APOD: Hanny s Voorwerp (2011 Feb 10)

[APOD Robot]

Hanny s Voorwerp

Explanation: Hanny's Voorwerp, Dutch for "Hanny's Object", is enormous, about the size of our own Milky Way Galaxy. Glowing strongly in the greenish light produced by ionized oxygen atoms, the mysterious voorwerp is below spiral galaxy IC 2497 in this view from the Hubble Space Telescope. Both lie at a distance of some 650 million light-years in the faint constellation Leo Minor. In fact, the enormous green cloud is now suspected to be part of a tidal tail of material illuminated by a quasar inhabiting the center of IC 2497. Powered by a massive black hole, the quasar suddenly turned off, leaving only galaxy and glowing voorwerp visible in telescopes at optical wavelengths. The sharp Hubble image also resolves a star forming region in the voorwerp, seen in yellow on the side near IC 2497. That region was likely compressed by an outflow of gas driven from the galaxy's core. The remarkable mystery object was discovered by Dutch schoolteacher Hanny van Arkel in 2007 while participating online in the Galaxy Zoo project. Galaxy Zoo enlists the public to help classify galaxies found in the Sloan Digital Sky Survey, and more recently in deep Hubble imagery.

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[bystander]

APOD: What is Hanny's Voorwerp? (2008 Jun 25)
http://asterisk.apod.com/vie ... hp?t=14249

HEAPOW: Unseen is Important (2010 Dec 13)
http://asterisk.apod.com/vie ... hp?t=22286

[br1an boru]

What would be the effect, if any, on life here on earth if there was a quasar in the centre of the Milky Way? Would the radiation be excessive and destructive to us?

[idahogie]

So Hanny's Voorwerp is illuminated by quasar that has shut off. Does that mean that we are seeing reflected light from a source that is closer to us, but has shut off? And does that mean that the Voorwerp will lose it's glow in the (galactically) near future?

[NoelC]

It's a great image of an interesting and certainly uncommon object!

Some obvious basic questions:

- Why should there be SO much oxygen? I thought oxygen was a byproduct of supernovae. Did a bunch of them explode all at once out there?

- Quasar or no, why should it be glowing as brightly as the entire galaxy next door? Could the amount of radiation really have been that great?

- Shouldn't there be even more hydrogen? Most gas clouds glow primarily reddish because hydrogen is simply more common...

- And oxygen emissions aren't this greenish-yellow color by themselves - they're more bluish-green. Is this object red-shifted?

- I have to assume someone's done spectral analysis of this cloud and characterized the emission line(s). What else is in there?

-Noel

[Ann]

Good questions, Noel.

Ann

[NGC3314]

It's a great image of an interesting and certainly uncommon object!

Some obvious basic questions:

- Why should there be SO much oxygen? I thought oxygen was a byproduct of supernovae. Did a bunch of them explode all at once out there?

There's not, relatively speaking - only about a quarter of the solar O/H ratio. It's just that doubly ionized oxygen is remarkably efficent at radiating in the optical emission lines at this temperature (13,000 K).

- Quasar or no, why should it be glowing as brightly as the entire galaxy next door? Could the amount of radiation really have been that great?

Its bright, alright, but the emission-line filters have been brought up in brightness in that representation to make the detail more visible. On the other hand, the quasar really did have a lot of power.

- Shouldn't there be even more hydrogen? Most gas clouds glow primarily reddish because hydrogen is simply more common...

See above - it's mostly a temperature effect in which hydrogen is less efficient at this temperature (as in planetary nebula).

- And oxygen emissions aren't this greenish-yellow color by themselves - they're more bluish-green. Is this object red-shifted?

Mildly redshifted. To my eye, [O III] at zero redshift is a remarkable pure emerald hue (based on my one experience doing visual acquisition with a 3-meter telescope.

- I have to assume someone's done spectral analysis of this cloud and characterized the emission line(s). What else is in there?

H, He, C, N, O, S, Ne have been measured in spectra. See this paper, and blog entries here, here, and here. (Being sort of terse for now because of class in 13 minutes).

[bystander]

What would be the effect, if any, on life here on earth if there was a quasar in the centre of the Milky Way? Would the radiation be excessive and destructive to us?

Jets from a quasar at the heart of the Milky Way would probably directed up/down towards the polar regions of the Milky Way. Here at the outer edges of the disk, little/no effect would be noticed. There may be evidence of such jets having previously been present.

