Saturday, 15 February 2014

Features of the Sun



Features of the Sun

Although I have loved astronomy my entire life, I only got really interested in solar observing about 2 years ago. The sun is really important; it is our nearest star so observations of its behaviour can give us an insight into what is happening within other stars. There are an increasing number of people observing the sun with a variety of specially adapted equipment, many of who are also posting their solar images online. But I have noticed that there is sometimes confusion about what people are actually seeing in these photos. I thought it would be a good idea to summarize the main features of the sun, and explain what the various filters will allow you to see.

Then Sun is a huge nuclear reactor. It is about 1,400,000km in diameter; that is 100 times the diameter of Earth and by fusing hydrogen into helium, it kicks out 3.84 x 1026 watts of energy per second. To put that figure into perspective, it would take 1.5 x 1017 power stations to match that output - that is 15 with 16 zeros after it! Although it may look it at first glance, the Sun is not solid. It is a huge sphere of gas which is split into several layers. At the centre is the core, where all the nuclear fusion reactions take place. It is extremely hot in the core; estimates put the core temperature at around 15,000,000 Kelvin (to convert Kelvin into Celsius you just subtract 273, so that still puts the core temperature at around 15 million degrees C!).  Next are the radiative and convective zones, which are involved with transferring the energy from the nuclear reactions to the outer layers. The closest thing the Sun has to a surface is the photosphere. This is the layer you see when you observe the Sun in visible or white light; I will use the term surface when referring to the photosphere, but the "surface" you are seeing is actually a gaseous shell.  Above the photosphere, the Sun also has an atmosphere and this too is made up of layers, the chromosphere and the corona.

The Photosphere
The photosphere is 500km thick and is where most of the Sun's light originates. This sounds like a very thick layer, but when you consider the diameter of the Sun, it is comparatively very thin. As a result of all the light it emits, it is very hot. The temperature ranges from around 9000 -  4500K but it averages at around 5800K. It has a very low density though, probably around 1000 times less dense than the air we breathe on Earth. Because there is so much light originating from the photosphere, it is necessary to use a filter to block out most of this light in order to study surface features. When you look at photos of the photosphere, the first obvious features you notice are limb darkening and sunspots. More of a challenge is granulation.

Limb Darkening
This is a phenomenon which causes the outer edge of the Sun’s disc, or limb, to appear darker than the rest. To understand this, you need to picture the spherical nature of the Sun surrounded by the gaseous shell of the photosphere. Because the photosphere is not solid, you are actually looking into it rather than just at it. The density of the photosphere increases with depth, so there is a limit to how far into it you can see. When you get towards the edges you can see further into it than you can in the middle. So why does it appear darker? In addition to the density increasing, the temperature also increases with depth. In the centre of the disc this means you are able to see into a region which is hotter, therefore brighter. At the edges, because of the geometry of the sphere, even though you are able to see deeper in terms of distance, you are only able to see into the cooler part of the photosphere, so it appears darker. This diagram should help to explain this a little better.

 
 



Sunspots
Sunspots appear as darker regions within the photosphere. They are darker because they are cooler than the surrounding area. Typically, the centre of a sunspot is around 4200K, about 1600K cooler than the rest of the photosphere. They are a transient phenomenon, usually lasting a few weeks. Because they last that long, it is possible to measure the rotational period of the Sun by tracking the movement of Sunspots across the surface, and this is exactly how early solar astronomers calculated the Sun’s rotational period and discovered that the Sun exhibits differential rotation.  At the equator the Sun takes just under 26 days to complete one rotation, however, at the poles it takes 36 days.  Sunspots vary greatly in size. While they may look small in comparison to the diameter of the Sun, each Sunspot is usually easily larger than the diameter of the Earth. The exceptionally large ones can measure a huge 200,000km! They are caused by localised changes in the magnetic field, which causes disruption in the convection currents in the regions beneath the photosphere. Because the heat cannot rise properly, it causes localized decreases in temperature and these regions appear darker than the surrounding area. Sunspots are usually made up of two parts, the centre or umbra, and outer area called the penumbra. The penumbra is slightly warmer than the umbra therefore does not appear to be as dark. Sunspots often appear in groups, as the lovely image below shows.  As they develop, they are assigned a number which begins with the initials AR which stands for “active region”.  This is because sunspots are a direct indicator of solar activity and are associated with solar flares. During periods of increased activity there are more sunspots, while during periods of low activity there are fewer, sometimes none at all. This is known as the solar cycle, and following many years of data collection it has been ascertained that the cycle is approximately 11 years long.  The predicted solar maximum should have been around 2011/2012, however, more recent predictions suggest the maximum will arrive during autumn 2013, but at the time of writing this article (September ’13) the sun’s surface is almost completely devoid of sunspots!


