APOD: Seyfert's Sextet (2013 Dec 10)

[smitty]

Thanks, Chris; and that's what suggested to me that the pair of stars we were discussing -- because they exhibit faint spikes -- might be relatively nearby, as also suggested in a comment Ann made regarding their relative motion.

[Anthony Barreiro]

Diffraction spikes are caused by any point or confined source of bright light. They are not a good indicator of distance.
http://en.wikipedia.org/wiki/Diffraction_spike

For the casual viewer of an astrophotograph, diffraction spikes are a good way to distinguish between relatively bright stars in our own galaxy and more diffuse objects that usually are farther away. The length of the diffraction spikes tells you how bright the star appears, not how far away it is -- but if you see diffraction spikes you can surmise that it's a star, in our own galaxy, and probably relatively close. I won't be surprised when somebody with more expertise than I have posts a contrary counterexample, but I still think this is a useful rule of thumb.

[neufer]

Regarding the distance to the pair of stars, I'd mentioned in a previous post that I took the faint specular "spikes" on the two to be an indication that they are relatively nearby. Is this a reliable indicator? Often, in other apods, nearby stars in the Milky Way appear with prominent spikes. I'm not sure about all the physics involved, but it seems quite common and predictable in apod images.

Diffraction spikes are caused by any point or confined source of bright light. They are not a good indicator of distance.
http://en.wikipedia.org/wiki/Diffraction_spike

• It is not a bad rule of thumb in general, however; ...OTOH:

[img3="Hubble picture showing a quasar's "host galaxy" with bright center blocked"]http://upload.wikimedia.org/wikipedia/c ... Hubble.jpg[/img3]

<<Quasars tend to inhabit the very centers of active, young galaxies, and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way, which contains 200–400 billion stars. This radiation is emitted across the spectrum, almost equally, from X-rays to the far-infrared with a peak in the ultraviolet-optical bands, with some quasars also being strong sources of radio emission and of gamma-rays. In early optical images, quasars looked like single points of light (i.e., point sources), indistinguishable from stars, except for their peculiar spectra. With infrared telescopes and the Hubble Space Telescope, the "host galaxies" surrounding the quasars have been identified in some cases. These galaxies are normally too dim to be seen against the glare of the quasar, except with special techniques. Most quasars cannot be seen with small telescopes, but 3C 273, with an average apparent magnitude of 12.9, is an exception. At a distance of 2.44 billion light-years, it is one of the most distant objects directly observable with amateur equipment.

[geckzilla]

Yes, neufer, that's exactly why I said that. There are times when I look at things where I'm not sure if I'm seeing a quasar or some other kind of active galaxy nucleus, a supernova, or just a local star. Some images get presented without captions so it is important to keep all those options in mind.

[Chris Peterson]

Yes, neufer, that's exactly why I said that. There are times when I look at things where I'm not sure if I'm seeing a quasar or some other kind of active galaxy nucleus, a supernova, or just a local star. Some images get presented without captions so it is important to keep all those options in mind.

Do you have a tool that lets you measure the FWHM, or view stellar profiles? Usually, quasars and bright galaxies are detectible as extended sources if you look at their profiles. It's rare for one of these objects to be a true point source and have the same shape as a star.

[Anthony Barreiro]

The double star looks quite bluish to me. But then, the galaxies in Seyfert's Sextet look quite bluish, too. However, these galaxies are certainly intrinsically yellow, since they are clearly evolved (except the "vertical" elongated one that looks grainy because it is full of young clusters, and also apart from the face-on spiral galaxy). The upper two galaxies, in particular, lack any sign of star formation, and the one on the right lacks any sign of dust. Both the upper galaxies are certainly intrinsically yellow, and yet they look quite blue. Is that a criticism? No, it is an observation, and it means that the double star, which appears to be about the same color as the galaxies, is likely also yellow.

Since this is mine, I can post whatever images I have of it. Anytime I grab some data, the first thing I try to do after getting the FITS files and converting them to TIFF is to get all of the darkest parts of the empty space to be as close to neutral black (actually a dark gray) as possible. Without fail, this is what I am looking at by the time that is done.

Seyferts_Sextet_yellow.jpg

Since it looks like we're viewing the picture through a ancient, yellowed piece of plastic, it's easy to see why I would make some major adjustments. Between all the filters, varying exposure times, redshifting and general mysteriousness of space I never know what an absolute color is. Can't say it's worth agonizing over.

