M27, the Dumbbell Nebula, has made regular appearances on this blog. It’s a large “planetary nebula” blown off from the surface of a dead star, which now lies as a white dwarf at its centre.
White dwarf’s themselves are fascinating objects. They have a mass similar to our sun, but compacted down to something the size of planet earth. They no longer generate energy and are in the process of cooling down. Eventually, they will become black dwarfs: dense, burnt out cinders. But since the time taken for a white dwarf to cool is calculated to be longer than the current age of the universe, no black dwarfs are currently expected to exist.
My previous best effort for the dumbell was limited by the use of a manual telescope and camera. The colours here are false.
The other night, for the first time, I pointed the smart telescope at it. I took two, 5 minute, stacked exposures. One with the telescope’s light pollution filter (which makes the nebula brighter) and one with it’s infra-red and ultraviolet filter (which shows more stars). The result is a combination of the two.
At about 1,000 light years distance, The Dumbbell Nebula is on our cosmic doorstep. This was a rather pretty distraction from the real effort this week though. That was Markarian’s Chain, a long string of galaxies about 50 million light years away. They’re named after the Armenian astronomer Benjamin Markarian, who demonstrated that they were all part of the same group, moving through space together.
This is too large an object to fit in most telescope and camera combinations. It takes multiple images, patched together as a mosaic to build the whole picture. Fortunately, the smart telescope knows exactly how to do this. The following image took it four hours to assemble fully. This took several attempts, but I’m fairly happy with the result.
I’m hoping to do even better some time soon. I’ve been experimenting with putting the smart scope on an “equatorial mount”. This allows it to track the apparent rotation of the sky by having a fixed axis that points at the north celestial pole. Initial efforts look good, but I need some clear, moon free, nights to really try it out properly. This would remove a lot of the rotation artifacts present in the above picture.
The “terminator” in this case is not some sci-fi sounding robotic killer. Instead, it is the line that marks the sunlit from the dark part of the moon. As it moves across the moon’s surface each month it has a dramatic effect on what is visible.
Here, for example, is Tycho crater, taken on 11 April.
Tycho is the centre of the large “splash” in the middle of this photo. Large rays of ejected material, resulting from the impact that caused the crater, are clearly visible, extending over a thousand miles in all directions.
Yet when we look at the same crater a few days earlier, the scene is completely different.
Tycho is more to the left in this picture. The rays are much less pronounced, with some of them barely visible. Instead, the crater walls and central mount are much clearer. These two photos barely even look like the same moon.
As I’ve pointed out before, even over a single night, the terminator moves visibly across the moon’s surface. It is this constant interplay of light and shadow that makes photographing the moon so endlessly fascinating.
About 45 years ago, when I was a young physics undergraduate, I spent a summer at the Royal Greenwich Observatory’s then base in Herstmonceux. I actually got to stay in Herstmonceux castle for a couple of months. Which sounds very exciting, except my room was actually a small cell in the attic. It was a small price to pay to have the privilege of accessing the wonderful range of telescopes and other equipment that they had.
The problem I was given was to develop a computer model of the sun’s corona.
The surface temperature of the sun is about 6,000K. As it’s atmosphere extends out into space, you would expect it to become colder and colder. But the sun’s corona does something different. In fact it heats up to millions of degrees before eventually cooling off. This seems to be quite contrary to the laws of thermodynamics that say that heat should always flow from hot to cold. But in the case of the sun’s corona, the opposite seems to be happening. Even today, I don’t think this process is fully understood. The most popular theory 45 years ago was that it was something to do with the sun’s magnetic field.
I did develop some models, although I think it’s safe to say these were not my greatest academic success. I predicted that the corona was composed of 90% lead. Even then I doubted that this was entirely correct. Fortunately for the world I never went on to become a professional astronomer.
Almost all of the energy that we see here on earth comes from the sun’s “surface”, it’s “photosphere”. Because of the huge density and temperature at the core, it takes about half a million years for the energy generated at the core to perform a random walk to the surface, and then a further nine minutes to get from the surface to us. (The exceptions are solar neutrinos, which barely even notice that the sun exists.)
