The classical idea was that the Sun was a ball of white-hot iron, and in the 18th century Sir William Herschel believed there was a dark, solid world, below the fiery clouds of the photosphere, which we saw through the vortices of sunspots.
The inhabitants carried sunshades to protect them from the heat overhead (at 6,000o, they’d need them).
“Dr. Elliott in 1787 upheld this view, and on his trial at the Old Bailey for the murder of Miss Boydell, his friends maintained his insanity, and quoted as proof of their assertion the pages of his book, in which this opinion was expressed.” (J.E. Gore, “The Worlds of Space”, Innes, 1894.)
Gore was amazed that it had been revived “in modern times”, but in 2006 I was blamed for my ‘closed mind’, when I rejected it on the obvious ground that the Sun is mostly hydrogen and helium and its gravitational pull would be much higher otherwise.
Until the 1940s it was still believed that iron atoms outnumbered hydrogen ones in the solar core, but that has long since been disproved. (Simon Mitton, “Fred Hoyle, a Life in Science”, Aurum, 2005.) ESA satellites have found enhanced levels of iron in the solar atmosphere above sunspots, but that’s a far cry from saying there’s a solid layer below. [Michael Mozina, ‘The Surface (Ferrite Layer) of the Sun’, e-mail text, 2006.]
After analysis of their light, stars are grouped by spectral type. The types were labelled before the processes of nuclear fusion were understood; now rearranged by surface temperature, the sequence from blue to red runs O, B, A, F, G, K, M, R, N, S (‘Oh be a fine girl, kiss me right now sweetie’).
The Hertzsprung-Russell diagram (above) plots stars by their light output (absolute magnitude) and surface temperature. The hottest, brightest stars (class O and B), are found at top left; the faint red dwarfs (class M and below) at bottom right. Between them runs a diagonal band called the Main Sequence, on which all stable stars fall: our Sun is near the centre of the graph, classed G0 or G2. Stars are stable while fusing hydrogen to helium in their cores; when the hydrogen is exhausted they move off the Main Sequence to upper right, becoming orange and red giants, increasingly unstable. The more massive stars go on to more violent fusion reactions; some explode as supernovae, synthesising the heavy elements which form planets like ours; the most massive collapse into black holes.
Storms on the Sun emit intense particle beams, associated with but not actually caused by the phenomenon of solar flares. Observations from the SOHO probe have shown that they aren’t shaped like jets from a hose, as we used to think. We now call them ‘Coronal Mass Ejections’, but they aren’t solid matter. They are plasma containing some nuclei of the heavier elements but consisting mostly of protons, with trapped electrons spiralling around the outside of the ‘jet’. Flares are huge releases of energy caused by the breaking of magnetic field lines and usually occur above sunspot groups, along with ‘coronal mass ejections’ which can have disruptive effects on Earth if they strike the magnetic field in the right position. For lunar astronauts they pose a major hazard when the Moon is outside the Earth’s magnetosphere, for about two-thirds of each month.
In the 1930s Sir James Jeans believed that the Solar System formed from such a streamer pulled out by a passing star, so rare an event that our planets would be unique. But nearly all the angular momentum in the Solar System is concentrated in the planets, where by Jeans’s mechanism it would still reside in the Sun. And now that we know of thousands of planetary systems, it’s clear that they tend to form along with stars and at the same time.
There’s a constant outflow of particles from the Sun called the Solar Wind, discovered by the Mariner 2 probe to Venus in 1962. Although very tenuous, that outflow from the ‘coronal holes’ in the Sun’s outer atmosphere has a major effect on the Earth’s magnetic field. Until Pioneer 10 passed Jupiter in 1974, it was thought that the boundary would be just beyond Jupiter’s orbit; but Voyagers 1 and 2 only reached the outer edge of it 24 years and more after launch, and have only now emerged into interstellar space.
Aurorae and magnetic storms on Earth are caused by particle streams from storms on the Sun. The older model was that when the streams hit the Earth’s magnetic field, they were diverted into the Van Allen Belts of trapped radiation, overloading them. It now seems that the aurorae are caused by backlash waves in the Earth’s magnetic ‘tail’, entering the upper atmosphere along the ‘rings of fire’ surrounding the magnetic poles forming to form rippling structures of arcs, sheets and curtains of ionised gas – usually white, but higher energies generate the green light of ionised oxygen. Still more intense radiation generates a red glow higher in the sky, again due to oxygen, and still deeper penetration ionises nitrogen, which glows red in the lower parts of auroral arcs.
The dying geomagnetic storms interrupt communications and disrupt power supplies; the worst recent one caused an 18-hour power cut in Quebec, in 1989. Recent evidence suggests that volcanic activity can also be affected. The mechanism for that isn’t obvious, although major episodes of continent-building in the distant past were correlated with resonances in the Earth’s outer core caused by the interacting gravitational pulls of the Sun and Moon.