Sunspot group, showing the central umbra and surrounding penumbra -Photo by Brian Ritchie








Solar Granulation
If you look closely at the photosphere, you will see that it is made up of a complex pattern of granules (you can just make them out on the sunspot photograph). Each granule looks a bit like a bright cell. They are ever-changing, typically only lasting about 5 to 10 minutes. So what are they? Each granule is the top of rising column of hot material which comes from deeper within the sun. As the hot material rises up and reaches the top of the column, it begins to spread out horizontally and then starts to cool and sink back down again. This forms the darker channels which can be made out between the brighter granules. Although they look small, each granule measures about 1000km across. They are a challenge to observe from Earth because our atmosphere causes interference in the images.

Faculae
Sometimes visible around the edges of white light images, you can see little bright channels snaking across the photosphere. They form in the canyons between solar granules, and are formed by a strong concentration of magnetic field lines on the solar surface.


The Sun’s Atmosphere
The light from the photosphere is so bright that it completely masks the small amount of light from the tenuous layers above. One time when it is possible to view the Sun’s atmosphere is during a total solar eclipse. The Moon acts to block out the photospheric light and allows the bright halo of the Sun’s atmosphere to shine through. It is then you can see the thin, pinky coloured layer of the chromosphere, and the much more extensive pearly white corona. However, solar eclipses only occur approximately every 18 months, and will only be visible from restricted locations across the globe. So we need an alternative way of blocking out the light from the photosphere to allow the observation of the chromosphere. One way of doing this is to restrict the wavelengths of light that are visible by using special filters. In order to understand how these filters work, it is necessary to understand the nature of light.
Light is just one part of the electromagnetic (EM) spectrum. At one far end of the EM spectrum we have gamma-rays with the shortest wavelength. Next come X-rays, then ultraviolet. Then we have visible light, which can be split into its component parts using a prism, or on a showery day you can see this at work in the form of a rainbow. At the shortest wavelength of the visible spectrum is violet light and the longest is red light. Beyond the visible range we have infra-red, followed by microwaves, and finally, with the longest wavelength of all, we have radio waves.  Wavelengths are often measured in nanometres (nm); 1nm = 0.000000001 metres, but you will also see the frequency of waves measured in Hertz (the shorter the wavelength the higher the frequency) or Angstroms (1 Angstrom = 1 10 billionth of a meter).  There are certain wavelengths at which the light from the chromosphere becomes more prominent than that of the photosphere. The two most commonly used are hydrogen-alpha (H-alpha), which restricts the view to light with a wavelength of 656.3nm, and calcium-K, which restricts it to 393.3nm. So why the names H-alpha and Calcium-K? It relates to the behaviour of atoms within the chromosphere.  Hydrogen atoms are very efficient at absorbing and emitting radiation. In this situation, the hydrogen atoms absorb a lot of the 656.3nm radiation coming from the photosphere, but at the same time they have very prominent emissions of their own at that same wavelength. These emissions are responsible for the chromosphere being reddish in colour. Hydrogen also behaves this way at shorter wavelengths, so in addition to H-alpha you can use H-beta, H-gamma or H-delta filters. However, H-alpha filters are the most widely used within amateur astronomy.  In a similar way, calcium atoms do the same thing but at a wavelength of 393.3nm for Calcium-K filters or 396.8nm for Calcium-H.
That’s the explanation of the filters out of the way, so let’s look at the features of the chromosphere.

