Starship Asterisk*

by **Tarree** » Wed Jul 27, 2011 6:07 pm

Got it, thanks.

by **Ann** » Wed Jul 27, 2011 5:57 pm

JLSmith wrote:To my view the spiral galaxy above NGC 474 mentioned in the article looks remarkably unaffected to be under going a collision/gravational interaction with the much larger NGC 474. I don't see any extended tidal tails or other effects; maybe a little bending in the outer arms. If these galaxys were actually interacting wouldn't the smaller galaxy be more affected?

These two galaxies aren't interacting very much. The shells and jets of NGC 474 were almost certainly caused by at least one past merger with a smaller galaxy, which has now been consumed.

Tarree wrote:

Any possibility that we are viewing the 474 with an intervening black hole and the visual effects are caused by gravity lensing?

An intervening black hole wouldn't cause that sort of visual distortions in a galaxy. Check out this page to see what kind of visual effects would be caused by an intervening black hole: http://www.sciencephoto.com/media/334002/enlarge.

Ann

by **Tarree** » Wed Jul 27, 2011 5:27 pm

Any possibility that we are viewing the 474 with an intervening black hole and the visual effects are caused by gravity lensing?

Yes, I am a newbie.

by **Scott Souza** » Wed Jul 27, 2011 5:06 pm

I wonder if this galaxy has a name. If not I'd like to propose 'The Mushroom Galaxy.' It has three or four large structures emerging from various points which look like mushrooms to my eye. - Scott Souza

by **saturn2** » Wed Jul 27, 2011 2:50 am

Elliptical Galaxy N G C 474 / Diameter 250,000 light years.
Elliptical Galaxy Milky Way / Diameter 100,000 light years.
The halos of large galaxies are the product of the interaction with ( and accretions of ) smaller nearby galaxies.
The accretion of matter is very forte.

by **neufer** » Wed Jul 27, 2011 1:51 am

alter-ego wrote:
To first order, the airy disk size (diameter of 1st minimum of an airy pattern) remains the same, and, as Chris said, the fraction of power in the central lobe (Strehl Ratio) goes down for obstructed apertures. Planetary viewing is the classic example for using refractors over obstructed reflectors. The important net result of brighter airy rings is a reduction contrast for details on extended objects, commonly planets with having low contrast featurs (bands, clouds, surface details). Thus it is harder to see low contrast details with an obstructed aperture. I imagine the same could be said for very close binaries or a globular cluster too. The contrast reduction problem applies to all extended objects, just some objects are affected worse. Planets are the classic examples.

This site shows nice diffraction simulations of obstructed apertures and the convolution of apertures with spiders

Very interesting, alter-ego, thanks.

Of course the largest refractor objectives are far smaller than the secondaries on most reflectors to begin with.

And then there is the critical importance of atmospheric seeing for ground based telescopes without adaptive optics.

by **JLSmith** » Wed Jul 27, 2011 1:17 am

To my view the spiral galaxy above NGC 474 mentioned in the article looks remarkably unaffected to be under going a collision/gravational interaction with the much larger NGC 474. I don't see any extended tidal tails or other effects; maybe a little bending in the outer arms. If these galaxys were actually interacting wouldn't the smaller galaxy be more affected?

by **alter-ego** » Wed Jul 27, 2011 12:47 am

neufer wrote:
Chris Peterson wrote:
neufer wrote:
Wouldn't the stellar halos be due to the round secondary mirror
(i.e., the one held in place by the 'linear structures in the optical path')
You'd get almost identical halos whether or not the aperture was obstructed. An obstructed aperture does produce a slightly different diffraction pattern- the secondary mirror diffracts additional light out of the central zone (the Airy disc) and into the surrounding diffraction rings. But the effect is small on stellar images.
The secondary mirror is 8 times smaller than the primary and therefore will produce faint but broad Airy disks 8 times wider (~ 0.4 arcsecond). These secondary induced Airy disks will likewise be 8 times wider than the width of the diffraction spikes...which looks about right to me.
http://www.answers.com/topic/hubble-space-telescope wrote:
The Hubble Space Telescope ... is a reflecting telescope with a primary mirror 2.4 m in diameter. The primary mirror directs light from astronomical objects to a 30-cm secondary mirror. Its angular resolution of 0.05 arcsecond at optical wavelengths.