Both flares and aurorae usually follow the 11-year cycle of sunspot activity, but 1645 to 1715 saw the ‘Maunder Minimum’, with ‘only a sprinkling’ of sunspots in the whole period. For the duration of it the northern hemisphere of the Earth had a mini-ice-age, with winter ice fairs on the Thames, and between 1400 and 1450 there was a similar drop now called ‘the Spörer Minimum’, during which the Viking colonies in Greenland were wiped out by the cold and the Inuit were forced to abandon a 4000-year old colony on Ellesmere Island, northern Canada, originally established from Mongolia.
Carbon-14 deposition in tree rings increased between 1640 and 1720, so we can use earlier tree rings to chart the Sun’s previous activity, and the record now goes back about 11,000 years. The rise in the 20th century corresponds to a marked increase in the numbers of solar flares up to 1970, after which it flattened out – in the run-up to the 2001 peak, sunspots in 1999 were well below predicted levels, and the most recent peak was even lower. The most intense solar activity since the Bronze Age was from 1150-1200 AD, peaking in the 1170’s with particularly warm weather. In 1186, for example, astrologers predicted storms and pestilence because of a planetary conjunction in Libra, “the season proving, in a more than usual degree, serene and benignant”. In North America, however, the solar activity may have caused the steep temperature rise and drought which forced the Anasazi people to abandon their rock dwellings in the Grand Canyon at that time.
It takes four minutes for the Earth’s rotation to change the position of the Sun in the sky by a single degree, but awareness of the alternation of day and night is programmed into us at a very basic level – in isolation, without external clues, human beings tend to drift towards a twenty-hour rhythm which goes back six hundred million years, to the time when life crawled from the sea on to the land. It is hard to say whether the dominant factor was the day/night cycle or the ebb and flow of the tides. But it is the braking effect of the tides which has slowed down the Earth since then, and it is the perception of the day/night cycle which resets our biological clocks.
The plane of the Earth’s orbit around the Sun is the Ecliptic, the center line of the Zodiac; as the Sun moves along it over the course of the year, its horizon position varies from its most southerly midwinter rise and set, when it’s overhead at the Tropic of Capricorn, to its most northerly midsummer rise and set, when it’s overhead at the Tropic of Cancer. The Stonehenge Avenue and the later structure both mark the midsummer sunrise.
The evidence suggests that the beginnings of agriculture did not come directly from the annual cycle of plants but from the movements of the great herds of game. From moving with the herds, as the Lapps do even today, domestication and nomadic herding comprised the next step and led to the first attempts at agriculture and fixed settlements. The very oldest towns, such as Jericho (c.8000 BC), came before the first crop-farming. The move to an annual cycle obviously required a true calendar, even if none had previously been attempted. But the year is harder to calibrate than the month – even today, a week is a long time in politics – and most societies, if not all, tried to fit their lunar calendar into the year. The cause is a lost one – the lunar and solar cycles are not commensurate; i.e., they do not fit together in any straightforward numerical relationship – and different cultures made different compromises to divide the year into approximate “months” of convenient length, while keeping religious ritual and agricultural practice in step with the solar year.
The Zodiacal Light
The Zodiacal Light is a cone which appears above the rising and setting Sun, best seen from the Tropics where twilight is short. On the opposite side of the Earth from the Sun keen-eyed observers sometimes glimpse a related glow called the Gegenschein (counter-glow). The Zodiacal Light is faint and the Gegenschein much fainter. Until the space age it wasn’t known whether the two effects were truly in interplanetary space or generated by a dust cloud surrounding the Earth, but the Pioneer 10 space probe found that both effects persisted as far out as the Asteroid Belt. The dust comes partly from there, and part of it is released by comets passing through the inner Solar System, and it spirals towards the Sun due to the Poynting-Robertson effect, in which light from the Sun exercises a slight but significant braking effect.
Artists often depict the Sun in space surrounded by the corona and the Zodiacal Light, all seen at once. The Zodiacal Light is seldom seen from Earth except in the Tropics, and then only after sunset or before sunrise, while to make the corona visible requires an occulting object in space – either the disc of the Moon, in a solar eclipse, or an occulting disc within a telescope like the ones on Skylab in 1973 and now on SOHO, currently between Earth and Sun at the L1 point. (See ‘New Discoveries…’ below.)
The element helium is named after helios, the Sun, because it was first discovered in the spectrum of sunlight, then in natural vents in Texas and Kansas (the first helium-filled airship was the US Shenandoah). The element exists in two isotopes with very different properties, both potentially useful to high-tech civilisations.
Helium-4 exists on Earth in small quantities, released by radioactive decay. In liquid form it’s the coldest substance in the Universe, only just above Absolute Zero. It has negative surface tension, so it will climb out of an open-topped container and flow down the sides; it has superfluidity, so you can pump it both ways along the same pipe at the same time; and when used as a refrigerant it promotes superconductivity, reducing the resistance of electrical conductors to zero. So it has many possible uses, for instance in power transmission and many systems requiring high-energy magnetic fields, including radiation shielding for manned spacecraft.