The Chromosphere
Images taken in H-alpha and calcium-K look very different to those taken in white light because you are looking at a totally different part of the sun.  In white light, you are looking at surface features of the photosphere. At these restricted wavelengths used with H-alpha and calcium-K, you are looking directly into the chromosphere. It is not uniform like the photosphere beneath it and it has been described as “a layer of froth stirred up by the photosphere”. One of the most surprising features of the chromosphere is that it is hotter than the photosphere, with a temperature that ranges between 4,500 - 100,000 K.

Plages
In both H-alpha and Calcium-K, the most prominent feature is the presence of bright specks littered across the surface, some of which group together to form really bright patches called plages. These bright regions are located directly above the active regions of the photosphere and they are closely connected to the faculae.  They appear brighter in Calcium-K images, as shown in the image below.

Calcium-K image of the Sun, showing the bright plages - Photo by Brian Ritchie.










Prominences
Visible best in H-alpha images, these are in my opinion the most spectacular solar feature to see both visually and photographically. They are huge clouds of relatively cool gas which are held up in arches by magnetic forces along the limb of the sun. When viewed anywhere other than the limb, they appear as dark winding features called filaments. It is difficult to emphasize just how big these features are, but in the second image below you can see a huge arching prominence with the Earth shown to scale.
 
H-alpha image showing a good example of a filament - Photo by Brian Ritchie






H-alpha image showing a huge arching prominence, with the Earth to scale - Photo by Brian Ritchie






The Corona
The final layer of the Sun’s atmosphere is the corona. As already mentioned, a solar eclipse allows us to see just how far reaching the corona is; it extremely far reaching, stretching millions of kilometres out into space.  It has an extremely low density, and only produces about a millionth of the visible light than the photosphere does.  This makes it all the more surprising that its temperature is an enormous 2,000,000K; compare this to the photosphere which has an average temperature of 5,800K.  It is still a subject of debate as to exactly what mechanisms are in place to allow such a tenuous layer of gas to have such a higher temperature than the surface below.  It is thought to be linked to the Sun’s magnetic field and magnetohydrodynamics (way beyond the scope of this article!).  Due to the comparatively dim light being emitted from the corona, the only way to observe it outside of a total solar eclipse is to use a coronagraph. This is a specially adapted telescope which uses an opaque disc to block out the light from the photosphere.  These are more difficult to build than they sound! All the best images of the Corona are taken from space-based coronagraphs.
The corona is not uniform; it forms distinct arches or rays, which alter position depending on the Sun’s activity.  At times of low activity it appears to be elongated at the equator leaving coronal holes at the poles, whereas during times of high activity it forms streamers which jut out in all different directions. The structure of the corona is best viewed in X-ray and space-based X-ray telescopes have shown the corona to have a varied structure.  It is fair to say that the corona is not accessible for amateur astronomers to observe outside of a total solar eclipse!


How to see the features discussed

The first thing to point out is an obvious one, but of vital importance. You must NEVER observe the sun without an appropriate filter or using a projection method. If you do, you risk permanently damaging your eyes, possibly even causing blindness. Even looking at the Sun through your telescope’s finder scope can damage your eyes.

This is not intended to be an article giving advice about solar observing and photography, mostly because I simply do not have enough experience to be in any kind of position to advise anybody on this subject! But I will give a very brief overview of what equipment you will need to see the features I’ve discussed.

The Sun in White Light
If you have a telescope or binoculars, you can safely observe the Sun with nothing more technical than a piece of white card. Using the projection method it is possible to view sunspots, and this is also a great way of observing a solar eclipse or planet transiting across the solar disc.


