Solar astronomy

The daytime seems at first glance quite a bad time for astronomy. There seems to be no stars, the Moon sometimes appears when close to the new Moon phase, but apart from that and the occasional daytime satellite flare (see Heavens Above for timings) there’s only one thing out there – the Sun. It provides warmth and light, particles and photons, both of which can produce unusual effects in the sky. It also has features to observe, when done carefully and with a full awareness of the dangers.

Daytime Moon

Daytime Moon

The Sun, a profile

Our star is roughly 5 billion years old. Its surface glows at a temperature of 6 thousand degrees celsius, and its centre one of six million. It is more than one and a quarter million kilometres in diameter and it derives it’s energy through the process of nuclear fusion.

It was discovered during the twentieth century that the atom was composed of protons and neutrons and electrons. But it was also discovered that the mass of individual protons, neutrons and electrons when added together in the appropriate amount was greater than the mass of the same protons, neutrons and electrons when they are bound together in an atom. This excess mass, it seems, is converted into energy as the nucleus fuses together. The famous equation energy = mass x the square of the speed of light describes how we convert from the missing mass to the energy produced by nuclear fusion. And it is through various chains of light atomic nuclei fusing to form other light atomic nuclei that keeps the Sun shining.

Nuclear fusion only happens in the core, when the temperature is high enough and the pressure great enough to crush enough protons and neutrons together for something to happen. Directly outside the core, less fusion occurs. In fact, fusion occurs sporadically at some level in any gas, but for a self sustaining reaction continuously producing significant amounts of energy, the kinds of conditions found in stellar cores are required.

Surrounding the stellar core, there is an area known as the radiative zone. There are three ways of transferring heat in a fluid – conduction, convection and radiation. Conduction of heat, where one bit of gas warms up the next bit of gas etc isn’t the most efficient way of covering the distances required in the Sun. Convection doesn’t work at this level either as the rate at which the heat drops off from the central core is stable. Then we are left with radiation, with photons of energy emitted and travelling through this region of the sun, bouncing off every electron and proton that they find in their way. They get absorbed and re-emitted, transferring energy outwards.

Eventually, the warmth of the core gets close enough to the cold of space for an unstable temperature gradient to form and so for convection to begin. This is the process of hot fluid rising up to cool close to the surface and cold fluid dropping down to the hot core.

The visible part of the Sun is called the Photosphere, ie the sphere that produces the photons. This is a very thick layer, which is cooler and darker the further from the core it gets. For this reason, the edge of the solar disk, known as the ‘limb’ is darker than the centre.

Above the photosphere, and invisible in normal circumstances, is the solar corona. This is effectively the solar atmosphere and can normally only be seen during an eclipse of the Sun. People studying the solar corona often have to resort to creating an artificial eclipse, such as a sun block on their instrument lens.

The Sun is a highly magnetically active thing, with a complicated magnetic field. At the start of a solar cycle, the magnetic field looks pretty much like the dipole field of a bar magnet. However, the lines of force in the field are tethered to the Sun’s plasma, and that plasma rotates at different speeds at different solar latitudes (the equator spins round once every 25 or so days, the poles closer to once every 34 days). This ‘differential rotation’ of the plasma leads to the magnetic field lines getting warped and wrapped tightly round the star. Over the course of roughly eleven years, the winding becomes so extreme the field is forced to reorder itself, flipping over so the north becomes the south in a new dipole configuration. The solar cycle therefore flips the field direction from north to south and back to north again over an average of 22 years.

