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Finally, Bortle 2 skies February 9, 2008

Posted by dorigo in astronomy, personal, science.
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It was sky transparency. We had suspected that Casera Razzo, the site I occasionally visit with a few amateur astronomer buddies in our deep-sky observing sessions, had the potential to offer a very dark, almost perfect sky; but only once in our dozen visits we had experienced it, while in all other cases the sky had been dark but had left something wanting.

We suspected that the factor that had been adverse in most cases was the transparency of the sky, but we needed some confirmation – after all, there are indeed many possible causes for a light-polluted atmosphere, some of them due to human activity and some others due to atmospheric conditions. Yesterday we reached the same level of quality of our formerly best night at the site – April 14th, 2007-, and we convinced ourselves that light scattered by particles and humidity in the atmosphere is a major factor affecting the darkness of the night in the eastern Alps. Light from towns 50 km away is masked well by the mountains surrounding the site, but the atmosphere needs to be transparent above your head if you want to avoid the photons from those far-away sodium lamps to bounce back  there and hit you.

Mauro’s sky quality meter -a calibrated exposimeter yielding a reading of the sky’s visual magnitude per squared arcsecond – started off at 21.42 at 10.50PM, and as lights in nearby towns were turned off it consistently grew to reach a 21.58 reading at 2AM yesterday night. The latter reading corresponds to a “Bortle-2” sky – just one notch below the best possible sky, which corresponds to readings of 21.9 or 22.0. Of course, those 0.4 mags of difference mean a whole lot when one observes faint galaxies visually, but 21.6 is probably the best one can hope for on italian territory. Better values can probably be found only far, far away from northern Italy; even professional sites do not often go above 21.6: for instance,  it is a typical reading at sites such as the Roque de los Muchachos, at La Palma – where several observatories are located. See for instance the following plot, taken from the site of the observatorio:

 

In the plot you see that the V-band magnitude (y axis) reaches 21.5 at zenith on a typical night. The x axis shows the azimuthal direction where the measurement is made, and the different curves refer to different altitudes in degrees. 

So, what did we see yesterday night ? Well, a lot indeed. The temperature went from minus 5 to minus 11 degrees during the four hours of observation, and the wind was almost absent -I had feared it a lot before arriving there. However, the fact that the temperature did not stay constant prevented the mirror of the telescope to reach thermal equilibrium, and this affected the resolution quite a bit, and with it our possibility to push the magnification. For that reason, we mainly observed extended objects at 120x or 200x. Indeed, we were naturally led to spending most of our time on the real showpieces of the winter sky, which -when observed under truly dark skies- show picture-like detail with a 16″ scope. So, little time was spent on the faintest objects, which are usually small and require magnifications in excess of 400x.

A list of observed objects is a rather dry way for a description of the night. Rather, I only mention what really impressed me. Messier 51, the whirlpool galaxy, was one object on which we spent several minutes. It showed filamentary detail inside the main spiral arms, and was a really glorious sight – you felt you could pick it up by one arm, peeling it off the eyepiece lens as you would remove a dead insect from your windshield. And Messier 101, another face-on spiral galaxy in Ursa Major, did not pale in comparison: it showed several H-II regions in the arms, and the details in its structure were the best I ever saw on this object.

M101 is an extended object – covering almost a fourth of a squared degree of sky – and light pollution can make it utterly invisible even with large instruments! In fact, one often sees threads on popular amateur astronomy forums where it is discussed whether M101 is visually observable at all… Quite ironic: under dark skies this galaxy is a true beauty. The picture below (taken by italian amateurs from Verona) represents a good approximation of what was visible through the eyepiece. It is sights like these that keep me wanting for more nights out, hands and feet freezing and nothing else around but snow and silence.

Comments

1. Phil Warnell - February 9, 2008

Hi D,

A very interesting and informative post! I find one of your focuses of interest, M101, as being particularly interesting in as it is a spiral galaxy that appears at first similar to our own and yet perhaps not. I have read a recent paper by researchers ( http://th-www.if.uj.edu.pl/acta/vol38/pdf/v38p3859.pdf ) out of The Henryk Niewodniczański Institute of Nuclear Physics (Poland) where they have found no evidence to suspect that this Galaxy contains Dark Matter as our own suggests it requires. The researchers involved synopsize this in saying:

“To sum up, in galaxy M101 dark matter is not only unnecessary but its introduction would even cause problems.”

It appears to be harder and harder to explain why dark matter would exist in some galaxies and not in others. However to account for this it seems we must surrender symmetry in some respect. That is gravity is not consistent as to distance, matter is not consistent (type) throughout the universe or matter (as to type) is not consistent as regards to time. None of these in one way or another seem plausible..

The other thing I find fascinating when looking at a galaxy that appears so similar to our own is to imagine if someone is perhaps looking back. With the 24 million year separation between the two this furthers the wonder. You are correct that it is all worth the trouble and discomfort.

Regards,

Phil

2. dorigo - February 9, 2008

Hi Phil,

very interesting to know that the pinwheel galaxy has this peculiarity. Indeed, dark matter is a good explanation of cosmological observations, but I retain some skepticism. If one were forced to conclude that dark matter is absent from some parts of our universe, any particle physics explanation would crumble miserably. No SUSY neutralinos could be invoked to explain it…

About the thought of somebody looking back, it is indeed inspiring. Even more so is to think that we are in fact observing back in time how things were, millions of years ago… The thought that a chain of events started that back in time in order to bring a tiny electromagnetic quantum of energy inside my pupil is mindboggling.