APOD: Huge Gamma Ray Bubbles Found Around Milky Way (2010 Nov 10)
http://asterisk.apod.com/vie ... =9&t=21968

Fermi Finds Giant Structure in our Galaxy
http://asterisk.apod.com/vie ... 31&t=21966

HEAPOW: Temporarily Blowing Bubbles? (2010 Nov 22)
http://asterisk.apod.com/vie ... 31&t=22083

[mexhunter]

Interesting subject and information.
Many greetings.
César

[hlwelborn]

It looks like a human hand print

[NoelC]

To my eye, [O III] at zero redshift is a remarkable pure emerald hue (based on my one experience doing visual acquisition with a 3-meter telescope.

Thanks for the insights!

I hope to have such an experience some day, where I get to look through a telescope with enough light gathering power to actually see color in astronomical objects. Even on the very brightest nebulae, I'm just not seeing color visually with my 10" Meade.

-Noel

[NGC3314]

Oops, that "Guest" above was from me. I didn't notice until later that I had not auto-logged in.

Bill Keel

[Ann]

It is not clear what "emerald green" really means. I googled "emerald color" and got, among others, these two samples:



These two shades of green are very dissimilar. Personally I wouldn't even use the word "green" to describe the cut stone. To me, the piece of jewellery represents what I think of as the color of ionized oxygen. The other sample, which I may call emerald green, is very far from what I think of as the color of OIII emission.

Ann

[bystander]

It is not clear what "emerald green" really means. I googled "emerald color" and got, among others, these two samples:

These two shades of green are very dissimilar. Personally I wouldn't even use the word "green" to describe the cut stone. To me, the piece of jewellery represents what I think of as the color of ionized oxygen. The other sample, which I may call emerald green, is very far from what I think of as the color of OIII emission.

Gachala Emerald

Neither example you give come even close to the color I imagine as emerald: the first too yellow, the second too blue. Even the Gachala Emerald, which I consider bluish, is greener than your cut stone. There is, however, a color defined as emerald: hex #50C878, RGB (80,200,120), or HSV (140, 60, 78). Closer, I think, to the color swath on the right than either of your choices.

[Chris Peterson]

I hope to have such an experience some day, where I get to look through a telescope with enough light gathering power to actually see color in astronomical objects. Even on the very brightest nebulae, I'm just not seeing color visually with my 10" Meade

Never underestimate the power of imagination!

Aperture is not generally relevant in seeing color in astronomical objects. That's because increasing aperture doesn't increase the brightness of an object. Increasing aperture only allows you to operate at a higher magnification without the object actually being dimmer than it would be with your naked eye. Occasionally, that might make a difference: if you can use more of your retina, your brain is better at interpreting color. But that really only applies to very small objects; most of the DSOs that are considered good visual targets are large enough that they appear reasonably large even at fairly low magnifications.

Up to 40X, your 10" scope provides as bright an image as a 3 meter telescope. I've visually used everything from a few inches up to 2.5 meters, and I can tell you there's not the slightest difference in how much color you can see in DSOs.

[Chris Peterson]

It is not clear what "emerald green" really means.

It doesn't mean much. "Color" is a physiological concept, not a physical one. One factor in defining color is brightness. If you look at a bright OIII source, such as a reference lamp in a lab, you'll see a brilliant green color, which many might consider similar to how emeralds look. If you look at a bright OIII source through a telescope, and you have good eyes, you'll see a pale gray-green color. If you have bad eyes, you might see something like bright green- this would be symptomatic of a type of color blindness. As you look at dimmer OIII sources, you'll quickly reach the point where you only perceive shades of gray.

The point being, the "color" of the object depends very much on the brightness of the object- and astronomically, all DSOs bright enough to show anything other than gray are only slightly brighter than the threshold for human color vision.

[NoelC]

I'd love to hear from more folks who have used large aperture scopes on this... Bill says he sees "emerald green" color, Chris says there should be no difference between tiny and large scopes up to 40x because of exit pupil size, but then goes on to say OIII color can be seen as anything from pale gray-green to bright green.

Open question:

Have you personally seen color through a telescope (without electronics)?

I know that actually using my dSLR OIII emissions look more like the teal green of the cut stones shown above than the more yellowish-green swatches shown, and the camera does a pretty good job of making accurate looking color in the daytime...

