Cygnus (The Swan)

Cygnus (The Swan)


Back in El Valle Lecrin for my summer sojourn in southern Spain at 36 degrees North where I can observe better the nearer equatorial constellations and stars eg. Scorpio and Sagittarius. However even here, although they’re very clear by eye, I am in a valley and the heat rising off the Sierra de las Guajaras makes the air very unstable at times and that of course makes the image very poor at high magnification in a telescope.

So, the best constellation to look at, not so affected by atmospheric scintillation is Cygnus right above me. Along with Vega in Lyra and Altair in Aquila it is referred to as “The Summer Triangle” and beloved by mariners in my day (pre-Sat Nav/GPS) as easy sextant targets for a good navigational fix.

For astronomers, there is of course a lot more going on. Deneb for instance is one of my favourite stars; a blue super giant estimated to be around 3220 light years distant and yet one of the brightest stars in the heavens to the naked eye; just try and imagine how incredibly powerful that star must be to shine so brightly at that distance. Within this constellation are some beautiful sites the “North American Nebula” (due to its shape), the “Pelican Nebula” and the “Veil Nebula”. But if you move down the neck of the Swan you see a not too bright star Eta Cygni, and just to the NE about a Moon’s width away is a much fainter though powerful blue star with an incredibly interesting name called HDE 226868 weighing somewhere between 10 and 20 times the mass of the Sun.

But, hey ho, there’s something wrong. This bloated blue star, massive as it is, is being swung around every 5.6 days. How can that be? It appears to be caught in the gravitational grasp of an immense object. Spectroscopic orbital analysis proves that its companion must weigh about 8.7 solar masses and lie about 30million Kms away from it, in fact relatively close to it, yet the object twirling this massive star around remains invisible. Usually massive stars are extremely bright but our most powerful telescopes today can find nothing there, so our first deduction is that it is a heavy and an under-luminous object.

We then find that this exact spot of sky emits an intense beam of X-rays, a powerful type of energy that’s always a signal of violence. Physics tells us that anything spiralling toward a black hole should be whipped up to speeds frenzied enough to cause X-rays, and sure enough this turns out be the highest energy X-ray source in the sky; a source so important that it is universally known in the X-ray catalogue as Cygnus X-1. Finally, tremendous changes occur in a millisecond, (a thousandth of a second), faster than an eye blink. Such near instantaneous variations prove Cygnus X-1 is no larger than 1/20th the size of the Moon. Put all the parameters together and you have an ironclad case for a black hole.

It can’t be a neutron star because infalling material would release energy on impact, but evidence of a black hole is because the infalling atoms only create X-rays while in orbit, after that nada. It’s hard to convey just what is required to be a black hole but maybe explaining that if you could compress the mass of Everest into an atomic nucleus you would have a black hole. When the Large Hadron Collider first went on line many people said it would create a black hole that would devour the Earth, but the Earth is hit every day by cosmic rays far in excess of any of the highest energies achievable in the LHC.

Yet black holes are scarce because matter does not voluntarily pack itself so firmly. There is one at the centre of our galaxy and probably most others but that still doesn’t make them common. Most stars will leave the “Main sequence” as bloated giants, then white dwarfs. Some will become dense neutron stars, a teaspoonful weighing as much as 14,000 tons. A star has to be at least 3.5 times heaver than the Sun and in its final life cycle to become a black hole.

When their nuclear furnaces no longer emit enough outward pushing pressure such stars cannot resist the gravitational urge to collapse. The smaller they get the smaller they want to be, until their gravitational escape velocity reaches 299,792km/ per/sec. Light itself, then cannot leave, and the stars effectively disappear.

Cygnus X-1’s singularity, the collapsed star at its centre, achieved black hole density when it became 6km wide and probably shrivelled even further to less than the size of a beach ball. The surrounding “event horizon” would be something like 26km; on this scale an invisible no-trespassing zone. Step across it and you are doomed. Fortunately, this will not be the fate of our Sun. In 5 billion years, it will have left the “Main Sequence” and absorbed the Earth as it swells into a Red Giant, before settling back to White Dwarf status. At the same time Andromeda will be colliding with the Milky Way, what a magnificent sky for astronomers that will be. On the assumption we’ve colonized other worlds.

Will we be around to witness it?

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