The Milky Way galaxy is our home. While we may think it is just Earth and the Sun, and maybe the planets of the Solar System too, we are members of a family of stars some 100 000 light years across, along with perhaps 400 billion other stars and their systems. And this family sits in its own family in the vastness of the observable universe.
So the first wonder of the Milky Way galaxy is itself: we see only the thick stream of stars across the sky on clear nights, the stream which led the ancients to compare it to spilt milk. Modern telescopes can see the galaxy much more clearly and we know now that we are in a spiral galaxy, with whirling arms around a thick, bright centre. The Solar System lies about halfway out on one of these arms. The shape is indescribably beautiful: elegant and strong, light and dense at the same time.
This galaxy, which has perhaps existed for 13.2 billion years – the age of the oldest known star within it and only half a billion years less than the age of the universe itself – is one of many, one of billions. Yet it contains so much that is magnificent.
It holds the only intelligent life yet discovered in the universe. The only places where beings made of the ash of ancient stars’ fires can look at the universe itself and decipher its mathematics. These beings look around them every day for signs that they are not alone, but so far there is only silence out there. There are, however, planets throughout the galaxy, the first of which outside the Solar System was discovered in 1992.
The Milky Way galaxy holds billions of stars. Most of these are unremarkable red dwarf stars, a few are yellow-white stars, like the Sun; and a very few are magnificent beasts. VY Canis Majoris is a hypergiant star and the largest known star in the universe. It is an old, red star, nearing the end of its life and is some 2000 times the diameter of the Sun – if placed in the Sun’s position, it would extend somewhere between Jupiter and Saturn. Betelgeuse, in the constellation of Orion, is a red supergiant and one of the brightest stars in the sky. It is 40 times more massive than the Sun and nearly 1000 times as wide. It is so huge that it has nearly exhausted its nuclear fuel after only eight or so million years – compared to the 4.6 billion years the Sun has been quietly burning – and will probably go supernova soon (within a thousand years). When it does, it will be the most brilliant, the most luminous object in the night sky, leaving the moon in its wake.
In between these stars are vast clouds of dust and gas. Some are simply agglomerations of hydrogen gas that have come together through gravity, and others are the remains of stars that have torn themselves to pieces in their final moments and strewn their wreckage across space. They can be stunning: like the Crab Nebula, which formed from a supernova explosion in 1054, or the Horsehead Nebula, a dark cloud of dust resembling a horse’s head and which is an active centre forming new stars. These huge, disparate clouds are only visible when lit up by radiation from nearby stars, and look much smaller or more compact than they really are. Still, they are true wonders of the galaxy.
The nebulae that are left after the explosions of massive stars are not the only things these monsters leave behind. When a star explodes and blows itself apart, it will often leave a tiny, super-dense object made entirely of immensely compacted material – atoms which have been crushed under their own gravity and squeezed into a mass of neutrons. These neutron stars are among the strangest things in the galaxy. They often spin rapidly, sending out radio pulses, and, being so dense, have enormous gravity and will suck any gas they can off a nearby star, if there happens to be one. Don’t get too close to a neutron star – you’d need to accelerate beyond a third of the speed of light to escape it.
Stranger still are black holes – the cores of the most massive stars which simply collapse beyond the point achieved by neutron stars and out of our visible range. They effectively punch a “hole” in the universe, and cannot be seen, except by the effects they have on nearby objects. Black holes have an escape velocity greater than the speed of light – namely, get close enough to it, and even light cannot escape. At the very centre of our galaxy there is a giant black hole, 4 million times more massive than the Sun, but covering a relatively small space – only around eight times the distance from the Earth to the Sun.
The Milky Way is host to so much that is strange and beautiful to us. It has inspired poets, novelists, mathematicians, physicists and ordinary people looking up at the sky and saying “wow”; it gives us all a greater sense of the brilliance of our universe.