The Sun doesn’t stop at the corona. Protons, neutrons and electrons from the Sun act as they would do in any other gas. They expand outwards from the source of heat. They carry the solar magnetic field with them in an arrangement called the solar wind. There are in fact two sources of solar wind, a slow wind from the equator and a fast wind from the poles, in regions known as coronal holes as little x-ray emission is seen from them. The solar wind’s two components being of different speeds run into each other and form a pulsed wind. Although the pulsing has been seen quite easily happening before the wind reaches the Earth, it isn’t until the wind reaches the gas giants that it has time to form really distinct pulses, with the material from the slow solar wind given out in about three days bunched into a small wavefront. This wavefront then slowly dissipates as the solar wind spreads further out into the solar system, losing its density. The point at which the solar wind reaches the winds of other stars as well as any interstellar flow is the heliopause – the point at which Helios, the Sun, ceases to have a direct effect beyond shining like any other star.

What kind of effects does the sun produce on the Earth?

The Sun has a number of direct effects on the Earth. Ignoring the more obvious ones such as the difference between day and night and the small matter of life on Earth, light from the Sun gets bent by the atmosphere in a number of ways. The atmosphere acts a little like a lens in a refracting telescope, like a lens in a refracting telescope, it will both absorb some of the light and bend the rest of it, splitting the light into a spectrum as it does so. During the day, the blue part of the spectrum is most noticeable. At sunset and sunrise, red is in the ascendant. The picture below shows red light reflecting off the Moon during a lunar eclipse, having been bent around the Earth.

Red Moon.

Red Moon.

Ice crystals and water droplets in the air can also cause further effects. These include Halos, such as that shown below around the Moon. More beautifully, circumzenithal arcs are rainbow like arcs that appear due to refraction through ice crystals. Circumhorizontal arcs are cirrus clouds refracting through ice crystals to provide a slab of rainbow coloured cloud. Finally, Sundogs are ‘false suns’ appearing one on either side of the sun, between 22 and 61 degrees from the solar centre in the sky. These are very common things.

Halo around the moon

Halo around the moon

Of course, the particles the Sun emits hit the Earth too. Or rather they strike the upper layers of our magnetic field, dividing that region into the heliosphere, where the Solar or Interplanetary Magnetic Field, is the dominant force and the Terrestrial Magnetosphere, where the Earth’s magnetic field is more dominant. Cycles of magnetic field reconnection and realignment force material gathered and accelerated in the magnetosphere to plunge back into the atmosphere. This can be seen in the Aurorae, either at the northern or southern polar circles. These will be discussed in much detail in many future posts as they are the subject of my own research.

Observing the Sun

As mentioned before, it isn’t a good idea to look at the Sun with the naked eye, never mind with a telescope. That is the easiest way to never look at anything else ever again. Nevertheless, there are ways to have a look.

The least technologically advanced way (and cheapest) is to put a pinhole in a piece of card and project the Sun onto another piece of card. A similar method, with similar results, is to point a telescope in the direction of the sun and project the image (focus the image onto a piece of white card or a screen, rather than looking through the telescope). You can use the shadow of the telescope to direct it (make the shadow as small as possible to have it pointing directly at the Sun) and remember to take off your finderscope rather than risk accidentally looking through it! This method will work for binoculars too.

What are you likely to see through this method? Well that depends on what is actually going on. During the most active phases of the solar cycle, when the magnetic field is highly twisted, the field buckles and loops of it come out of the body of the Sun. The feet of these loops are areas of high magnetic density that prevent plasma from moving sideways – it effectively cools it down. As a result, the plasma shines less brightly and appears black against the more bright rest of the surface – you have a sunspot. A good sunspot (and they can be good, with a fairly recent one filling a quarter of the sun’s visible disk at one point) will be seen to have an umbra and a penumbra – a middle dark bit and an outer dark bit. They may, if your image is good enough, also be associated with plages, which are the opposite in terms of being whiter than the rest of the solar surface. Also, if there is something transiting the solar surface – a planet (either Mercury or Venus) passing between the Earth and the Sun, for example – that will show as a big black dot too. The photograph below shows something a little closer to home making a partial transit of the Sun as shown project onto a screen – the Moon. Solar eclipses happen when the Moon is in the New phase and happens to be at a part of its orbit where it crosses the path of the Sun. This is as opposed to the Lunar eclipse, where the Moon is full and the Earth passes between it and the Sun, due to the Moon being on the right part of its orbit. The Moon’s orbit is tilted with respect to the Earth’s orbit of the Sun, meaning usually, the full and new Moons go ahead with no eclipses bothering them at all.