To first order, the airy disk size (diameter of 1st minimum of an airy pattern) remains the same, and, as Chris said, the fraction of power in the central lobe (Strehl Ratio) goes down for obstructed apertures. Planetary viewing is the classic example for using refractors over obstructed reflectors. The important net result of brighter airy rings is a reduction contrast for details on extended objects, commonly planets with having low contrast featurs (bands, clouds, surface details). Thus it is harder to see low contrast details with an obstructed aperture. I imagine the same could be said for very close binaries or a globular cluster too. The contrast reduction problem applies to all extended objects, just some objects are affected worse. Planets are the classic examples.

This site shows nice diffraction simulations of obstructed apertures and the convolution of apertures with spiders

by **Chris Peterson** » Tue Jul 26, 2011 8:42 pm

neufer wrote:The secondary mirror is 8 times smaller than the primary and therefore will produce faint but broad Airy disks 8 times wider (~ 0.4 arcsecond). These secondary induced Airy disks will likewise be 8 times wider than the width of the diffraction spikes...which looks about right to me.

I believe this image was captured with the CFHT on Hawaii, which in this configuration does not have a secondary mirror, but utilizes the MegaCam wide field camera mounted directly at the prime focus. The silhouette of the camera therefore becomes the aperture obstruction, and I don't know the details of its size or shape.

In any case, with an image pushed this hard to get very deep structure, scattered light around the stars is probably a bigger effect than diffraction.

by **neufer** » Tue Jul 26, 2011 7:31 pm

Chris Peterson wrote:
neufer wrote:
Wouldn't the stellar halos be due to the round secondary mirror
(i.e., the one held in place by the 'linear structures in the optical path')
You'd get almost identical halos whether or not the aperture was obstructed. An obstructed aperture does produce a slightly different diffraction pattern- the secondary mirror diffracts additional light out of the central zone (the Airy disc) and into the surrounding diffraction rings. But the effect is small on stellar images.

The secondary mirror is 8 times smaller than the primary and therefore will produce faint but broad Airy disks 8 times wider (~ 0.4 arcsecond). These secondary induced Airy disks will likewise be 8 times wider than the width of the diffraction spikes...which looks about right to me.

http://www.answers.com/topic/hubble-space-telescope wrote:
The Hubble Space Telescope ... is a reflecting telescope with a primary mirror 2.4 m in diameter. The primary mirror directs light from astronomical objects to a 30-cm secondary mirror. Its angular resolution of 0.05 arcsecond at optical wavelengths.

by **Ann** » Tue Jul 26, 2011 7:09 pm

British-Australian astrophotographer David Malin was one of the first to notice that many large elliptical galaxies have "shells" of halos, like onions. He first noted shells around NGC 5129, also known as Centaurus A or Cen A. In his book, A View of the Universe, David Malin writes about how he himself and two of his colleagues, Peter Quinn and John Graham, made sense of the shells:

From the distibution and size of the shells, Peter Quinn was able to calculate that Centaurus A had encountered another, less massive galaxy, perhaps something the size of M33. The merger is still far from consummated and the intense activity that makes Cen A such a strong radio source is only the latest manifestation of an interaction that has been under way for about a billion years. The interaction also led to a very nice paper by Malin, Quinn and Graham on the dynamics of this fascinating southern galaxy.

This is what NGC 5128 or Cen A usually looks like:

Credit: ESO

This is what it looks like when the Blanco Telescope reveals its shells:

: Credit: E. Peng and H. Ford ([url=http://www.pha.jhu.edu/]JHU[/url]), K. Freeman ([url=http://www.anu.edu/]ANU[/url]), R. White ([url=http://www.stsci.edu/]STScI[/url]), [url=http://www.ctio.noao.edu/]CTIO[/url], [url=http://www.noao.edu/]NOAO[/url], [url=http://www.nsf.gov/]NSF[/url]

The point made by David Malin is that mergers cause the shells. Therefore, shells around large elliptical galaxies speak of past mergers.

Ann

by **Sam** » Tue Jul 26, 2011 5:38 pm

neufer wrote:"Dance of the seven veils"...

Click to play embedded YouTube video.