Images of the August ’99 partial eclipse, viewed using the projection method. Photos by Mark McIntyre

 


 If you have a telescope and want to see more detail, it is easy and relatively inexpensive to make a home-made solar filter using Baader AstroSolar Safety Film. This Film is available for less than £20 per sheet.  Care must be taken not to damage the film in any way, and to also ensure that it makes a snug fit with your telescope; otherwise too much light may get through and damage your eyes. The picture below shows the home-made Baader filter I use on my Helios 102mm refractor. We used the end cap of the telescope as a housing to ensure that the film remains mostly covered in rigid plastic. If I hadn’t been able to do this I would have made sure that I had a rigid container to keep it in when not in use to prevent it from becoming damaged. Baader AstroSolar Safety Film blocks out the majority of the light from the photosphere and allows the observation of sunspots and limb darkening.  Visually the image looks grey in colour, but when people take photographs with AstroSolar Film, they sometimes artificially add colour back in again using image processing software.  For around £80 it is possible to buy glass solar filters such as those made by Thousand Oaks. These are obviously much higher quality. Rather than grey, the image looks a sort of dark yellow in colour.








The Sun in H-alpha & Calcium-K
It is possible to convert an existing telescope to be fully functional H-alpha or Calcium-K telescope, but it is not as straightforward as just popping a filter onto the front of you telescope. You can buy a filter module which fits into your telescope, but it is easier to buy a dedicated solar scope. The design of this kind of filter is much more complex than the white light filters. Each type of solar telescope will have slightly different internal components which are arranged in slightly different configurations, but there are a number of optics which will be common to most. First of all is an energy rejection filter. This cuts out all the unwanted heat and damaging UV radiation. Next is the important component which is at the heart of every solar scope; an etalon. In simple terms, an etalon is made of 2 highly reflective plates which are mounted very close together, and light bounces back and forth between the 2 plates.  This is the component that ensures you are only viewing light of the correct wavelength, whether that be 656.3nm for H-alpha or 393.3nm for calcium-K. In some solar telescopes it is possible to adjust the etalon and view light at wavelengths on either side of the H-alpha line; this slightly changes which part of the sun’s atmosphere you are looking at. In the Coronado PST model which we have, it is possible to view the photosphere and see sunspots at one end, and then as you alter the wavelength you effectively zoom out into the chromosphere, and then the plages and prominences become visible.  Some higher end solar telescopes have 2 etalons in them, and this is commonly referred to as a double-stack. In this case, the 2 etalons are tuned to slightly different frequencies, and this causes the transmission curves to overlap. This makes the image slightly dimmer, but has the advantage of giving you a sharper resolution.  Placed either before or after the etalon, depending in the design, will be an objective lens (if this is placed before the etalon there will also be a re-converging lens after the etalon) and finally a trim filter. There are several dedicated solar telescopes on the market, the most popular being made by Lunt and Coronado.  Because the Sun is so bright, you don’t need a particularly large aperture telescope to see the main features. But as with most telescopes, you will get more impressive results if you go for a larger aperture.  The Calcium-K telescopes tend to be larger aperture than the entry level H-alpha telescopes. Interestingly, when viewing in Calcium K it can be difficult to see the features visually, but your camera will pick up stunning detail.




Photographing the Sun can be tricky, but it is possible to get decent results afocally (by basically holding your camera up to the telescope eye-piece) with a bit of patience.  If you can mount a DLSR camera at your telescope’s prime focus point you will probably get better focus (and photographing the Sun with a DSLR is a great way to find any dust bunnies on your sensor!). But by far the best results from an imaging point of view is to use a dedicated webcam, such as the range made by DMK or Atik.  There are many articles online which can advise you about which imaging device will best suit your needs. Don’t be tempted to try and photograph the Sun without the correct filters in place, because it will cause permanent damage to your camera.
The most important thing when solar observing is to stay safe. If your equipment develops a fault, never try to repair it yourself.  But as long as you have the correct equipment, solar observing is a very rewarding pastime. And you don’t get nearly as cold as you do when observing at night!



Thanks to Brian Ritchie for permission to use his images. If you want to view more of his fantastic photos, follow this link:

Thanks also to Mark McIntyre for permission to use his eclipse images.


Further information:
A really good article and video blog all about solar observing, photography and image processing is available from the Astronomy Now website at this address:

If you want to learn more about the Sun and how to get the best out of your solar photography, there is a great short course that you can download from http://www.astronomyknowhow.com/solar-imaging for the great price of just £35 

More information about how solar filters work can be found here: