GCSE Astronomy

The Sun

Introduction

On average, the Sun is 149,597,870 km away. Light takes about 8m 20s to reach us.

Some Data

Radius   696,000 km

Surface Temperature 5,800 K

Core Temperature about 15 million K

Rotation Period about 25 days at the Equator, about 35 days at the Pole (sidereal); 27 days at the equator (synodical).

If the Sun is viewed through a telescope, the concentrated heat will cause instant damage to your eyes. During the eclipse of 1999, there were genuine cases of people ignoring related advice about looking at the Sun for too long and thereby damaging their eyes. I am assuming that just a single mention here will suffice for anyone who has the slightest intelligence. There is no sensation of pain to act as a warning.

The standard way to observe the Sun is to project the image onto a screen, or similar. However, if you have a thousand pounds or so available, you could always acquire a hydrogen alpha (Hα) filter. In the literature you might sometimes read about the effectiveness of welder's glass (although seemingly stronger than welders would use in practice).

But please make certain any filter is a reliable filter. Superficially effective filters could still allow enough Infra-Red and Ultra-Violet radiation through to damage the eyes without the eye being able to detect this dangerous radiation beforehand. And cracks can cause damage in an instant.

Hα is a red 'line' in the visible spectrum, one of the lines in the Balmer Series, which are designated α, β, γ.. etc.. Hα is at 656.3 nm, often the strongest of all Hydrogen lines - it emanates from cooler material.

The Sun is believed to be about 75% Hydrogen and 25% Helium in the outer regions although fusion reactions in the core has converted hydrogen to helium, so there the proportion may be 60:40 respectively.

Visible Regions

The visible regions of the Sun are

Photosphere This is the origin of the light you observe from the Sun in the normal run of things. The light is not actually coming from a single surface but a layer about 500 km thick. This explains the phenomemon of limb darkening - when you look (figuratively speaking) at the Sun directly then you are 'looking' as much as 500 km into the Sun where the bottom layer will be hotter than the surface layer; when however you look at the edges, you are still 'looking' thru 500 km of photosphere but this will be 500 km at an angle so no light will actually be originating from as far down as 500 km from the surface. Because this light is coming from cooler regions overall, it appears darker than light coming from the center of the disk.

Chromosphere This extends a couple of thousand kilometers above the photosphere but is normally overwhelmed by light from the photosphere itself, and is therefore not normally observable. It can be viewed during a Solar Eclipse.

Corona This is the outer region, again normally viewed during an eclipse, although it is best seen during those eclipses where the chromosphere is blocked out as well (the apparent size of the Moon will vary from eclipse to eclipse). Its extent varies with the solar cycle - at maximum, the corona becomes more extended and is more 'circular', whereas at minimum it is mainly restricted to around the equatorial regions. At maximum helmet streams, so-called because they resemble the spikes on spiked-helmets, are arranged like petals on a flower, whereas again they are more restricted in extent at other times. The corona is a very low density region but has a temperature measured in millions of degrees. This is actually a great mystery because the Second Law of Thermodynamics forbids such a scenario - the extremely high temperatures of the Corona cannot be produced by the lower temperature regions immediately below it by means of a straightforward flow of heat. See here for an explanation of temperature from a scientific standpoint.

Core

Large numbers of neutrinos are emitted by the core which, due to their low reactivity, exit the Sun immediately and are therefore a means of examining processes in the core at the present (energy transmitted by photons, on the other hand, takes about one million years to reach the surface). Needless to say, the low interactivity of neutrinos makes detection on Earth difficult but not impossible. For a long time, the neutrino detection rate was only about a third of what the theories expected, a state of affairs which seems to have been mainly tackled, if at all, by trying to invoke new particle theories that correspond with this low detection rate, rather than modifying the astronomical theories.

Sunspots

Sunspots were recorded by the Chinese from as early as 800 BC. The fact that they they are recorded for the first time in Europe only from the work of Galileo is a sad indictment of the strength and nature of prevailing Christian teaching which forbade such objects from existing (compare George Orwell's 1984 - 'the party instructs you to disregard the evidence of your eyes and your ears').

The typical sunspot has about the same diameter as the Earth. They only appear dark to the human eye because of a contrast effect - they are at a lower temperature than their surroundings. The larger spots contain a dark center - the umbra (at about 4200K), surrounded by a less dark area called the penumbra (at about 5700K).

Most spots are depressions with respect to the Sun's surface. This produces the Wilson Effect when a sunspot is near the Sun's limb - one side of the penumbra (on the 'inside') will be narrower than the other side (on the side nearer the limb).

They take about 2 weeks to move across the Sun (if they exist for that long). They move from Left to Right as seen by us, or East to West.

They are associated with strong magnetic fields, in localities where the magnetic field brings the upward convection of material to a virtual standstill. In fact, sunspots always occur in pairs, one being a magnetic North Pole, the other a Southern Magnetic Pole.

Most spots tend to be associated into groups lead by a 'flagship' pair.