--
"Sweet Holy Moses" Sam

by **Chris Peterson** » Tue Jul 26, 2011 5:26 pm

neufer wrote:Wouldn't the stellar halos be due to the round secondary mirror
(i.e., the one held in place by the 'linear structures in the optical path') :?:

You'd get almost identical halos whether or not the aperture was obstructed. An obstructed aperture does produce a slightly different diffraction pattern- the secondary mirror diffracts additional light out of the central zone (the Airy disc) and into the surrounding diffraction rings. But the effect is small on stellar images.

by **neufer** » Tue Jul 26, 2011 5:12 pm

Chris Peterson wrote:
Charlie Mercer wrote:
It appears to me that ALL stars ( and faintly ) even the other galaxy have HALOs, not just spikes. this is the only one I have seen with this anomaly.?? Actual or camera induced.?
The stellar halos are artifacts. They are some combination of scatter off the optics and diffraction. Whereas diffraction spikes are produced by linear structures in the optical path, the aperture itself (which is round) is the major source of diffraction, and that diffraction is seen as stars with a finite diameter (optically, they are point sources), and surrounded by rings (which tend to blur together into a halo with long exposures through the atmosphere).

Wouldn't the stellar halos be due to the round secondary mirror
(i.e., the one held in place by the 'linear structures in the optical path')

by **Chris Peterson** » Tue Jul 26, 2011 4:58 pm

Charlie Mercer wrote:As one who is unable to comprehend the distances and time of our beautiful universe one thing in the shot of NGC474 stands out and begs explanation. It appears to me that ALL stars ( and faintly ) even the other galaxy have HALOs, not just spikes. this is the only one I have seen with this anomaly.?? Actual or camera induced.?

The stellar halos are artifacts. They are some combination of scatter off the optics and diffraction. Whereas diffraction spikes are produced by linear structures in the optical path, the aperture itself (which is round) is the major source of diffraction, and that diffraction is seen as stars with a finite diameter (optically, they are point sources), and surrounded by rings (which tend to blur together into a halo with long exposures through the atmosphere).

by **Charlie Mercer** » Tue Jul 26, 2011 4:18 pm

As one who is unable to comprehend the distances and time of our beautiful universe one thing in the shot of NGC474 stands out and begs explanation. It appears to me that ALL stars ( and faintly ) even the other galaxy have HALOs, not just spikes. this is the only one I have seen with this anomaly.?? Actual or camera induced.?
This is a facinating and wonderful website.!!!!
Charlie

by **8Khigh** » Tue Jul 26, 2011 4:12 pm

Well... my eyes are highly likely to be more untrained than anyone here and likely older than many.

However, my brain immediately saw a time-dependant pattern I wonder if others saw... Consider a small galaxy/cluster starting from the left of the picture, approaching parallel to 474 and above it. As it approaches 474's disc, the gravitationally spread density of 474 starts to spread it AND capture it. It's an expanding 'sheet' as it then is captured and drops down and behind 474 on the right and wraps around to the left. By the time it reappears on the left side of 474, it's now wrapping top to bottom. Ultimately, it will form a shell surrounding 474. All of what I see now my brain won't let go of this model. Anyway, thought I'd share. Nope... it's not the heat getting to me, either.

by **Beyond** » Tue Jul 26, 2011 3:03 pm

Neufer posted " Strauss: Salome Dance of the 7 Veils, part 2"

Well... At least we got to see all the Veils... I think.

by **orin stepanek** » Tue Jul 26, 2011 2:11 pm

what an interesting looking galaxy. It kind of looks like a CD immersed in a cloudy liquid.

The star trails don't seem to lead to another galaxy; so it seems to have about finished digesting the smaller galaxy!

by **neufer** » Tue Jul 26, 2011 1:30 pm

biddie67 wrote:
Dear old NGC 474 - she's like a dancer out in the garden under the moonlight - slowly turning and swirling her veils to her own secret music.

http://en.wikipedia.org/wiki/Dance_of_the_seven_veils wrote:
[youtube]http://www.youtube.com/watch?v=yS_Z68XpB-o&NR=1[/youtube]
<<The Dance of the Seven Veils is thought to have originated with the myth of the goddess Ishtar and the god Tammuz of Assyrian and Babylonian lore. In this myth, Ishtar decides to visit her sister, Ereshkigal, in the underworld. When Ishtar approaches the gates of the underworld, the gatekeeper lets Ishtar pass through the seven gates, opening one gate at a time. At each gate, Ishtar has to shed an article of clothing. When she finally passes the seventh gate, she is naked. In a rage, Ishtar throws herself at Ereshkigal, goddess of the underworld; but Ereshkigal orders her servant Namtar to imprison Ishtar and unleash sixty diseases against her. After Ishtar descends to the underworld, all sexual activity ceases on earth. Papsukkal, the messenger-god, reports the situation to Ea, king of the gods. Ea creates a eunuch called Asu-shu-namir and sends him to Ereshkigal, telling him to invoke "the name of the great gods" against her and to ask for the (doy?) bag containing the waters of life. Ereshkigal, having promised to grant Asu-shu-namir's wish, is enraged when she hears the demand, but she has to give him the water of life. Asu-shu-namir sprinkles Ishtar with this water, reviving her. Then Ishtar passes back through the seven gates, getting one article of clothing back at each gate, and is fully clothed as she exits the last gate. Her release is, however, granted only under the condition that she find someone to replace her in the underworld. Tammuz, Ishtar's husband, has been making merry while she has been dead, and so the goddess sends Tammuz to Ereshkigal.>>

