Our home galaxy the Milky Way, a barred spiral galaxy, a colossal 100,000 light year wide disk of hundreds of billions of stars, planets, dust, gas, and dark matter slowly turning in interstellar space. On a clear night from a location without light pollution, the Milky Way arching overhead is an absolutely astounding and amazing sight. Star clouds, dark nebulae, and star clusters are packed into a narrow band of light that is the Galaxy of which we are part. If you view the Milky Way with imagination and see the scene as it actually is, the plane of our Galaxy and not just a band of stars, then it really gives a feeling of perspective and insignificance as we are looking into our Galaxy from a small rocky planet that orbits just one of the hundreds of billions of stars contained in it’s immense disk. The Milky Way is of course just one of hundreds of billions of others scattered throughout the universe.
We are looking edge on through part of a spiral arm, that is why the stars are packed into a thin band. Just like looking at a dinner plate edge on, but we are “inside” the dinner plate so we cannot obviously see the Milky Way from outside as an entire galaxy. So astronomers and scientists try to map it as well as possible from our vantage point inside it. When you look into the night sky the region of Milky Way that is the densest lies towards the constellation of Sagittarius, as this marks the galactic centre. But this night sky scene is very vulnerable to light pollution and can disappear completely, being washed out by the lights of our towns and cities. It is only when you see this spectacle in all it’s glory from a truly dark sight that you realise that this is denied to so many people.
The Milky Way is a giant, being average to large as galaxies go. It is 100,000 light years across, 1,000 light years thick, and with a circumference of between 250,000 to 300,000 light years. These measurements only take into account the visible part of our Galaxy, the dark matter percentage will take these figures much higher. If you could see the Milky Way from deep space it would be as striking as any other, with two major spiral arms that encircle the nucleus in a graceful spiral structure. It is now thought that our Galaxy has a central bar that stretches 27,000 light years from end to end according to latest observations from the Nasa Spitzer Space Telescope. The two major spiral arms are connected to the ends of this central bar structure, and it is thought that the bars of such galaxies like our own act as a channel for dust, gas, and other interstellar material to get transported towards the core, eventually ending up in a super-massive black hole in the centre.
An artist's impression of our Milky Way Galaxy with a 27,000 light years central bar and two major spiral arms connected to the ends. Note the position of our Sun
As you can see from this artist’s impression, the two main arms are the Scutum-Centaurus arm and the Perseus arm. The solar system is contained in a minor arm called the Orion-Cygnus arm. The Milky Way is thought to contain between 200 and 400 billion stars, each one of these stars is a sun in it’s own right that could have a family of planets. It does not require a mental leap for you to realise the sheer unimaginable numbers of potential planets out there in our own Galaxy that could have life and maybe other civilisations. As in other galaxies, apart from stars it contains large numbers of interstellar dust clouds that are collapsing due to gravity to form new solar systems, and therefore star forming regions or stellar nurseries can be observed. Recently vast star factories have been discovered in our Galaxy by the Herschel Telescope that can see through the obscuring dust. But probably the most famous and easily observable of these stellar nurseries is the Orion Nebula (M42)
Our Sun lies 24,000 to 26,000 light years out from the core, and orbits around the centre of the Galaxy. The Milky Way is so vast that the time it takes the solar system to do just one orbit of the Milky Way’s centre is 225 million years, this is known as a galactic year. The Sun has been in existence for 20 galactic years, so it has only done 20 orbits in all of it’s life so far. It was 4 galactic years when oceans first appeared on Earth, at 7 galactic years the first bacteria appeared, at 16 galactic years the first multi-cellular organisms evolved, and at 19.999 galactic years the very first humans were walking on the planet. This puts into perspective just how old the Earth and solar system really is, and makes you realise that we are the late comers on our planet.
The solar system travels through the interstellar space of our Milky Way at 251 kilometres per second. It is surrounded by the heliosphere, a protective “bubble” that protects us from dangerous cosmic radiation. This protective bubble is produced by the solar wind, a constant stream of particles blowing out from the Sun. As the Sun and it’s planets travel around the galactic centre it does not just stay level as it makes it’s trek, it actually bobs up and down in the 1,000 light year thickness of the Galaxy’s plane. This up and down motion amounts to an oscillation of 3.7 times per orbit, putting the solar system through different regions of interstellar dust and gas and maybe near potential hazards. This up and down motion along the galactic plane has been noticed to coincide with major extinction events. Surrounding our Galaxy is a halo of globular clusters, the stars in these clusters are amongst the oldest in the Galaxy, being between 10 and 13 billion years old. This makes them some of the oldest objects in the universe. They reside far above and below the main arms of the Milky Way. These clusters are balls of stars and can number hundreds of thousands of members. The number of globular clusters in the Milky way is thought to be 158. Famous globulars are M13 the Great Hercules Cluster in the constellation of Hercules, and M92 also in Hercules.
We do have a few smaller galactic neighbours nearby that orbit our Galaxy. The Large Magellanic Cloud is a satellite galaxy of the Milky Way, with a diameter of 20,000 light years, aproximately a fifth the size of our main Galaxy. The Large Magellanic Cloud has an even smaller close companion called the Small Magellanic Cloud 20,000 light years away, with a diameter of just 7,000 light years, and containing a few hundred million stars. Some think the LMC and the SMC were originally small barred spirals that have been captured by the Milky Way’s gravity and pulled out of shape. The central bars are still evident in their structures.
As with all galaxies we also have a monster at the centre of ours, Sagittarius A is the name for the energetic region at the Galaxy’s core. This region is an intense radio source, it marks the centre of the Galaxy, and the motion of stars and material around it gives it away as an object of very large mass, a super massive black hole. This is a turbulent and dynamic area of bright star clusters, very hot gas, intense x-rays, and huge magnetic fields. The black hole at the centre of all this activity has a mass of 2.6 million to 4 million times the mass of our Sun. Although this black hole seems to have given up feeding for the time being, possibly due to exploding stars blowing dust and gas away from the black hole’s gravitational grip.
The Milky way is not alone in our part of the universe, we are part of a collection of galaxies called the Local Group which numbers around 30 or so members that include dwarf galaxies. Although we are one of the major members we are not the largest galaxy, we come second in size to the Andromeda Galaxy at 2.5 million light years away. The Milky way and Andromeda dominate the heart of the Local group, the central point being somewhere between us and Andromeda. The Local group is in turn part of a larger galaxy grouping called the Virgo Super cluster. The Andromeda Galaxy is a giant maybe twice the size of our Milky Way and contains up to a trillion stars. The Milky Way and Andromeda are destined for a violent collision, Andromeda is heading in our direction at 313,200 miles per hour. When this collision happens both galaxies will be distorted, and could have hundreds of thousands of stars ripped far from each galaxy as they converge, due to immense tidal forces. Over time, spanning hundreds of millions of years from the point at which we collide with Andromeda, the two galaxies will eventually form into one huge elliptical galaxy.