Sunspot activity appears to follow an 11-year circle. When they are most numerous the cycle is said to be at maximum. At minimum, it is possible that the sunspots could be so few as to be non-existent. At the start of the sunspot cycle, the sunspots lie on average about 28 degrees from the Equator. As the cycle proceeds they move closer to the equator, until at the end of the cycle they lie on average about 7 degrees from the Equator. The maximum of the solar cycle occurs when the sunspots are about halfway between these extremes. This is sometimes known as Spörer's Law.

The apparent 11-year cycle is actually half of a 22 year cycle - there is a switch in magnetic polarity in each 11-year half cycle. Sunspot activity can be plotted schematically in a Butterfly Diagram.

In any 11-year half-cycle, all the leading spots of a pair in one hemisphere will be of the same polarity, with the situation reversed in the opposite hemisphere. All these polarities are reversed in the next half-cycle.

The usual way of viewing sunspots is to project the image of the Sun from a telescope onto a screen of some description. Observing this way, you can also appreciate how fast the Sun moves across the sky.

(Newtonians and Schmidt-Cassegrains are not recommended for projection because 'hot-spots' inside the telescope itself could cause damage)

Under very good seeing, short lived tiny spots can be detected. These are called pores. Some pores do grow into full size sunspots

Other Solar Phenomena

Plages These are bright areas of solar activity, surrounding a sunspot. The plages themselves stem from the chromosphere. Under certain circumstances plages are also called 'flocculi' or 'faculae'.

Prominences are masses of gas rising hundreds of thousands of kilometers above the Sun into the corona. They appear to be connected with sunspots and are viewed best when rising from the limb of the Sun (as above) where they can be seen to be following the magnetic field. On the disk of the Sun, they appear dark due to them absorbing light from below and are then usually known as filaments.

Prominences are less frequent around maximum. The latitude where they occur most frequently varies over the solar cycle.

Flares are sudden, enormous release of energy into the corona, brightening the chromosphere. They last typically about 20 minutes, and can cause enormous disruption on Earth. They are not normally seen in visible light, except for the larger ones which can cause brightening of the photosphere. Most flares appear to occur at maximum and therefore, as expected, they appear to be connected with sunspots.

Inhabitants of Earth are protected from the effects of flares (X-rays and increased solar wind) but they would be fatal to any astronauts traveling to the Moon for example. Permanent moonbases would need to be protected in some manner.

Solar Wind

A steam of energetic, electrically charged particles that flows out from the Sun's corona at all times, primarily protons, alpha particles and electrons. Large blasts of solar wind occur during solar flares. The wind seems to come from regions in the Sun's corona called coronal holes.

Auroras/Aurorae

These are caused by the fast-moving (up to 1000 km/sec), electrically-charged particles in the solar wind , cascading towards the Earth's magnetic poles, stimulating atmospheric particles to emit light. Auroras are known to be visible about 2 days after a solar flare, so in this scenario they can be predicted, although other aurorae not caused by a flare are difficult to predict. They are affected by the Sun's magnetic cycle and reach a peak of occurence about 2 years after sunspot maximum.

Fraunhofer Lines

Fraunhofer and Kirchoff were responsible for pioneering spectral analysis in astronomical investigations. It seems that Fraunhofer noticed the dark lines in the spectrum of the Sun, about 600 of them in total but the most famous are lines in the yellow part of the spectrum which are now known to be absorption lines produced by Sodium in the Chromosphere.

After Fraunhofer's premature death, it was Kirchoff who worked out the theory behind Fraunhofer's discovery - this was in 1859.

Altogether about 70 elements (out of 92) have been discovered in the Sun. Helium was discovered in the Sun before it was discovered on the Earth.

Solar Eclipse

Eclipses occur at new moon, and there can be between two and four in any one year. Because of the Moon's (elliptical) orbit, the relative size and position of the moon can vary, so by no means all these eclipses are total (by total, we mean the Sun's disk is completely obscured).

When the Moon is closest to Earth, a total eclipse can last as long as 7 minutes 40 seconds although it is rare to have eclipses of this order of duration. In Britain, the maximum theoretical length is 5.5 minutes but, for example, the total eclipse of 1927 (visible in Northern England) lasted 20 seconds.

When the Moon first 'touches' the Sun, this is called first contact. It takes about an hour from then until second contact when the Sun is covered. It is only when a thin portion of the Sun remains uncovered that the light becomes noticeably darker.

Third contact refers to the time when the Moon starts to move off the Sun. At either the second or third contact, a Diamond Ring Effect often occurs when just a small portion of the Sun's photosphere is visible. Also at second and third contact, light shining thru lunar valleys can produce an effect called Baily's Beads

1999 Eclipse

Probes

Solar and Heliospheric Laboratory (SOHO) was launched in December 1995, exceeded its plneed two year planned life. Its intended two-year mission extended planning to cover the period of solar maximum around the turn of the millennium.