The superfluidity of liquid helium II was discovered, named and explained by Peter Leonidovich Kapitza (1894-1984), who did so at Cambridge in 1930-37 (Simon Mitton, “Fred Hoyle, A Life in Science”, Aurum, 2005). Kapitza received the Nobel Prize in 1978. Outside the Sun the largest repository of helium is in the atmosphere of Jupiter, and Isaac Asimov suggested that helium would be the planet’s major export to the Solar System. (‘The Element of Perfection’ in “View from a Height”, Dobson, 1964).
However the lighter helium-3 may prove to be even more important. If we ever master controlled fusion, for energy generation or spaceship propulsion, the most promising reaction seems to be the fusion of deuterium (heavy hydrogen) with helium-3. The theory was examined in detail by the British Interplanetary Society’s interstellar probe study, “Project Daedalus” (BIS, 1978). Deuterium is plentiful on Earth, particularly in sea-water, and Helium-3 is found in small quantities in solar wind deposits on the lunar soil, but claims that it could solve the USA’s energy problems seem highly questionable. To meet even 10% of the US energy requirement, so much lunar soil would have to be strip-mined that the scar would become visible from Earth with the naked eye, in only three years! (“America at the Threshold”, US Goverment Printing Office, 1991.)
Helium-3 can be manufactured in nuclear reactors, but that would generate so much waste energy that the plant would have to be on the Farside of the Moon to protect the Earth! Extracting it from Jupiter would be much more practicable, and I outlined ways to do it in my books Man and the Planets and Incoming Asteroid!, as well as in several scientific papers and articles.
New Discoveries under the Sun – and on it – and over it.
After a decade of discoveries pouring in from spacecraft orbiting Mercury, Venus, the Moon, Mars, Vesta, Ceres and Saturn, with the Juno mission now orbiting Jupiter and the Curiosity and Perseverance rovers hard at work on Mars, and Lunar Reconnaissance Orbiter still circling the Moon, it’s easy to forget that there’s a flotilla of spacecraft dedicated to observing the Sun. Among them Europe’s veteran SOHO mission remains on-station at the Sun-Earth L1 point, between us and the Sun, watching what happens on it, what comes off from it and giving early warning of what’s coming our way, including the violent Coronal Mass Ejections which cause aurorae and other disturbances of Earth’s magnetic field. The Japanese Hinode, launched in 2006, is still hard at work. STEREO, a twin mission launched in 2006, sent probes to opposite sides of the Sun to give us 24/7 coverage of all events on it, and one of them is still operational.
Not that it should be necessary, but STEREO finally laid to rest the myth of a planet twinned with Earth, orbiting on the far side of the Sun, featured in a novel by Edgar Wallace, the feminist ‘Twin Earths’ comic strip of the 1950s, and in the ‘Gor’ novels of John Norman, diametrically opposite in sentiment as well as location. Relative to Earth the planet would have been at the Sun-Earth L3 point, which is a condition of unstable equilibrium: the pulls of the other planets would quickly have brought the ‘counter-Earth’ into view.
More importantly, the images from SOHO and the other probes are complemented by higher-resolution ones from SDO, the Solar Dynamics Observer launched in February 2010. Big discoveries are still being made: for the first time scientists have been able to see the predicted Alfvén waves in spicules projecting from the surface of the Sun, compared to the action of the wind on stalks of wheat. Not only can they now be seen, but they prove to carry so much energy that they can explain the heating of the corona, the Sun’s outer atmosphere, which has been a mystery for decades – and also the energy of the Solar Wind, which can reach speeds of 1.5 million miles per hour as it streams through holes in the corona, discovered by the Apollo Telescope Mount on the Skylab space station in 1973. (Tammy Plotner, ‘Amber Waves Of Energy’, Universe Today, July 29, 2011.)
SOHO’s camera resolution is high enough to see seismic waves on the surface of the Sun, and a great deal has been discovered from them about the behaviour of the Sun and other stars. Combining data from SDO and from SOHO, a team at Stanford University have computed the behaviour of seismic waves below the solar surface and discovered that they can detect disturbances 60,000 km below the surface, travelling upwards at 1,000 to 2,000 kilometres per hour, which evolve into huge vortices as they reach the surface – sunspot groups. It allows their outbreaks to be predicted one to three days in advance. (Tammy Plotner, ‘Scientists Detect Sunspots Before They Emerge’, Universe Today, August 23, 2011.)
At the same time as we’re explaining the newest features of the Sun to be discovered, we’re finding the origins of the ones which have been known for longest. The first naked-eye sunspot sighting on record is prior to 800 BC, in the Chinese Book of Changes, and the first known drawing is by John of Worcester in 1128-29. (Chris Kitchin, ‘Rhythms of the Sun’, Astronomy Now, November 2001.) For as long as we have satellites like SOHO, STEREO and SDO operational, there’s now no danger that astronauts on the Moon, unprotected by the Earth’s magnetic field from last to first quarter, can be surprised by sunspots and flares coming round the Sun as they did during the Apollo 16 mission, fortunately without the particle streams coming this way.
- Astronomy Beginner’s Guide: Part 3 Co-Ordinate Systems
- Astronomy Beginner’s Guide: Part 2 Compass Points in the Sky
- Amateur Involvement in Astronomy: Part 1 of Our Beginner’s Guide
- The Sky Above You – April 2021