by **Chris Peterson** » Tue Jul 26, 2011 1:25 pm

Mikey wrote:I often wondered why the spikes always seem to be perpendicular to the viewing frame (perfectly vertical and/or horizontal) and not eskew.

The spikes are perpendicular to the structures that produce the diffraction, which in most cases are the spider arms supporting the secondary mirror. If the spikes lie on the horizontal and vertical axes of the image, it only means that the camera was aligned with the mirror supports. But that is often not the case, and you'll see many astronomical images here on APOD where the diffraction spikes are not orthogonal with the image axes.

You sometimes see images made with telescopes having three mirror support arms, which results in six diffraction spikes, not four. Each arm produces a pair of opposed spikes. Most telescopes support their secondary mirror with four arms. That produces eight spikes, but since opposite arms are parallel, their spikes lie on top of each other, and we only see four discrete spikes.

by **Mikey** » Tue Jul 26, 2011 1:02 pm

"Their light arrives into the camera "photon after photon" along a straight line, so to speak." - Ann

I often wondered why the spikes always seem to be perpendicular to the viewing frame (perfectly vertical and/or horizontal) and not eskew. I like your explanation. However, you are incorrect about not being the right person to offer the explanation. Thanks.

by **Chris Peterson** » Tue Jul 26, 2011 12:56 pm

Ann wrote:A galaxy is a diffuse patch of light. In most cases, the galaxy is too far away for any of the individual stars to be resolved. But even when the stars are resolved, they are very faint. The photons from these resolved stars arrive very intermittently. At the same time, the camera is detecting photons from all over the face of the galaxy. You simply don't have the concentrated narrow "jet" of photons that causes diffraction spikes.

The entire galaxy also produces diffraction artifacts. But because a galaxy is an extended source, they aren't "spikes" as such, but big broad zones. And because the amount of light that ends up being diffracted is only a small percentage of the total, and the galaxy is pretty faint to begin with (typically orders of magnitude less brightness than the stars with visible spikes), the diffraction is simply not visible.

However, some galaxies have very bright and star-like nuclei. It might be possible for such a bright galactic nucleus to cause diffraction spikes similar to diffraction spikes around stars.

And so they do. But again, even the brightest galaxy cores are dim compared with the stars we see spikes around. It doesn't always seem so in images, because once you get to white, nothing appears brighter. But in a typical galaxy image, stars with diffraction spikes are many times whiter than white. To see diffraction spikes around a galaxy core, you have to really overexpose the galaxy, which hardly ever happens. Where you see galaxies with diffraction spikes is in very deep scientific images, where distant background galaxies are almost point-like, and are not the targets of interest.

by **biddie67** » Tue Jul 26, 2011 12:07 pm

Dear old NGC 474 - she's like a dancer out in the garden under the moonlight - slowly turning and swirling her veils to her own secret music.

by **neufer** » Tue Jul 26, 2011 11:22 am

Ann wrote:
seeley wrote:
......in this pic the stars have the characteristic lines pointing N,S,E,and W which I am informed is due to elements within the telescope. But the galaxy pictured does not display this feature, although as far as I know the telescope is simply picking up its light, same as with the stars. Can anyone tell me why a star appears like this but not anything else picked up by the same telescope?
A galaxy is a diffuse patch of light. In most cases, the galaxy is too far away for any of the individual stars to be resolved. But even when the stars are resolved, they are very faint.

'They are very faint'...is the answer.

None of the stars here (near or far) are resolved and they are all producing characteristic but faint diffraction pattern of lines 'pointing N,S,E,and W.' It's just that such faint diffraction patterns can only be perceived on the extremely over exposed stars. These over exposed stars are also the ones that show up as large bright dots with halos rather than small points of light. If the exposure times were reduced so that even bright nearby stars showed up just as small points of light then their diffraction patterns would also be indiscernible.

Starship Asterisk*

APOD: Galaxy NGC 474: Cosmic Blender (2011 Jul 26)

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