Partially eclipsed Sun, with added cloud, projected onto a screen

Partially eclipsed Sun, with added cloud, projected onto a screen

The next stage up from viewing the Sun through a projection would be to view the Sun through some sort of filter. The easiest filters to use are solar eclipse glasses, such as those lovingly modelled by two passing policemen in the picture below. The image of the Sun through these glasses is also shown in the picture below that. It is very similar to the image projected, but a lot smaller, which is why they tend to be saved for eclipses, and maybe a transit or unusually large sunspot for the eagle eyed.

Ello, ello, ello, whats all this then?

Ello, ello, ello, what's all this then?

Quick look at the tiny Sun

Quick look at the tiny Sun

Telescopes too can have filters attached to them. These can include Mylar filters, with arrive in sheets and have to be cut to shape, as well as more expensive glass filters, such as those available here. Before using a filter on a telescope, camera or binoculars, it is best to hold it up to the Sun and make sure that there are no small or large holes that may then allow dangerous amounts of light through. Also ensure you only ever by filters that cover the main aperture, not ones that get stuck onto the eyepiece. These second ones promote build ups of heat, which can be very dangerous in enclosed tubes. The advantages in using filters include the ability to use the telescope as a normal telescope with all the comfort that entails and the ability to see more than projection will give. As well as sunspots, plages, faculae(bright areas) and filaments (long wirey things), you may see granulation, which is caused by the convective cells of material spewing out of the centre of the cell and falling, when cooled back down the sides. In fact, what you’ll see is supergranulation, which is groups of these granules. Other things to see include solar flares and coronal mass ejections, where matter moves from the surface of the sun outwards in apparently small ejections that nevertheless dwarf something like the Earth. Then there are coronal loops, these are the loops of magnetic field that sprout from sunspots. Electrons and protons whizz up and down them, creating radio emissions, which can also be observed using something like Radio Jove‘s equipment.

Of course, you might want to go one further than that and purchase a dedicated solar telescope, a thing which has become quite popular in recent years. The Coranado Personal Solar Telescope is shown below at the same event as the photos above. This is a telescope with filters already built in optimized for solar viewing in a few selected wavelengths (to bring out certain features). The PST is just a small refractor, which gives an indication of how smaller instruments can be well used in solar astronomy.

Personal Solar Telescope

Personal Solar Telescope

If you want to know what the Sun’ll look like right now (perhaps to check for sunspots in advance), then SOHO provides this website. MDI continuum is best for seeing how the sun looks through projection, with the different filters in the top row showing a very high resolution version of what you might see through filters (the actual view is a lot less high quality from the Earth).

Professional Observations

Of course, it isn’t just the amateurs at it, professional astronomers point telescopes at the Sun as well. The Swedish Solar Telescope is one of the largest refracting telescopes in the world, with a lens a metre across. Most observations are now space based, with SOHO – the Solar Heliospheric Orbiter and STEREO – the Solar Terrestrial Relations Observatory spacecraft leading the way in light collecting. In the case of STEREO, as its name suggests two telescopes are used to provide a stereoscopic picture of the Sun, with the various solar flares and coronal loops shown in three dimensions, where previously we’d have to wait to view them off the limb. Numerous Earth and near Earth orbiting satellites also examine conditions in the solar wind and magnetosphere.

All this activity feeds into our knowledge of our closest star, how stars work, what they do things we can apply to unusual phenomena suggested by observations of other stars. There is also that small point about the Sun’s influence on life on Earth and the planet in general. Always nice to keep an occasional eye on that…


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