Cheers,
T.

3. Amara - February 9, 2008

I like M51 for a lot of reasons, including that it is an object that can be photographed and appreciated well by amateur astronomers too. Check out this awesome photo by Tony and Daphne Hallas:
http://antwrp.gsfc.nasa.gov/apod/ap020710.html

That photo shows another reason why I like it so much: it is colliding with galaxy: NGC 5195. When I taught astronomy, I used this pair of galaxies to explain (idea I lifted from Andrew Fraknoi in one of his texts):

“Why do galaxies collide and stars rarely do?”

If you compare the distances between stars, they are *huge* compared to their physical sizes. For example, our Sun is about 17 million of its own diameters from its nearest neighbor. However, the distances between galaxies is often nearly that large compared their diameters. There are three satellite galaxies that are just one or two Milky Way diameters from us. The closest major spiral galaxy to our Milky Way is M31, just 24 of our galaxy diameters from us. And galaxies in rich clusters are even closer together than the members of our poor Local Group. Therefore, the chances of galaxies colliding are far greater than the chances of stars in the disk of a galaxy colliding. Neat piece of science, huh?

4. Phil Warnell - February 9, 2008

Hi T,
“Indeed, dark matter is a good explanation of cosmological observations, but I retain some skepticism.”

Yes, I to would like to hold on to this concept for more then one reason. In fact if you consider MOND in regards to what those in this paper propose it doesn’t stack up very well. Then on the other hand with many cluster galaxies and now as it seems some spirals not requiring either, one is left even more confused. Let’s hope you and you fellow particle smashers can lend some insight into all this.

Another thing I noticed is that many if not all of the stars in this galaxy appears blue. Do you know if this is considered as an indication of the M101 being young (star forming) or simply that it is approaching us (blue shifted)?

“The thought that a chain of events started that back in time in order to bring a tiny electromagnetic quantum of energy inside my pupil is mindboggling.”

Yes it is, then again everything we perceive as our reality is actually the past; this is true whether considering just across the room or across the (visible) universe. It is only differentiated by time and resolution. The (relative) past is our reality, while the future and the now remain ahead and beyond, more then just in manner of speaking.

Regards,

Phil

5. Amara - February 9, 2008

Looking further at the Hallas’ web site, I see that they have a strong
artistic flair: combining their photos in imaginative scenes.
http://www.astrophoto.com/images.htm

But, in keeping with the object described here, the Hallases
give wide field images of M51 and M101 that are jaw-dropping:

M101
http://www.astrophoto.com/M101LRGB.htm

M51
http://www.astrophoto.com/M101LRGB.htm

P.S. to Phil: the blue in the images is due to reflectance from dust (notice the haze around the Pleiades star cluster, for example). The dust particles are less than a micron which direct light most effectively _forward_ (this is called ‘forward scattering’). When we see blue starlight scattered off dust, then what we are seeing is a lot of dust on the front side of the object.

6. Amara - February 9, 2008

Oops, copied and pasted too fast: the wide scale M51 from the Hallases is here: http://www.astrophoto.com/M51WIDELRGB.htm

7. dorigo - February 10, 2008

Hi Amara,

yes, the physics of galaxy collisions is quite interesting! And indeed, those are the objects I most often aim my telescope at: interacting pairs of galaxies. The whirlpool is of course the best example, but there are many more as you know… I bought a copy of the Arp atlas and every night out I try to view a few new peculiar objects. Some are real showpieces: the antennae (Arp 244), the coccoon (NGC4485-4490), the “whale” (NGC4631-4627)… the list is very long. The other night I also gave a look at a very interesting pair in UMA: NGC3690, a very close pair of 12th and 13th magnitude galaxies.

Cheers,
T.

8. dorigo - February 10, 2008

Hi Phil,

I have grown wary to give too much weight to the color in a picture. Usually the best ones try to reproduce as faithfully as possible the spectrum of frequency with which the galaxy shines, but it is simply not possible to match the response function of our detection system (eye+brain) convoluted with the continuous emission spectrum of an object in a picture… However I think Amara’s explanation is a good answer as far as blue is concerned in the m51 pic.

Cheers,
T.

9. Phil Warnell - February 10, 2008

Hi Amara,

“When we see blue starlight scattered off dust, then what we are seeing is a lot of dust on the front side of the object.”

Thanks one again for the explanation. So you are saying that the colour is more telling of the density of the dust and not so much the nature of the stars or relative direction. Would also a large quantity of dust be consistent with a lot of potentially coalescent matter which in turn provides a good environment for star formation?

Regards,

Phil

10. Phil Warnell - February 10, 2008

Hi T,

“…but it is simply not possible to match the response function of our detection system (eye+brain) convoluted with the continuous emission spectrum of an object in a picture”

Sorry, once again I may appear to be a bit thick here. Are you saying that our eye/brain interpretation is contextually subjective and therefore unreliable or are you saying the spectra presented in the photo is not accurate?

Regards,

Phil

11. Amara - February 10, 2008

Dear Phil, Yes to the first question, and of the second, it depends how the colors in the images (these are visual wavelengths) are ‘mapped’ to a color scale. Our host has a nice post about this topic: here.

12. The continuing quest for dark skies « A Quantum Diaries Survivor - June 3, 2008

[…] 3, 2008 Posted by dorigo in physics. trackback I reported here several times in the past on my ongoing search for good sites for deep-sky observations in north-eastern Italy. A site which I had never tested and which was high in my priority list was […]


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