Pole stars of the future in
The Sun’s southerly motion since last week’s solstice has yet to gain speed and not until the 12th does it lie sufficiently far south for Edinburgh to enjoy any so-called nautical darkness, with the Sun more than 12° below the northern horizon in the middle of the night. Even then, moonlight is troublesome for a few more days to delay our first views of a dark summer night sky.
If there is one star-pattern that dominates our skies over the summer, it is the Summer Triangle. Formed by the bright stars Vega in the constellation Lyra, Deneb in Cygnus and Altair in Aquila, it occupies the upper part of our south star map, though its outline is not depicted. In fact, the projection used means that the Triangle’s proportions are squashed, because Vega and Deneb are significantly closer together in the sky than either are to Altair.
The leader and brightest of the Triangle’s stars is Vega which moves from high in the east at nightfall to stand even higher in the south at our map times. Blazing at magnitude 0.0 from a distance of 25 light years, it is a white star, twice as massive as our Sun but very much younger. Excess heat revealed by infrared astronomy indicates that Vega is encircled by a disk of dust which may be evidence that a planetary system is forming around it.
Set your time machine for about AD 13,700 and you will be able to glimpse Vega close to where we currently find Polaris, our current Pole Star. This is because the Earth’s axis is slowly toppling in space, taking 26,000 years to complete a 47° circle in the sky and carryingthe axis to within 4° of vega. Polaris happens to lie within 0.8° of the axis at present so that, as the Earth rotates once each day, it stays almost fixed in our sky and the other stars appear to circle counterclockwise around it
Altair is the second brightest of the Triangle’s stars and one of the closest bright stars at “only” 16.7 light years. Shining at magnitude 0.8, half as bright as Vega, it is 80% more massive than our Sun but, remarkably, spins on its axis in about nine hours as compared with the more leisurely 25 days taken by the Sun. As a result, it is noticeably oblate, measuring 20% wider across its equator than it does pole-to-pole.
Deneb’s magnitude of 1.2 makes it the dimmest of the Triangle’s corner stars but it is also one of the most luminous stars in our Milky Way Galaxy. Because its distance may be around 2,600 light years, it very difficult to measure the minuscule shift in its position when viewed from opposite sides of the Earth’s orbit around the Sun – the parallax technique that gives us accurate distances to Vega and Altair. Indeed, estimates of Deneb’s distance differ by well over 1,000 light years.
White-hot and shining at some 200,000 Sun-power, Deneb is large enough to engulf the Earth were it to swap places with the Sun. It is also burning its nuclear fuel at such a rate that it seems destined to disintegrate in a supernova within a few million years, although it should survive to be another of our