A Voyage of Discovery
It’s often said that there are more stars in the Universe than there are grains of sand on Earth! Now that may seem like a crazy fact to get your head round but consider also that the Universe itself is thought to have popped into existence out of nothing, in the so called ‘Big Bang’ event. Before that, neither time nor space existed! The Universe today is so vast that it takes the light from the most distant objects many billions of years to reach us traveling at 300,000km/s which means it left on its long journey across the Cosmos before the Earth was even here. Welcome to the crazy world of Space.
Light is considered to be made up of tiny packets that we call photons. The photon of light which just reached Earth from the most distant object visible has been journeying for best part of 13 billion years. Its been on an incredible journey sweeping by great galaxies, glowing dust clouds and exploding stars. Lets just rewind the journey and follow the photon of light back into deep space.
Leaving the Earth behind, the first object we come across is the Moon, our nearest neighbour in space. At 300,000km/s it takes just 1.3 seconds before we hurtle passed the Moon. It’s a small almost airless place with its surface peppered by the scars of countless meteorite impacts. It takes us another 8 minutes before we enter the violent domain of the Sun. As we draw closer, its clear to see why its in charge of things in the Solar System as its possible to fit one million Earths inside it. Deep in the core of our local star, the Sun is converting hydrogen to helium through the process of nuclear fusion giving us the heat and light needed for life to thrive here on planet Earth.
Heading out of the Solar System again, we pass Mercury and Venus, the two inner planets before heading out to Mars which has a strange yet distinctive red colour to it! Unlike astronomers who first studied Mars through a telescope there is certainly no sign of great canals and civilisations inhabiting this world. The red colour comes from a reaction between iron and oxygen turning the surface into iron oxide, which you may know by its common name, rust!
Beyond Mars we pass a belt of asteroids and start on the long journey between the great outer planets. It takes about 40 minutes for our photon of light to reach the first of the gas giant planets, Jupiter. And it really is a giant, large enough to swallow up all of the planets in the Solar System put together, twice! Its easily visible in the night sky and even small telescopes reveal it as a huge swirling ball of gas with a great storm raging high in the atmosphere. It takes another 40 minutes before we swing by majestic Saturn and eventually on passed Uranus and Neptune. It’s taken us about 4 hours to journey from the Sun to the edge of the Solar System and, leaving the familiar sight of the planets far behind, we start a very, very long journey through interstellar space.
We have to wait patiently for about 4 years and 2 months, passing the odd lump of stray rock on the way, before we reach the nearest star to our own, Proxima Centauri. Bright stars are named after the constellation in which we see them, so this one is in the constellation of Centaurus. Its a small red dwarf star which is just 1/7th the diameter of our Sun and on occasions gives off flares of energy as a result of changes in its magnetic field. Because it takes light 4.2 years to reach us from this star, we say its 4.2 light years away, this equates to a distance of 39.7 million million km! We use light years as measurements because the numbers are smaller and easier to deal with!
Leaving Proxima Centauri behind, our photon of light now has to travel for 1344 years before it reaches our next destination, the Great Orion Nebula, not surprisingly, found in the constellation of Orion. To the naked eye from Earth it looks like a pretty inconspicuous smudge just below Orion’s famous three star belt but from close up its true gaseous nature is revealed. This great cloud of gas and dust, or nebula, is a fine example of the process of the birth of stars. The Universe is peppered with clouds of gas and dust and eventually, at a local level, gravity takes command and starts to compress the cloud. The temperatures and pressures in the centre become greater and greater and after many millions of years, hydrogen atoms in the cloud start crashing together to produce helium atoms. This is the fusion process we spoke of earlier and when it starts, it marks the birth of a star. Not only are stars born but stars also die; for low mass stars in a fairly gentle way that we see as a planetary nebula or for more massive stars as a supernova explosion.
So far, everything we have looked at is found within our Galaxy, the Milky Way. Depending on the direction we go it will take a varying amount of time to reach the edge of the Galaxy. Even the shortest route will take us 90,000 years before we enter intergalactic space. Before we get there though, a glance to the side as we break free from our Galaxy will reveal some strange spherical concentrations of stars. These globular clusters are believed to have formed at the same time as our Galaxy and seem to contain some of the oldest stars in the Universe.
If we glance behind us as we venture into the inky blackness we can see for the first time, the shape of our Galaxy. It seems to be roughly spiral in shape with a bar across the centre. Looking at it from the edge would be like looking at two fried eggs, back to back, with the yolk in the middle representing the bulge at the galactic centre and the egg white representing the spiral arms with the Sun about 30,000 light years from the centre. Eventually, after 2.3 million years of emptiness, we see another galaxy swing into view, the Andromeda Galaxy, our nearest major galactic neighbour. Its broadly the same shape as our own Galaxy and will be home to globular star clusters, nebulae, stars and planets, just like our own.
If we continued our journey on for another 13 billion years or so we would, on occasion pass other galaxies drifting silently in space and perhaps even notice that some of them seem to be gravitationally bound into groups. The Milky Way and Andromeda Galaxy are members of our Local Group of galaxies which is thought to contain at least 30 members. You would think though, that we would eventually reach the edge of the Universe but it has one more trick up its sleeve to keep some secrets hidden forever. Just after the Big Bang, there was a brief period of time called the Inflationary Period where the Universe briefly expanded faster than the speed of light. This means that no matter how long we travel with our photon of light, we will never be able to reach the edge of the Universe, it will remain hidden from our view for all eternity.