HomeSkyWhere Are We in the Universe? A Brief Tour of Our Cosmic Address Charles December 5, 2016 Sky Today, I’m going to explore our place in the universe by defining our cosmic address and demonstrating just how insignificant we are in the greater scheme of things. I consider this article to be a primer for a series I have in the works where I’m going to take a gradual journey towards the furthest reaches of space. As always, any feedback would be much welcome. Andrew Z. Colvin We all know that space is unimaginably vast and possibly even infinite. Nonetheless, we’re right in the centre of what we describe as the observable universe. This region refers to the portion of space that we can see because the light from those distant galaxies has had enough time to make its incredibly long journey to our eyes. Even in this immense ocean of galaxies, stars, nebulae, planets and empty voids, an analogy that considered Earth as nothing more than a single grain of sand on all the beaches on the planet would pale in comparison to our place in the universe itself. So where exactly do we fit in? What is our place in the universe, and just how small a part of space do we really occupy? Let’s take a look at our cosmic address: Earth Solar System Milky Way Galaxy Local Group Virgo Supercluster Observable Universe ? From small to large and from relatively near to unthinkably far, the above shows our place among everything in existence, but let’s put it into context. The Earth PixabayThe Earth and Moon are just a tiny part of our own galaxy, let alone the wider universe. Our Earth, sometimes affectionately referred to as the Blue Marble by astronauts who have the luxury of being able to see it in all its glory, is an almost perfectly round ball of rock and molten iron with a diameter of 7,926 miles and a circumference of 24,902 miles. If fuel were not a concern, it would take around two days to make a full trip around earth at the speed of a typical commercial jet airliner. Although there is not really any firmly established boundary as to where space begins, it is generally considered to be 62 miles above sea level, though most people will fall unconscious from lack of air within 6 to 9 seconds after about nine miles (14.5 km) of altitude. By contrast, the International Space Station orbits the Earth at an average distance of 267 miles (430 km), while GPS satellites orbit at around 12,550 miles (20,200 km). The Moon is, of course, Earth’s closest cosmic neighbour, and it is responsible for our lunar tides, giving it an extremely important role in the evolution of life while also protecting us from many potential impact events. At an average distance of 238,855 miles (384,400 km) away from us, it took around three days for the Apollo missions to reach the Moon, although current spacecraft, such as the New Horizons probe that reached Pluto last year, zipped past the moon in as little as eight hours. The Solar System PixabayThe planets of the solar system from the closest to the furthest from the Sun (not to scale). Things start getting exponentially bigger once we leave our little vicinity of the Solar System behind. After all, Venus, the nearest planet to Earth, is 25 million miles (40 million km) away when it’s at its closest distance. Mars is the next closest planet to Earth, with a minimum distance of 34 million miles (55 million km), but even an optimally planned journey to the Red Planet takes six to nine months with current technology. By contrast, the Sun is 93 million (150 million km) miles away, but still far enough for its light to take eight minutes to reach us. Our Solar System has at least eight planets, five dwarf planets (now including Pluto), hundreds of moons and countless asteroids and comets. Closest to the Sun is the airless world of Mercury, constantly blasted with the heat of our host star, followed by Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Between Mars and Jupiter lies the asteroid belt. At minimum, Jupiter is over twelve times further away from Earth than Mars, and Neptune, the furthest planet from the Sun, is almost eighty times further away than Mars. The vast distances between the outer planets are precisely why planetariums are rarely built to scale. After all, if Mars were across the street, Neptune would probably find itself in another district. Neptune is not nearly as far from the Sun as many other objects are, however. Currently, the minor planet Sedna holds the record as being the most distant known celestial object from the Sun. The 621-mile-wide (1,000 km) reaches a maximum of 31 times further away than Neptune, so far away that it takes 11,400 years to complete an orbit of the Sun. By contrast, the most distant manmade object is the Voyager 1 space probe which, in December, 2016, was almost 12.8 billion miles (20.6 billion km) and continuing its eternal journey into interstellar space. The Milky Way Galaxy PixabayAn artist’s representation of a typical barred spiral galaxy such as our own Milky Way. If we consider our Solar System to be our cosmic cul-de-sac, then the Milky Way would be an impossibly vast metropolis spanning millions of miles. It becomes completely impractical (and tiresome) to speak of distances beyond our solar system in terms of miles or kilometres, simply because they are so large. From now on, we’ll have to use light years, which refers to the distance that light can travel in a single year or, in layman’s terms, almost 6 trillion miles (9.5 trillion km). Our Solar System is not very special in the greater scheme of things, other than the fact that it is the only place in the universe confirmed to be home to life. It’s a mere pinpoint in the Milky Way Galaxy, and there are millions more just like it. In fact, it is just one of at least 100 million stars in