Altair

Last month I tried a few different ideas with my Star Analyser diffraction grating. One of the bigger successes was taking some black and white images as well as the usual colour shots. Since it's usually best to experiment on something that's easy to find and that this was still summertime, I settled on Altair.

Altair is one of our near neighbours, about twice the mass of the Sun and ten times as luminous. Similarly to the other two points of the summer triangle, Altair is of spectral class A so has some fairly strong Balmer lines that should show up nicely using the Star Analyser. Here's what I ended up with:


Both sets of spectra have their merits but the black and white trio show much more detail. The colour pictures show a clear H-Beta absorption line (in the light blue) but the H-Alpha and H-Gamma lines (in red and blue, respectively) have to be searched for. The black and white shots not only clearly show these three absorption lines, but also show the H-Delta line.

It is also much easier to locate the peak luminosity when looking at the black and white images. Looking at the colour images the peak luminosity could lie anywhere in the light blue, green or yellow. The black and white images quite clearly show luminosity peaking in yellow.

I Got The Sun In The Morning...

...And all of the rest of the day since I now have an Orion Solar Filter. Simply fit it over the front of the telescope and it cuts out 99.999% of incoming light. This makes the Sun a safe observing target and shows up sunspots, prominences and granulation.

Sunspots are areas on the surface of the Sun where the magnetic field has become tangled up making one patch cooler than the surrounding area. The temperature of the sunspot will be around 4,000 degrees compared to around 6,000 degrees for the surrounding area. The cooler area shows up as dark spot on the surface. Sunspots can be very big - the biggest can be up to around 50,000 miles across.

It has been well publisized that the Sun has been rather quiet over the last few years. The Sun goes through a fairly regular cycle every 11 years or so where it changes from having very few sunspots to lots of sunspots and back again. The current low period has been going on for longer than usual but there are signs that activity is beginning to pick up again (last year 71% of days showed no sunspots compared to 16% so far this year).

This increase in activity left me optimistic that once the recent spell of rain had abated I would be spotting spots straight away. Alas no. The clouds cleared at the start of a mini-streak of 5 spot-free days. This morning was a different story. After consulting the latest space weather, I knew that I'd have at least one sunspot to look at, and here it is:



Sunspot 1101 is clearly visible in the bottom-right quadrant. There was quite a lot of whispy cloud around (as can be seen in the picture) so I settled for a few afocal snaps rather than getting out the webcam. It will be interesting to see what resolution can be achieved on a clearer day and what happens to sunspot 1101 in the future.

Gamma Cassiopeiae

With autumn approaching I felt it was about time to turn my spectroscopic attention to Cassiopeia. The first stellar spectrum I ever observed was that of Gamma Cassiopeiae and 15 years later I couldn't wait to see how it would look through my Star Analyser.

Back in 1866, Gamma Cassiopeiae was the first star ever observed with emission lines in its spectrum. This made it the prototype Gamma Cassiopeiae Variable star, the first known Be star (a type B star with emission lines), and one of the most popular targets for spectroscopy ever since.

One of the most common features of a stellar spectrum is to see hydrogen absorption lines. These are caused by hydrogen atoms in the star absorbing light at a wavelength corresponding to the energy required to excite an electron between different energy levels. For example, the spectrum of Sirius that I posted last month shows a clear H-Beta absorption line caused by exciting electrons between the second and fourth energy levels.

Here are three spectra I took of Gamma Cassiopeiae and instead of absorption lines we can see a clear H-Alpha emission line (in the red part of the spectrum).


Clearly some other process must be taking place. Some of the great minds of the early 20th century found that these emission lines must be coming from material around the star rather than the star itself. This material is a disc produced by a combination of very rapid rotation, magnetic fields and stellar pulsing. The cooling of hydrogen atoms in this disk then produce the emission lines seen in the stellar spectrum.

When I observed this star at university (with considerably better equipment!) I could produce a much more detailed spectrum. It was possible to measure the rotation speed of the disc by calculating the Doppler broadening of the emission line. It was also possible to see a narrower absorption line in the middle of the broad emission line caused by the disk absorbing light from the central star.

Copernicus

It doesn't take many lunar observing sessions before ending up with a picture of Copernicus. Here's a picture I took during an observing session in March. It was a near full moon so the surface is looking rather flat.