-Noel

[Chris Peterson]

I'd love to hear from more folks who have used large aperture scopes on this... Bill says he sees "emerald green" color, Chris says there should be no difference between tiny and large scopes up to 40x because of exit pupil size, but then goes on to say OIII color can be seen as anything from pale gray-green to bright green.

We need to distinguish the possible range of perceived color when viewing an OIII source with the range that is possible when viewing an OIII astronomical source. The example of bright green I gave was looking at a reference tube in the lab. And different people would describe that color differently- your example of "teal" is a good one, because the OIII triplet definitely lies in the green-blue range.

Astronomical targets provide a very different situation. First, as previously mentioned, the sources are orders of magnitude dimmer than a reference lamp. Second, you seldom have discrete OIII sources. The effect of this is apparent when viewing something like the Veil Nebula. Unfiltered, it appears gray (to me, anyway, and to others I've viewed with). But it is frequently viewed with an OIII filter to increase the contrast against the sky background. With the OIII filter the nebula appears faintly gray-green, meaning it is actually bright enough to trigger color vision. But that green isn't seen without the filter, because other emissions like hydrogen shift the color (Ha in this nebula isn't bright enough to trigger color response; you can view it with a red Ha filter, but it still only looks gray).

I know that actually using my dSLR OIII emissions look more like the teal green of the cut stones shown above than the more yellowish-green swatches shown, and the camera does a pretty good job of making accurate looking color in the daytime...

Color cameras are notoriously bad at recording anything close to "accurate" color for astronomical emission sources. There are two problems- the color filters over the pixels are leaky, meaning they let through a fair bit of out-of-band energy, and their overlap regions are optimized for accurate color on continuum sources, which is what terrestrial imagers care about. Astronomical filters used for color imaging have very low transmittance out-of-band, and their crossover regions are optimized for common emission lines like OIII.

[zloq]

We need to distinguish the possible range of perceived color when viewing an OIII source with the range that is possible when viewing an OIII astronomical source. The example of bright green I gave was looking at a reference tube in the lab.

You saw the green of [OIII] glowing in a tube in "the lab?"

Do you know where I can buy a tube like that?

zloq

[neufer]

[size=150][color=#00FF00]GREEN OXYGEN[/color][/size]

---

We need to distinguish the possible range of perceived color when viewing an OIII source with the range that is possible when viewing an OIII astronomical source. The example of bright green I gave was looking at a reference tube in the lab.

You saw the green of [OIII] glowing in a tube in "the lab?"

Do you know where I can buy a tube like that?

[Chris Peterson]

You saw the green of [OIII] glowing in a tube in "the lab?"

Yes. I have a set of spectroscopic reference tubes, which are interchangeable in a lamp fixture.

Do you know where I can buy a tube like that?

IIRC, mine came from Melles Griot about 20 years ago. The lamps themselves have "Siemens" etched on their edges.

A more common instrument found in many labs is a monochromator, which allows you to generate a narrow band output of any desired wavelength. That's another simple way to see the "color" of a bright OIII source. Finally, if you have an astronomical OIII filter you can view a bright white source like the Sun through it.

[zloq]

You saw the green of [OIII] glowing in a tube in "the lab?"

Yes. I have a set of spectroscopic reference tubes, which are interchangeable in a lamp fixture.

Do you know where I can buy a tube like that?

IIRC, mine came from Melles Griot about 20 years ago. The lamps themselves have "Siemens" etched on their edges.

OK - you are describing something that does not exist and never has. The brackets around the [OIII] indicate it is a so-called forbidden line, meaning the conditions needed for its emission require extremely low density and high excitation energy - as in a nebula. Some forbidden lines are less "forbidden" than others. Oxygen has forbidden lines in the upper atmosphere that appear in skyglow and aurorae, and they have been synthetically generated in elaborate discharge tubes and pulsed-microwave devices - but the [OIII] line requires an extremely long lifetime between collisions for it to be spontaneously emitted. I know of no report of anyone ever creating a pure green source of [OIII] emission visible to the eye in a lab ever - let alone in a discharge tube you can buy. If you have a reference to anyone actually seeing the pure glow of [OIII] in a laboratory environment - I would be very interested to see it.

