Why is the Sun so round?
Nasa’s Parker Solar Probe is now the fastest object ever built, hitting 101 miles per second as it passed within six million miles of the Sun last month, setting a record for the closest approach to our star on its tenth planned orbit. The views during its slingshots are spectacular, with Parker’s camera catching Mercury, Venus, Jupiter, Saturn, the Milky Way and the Earth sliding past the window on a recent passage.
These familiar objects are seen in an unfamiliar setting. The flickering light in the background is from the Sun’s outer atmosphere, the corona, through which Parker is flying. It is the first spacecraft to enter this region of tenuous hot gas, normally only visible from Earth during the special conditions of a total solar eclipse.
That such opportunities exist at all is the result of what seems to be a cosmic coincidence, that the Moon and the Sun are approximately the same size in the sky. If the Moon were any larger, or any closer to Earth, during an eclipse it would block the corona as well as the main disc of the Sun. If it were any smaller, or more distant, we would enjoy only annular eclipses. Instead, a celestial configuration that may be extremely rare has made the corona an object of fascination at least modern eclipse science began in 1715.
The eclipse in April of that year had a track which crossed Southern England, and it was much anticipated. Rival astronomers vied to make predictions of its circumstances. The Painted Hall of the Royal Naval College in Greenwich has a contemporary portrait of the astronomer royal John Flamsteed hiding in the ceiling’s baroque splendour, waving a chart with his forecast.
Repeated observations of eclipses throughout the eighteenth and nineteenth centuries made it clear that the Sun was surprisingly dynamic, with the shape of the corona changing each time. The Antarctic eclipse enjoyed by a few hardy tourists and several million penguins earlier this month displayed the symmetrical form typical of periods where the Sun is less active. Eclipses near solar maxima reveal sculpted, extended shapes. The Parker images reveal this dynamic region in detail, showing myriad small streamers threading the corona.
Observing such fine structures may, it’s hoped, solve a longstanding problem. The corona is several million degrees hotter than the solar atmosphere beneath it, and the source of this energy is unclear. Future voyages close to the Sun’s surface may help, but it’s still the case that despite its familiarity, much about the Sun is confusing. It is, for example, too round; a rapidly rotating gaseous body should be flattened and yet our star is the most perfect sphere found in nature.
Such mysteries have inspired a generation of new instruments, including a giant telescope in Arizona dedicated to solar science and the European Space Agency’s Solar Orbiter. These projects are technological marvels, and of great scientific importance. The fact that studying the Sun produces images like those from Parker, full of wonder and strange beauty, is a welcome side-effect.