Chapter II: Where Stars Live
The Organisation and Scale of the Universe
We live on a beautiful blue oasis, orbiting a rather average star along with seven other major worlds. Together, the eight major planets and the Sun make up the bulk of the Solar System. Although most of us know of the planets in the Solar System, most other stars in the universe also harbour their own planetary systems. As of May 2016, more than 2,000 planets have been discovered around other stars, although mostly too distant to see a direct view of.
The fact that most other stars harbour their own planetary systems tells us something very important in our place in the Universe: which is that we are located nowhere special. This fact becomes even more evident as we travel up the hierarchy of structure in the cosmos.
In order to get a perspective of how the universe is organised on the largest scales, we need to first start by looking up.
Looking up: The Milky Way
For many of us city dwellers, seeing a starfilled night sky is not something that we are accustomed to. However, for those who have had the opportunity to travel to a rural area or dark sky site (or better yet, the fortune of living in such an area), and looked up on a clear moonless night, one would see a vista of thousands of stars filling up the black velvet sky. If the time is right, a glowing band of light may also be seen arching across the firmament above.
This is the Milky Way, a collection of about 300 billion (300,000,000,000) stars, of which the Sun and Solar System are part of. All the stars that you can see as individual points of light in the night sky are part of the Milky Way, and even then, these constitute only the stars nearest to us. Most of the stars in the Milky Way lie beyond these stars in our immediate neighbourhood. Since they are too distant to be seen as points of light by our naked eye, they appear as a continuous glowing band of light arching across the sky. This is the bulk of the Milky Way that we can see from our vantage point on Earth.
The image above is a long exposure photograph taken with a DSLR and 50mm lens, which gives you a field of view similar to the magnification of the naked eye. Although the Milky Way is not as colourful when seen with the naked eye due to its faintness, astrophotography captures light over the span of many minutes to hours to reveal the colours of the stars.
Because we are located inside the Milky Way galaxy, it can sometimes be difficult to picture why we see it as a band of light that arches across the sky. This is because the Milky Way is shaped like a flattened disc, and we are located somewhere about halfway between the centre and the edge of this disc. If one were to picture the Milky Way as a pan of pizza, our vantage point would be analogous to how a piece of pepperoni would see the rest of the pizza: a flattened band that encircles our surrounding view.
In order to get a picture of how the Milky Way looks like from the outside, we simply cannot send a space probe to send a picture back to us. Even with our fastest spacecraft, it would take over a trillion years to travel far enough to obtain a bird's eye view. Thankfully, the Milky Way is not the only galaxy in the Universe, which allows us to look at other galaxies with similar structures to our own. As if having 300 billion Suns weren't enough, there are yet another several hundred billion or so other galaxies in the Universe, each of which also containing stars that number far into the billions.
One such nearby (relatively speaking) galactic neighbour is the Andromeda Galaxy, which is pictured above. The Andromeda Galaxy is a naked eye object when seen under dark skies, and has a structure similar to our own galaxy. Although it is in many ways like a sister galaxy to the Milky Way in terms of its structure, it is much more massive, containing within itself about a trillion stars (1,000,000,000,000). Despite its distance, the sheer intensity of all its 1 trillion stars shines so strongly that even at 2.5 million light years away, our feeble eyes can still clearly detect its light.
To get a sense of how immense this distance is, we need to first move beyond our scales measured in metres and kilometres, and enter the realm of light years. Light is the fastest thing in the Universe, and the laws of physics forbid anything to travel at any speed greater than that. It travels so rapidly that a beam of light could travel 7 times around the Earth in one second. For us to travel that same distance on board a plane, it would take over 13 days. Despite light's incredible speed, it travels at snail's pace in the realm of the stars: the same ray of light can circle the Earth 7 times in one second would take more than two million years to travel from the Andromeda Galaxy to the Earth. Yet, on intergalactic scales, the Andromeda Galaxy is one of the nearest galaxies to us.
The Local Group
In space, the force that holds the planets to their stars, and for stars to congregate into galaxies is the same. This is the force that also holds us down on the Earth: gravity. Although gravity diminishes rapidly with distance, the sheer mass of galaxies allow gravity to exert its influence even over incredible scales.
In fact, the effects of gravity are not limited to within galaxies. Due to the large spheres of influence around entire galaxies, galaxies themselves tend to gather into clusters, in a similar manner to how stars gather in clusters on much smaller scales. Accordingly, the Milky Way also exists in a cluster of galaxies known as the Local Group. The Local Group of galaxies also consists of the Andromeda galaxy, which is incidentally the largest. The Milky Way weighs in at the runner's up, being the second most substantial galaxy after the Andromeda Galaxy.
The Small Magellanic Cloud
The Small Magellanic Cloud
(Satellite of the Milky Way)
The Triangulum Galaxy
The Large Magellanic Cloud
(Satellite of the Milky Way)
(Satellite of Andromeda Galaxy)
The images above show some of the other members of the local group which I have photographed. The Triangulum galaxy is the third largest galaxy in the local group, after the Milky Way. In total, the local group has 54 galaxies, although most of which are relatively small. The other three galaxies pictured above are some of these smaller ones, and also happen to be satellite galaxies.
In astronomy, satellites are objects that orbit around other objects. For example, the Moon is a satellite of the Earth, while the Earth is a satellite of the Sun. Because the Milky Way and Andromeda Galaxies are much larger than these smaller galaxies pictured above, these smaller galaxies enter an orbit around their larger counterparts.
For the case of the Milky Way, the larger of these satellites are the Large and Small Magellanic Clouds, pictured above. The Magellanic Clouds can be seen with the naked eye in Southern Latitudes, appearing like large, broken pieces of the Milky Way.
Beyond the Local Group
Beyond galactic clusters, clusters of galaxies in turn group together into superclusters. Superclusters in turn, are arranged in thread-like formations known as filaments. Filaments are the largest structures known in the Universe. At this scales, light takes a whopping several hundred million years to travel from one end to the other.
At this point, we have travelled up many hierarchies of celestial organisation. Stars like the Sun group into galaxies (or sometimes found in smaller groups called clusters within galaxies), galaxies group into clusters, clusters group into superclusters, and superclusters are arranged in filaments. Accordingly it is estimated that in total, the total number of stars are:
Or a "1" with 24 zeros after it, remembering that each star is an entire Sun in its own right, possibly harbouring its own planets in their own star systems. More stars exist than the number of grains of sand on the Earth combined, and the scale of things beyond our tiny planet is beyond any ordinary human comprehension.
We are truly located in a very unremarkable place in the Universe, a fledgling species that is only beginning to look beyond the shores into the 'grand cosmic ocean'. While we will almost certainly never visit all but the closest worlds out there, this picture of the cosmos given by modern science is a testament to our desire to explore the great unknown.