The following pages contain remarkable images of the Aurora Australis, Moon, and our Milky Way under dark Otago skies.

Named after the Roman goddess of the dawn, the Aurora is caused by energetic protons and electrons from our sun, the solar wind, colliding with atoms of oxygen and nitrogen between a hundred and a thousand kilometres up in our atmosphere.   Aurorae are centred over the geomagnetic poles of the Earth, where the shape of the earth’s magnetic field, the magnetosphere, briefly traps the solar wind.   The different colours in the aurora represent the differing energies of particles interacting with differing atoms in our atmosphere.   In the southern hemisphere, the aurora Australis is centred over the south geomagnetic pole at 64 degrees south, 135 degree E, or 26 degrees away from due south.  The centre of most aurorae seen from Otago will therefore be offset by this amount from due south, close to the horizon.  Really bright aurorae, caused by strong solar winds from activity on the Sun, will be seen much brighter and higher in the sky.  From a minimum in 2020, astronomers are now seeing a marked increase in solar activity, predicted to peak in 2025, consistent with the 11-year solar activity cycle.  Over the next two-threes we can look forward to many more aurorae visible under the dark skies of Otago.

In the Otago winter months, the centre of our Milky Way galaxy rises majestically at dusk and passes overhead at midnight.  Our galaxy in a spiral galaxy with a bulge in the middle (think two fried eggs back to back).  Under dark skies the central galactic bulge is easily visible in a broadening in the milky way complete with a dark central lane of galactic “dust” obscuring the light from the stars behind.   Throw in a few glowing nebulae of ionised hydrogen and dusty star-forming cocoons and you have the sight of the milky way featured in many of the images here.

But as you marvel at these photographs, here are some numbers to ponder.

There are between 100 and 400 billion stars in our Milky Way galaxy.  By a curious coincidence this lower limit is consistent with estimates for the number of humans that have lived on planet Earth.  So, there is at least one star in our galaxy for every man, woman and child who have lived on our planet.   Of all those stars, it is currently estimated that at least 10% have Earth-like planets in orbit around them.  In other words, there are somewhere between 10 and 40 billion other potential Earths out there in our Milky Way alone.  Add to that, the fact that there are more galaxies in the Universe than stars in our own galaxy and the question is not whether life exists elsewhere in the Universe, but how can life not exist elsewhere in the Universe?

But while the galaxy is teeming with stars and planets, the vastness of space leads to the paradoxical position that it is also almost empty.  It is 150 million kilometres from the Earth to the Sun, but 40 208 000 000 000 (40.2 trillion) kilometres from the sun to the next nearest star (Proxima Centauri).   If the sun scaled down to the size of a tennis ball, then the nearest star would be the size of a garden pea some 1500km distant.  Space is big.  And (mostly) empty.

If you are not feeling a little insignificant yet, try this for size.   It is a further 250 000 000 000 000 000 (250 quintillion) kilometres, or about 25000 light years to the centre of our Milky Way galaxy.  The Milky Way itself is about 100 000 light years across, and we are still very much in our Universe’ backyard.  The entire visible Universe extends 13.6 billion light years in all directions – more than 100 000 times the size of our milky way galaxy.   And here we stop.

Professor Brian Boyle

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