I got the webcam orientation a bit wonky so north is roughly pointing towards 4 o-clock. Copernicus is a relatively new crater being around 800 million years old. Typical of many Copernican period craters it has a prominent ray system and the crater hasn't been flooded with lava - some of the features inside the crater are obvious even with this little shadow.

The relatively bright crater in the bottom-right corner is Pytheas, a crater of similar age to Copernicus and located in the southern part of Mare Imbrium. To the right of Copernicus in this picture is the mountain range Montes Carpatus which is 2-3 billion years older than the two craters just mentioned.

The two medium-sized craters situated at 11 o'clock are Reinhold and Reinhold B, both of which are much older craters than Copernicus.

It's a shame I didn't pick a slightly shifted field of view - a little more towards the top-right would have given a better view of Montes Carpatus, but just out of shot in the bottom-left is Eratosthenes which would have made a great target alongside Copernicus. Maybe next time.

Mersenius and Gassendi

Mersenius and Gassendi are two large craters in the southwestern part of the Moon and are the next step in my quest to image the entire lunar surface.


Of the two large craters in the middle of the picture, Mersenius is on the left and Gassendi on the right. The large, smooth, dark region at the bottom of the picture is Mare Humorum and North is roughly towards one o'clock.

Mersenius is 84km in diameter, 2.3km deep and was formed nearly 4 billion years ago. The interior of the crater has been flooded by basaltic lava which has solidified into a central dome shape and covered many other features. The rim of the crater is heavily worn particularly at the northern edge. The small crater Mersenius N can be seen lying across the southwestern rim.

Gassendi is a larger and apparently shallower crater. Similarly to Mersenius, Gassendi has been filled with lava but some of the multiple central peaks are still visible. The crater situated on the northern rim is Gassendi A and the appearance of the two craters has been likened to a diamond ring.

Spectroscopy

At the beginning of this year I added a new toy to my stargazing kit - the Star Analyser 100 from Paton Hawksley. I'd always had a hankering for getting into stellar spectroscopy and this looked like the perfect starting point - spectrometers are VERY expensive, the Star Analyser could get me started for under £100.

Spectroscopy is essentially analysing the light from an object and seeing how the intensity of the light varies as a function of wavelength (i.e. looking at the spectrum of the object). This can reveal all manner of things such as the temperature of the object, what it is made of and how fast it is moving. This is detailed analysis that needs finely tuned equipment and the Star Analyser tries to fill a gap in the low end of the market - it won't show the detail but it's fun, easy to use and informative.

The Star Analyser is just like a standard filter - it screws into any eyepiece and off you go. When combined with my webcam it means I can take pictures of stellar spectra for further analysis. Here are some spectra I took of Arcturus:



As you can see, the results are very consistent and show some detail - the red appearance of Arcturus is obvious and the dark line on the middle-right is one of the Fraunhofer absorption lines due to oxygen in the Earth's atmosphere.

To show the difference between a relatively cool K-type star like Arcturus and something a bit hotter, here's a spectrum taken of Sirius an A-type star:



The spectrum is much more green/blue and other features are visible such as the H-Beta line in the light-blue section.

Another interesting use of the Star Analyser is to compare stellar magnitude. I find it much easier to compare the brightness of two spectra rather than looking directly at the stars. Providing I keep the webcam settings the same and compare stars of the same spectral type then I get a decent comparison of magnitude. I tried this with the main stars in the Plough (which are mostly A-type stars) and got a magnitude comparison and therefore distance approximation. I haven't had much chance to play around with this yet and feel it needs a new post anyway! I'll also follow this post with a description of how to get a spectrum from using the Star Analyser.

Saturn

It's been a while since my last post and the almost permanent twilight of this time of year means I've produced no new material for about 6 weeks. Still, this gives me the chance to catch up on some webcam pictures from earlier this year.

For stargazers like myself who aren't that interested in starting an observing session at two in the morning, Saturn came back into range in around early March. For pre-midnight observing it was still lurking in the haze near the horizon and, for my location, in the direction of Birmingham city centre but Saturn is Saturn and it was my first chance to see it through the new 'scope.

It's just not possible to tire of seeing those rings and I was at the eyepiece for most of session - it's not the same looking at a laptop screen! I recorded a couple of decent videos, the best of which produced this:



The rings are still quite narrow but they should open out nicely throughout the year. And I'll have to try and make the most of it - after more than a decade in the northern sky Saturn is now most definitely heading south which will soon mean about 13 years of less favourable viewing.