June 19, 2018

Starlight: the muse, compass and human right

The spectacular splashes of constellations across the heavens have been the muse of story-tellers before humans even learned to write, at times now lost in the mist of history.

Around flickering flames and within the ornate walls of the courts of kings, tales were woven around patterns in the stars. The sky then burst with stories of great goddesses and gods, of chained queens, of hunters and heroes suckling on divine milk.

Celestial myths are infused with sacred cultural traditions and worldly views of the natural cycles and human condition: plunging into the autumn equinox, toiling throughout the long winter nights and emerging with the rebirth at the spring equinox. For the Egyptians, Orion summoned the flooding of the Nile and sprouting of vegetation. The Summer Triangle announced spring or summer. For Australia’s Aboriginal astronomers, the dark clouds of our Milky Way manifest a huge emu, the Great Celestial Emu, whose position in the sky signals the time of emu egg collection.

 

This is how constellations were calendars that marked the cycles of agriculture. The personified patterns of the stars made them easy to remember and informed ancient people when to sow and harvest their crops.

 

Constellations also facilitated navigation. Tracing Ursa Minor, the long-tailed baby bear known as the Little Dipper, and Polaris, the North Star, helped travellers navigate their course as they travelled over land and ships to find their way on the open seas. This opened up trade, world exploration and cultural exchange.

 

Some animals use stars such as Betelgeuse and the North Star for navigation. Mallard ducks can find north using stars. Sun compass orientation beckons fish, butterflies, bees, birds, sea-turtles and ants. Birds (warblers) navigate by starlight, and certain beetles orient themselves by the Milky Way.

 

Don’t we ourselves orient our lives by our Sun? She decides when our active days start and when they end. Without her intense heat and energy, life on Earth would wither and die. Through photosynthesis, she powers and sustains the food chain. She also keeps our planet in orbit, otherwise it would be hurled into space like a slingshot.

 

Beyond navigating the land and the seas, stars are now used to navigate the Galaxy and calibrate our knowledge of the skies.

By analysing the star’s light, or stellar spectrum, we can determine its atmospheric parameters such as surface temperature, gravity and chemical make-up. This is called stellar spectroscopy. Recently, with the growth in astrophysical data, the task of characterising stars has been automated using “processing pipelines” for efficiency. However, these methods need to be calibrated to evaluate their accuracy and ensure reliable measurements. This calibration is possible with a set of well-known stars, called “benchmark stars”. These have been observed not only with spectroscopy but with other independent methods, such as interferometry or astrometry, allowing us to know much more about them than a star for which only a spectrum is available. This makes us believe that their atmospheric parameters are much better constrained, and can thus be used for this purpose. A team of astronomers, led by Paula Jofre, created such standardised set of guidelines. These bright nearby stars they studied, including our Sun, were selected to be very different yet each representing a typical Milky Way star. They became one of the favourite reference samples, and in a sense a “spectral ruler” which provides a unified system of reference to classify and compare data for other stars. This is essential to analyse data from Gaia for example, the richest spectroscopic dataset to date, and critical for its mission to map our Milky Way. It also allows astronomers to compare the results of different survey pipelines that complement Gaia from the ground with high resolution spectra. As Paula tells Scheherazade,

What is really cool about this work is that it’s possible to see many of the Gaia benchmark stars (Aldebaran for example) with the naked eye even from the largest cities on the planet. They shine on as a reminder of how they served us to navigate since ancient times.

Preserving this skyward perspective has driven us to protect the night sky and establish starlight reserves. The International Dark-Sky Association recognises 12 protected skies worldwide, of which the UK has 4. Michigan was the first state in the United States to designate an area as a “Dark Sky Preserve” in 1993 and there are calls for international dark sky reserve in Central Australia. The Spanish island of La Palma, one of the Canary Islands in the Atlantic, was declared a “Starlight Reserve” more than a decade ago. It is protected by law against air and light pollution to guarantee its astronomical observatories clear dark skies. These strict anti-light pollution laws enforce outdoor lighting standards and maximum levels of light flux. The “Declaration in Defence of the Night Sky and the Right to Starlight, adopted in 2007, is promoted by UNESCO, the International Astronomical Union, the UN-World Tourism Organisation and the Instituto de Astrofísica de Canarias, among others.

 

From influencing religious beliefs and architecture, to defining societal norms and marking calendars for agriculture, these heavenly bodies have inspired philosophers, beckoned travellers and now help us navigate the Galaxy and grow our knowledge of the skies.

 

But if you think this is an anthropocentric view of stars, you are right. They actually play important roles not only in our lives and the lives of ancient people but on galactic and cosmological levels as well. They are the atoms of our Universe and drive many of its rattling events.

 

The dawn is now breaking and the stars are retreating, so my story will rest until they are revealed once again…

 

Image credit: IoA/Amanda Smith

 

 

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