Saturday, June 27, 2009

The Planetaries of the Summer Triangle

Planetary Nebulae are the remnants of Sun-like stars. When our Sun is going to run out (literally!) of hydrogen gas in its core in approximately 5 billion years it will first increase in size up to 100 times its current diameter and then shed its outer layers while shrinking to the size of a planet like Earth and "retire" as a white dwarf star. The inner planets Mercury and Venus will be destroyed in the process. Mars will certainly survive whereas the destiny of Earth is uncertain. Some computer models indicate that Earth should make it, but the proximity to the giant dying Sun will obliterate any form of life still present at that time.

The material ejected by the dying star gives birth to a compact structure called Planetary Nebula. The name is misleading: these nebulae have nothing to do with planets. 18th century astronomers coined the term "planetary nebula" because of the similarity in appearance of these objects to giant planets (albeit much fainter!) when viewed through small telescopes.

Despite the grim Grand Finale, Planetary Nebulae constitute one of the most beautiful type of objects populating the night sky. The elaborated and colourful structure of Planetary Nebulae has been popularized in several famous images taken by the Hubble Space Telescope.

Three of the most famous Planetary Nebulae are located in the region of the Summer Triangle, the portion of the sky within the imaginary lines connecting Vega (α Lyrae), Deneb (α Cygnii) and Altair (α Aquilae). They are the Ring Nebula (Messier 57), the Blinking Nebula (NGC6826) and the Dumbbell Nebula (Messier 27):

I decided to image the three planetaries using my MallinCam HYPER Plus. Summer solstice has just gone by and Edmonton is immersed in perpetual twilight. Whatever is left of the night is very short and not exactly very dark. From my backyard on a moonless night, "darkness" these days lasts between 1am and 3am. The advantage of the MallinCam is that it allows for very short imaging sessions thanks to its superior sensitivity. My Canon DSLR is superior when it comes to overall image quality (after processing), but it requires data collection sessions many minutes long, very good polar alignment, autoguiding and calibration frames (darks and flats). No way I can image three targets in only two hours and so late during the night (I do have to go to work in the morning, after all...). With the MallinCam each planetary nebula was imaged by recording a 1000 frame video (AVI format) which, at a 30 frame per second rate, lasted about 33 seconds! So total imaging time: about 1 minute and 40 seconds. Not bad! The vast majority of the time was spent star hopping to the targets since my Losmandy G-11 is not equipped with a go-to system. The biggest challenge with the MallinCam is to find the right combination of settings to get the most out of your telescope. The focal ratio is definitely the most important parameter, but so is the brightness of your sky. At any rate, the best way to find the optimal settings for your specific situation is to try.

I started with the Ring Nebula (Messier 57). Using a 25mm Plossl eyepiece in my 10" f/4.7 Newtonian, I zeroed in on β Lyrae which, at magnitude 3.4, is visible naked eye in my 4.2 magnitude skies. Using Cartes du Ciel and superimposing the field of view (FOV) of the 25mm eyepiece, I could see that if I placed β Lyrae close to the border of the FOV, M57 should be visible at the same time:

So no hop in this case. After centering on the nebula and carefully replacing the eyepiece with the camera, I began to play with the settings. Focusing wasn't an issue simply because I had already determined in the past where the best focus is on a bright star like Vega and putting a mark on the draw tube of the focuser. By the way, I use a Pinnacle Dazzle framegrabber to stream video frames from the MallinCam to my laptop. To display the frames I use
VirtualDub. Anyway, the first thing I did was to set SENSE to 128x at 2.1sec integration (I didn't touch the switches on the side of the camera). That was barely enough to make out the contour of the nebula, so I bumped the integration to 7sec by acting on the corresponding switch on the side of the camera. At that point the Ring became obvious, although quite dim against the bright background sky that looked blue on my laptop because of twilight (it was past midnight!). Then I set AGC (the gain) to MAN (manual) and bumped it up one notch. The camera is very sensitive and if the gain is set too high, the live image displayed on the laptop would start flashing. So baby steps...As soon as I moved the gain up on notch, I noticed that the image would get a green cast for about 7sec, then disappear for 14sec and eventually display a stable image. This behaviour has been reported and discussed a number of times on the MallinCam Yahoo!Group. You just have to let the camera adjust to the increased gain, that's all. Once the image was stabilized, I moved up the gain another notch and so on until I would get the best possible image in terms of noise, colour and detail. In my case I moved the gain two notches to the left of the midline. At that point the background sky looked really bright, so I set GAMMA to 1.0 and that solved most of the problem. As I mentioned earlier, a 33 sec AVI was captured using VirtualDub before moving to the next subject, the Blinking Nebula (NGC6826).