All the pictures of the sun that I’ve ever posted are of the photosphere. To take pictures of the photosphere you need a really good filter. One that blocks 99.99% of the sun’s radiation at all wavelengths. These are relatively cheap. You can buy a sheet of filter material for a few pounds.
With this filter in place, you can see the sun’s disk and any sunspots that might be present, but that’s about all. In order to explain how we can get better pictures, and more information, we need a little bit of physics.
The sun is mostly a big ball of hydrogen. Hydrogen atoms are as simple as an atom can be, with a single proton at the centre and a single electron in orbit. Left to itself, the electron will occupy the ground state in the atom. It can gain energy, either by absorbing a photon of light, or through thermal excitation – basically bumping into other atoms.
An H atom doesn’t stay excited for very long before the orbit decays back to the ground state. When it does so it emits a single photon of light at a very specific frequency that is called “Hydrogen Alpha”.
This picture is more of a classical representation of an atom. Both the proton and the electron should by fuzzy. And the scale is all wrong. If the proton was the size of a tennis ball, then the electron should be an infinitesimal spec of dust a couple of miles away. But just ignore all that.
The sun’s core generates electromagnetic energy with a very specific distribution of frequencies called “black body radiation”. It may seem odd to describe the brightest object in the sky as a black body, but the name comes from the study of the surface of furnaces that are perfectly black on the inside. The need to explain the measured intensity of frequencies in black body radiation was what led Max Planck to his famous hypothesis of quantisation of energy, but that’s a whole different story.
When this black body radiation from the core reaches the surface it encounters the hydrogen in the photosphere. Most of the energy passes straight through, but when the bits at the H Alpha frequency encounter an H atom, it gets absorbed and raises it’s electron to the first excited state. The atom then decays and emits a photon of energy, again at the H alpha frequency, but it does so in a random direction, some of which is back into the core. The photosphere therefore appears darker at the H alpha frequency compared to all nearby frequencies.
Of course there are other excited states of hydrogen and other trace elements in the photosphere. These all lead to dark bands in the sun’s spectrum. But H alpha is the most important example. The important thing to note is that the photosphere is very bright at a wide range of frequencies, except at H alpha.
In between the corona, at millions of degrees, and the photosphere, at 6000K, there is a thin layer of hydrogen called the Chromosphere. This is hotter than the surface but not as hot as the corona. It’s typical temperature is about 20,000K. This is hot enough to excite H atoms by thermal means which then decay via H alpha radiation. So the Chromosphere actually generates the very frequency that is absent in the photosphere. It is also close enough to the photosphere to share many of its characteristics.
All you have to do is isolate the H alpha frequency and you’ll get a detailed image of the chromosphere and many of the surface features that it replicates. The only catch is, these filters are extremely expensive – of the order of thousands of pounds. There are specialist telescopes with these filters built in, but they’re equally expensive. You can get cheaper, wide band, H alpha filters for nebula photography, but they just won’t do for solar imaging. (I’ve tried – you just get a red picture of the sun.)
If you want to use a reflector telescope, like mine, you also have to block out most of the other energy from the sun, particularly infrared, before it hits the telescope mirror and starts heating it up. Small aperture refractors are best.
It’s also best to use a mono camera. Most colour cameras are at their most sensitive in green light. H alpha is red.
There is another option. There’s a frequency called calcium K. This is in the ultraviolet and leads to spooky pictures like this.
But, you guessed it, calcium K filters are even more expensive.
So if I wanted to take nice pictures of the chromosphere I’d have to buy a new telescope, a new camera and an expensive filter. All to take pictures of just the sun. It would have to be a very serious hobby for me even to consider it. So next time you see an amateur picture of solar flares and detailed surface features, just consider how much effort and expense has gone into it (and how much science has gone into making it possible).
Here are two very similar pictures of the moon. One taken with my 6 inch Skywatcher reflector and a Lumix GF7 camera. The other taken with the Seestar S50 2 inch refractor.
Do you have a preference? It might not be easy to decide as I’ve tried to make their size, orientation, contrast and colour as similar as possible. Here’s a higher resolution version. (I recommend that the high res pics are viewed on something a bit bigger than a phone.)
I’ll come back to this later. To maintain the air of mystery and suspense, I won’t say yet which picture came from which telescope.
