It hasn’t been the ideal summer for observing the skies, but as we move into September we live in that almost annual hope of an Indian summer, writes Niall Smith.
Happily, we still have many of the bright summer stars with us for a while yet.
Possibly the easiest to spot is the giant star Vega, directly overhead and with a very distinct blue tint that comes because of its high temperature — almost twice that of our Sun. Vega is a very close neighbour in astronomical terms.
Light has taken just 25 years to cross the gap between it and ourselves (compared to billions of years for some stars, though such stars are so faint due to their distance that they cannot be seen without a large telescope).
It is also a very young star, about 450 million years old, compared to our Sun which has already clocked up 4,500 million years. Vega is 2.5 times more massive than the Sun and 40 times brighter, so it’s just as well we don’t orbit it as our surface temperature would then be hot enough to melt lead.
Vega was potentially important to our ancestors not only because it is bright and easy to spot in the northern hemisphere, even under moonlit skies, but also because of its location in the sky.
The Earth’s axis wobbles like the axis of a spinning top and right now it points to a rather unremarkable and faint star we call Polaris (Pole Star) in the constellation of Ursa Major. Polaris is always visible from Ireland, weather permitting of course. However, around 12,000 BC, just before the first recorded evidence for human habitation in Ireland, the Earth’s axis had tilted conveniently so that the Pole Star was Vega.
That must have been an impressive sight and the coincidence of having the fifth brightest star in the sky as the pointer to north must have seemed destined to be.
In another 13,725 years Vega will again be the Pole Star as the Earth’s axis continues to spin. One can only wonder how humanity will view it then compared to how we did in 12,000B.C. Will we perhaps have found a way to reach Vega?
Will humans have viewed it from orbit, en route to other destinations of greater interest? As a young hot star, Vega emits massive amounts of ultraviolet radiation, much more severe than that which gives us
sunburn here on Earth, and it likely emits copious amounts of other radiations that would sterilise any biological entity that was unfortunate enough to be exposed to it.
So as a destination it’s unlikely to top the list of most desirable places.
However, we do learn a lot about how stars evolve by studying young stars such as Vega. It is rarely, if ever, the case in science that the chance to observe something up close brings no benefits.
Vega is very well placed to view during September (and from Ireland it never sets, but at the moment it is particularly well placed for observing at a reasonable time of the night).
September is also the month of the autumnal equinox. On September 23 the Sun will be directly over the Earth’s equator, meaning day and night will be almost equal almost anywhere on the planet. After that date the nights are longer than the days.
On September 6 and 7, the Moon is nicely placed between the bright planet Jupiter (to its right) and the fainter planet Saturn (to its left).
While Jupiter is not quite as bright as it was in August, it would take a careful observer to notice the difference, so don’t be deterred from looking for it.
Further information on September’s night sky can be found on the CIT Blackrock Castle Observatory website at www.bco.ie