To get to the Blinking I reverted back to the 25mm eyepiece, slewed to δ Cygnii (visible naked eye) and then hop from there. I noticed that δ Cygnii and the Blinking have almost the same Right Ascension so hopping was easy: I just kept slewing in one direction and checked once in a while how far I was to make sure I would not go past the target:

Again, replacing the eyepiece with the camera, quickly achieving focus and using the same set of settings for the Ring, the Blinking (which blinks only visually) showed up on the computer screen. The main difference between the Ring and the Blinking is the relative size: the Blinking is so small! After collecting a 1000 frames video file, I put back the 25mm eyepiece and slewed to γ Sagittae (3.5 magnitude) to bag the last target of the night: the Dumbbell. The hop was also quite easy in this case, since the Dumbbell and γ Sagittae are very close in Right Ascension:

The first thing I noticed when I put the camera back and started collecting images on the laptop screen was how much dimmer the Dumbbell was as compared to the the Ring or the Blinking. The integrated magnitude is lower (7.5 against 8.8 for the Blinking and 9 for the Ring), but the Dumbbell is much bigger so its magnitude per arcsec square is higher. In other words, on average an arcsecond square of the Dumbbell is dimmer than an arcsecond square of the other two planetaries. To make up for the lower surface brightness I had to increase the integration of the MallinCam to 14 seconds and lower the gain a couple of notches.

Here are the images:
Ring Nebula (Messier 57)
Blinking Nebula (NGC 6826)
Dumbbell Nebula (Messier 27)

Remember: these images are the result of only 33sec data collection sessions! Each of them was obtained by aligning and stacking all the frames in their respective AVI files using Registax 5 and then processing the final images in Photoshop.

It is intersting to notice that the surface magnitude of the three nebulae is quite different:

Name Dimensions (arcsecxarcsec) Integrated Magnitude Surface Magnitude
Ring 84"x60" 9 18.25
Blinking 27"x24" 8.8 15.83
Dumbbell 480"x336" 7.5 20.52

The images above confirm that the Blinking has indeed the highest surface brightness: it appears almost overexposed. The Dumbbell is definitely the dimmest: 14sec integration were not enough to get the same brightness level of the Ring. An interesting question to ask would be: why is the surface brightness of these three planetaries so different? Has that anything to do with their age? Initially I thought that perhaps young, smaller planetaries would be brighter than older, more diffuse ones. The implicit assumption I was making was that the overall brightness of the nebula remains somewhat constant over its lifetime. Well that does not seem to be the case. A quick research showed that the evolution of a planetary nebula is a very complicated business in which a multitude of factors contibute to determine chemical and physical characteristics of this class of objects. So let's leave the answer to the professional astronomer and ask a more mundane (but interesting nevertheless) question: what would the angular size of these three nebulae be if they were placed at exactly the same distance from us? Ignoring for a moment the uncertainty in their actual distances, it turns out that the Blinking is located at 2,000 ly from Earth, the Ring 2,300 and the Dumbbell 1,360. Since the angular dimension is inversely proportional to the distance, if we placed the Ring and the Dumbbell at the same distance of the Blinking (2,000 ly) then the Ring would look a bit bigger (15%) and the Dumbbell 32% smaller. Here's a comparison of what they look like in the sky and what they would look like if they were all 2,000 ly away from us.



  1. Nice story Massimo. I enjoyed it and the images as well. The Blinking Nebula image looks like it has a small galaxy graphic inside it.

    Luca Vanzella

  2. Good stuff, Massimo. I especially enjoyed the comparison image showing the three at equivalent distances. Clearly they are at different stages of their evolution. Do you know, is there a method to determine the age of these objects? More specifically, how long since they enetered the planetary nebula stage?

  3. Hi Luca,

    thanks! The "small embedded galaxy" look you are mentioning is actually caused by so-called FLIERs (Fast Low-Ionization Emission Regions):
    Other planetaries have FLIERs, the Blinking being the most famous.


  4. Hi Bruce,

    as I was mentioning at the end of my article, the time evolution of the physical and chemical characteristics of a Planetary is a complicated process. Many factors contribute to determine the expansion rate of the nebula. The way I understand it, current models indicate that the dynamical evolution of a planetary is due to the interaction between two types of stellar winds:
    1. The so-called FAST WIND emitted by the central star after reaching the stage in its evolution where it stops growing as a Red Giant and starts moving back towards the Main Sequence on its way to become a white dwarf;
    2. The SLOW WIND emitted earlier by the star during the Red Giant phase.
    When the fast wind catches up with the slow wind it creates a compression shock. The shock front thickens the material of the winds creating a shell that makes up the planetary nebula.

    The problem is that the speed of the two winds depends on many factors, such as the metallicity of the central star, its luminosity and periodicity during the last stages as a Red Giant.

    That's why there is no simple relationship that links together age and size of Planetary Nebulae. Is the Dumbbell older than the Blinking because it is bigger? Hard to say. To answer that question we need (at the very leasy) to know the physical and chemical properties of the central star