I was hoping to show some splendid pictures of the total lunar eclipse from Friday morning. It was a beautiful clear sky all night with the forecast to remain fine. The moon was full and looked absolutely glorious, all ready to move into the Earth’s shadow. I had both telescopes set up and ready to go (even though my back pain was giving me hell). The S50 was going to do time lapse movies, while the Skywatcher would record high resolution pics. Then, with ten minutes to go, this happened…
And here’s a short movie of the clouds doing their thing, getting in the way.
I waited for about half an hour, hoping that the cloud was just temporary, but it just got thicker and thicker, until the moon was completely obscured. So I put all the equipment away and came back inside. Just after dawn, when the moon had set, the sky cleared again. If I was so inclined, I might suspect that someone was trying to tell me something.
Here are some pics from those who were more fortunate.
Still, we had some lovely clear nights over the last week or so. I managed to enlarge my set of waxing moon phases. Adding in some from January, it now looks fairly complete.
On the night of the thinnest crescent moon, I also took an overexposed shot to reveal the part of the moon in shadow.
And on the night of March the 7th to 8th I took one of those comparison photos that shows the terminator moving overnight.
The waxing moon is the easy bit though. It appears from early evening and usually means going out either just before, or just after, tea. The waning moon is very different. It’s OK getting the initial stages, but the later crescents mean being out early in the morning close to dawn. Let’s see how dedicated I can be.
Now back to the telescope comparison at the top. Personally, I can’t tell a lot of difference between the two when viewed on a web page. However, when you zoom in, the difference becomes more obvious.
In both cases, the right hand picture comes from the larger, six inch, scope. The close up illustrates the much higher resolution that the larger scope is capable of. This is what I’ve found using the smaller smart telescope over the last couple of months. It’s absolutely fantastic for just putting outside and telling it to go take a picture of something. Incredibly easy. It produces amazing results for publishing on the web. But take a closer look and its pictures are all a bit fuzzy. It wets the appetite, making you want to try to do something better.
The larger scope can produce much more detailed pictures, but it isn’t “smart”. It takes a lot of fiddling about to keep it on target and to keep taking images.
Oddly enough, the manufacturer of the smart scope also supplies a lot of higher resolution, and much more expensive, equipment. If this is a clever marketing strategy then I have to say it’s working. I’m looking at their more expensive gear. This little box of tricks, for example, can turn almost any scope into a smart scope.
Look south just after sunset today and you’ll see the unmistakable constellation of Orion.
Follow Orion’s belt left and down and you find Sirius, the brightest star in the sky. Sirius is so bright for two reasons. First, it really is a bright star, shining some 25 times brighter than the sun. Second, Sirius is on our cosmic doorstep. At a mere 8 light years away it’s one our next door neighbours in space.
Sirius is surrounded in our view of the sky by a bunch of light smudges. These are open clusters of stars. A closeup map from Stellarium shows a few of the brighter ones. Many of these are visible with binoculars.
And here are a few of the brightest ones: M46, M47 and M50.
I particularly like M46 because it includes a bright orange foreground star, 140 Pup, a red giant 700 light years away. This contrasts with the main cluster which is nearly 5,000 light years away.
Open star clusters tend to show mainly young, bluish stars. They’re usually regions of recent star formation where the constituents haven’t yet dispersed. We mostly see the brightest members which tend to be blue-white in colour. Their dimmer, yellow-red companions are outshone by these blue-white stars.
M46 has another surprise though. It also includes a planetary nebula, shown near the top left of this photo. It goes by the charming name of NGC 2438.
Again, this is a foreground object about 1,300 light years away. It’s not a true member of the cluster. Planetary nebulae have nothing to do with planets. They just look a bit like planets. They’re the remnants of old stars that have blown off their outer atmosphere and are now illuminated by the remaining core at their center. Given that this is an older star, it should now be obvious that it can’t truly be part of the open cluster M46.
On the opposite side of the sky, I’m still stacking photos of M13, the Great Globular Cluster in Hercules. This is three night’s worth of photos stacked. I’m not sure if adding any more to this will improve the quality or not. Only one way to find out…
Platitude of the Day 