This is much closer to how these galaxies would appear through the eyepiece of a large telescope under excellent observing conditions than the brilliant reds and blues of a typical astrophotograph. I'm not saying one is better or worse than another. But visitors to the observatory where I volunteer are often underwhelmed by views of galaxies. They expect to see bright colors and fine detail. They need a bit of coaching to comprehend that they're seeing the combined light of hundreds of billions of stars that has been traveling through space for tens of millions of years.

[geckzilla]

Yes, neufer, that's exactly why I said that. There are times when I look at things where I'm not sure if I'm seeing a quasar or some other kind of active galaxy nucleus, a supernova, or just a local star. Some images get presented without captions so it is important to keep all those options in mind.

Do you have a tool that lets you measure the FWHM, or view stellar profiles? Usually, quasars and bright galaxies are detectible as extended sources if you look at their profiles. It's rare for one of these objects to be a true point source and have the same shape as a star.

I'm afraid all I've got are my two eyes. I use the FITS Liberator because it's free and it only converts FITS files to TIFF. What do you use?

Just as a side note because I think it's interesting, sometimes diffraction spikes have uses, which I'm sure is old news to anyone with experience. I was messing with a picture of the Red Rectangle and the center was blown out but the diffraction spikes told a story of how the star has a band of dust across it.

This is much closer to how these galaxies would appear through the eyepiece of a large telescope under excellent observing conditions than the brilliant reds and blues of a typical astrophotograph. I'm not saying one is better or worse than another. But visitors to the observatory where I volunteer are often underwhelmed by views of galaxies. They expect to see bright colors and fine detail. They need a bit of coaching to comprehend that they're seeing the combined light of hundreds of billions of stars that has been traveling through space for tens of millions of years.

I'd love to look through one of those someday. I thought the people operating big telescopes have better things to do than let curious visitors peer confusedly through them and then deal with their disappointment.

[Chris Peterson]

I'd love to look through one of those someday. I thought the people operating big telescopes have better things to do than let curious visitors peer confusedly through them and then deal with their disappointment.

Having looked (visually) through several very large telescopes, I can tell you that practice does conform to theory, and the view through these scopes offers no brighter or more colorful view of galaxies than you see through typical amateur telescopes. All the large scopes offer visually is the possibility of more magnification. With many objects, that allows us to see more detail, because more of the retina is involved. And in some cases, there is a perception of greater brightness for the same reason. But for galaxies, the view is seldom any better with a large scope than one in the 8-12 inch class. I've visually used both the 60-inch and 100-inch telescopes at Mt Wilson, the 200-inch telescope at Palomar (a rare treat), two 40-inch telescopes (one amateur), and quite a few 20-24 inch telescopes.

[Chris Peterson]

Do you have a tool that lets you measure the FWHM, or view stellar profiles? Usually, quasars and bright galaxies are detectible as extended sources if you look at their profiles. It's rare for one of these objects to be a true point source and have the same shape as a star.

I'm afraid all I've got are my two eyes. I use the FITS Liberator because it's free and it only converts FITS files to TIFF. What do you use?

I have an assortment of astronomical image processing tools. Mainly, I use Maxim. For some more esoteric processing I use IRAF, which is freely available. Sextractor, also free, will extract all the sources in an image and perform profile analysis, and will usually distinguish stars from extended objects that appear stellar visually (and provide both a size and an axis of elongation, if there is one).

[Ann]

Thanks, geckzilla. The slight specular spikes on the pair suggests that they're nearer us, perhaps within our Milky Way? I recall seeing a beautiful yellow-purple pair of binary stars somewhere in the night sky, but don't recall offhand exactly where. Lyra perhaps?

Albireo, I would guess, in Cygnus. The contrasting colors, yellow and blue, make the color much more apparent than we'd normally notice if the stars were isolated.

Albireo is certainly the most famous of all "color contrast" stars. A very good thing about Albireo is that it is bright and very easy to find, and it can be found almost all year round for those in the northern hemisphere.

Personally i am somewhat underwhelmed by Albireo. The K-type primary is relatively pale for a red giant - possibly because its color has been diluted by a hot spectroscopic companion - and the B-type secondary is not strongly blue, either. The B-V values for the primary and the secondary are +1.09 (Johnson) or 1.278 (Tycho), and -0.095 (Johnson) or -0.072 (Tycho).

To those of you would like to see a really good color contrast "triple", 30 and 31 Cygni. 31 Cygni consists of a K-type primary whose B-V index is +1.279 (Johnson) or +1.401 (Tycho), and a B-type secondary whose B-V index is -0.133 (Johnson) or -0.129 (Tycho). In any case, the red primary of 31 Cygni is redder than the red primary of Albireo, and the blue secondary of 31 Cygni is bluer than the blue secondary Albireo.

