in the scope with a filter.

It’s not difficult to locate the Helix starting from first magnitude Fomalhaut in Piscis Austrinus. Swing about 10 degrees northwest to fifth magnitude Upsilon Aquarii (the brightest at the northern end of a distinctive row of three stars) then move a little more than a degree west and you’re there. Currently, the Helix is about 10 degrees south-southeast of Neptune.

Joe, thank you for the encouragement and clear directions. I've seen the helix from a dark location through binoculars and a 6-inch newtonian with a nebula filter, but from my urban back yard it's impossible.

[500pesos]

Anthony Barreiro wrote:

And, come to think of it, is it a malapropism to talk about the geometry of an object 700 light years from Earth?

Good point! I haven't googled it, but I think that "geometry" means "geo" + "metry" which might mean "Earth" + "measure" as in "measuring the Earth". It's kind of strange to talk about measuring the Earth when we try to work out the structure of a planetary nebula. Perhaps in this case we shouldn't talk about "geometry" but, maybe, "planetary-nebula-metry"?

Similarly, it's a bit strange to talk about the "geology" of, say Mars. But my favorite groan-inducing expression is when astronomers talk about galaxies having "global", that is, "overall" properties. But even though most galaxies do have a spherical component (as in the halo of the Milky Way, and of course the overall shape of elliptical galaxies), it still feels to me as if someone is trying to fit the shape (and size!!) of the Earth onto galaxies, when galaxies are said to have "global" properties!

Ann

Indeed, geometry is the art of measuring the Earth; Gaia or Gee being the Greek name for Earth (in this case 'gee' with a small 'g', meaning the ground, not the planet) and 'metro' being the verb 'to calculate' or 'to measure' = geo-metria (geometry in Greek). I guess the first reason geometry was developed was to measure land in order to settle (or prevent) land disputes among people.

Global on the other hand is just an adjective (Latin origin this one) meaning 'spherical' or metaphorically 'overall'. The word is not referring exclusively to the Earth. Yes, the Earth happens to be a globe but so are all spherical things.

[neufer]

It is interesting to note....How much Oxygen must there be there???

As it looks like an Eye....

and the top looks like a Brain.....

it looks like "THE THING FROM 700 LIGHT YEARS".....

a B grade Sci-Fi movie Starring Mike "Touch" Connors, and Peter Graves....
with Lee Van Cleef as the bad guy of course....

[b][color=#0000FF]The eye that washed ashore on Pompano Beach, Fla.[/color][/b]

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Giant eyeball mystery solved:
By Megan Gannon and Stephanie Pappas, Associated Press / October 15, 2012

<<Wildlife officials in Florida are examining a lone blue eye the size of a softball that washed ashore on Pompano Beach this week. While test results are pending, some researchers have speculated that the mysterious eye belonged to a large swordfish. Though some had also suggested it came from a deep-sea squid, experts contacted by LiveScience lean toward a swordfish as the likely eyeball owner.

Marine scientist Heather Bracken-Grissom, of Florida International University in Miami, told LiveScience that the shape of the eyeball's lens and pupil is similar to that of a giant squid. "The eyes of squids do dislodge quite easily during dissection," Bracken-Grissom said in an email, though she added that "it would be very rare for a fresh squid eyeball to wash ashore a Florida beach."

Other experts saw a "fishier" explanation for the eyeball. "I have not seen a squid with blue eyes, but I am not an expert," said Trevor Wardill, a research associate at the Marine Biological Laboratory (MBL) at Woods Hole, Mass. "My guess is that it is from a vertebrate, as the iris is not fully open, but still very round."

Squid have w-shaped irises when they aren't fully open, Wardill told LiveScience.

Eric Warrant, a vision scientist at the University of Lund in Sweden who has worked extensively with swordfish eyes, said if the eye is not fake, it is almost certainly a swordfish. But that still wouldn't answer a key question: How would a single swordfish eye wash up on a Florida beach?

"You usually don't find random floating eyes of any animal," said biologist Sönke Johnsen of Duke University. Johnsen was cautious about making a judgment based on the photos but said, "I'm fairly sure it's just the eye of a large xiphid, likely a swordfish or marlin. They get seriously big, but people don't realize it because most of the eye is inside the head," he wrote in an email to LiveScience. (Squid eyes can also get seriously big, with scientits finding the giant squid can have basketball-size peepers, likely as a way to spot predators like sperm whales in their dim undersea homes.)>>

[owlice]

::shakes head::

No proofreading at the CSM either, apparently.