our galaxy alone. We’re located in a part of the galaxy that is known as the Orion Arm, approximately half way between the edge of the galaxy and its centre. The Milky Way is a vast spiral formation consisting of countless stars (many with their own planetary systems), nebulae, rogue planets and other celestial bodies. Aside from the Sun, the nearest star to us is the binary system consisting of Proxima Centauri and Alpha Centauri, some 4.3 light years away. It would take some 72,000 years to reach using current space rocket technology. There are some 53 stars within 16 light years of us, but for comparison, the centre of the galaxy, located in the constellation of Sagittarius, is around 25,000 light years away, and the galaxy has a total diameter of around four times that. The Local Group NASAAndromeda is our nearest galactic neighbour excluding the numerous satellite galaxies that are gravitationally bound to our own. If the Earth is our house, the Solar System our street and the Milky Way our (impossibly large even on this scale) city, then the Local Group could be said to represent our county or province. The Local Group is a cluster of over 54 galaxies, including many dwarf galaxies that are actually satellites of larger ones such as our Milky Way, in the same manner that the Moon is a satellite of Earth. The Milky Way is host to around 50 of these satellite galaxies. The nearest galaxy of comparable size to the Milky Way is Andromeda, some 2 million light years away. In fact, the Milky Way and Andromeda are by far the largest and most massive members of the Local Group, which in total, sports a diameter of around 10 million light years. Interestingly, our nearest galactic neighbour is visible with the naked eye, and it is the most distant celestial object that you can see without the assistance of a telescope. You should be able to locate it by starting at the north-eastern corner of the constellation of Pegasus. Through my Celestron 130 SLT telescope, it appears as a large white cloud that looks distinctly different from any star. The Virgo Supercluster NASAThe famous Hubble Deep Field photo actually only shows a relatively tiny region of the sky (a single 24-millionth in fact). Spanning some 110 million light years is the Virgo Supercluster, or what we could call our cosmic realm or continent. Consisting of dozens of galaxies concentrated into what is, in terms of the size of the Universe, a relatively small area. In turn, the Virgo Supercluster is part of an even larger entity called the Laniakea Supercluster, which is some 520 million light years in diameter. At the centre of the Laniakea Supercluster, one can observe one of the most bizarre phenomena of all, the Great Attractor. In this mysterious part of the heavens, there’s something thousands of times more massive than a typical galaxy with a powerful enough gravitational pull that it has a significant influence on the entire cosmic region. What it is we simply don’t know, but there are various theories ranging from monster black holes to entire densely packed clusters of galaxies obscured by the Milky Way. The Observable Universe PixabayFor every pinpoint of light you can see in night sky, there are countless trillions more that you can’t see. Finally, we have the Observable Universe, which refers to absolutely everything that we can see at the present time, since light has had enough time since the formation of the Universe with the Big Bang of 13.8 billion years ago to reach us. Anything further away is simply still invisible to us, since light is still making its unimaginably long journey towards Earth. Things start getting a bit complicated at these distances, not least because the Universe is constantly expanding. The most distant objects we can see are much far further away now than how we are seeing them, because we are looking into the extremely distant past. The edge of the observable universe is approximately 46 billion light years away in any direction from us, though the furthest visible objects were under 13.8 billion light years away from us when light left them. The fact the furthest reaches of the Observable Universe are so far away is precisely how we can learn so much about the history of the universe, because we can see how these galaxies looked when the universe was very young. However, those distant galaxies and the many stars and planets that they no doubt host will be very different to what we’re seeing today: many of them will be long dead or merged to form larger galaxies and clusters. It is simply impossible for us to know their fate. Beyond the Observable Universe PixabayAccording to the many-worlds theory, there are an infinite number of different universes leading to the only logical conclusion that everything that can happen has happened, is happening and will happen an infinite number of times. The observable universe only refers to what we’re able to see directly from where we’re standing. As such, the observable universe is different depending on where you are. For example, someone living 13.39 billion light years away in GN-Z11, the most distant galaxy known will see our galaxy as it was 13.39 billion years ago. Any observer on such a distant world would define their observable universe very differently, and all we can do is imagine what they’re able to see. Where the entire universe ends is anyone’s guess, but there are various theoretical models that describe the possibilities. Some of these theories hold that the universe is infinitely large, while another considers it as a sphere with a minimum circumference of 760-billion light years. If that were the case, you could theoretically travel the longest distance in any direction and eventually get back home. 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