of different systems that interact and influence each other. The Arches Cluster is the densest known star cluster in the galaxy. Formed from about 150 young, intensely hot stars that dwarf our sun in size, these stars burn brightly and are consequently very short-lived, exhausting their supply of hydrogen in just a couple of million years. The Quintuplet Cluster contains one of the most luminous stars in our galaxy, the Pistol Star, which is thought to be near the end of its life and on the verge of becoming a supernova (see Chapter 2). It is in central clusters like the Arches and the Quintuplet that the greatest density of stars in our galaxy can be found. As we move out from the crowded galactic centre, the number of stars drops with distance, until we reach the sparse cloud of gas in the outer reaches of the Milky Way known as the Galactic Halo.
This artist’s impression shows the Arches Cluster, the densest known cluster of young stars in the Milky Way Galaxy.
NASA
Along with the Arches Cluster, the Quintuplet Cluster is located near the centre of the Milky Way Galaxy.
NASA
The bright white dot in the centre of this image is the Pistol Star, one of the brightest stars in our galaxy.
The distance between the Sun and the outermost planet of our solar system, Neptune, is around four light hours – that’s one-sixth of a light day. You would have to lay around 220 million solar systems end to end to cross our galaxy.
In 2007, scientists using the Very Large Telescope (VLT) at the Paranal Observatory in Chile were able to observe a star in the Galactic Halo that is thought to be the oldest object in the Milky Way. HE 1523-0901 is a star in the last stages of its life; known as a red giant, it is a vast structure far bigger than our sun, but much cooler at its surface. HE 1523-0901 is interesting because astronomers have been able to measure the precise quantities of five radioactive elements – uranium, thorium, europium, osmium and iridium – in the star. Using a technique very similar to carbon dating (a method archaeologists use to measure the age of organic material on Earth), astronomers have been able to get a precise age for this ancient star. Radioactive dating is an extremely precise and reliable technique when there are multiple ‘radioactive clocks’ ticking away at once. This is why the detection of five radioactive elements in the light from HE 1523-0901 was so important. This dying star turns out to be 13.2 billion years old – that’s almost as old as the Universe itself, which began just over 13.7 billion years ago. The radioactive elements in this star would have been created in the death throes of the first generation of stars, which ended their lives in supernova explosions in the first half a billion years of the life of the Universe (see Chapter 2)
THE SHAPE OF OUR GALAXY
As well as being vast and very, very old, our galaxy is also beautifully structured. Known as a barred spiral galaxy, it consists of a bar-shaped core surrounded by a disc of gas, dust and stars that creates individual spiral arms twisting out from the centre. Until very recently, it was thought that our galaxy contained only four spiral arms – Perseus, Norma, Scutum–Centaurus and Carina–Sagittarius, with our sun in an off shoot of the latter called the Orion spur – but there is now thought to be an additional arm, called the Outer arm, an extension to the Norma arm.
Close to the inner rim of the Orion spur is the most familiar star in our galaxy. The Sun was once thought to be an average star, but we now know that it shines brighter than 95 per cent of all other stars in the Milky Way. It’s known as a main sequence star because it gets all its energy and produces all its light through the fusion of hydrogen into helium. Every second, the Sun burns 600 million tonnes of hydrogen in its core, producing 596 million tonnes of helium in the fusion reaction. The missing four million tonnes of mass emerges as energy, which slowly travels to the Sun’s photosphere, where it is released into the galaxy and across the Universe as light
The Andromeda Galaxy is our nearest galactic neighbour, and our own Milky Way Galaxy is believed to look very much like it.
Located 5,000 light years away, the Lagoon Nebula is one of a handful of active star-forming regions in our galaxy that are visible from Earth with the naked eye.
NASA
A STAR IS BORN
Our sun is in the middle of its life cycle, but look out into the Milky Way and we can see the whole cycle of stellar life playing out. Roughly once a year a new light appears in our galaxy, as somewhere in the Milky Way a new star is born.
The Lagoon Nebula is one such star nursery; within this giant interstellar cloud of gas and dust, new stars are created. Discovered by French astronomer Guillaume Le Gentil in 1747, this is one of a handful of active star-forming regions in our galaxy that are visible with the naked eye. This huge cloud is slowly collapsing under its own gravity, but slightly denser regions gradually accrete more and more matter, and over time these clumps grow massive enough to turn into stars.
The centre of this vast stellar nursery, known as the Hourglass, is illuminated by an intriguing object known as Herschel 36. This star is thought to be a ‘ZAMS’ star (zero ago main sequence) because it has just begun to produce the dominant part of its energy from hydrogen fusion in its core. Recent measurements suggest that Herschel 36 may actually be three large young stars orbiting around each other, with the entire system having a combined mass of over fifty times that of our sun. This makes Herschel 36 a true system of giants. Eventually Herschel 36 and all the stars in the Milky Way will die, and when they do, many will go out in a blaze of glory.
Eta Carinae is a pair of billowing gas and dust clouds that are the remnants of a stellar explosion from an unstable star system. The system consists of at least two giant stars, and shines with a brightness four million times that of our sun. One of these stars is thought to be a Wolf-Rayet star. These stars are immense, over twenty times the mass of our sun, and are engaged in a constant struggle to hang onto their outer layers, losing vast amounts of mass every second in a powerful solar wind. In 1843, Eta Carinae became one of the brightest stars in the Universe when it exploded. The blast spat matter out at nearly 2.5 million kilometres (1.5 million miles) an hour, and was so bright that it was thought to be a supernova explosion. Eta Carinae survived intact and remains buried deep inside these clouds, but its days are numbered. Because of its immense mass, the Wolf-Rayet star is using up its hydrogen fuel at a ferocious rate. Within a few hundred thousand years, it is expected that the star will explode in a supernova or even a hypernova (the biggest explosion in the known Universe), although its fate may be sealed a lot sooner. In 2004, an explosion thought to be similar to the 1843 Eta Carinae event was seen in a galaxy over seventy million light years from the Milky Way. Just two years later, the star exploded as a supernova. Eta Carinae is very much closer – at a distance of only 7,500 light years – so as a supernova it may shine so brightly that it will be visible from Earth even in daylight.
Out in the Milky Way we can see the whole cycle of stellar life