On the other hand - yes, a monochromator can easily provide a bright source of that color - and you can hold a narrow band OIII filter to a white light and see it there, too. You can also take a DSLR image of those sources and compare them to what you see with your eyes - and it will match very well. DSLR's and their Bayer arrays do very well to match color perception - and narrow band emission is no different.

zloq

[Chris Peterson]

OK - you are describing something that does not exist and never has.

Rubbish. I built professional astronomical spectroscopes for years, and many had these gas tubes built right into the optical train, to directly create reference lines alongside the measured spectrum. The tube provides the 495.9/500.7 doublet, with the peaks distinctly resolvable using a good spectroscope.

DSLRs generally don't do a very good job of creating the same color green that you'll see through an OIII filter, for the reasons I previously mentioned. It is possible to do better if you access the raw data and de-Bayer it yourself, but that is difficult, and most people use the de-Bayering routine in the camera or their raw importer to do that. Those routines are optimized for terrestrial sources.

[neufer]

<<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. In formal physics, this means that the process cannot proceed via the most efficient (electric dipole) route. Although the transitions are nominally "forbidden", there is a small probability of their spontaneous occurrence, should an atom or molecule be raised to an excited state. More precisely, there is a certain probability that such an excited atom will make a forbidden transition to a lower energy state per unit time; by definition this probability is much lower than that for any transition permitted by the selection rules. Therefore, if a state can de-excite via a permitted transition (or otherwise, e.g. via collisions) it will almost certainly do so rather than choosing the forbidden route. Nevertheless, "forbidden" transitions are only relatively unlikely: states that can only decay in this way (so-called meta-stable states) usually have lifetimes of order milliseconds to seconds, compared to less than a microsecond for decay via permitted transitions.

Forbidden emission lines have only been observed in extremely low-density gases and plasmas, either in outer space or in the extreme upper atmosphere of the Earth. Even the hardest laboratory vacuum on Earth is still too dense for forbidden line emission to occur before atoms are collisionally de-excited.

However, in space environments, densities may be only a few atoms per cubic centimetre, making atomic collisions unlikely. Under such conditions, once an atom or molecule has been excited for any reason into a meta-stable state, then it is almost certain to decay by emitting a forbidden-line photon. Since meta-stable states are rather common, forbidden transitions account for a significant percentage of the photons emitted by the ultra-low density gas in space.

Forbidden line transitions are noted by placing square brackets around the atomic or molecular species in question, e.g. [O III] or .

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.>>

<<Forbidden line: An emission line found in the spectrum of a rarefied gas under special conditions, such as those found in some nebulae, the solar corona, and parts of active galactic nuclei. Although not seen on Earth – hence their name – forbidden lines may account for 90% or more of the total visual brightness of an object such as a planetary nebula.

A forbidden line arises when an electron in an excited (energized) atom jumps from a metastable state to a lower energy level. Under normal circumstances, when particle densities are higher (greater than about 10⁸ per cm³), such an electron would almost immediately be knocked out of its metastable state by collision and not be given time to emit a photon. But in an environment like that of a planetary nebula, the time between collisions averages 10 to 10,000 seconds. Consequently, when ions such as O+, O2+ (singly and doubly ionized oxygen), or N+ (singly ionized nitrogen) go into metastable states by allowed transitions from higher states, they remain there undisturbed until they radiate spontaneously. A large fraction of the more highly excited ions eventually drop into these states and, in a nebular environment, practically every ion goes from them to the ground state by forbidden radiation.

Forbidden lines are denoted by enclosing them in brackets. The strongest are two lines of doubly ionized oxygen [O III], in the green part of the spectrum (to which the human eye is most sensitive) at 495.9 nm and 500.7 nm. When these lines were first seen, in the spectra of planetary nebula in the 1860s, their true nature wasn't recognized and it was thought they might be due to a new element, which was dubbed "nebulium." More than half a century passed before Ira Bowen provided the right explanation. Besides forbidden lines of ionized oxygen, others of neon, nitrogen, and other relatively abundant elements are seen making up the light of nebulae, as well as the ordinary, permitted lines of hydrogen and helium.>>

[DavidLeodis]

I was very surprised to read that Hanny's Voorwerp "is about the size of our own Milky Way Galaxy". I have heard of Hanny's Voorwerp before (it has even been the subject of a previous APOD) but I had clearly forgotten that it was so big. I thought it was just a small but strange object! and