Where Are We?
Let’s start simple: apart from a few astronauts, we all live on a planet named Earth. Earth is one of the four inner rocky planets of the Solar System, located at about 150 million kilometres from the Sun, and is the third closest planet to the Sun. As of yet, it is the only planet known to host life.
The Solar System is host to seven other planets, with masses ranging from 0.05 times to 300 times the mass of the Earth. Nevertheless, most of the mass of the Solar System is contained within the Sun, which holds about 99.9% of it.
Up until relatively recently, we were not sure whether there were other planetary systems out there, but now we know of a little over 3000 of such systems, with almost 4200 confirmed exoplanets! The closest star to us, apart from the Sun, is Proxima Centauri which translates to nearest of Centaurus. This star is about 40 trillion kilometres away from us! Now we have arrived at a scale at which kilometres is not really the easiest unit of distance to use. An often-used measure of distance is the light-year. Light travels as fast as it is possible to travel: at the speed of light of about 300 thousand kilometres per second! A light-year is the distance light travels in exactly one year. Proxima Centauri is 4.2 light-years away from us: it would take light 4.2 years to cross the distance to the nearest star.
The Solar System itself is located in the Milky Way galaxy, which gets its name from its appearance in the night sky as a hazy band of light formed from stars that cannot be individually distinguished by our eyes. The Sun is only one out of about 250 billion stars that make up our entire galaxy! To be more precise, the Solar System can be found in the Orion–Cygnus Arm, a minor spiral arm of the Milky Way Galaxy that is 3,500 light-years across and approximately 10,000 light-years in length. The galaxy itself has a diameter of over 150000 light-years. It would take light 150 millennia to cross the entire length of the galaxy!
Moving even further out, we are now on the scale of the so-called Local Group, a group of galaxies that includes our own. The two most massive galaxies are the Milky Way and Andromeda, which actually can be seen with the naked eye in the constellation of the same name. Currently, Andromeda and the Milky Way are about 2.5 million light-years apart: it would take light 25000 millennia to cross from our Galaxy to Andromeda. The closest galaxy to us is the Canis Major Dwarf Galaxy, which is ‘only’ 25000 light-years from us. This galaxy is actually so close to us that it is being pulled apart by the Milky Way’s gravity. Since 2003, we think that the Local Group comprises of 36 galaxies. In total, the Local Group has a mass of over 2 trillion suns!
A group of galaxies, such as the Local Group, is made up of about 50 galaxies or fewer. There exist collections of galaxies even larger than groups called galaxy clusters. However, groups and clusters of galaxies can themselves be part of even bigger aggregations called superclusters. Our Local Group resides in the Virgo Supercluster. This supercluster contains at least 100 galaxy groups and clusters within its diameter of 110 million light-years! We estimate that the Virgo Supercluster is only one of about 10 million superclusters in the observable Universe.
Even now, we have not reached the largest scale yet! If we zoom out as far as we can, we can now see the entire observable Universe. This is the region in our Universe that we can observe from the earth at the present time because the light from these objects has had time to reach the Solar System. Everything beyond this spherical region is so far away that any light from it would take longer than the current age of the Universe, about 13.8 billion years, to reach us! Its diameter is about 93 billion light-years. (Would that take light not 93 billion years to cross? Technically yes, but we need to take the Cosmic Expansion into account!)
At this grandest scale, the large-scale structure of the Universe becomes apparent. The Virgo Supercluster is but a small part of a network of 100 billion galaxies. We call this network of galaxies the Cosmic Web. This Cosmic Web is composed of interconnecting filaments of clustered galaxies and gases stretched out across the Universe and separated by giant voids. The cover photo of the homepage shows what this structure looks like. These superclusters of galaxies form at the intersections of these filaments.
Check out this clip from the TV show Cosmos (2014)
below to discover our cosmic address!
What Makes Up a Universe?
So we know where we are in the Universe, and we have already come across a few of the objects that litter space. But what exactly makes up a Universe? What are the ingredients that go into creating the diversity of structure that we see around us? We can divide these ingredients into three main groups:
- Baryons: All material made up of protons, neutrons and electrons is called ‘baryonic matter’. We are ourself made up of baryons, as well as the computer you are using to read this, the Earth itself and the stars! Most of the mass of ‘normal’ (not dark!) matter is dominated by baryons.
- Dark Matter: This type of matter accounts for approximately 85% of the matter in the Universe. The name refers to the fact that dark matter does not interact with light, it is ‘dark’. This means that we cannot actually see it. However, its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained unless more matter is present than can be seen.
- Dark Energy: An unknown form of energy that is thought to make up 68% of the total energy in the present-day observable universe. Dark energy is responsible for the accelerated expansion of the Universe.
Check out the video below to learn more
about the ingredients of the Universe and Cosmic Expansion!
Image Credits: Cover – ESO/S. Brunier. Solar System – Moon Blink. Milky Way – NASA. Virgo Supercluster – Richard Powell. Contents Universe – WMAP/NASA.