Interestingly, 30 Cygni is located quite close to 31 Cygni, and this A-type star is redder than 31 Cygni B but a lot bluer than 31 Cygni A. When I looked at this stellar trio, I saw one orange-yellow, one blue and one cyan-tinted star. Very interesting!

Ann

[dwainwilder]

The 2002 October 4 photo of NGC 6946 is shown as an example of a face-on spiral galaxy; it is not an image of the spiral galaxy that is captured in the image of Seyfert's Sextet. NGC 6946 is in the constellation Cepheus; Seyfert's Sextet is in Serpens. The small spiral galaxy in the background of Seyfert's Sextet has been designated NGC 6027d.

Thanks for the clarification. But, to the nub of my inquiry, any comment on how the face-on galaxy shown with (but not of) the Seyfert's Sextet has a somewhat square perimeter, with major arms parallel to each side? Most peculiar, and wondering whether there are some unusual dynamics at play there.

[Ann]

The 2002 October 4 photo of NGC 6946 is shown as an example of a face-on spiral galaxy; it is not an image of the spiral galaxy that is captured in the image of Seyfert's Sextet. NGC 6946 is in the constellation Cepheus; Seyfert's Sextet is in Serpens. The small spiral galaxy in the background of Seyfert's Sextet has been designated NGC 6027d.

Thanks for the clarification. But, to the nub of my inquiry, any comment on how the face-on galaxy shown with (but not of) the Seyfert's Sextet has a somewhat square perimeter, with major arms parallel to each side? Most peculiar, and wondering whether there are some unusual dynamics at play there.

M61.
Adam Block/NOAO/AURA/NSF

It is not entirely unusual for spiral galaxies to have arms that are "oddly shaped" one way or another. Messier 61, a well-known galaxy in Virgo, resembles the face-on spiral galaxy seen in the field of Seyfert's Sextet in that it has a somewhat "square" shape.

Ann

[Anthony Barreiro]

This is much closer to how these galaxies would appear through the eyepiece of a large telescope under excellent observing conditions than the brilliant reds and blues of a typical astrophotograph. I'm not saying one is better or worse than another. But visitors to the observatory where I volunteer are often underwhelmed by views of galaxies. They expect to see bright colors and fine detail. They need a bit of coaching to comprehend that they're seeing the combined light of hundreds of billions of stars that has been traveling through space for tens of millions of years.

I'd love to look through one of those someday. I thought the people operating big telescopes have better things to do than let curious visitors peer confusedly through them and then deal with their disappointment.

If you're ever in Oakland CA on a Friday or Saturday night when the weather is decent, visit Chabot Space and Science Center for free viewing through an 1883 8-inch Alvan Clark and Sons refractor, a 1915 20-inch Warner and Swasey refractor with Brashear optics, and a modern 30-inch Cassegrain that was built by Chabot volunteers. The 20-inch refractor was the only telescope in the right place to track the final approach of the disabled Apollo 13 spacecraft. When NASA called, the telescope was being used by local high school students, including Tom Hanks!

The other nights of the week the Cassegrain is used to search for near-Earth objects and to confirm exoplanets, as well as for other research projects, but on Friday and Saturday nights Chabot offers the public a view to the stars.

As for visitors' level of understanding and appreciation, that depends much more on the interpretation provided by human volunteers than on the telescopes themselves. In many ways the telescopes are just the hook that draws visitors in. Much of what they remember comes from conversation with volunteers.

So you could take Chris' word for it, and just stay home with the Hubble archive ... . But seriously, there is a special magic in seeing a galaxy for yourself in real time.

[Chris Peterson]

So you could take Chris' word for it, and just stay home with the Hubble archive ... . :ssmile: But seriously, there is a special magic in seeing a galaxy for yourself in real time.

I didn't say that! All I said is that a large telescope doesn't generally offer much better views of galaxies than small ones- and viewing opportunities through small telescopes are much easier to find.

[Anthony Barreiro]

So you could take Chris' word for it, and just stay home with the Hubble archive ... . But seriously, there is a special magic in seeing a galaxy for yourself in real time.

I didn't say that! All I said is that a large telescope doesn't generally offer much better views of galaxies than small ones- and viewing opportunities through small telescopes are much easier to find.

Sorry for my sloppy misinterpretation.

[Joe New]

What's the apparent double star just to the right of the main cluster, at about the two o'clock position? Thanks!

[geckzilla]

What's the apparent double star just to the right of the main cluster, at about the two o'clock position? Thanks!

There's some discussion on it/them throughout the thread.

[Joe New]

What's the apparent double star just to the right of the main cluster, at about the two o'clock position? Thanks!

There's some discussion on it/them throughout the thread.

Oops, sorry.