1 Our place in space

We know that the Sun is a star, one of several hundred billion stars that make up our galaxy, the Milky Way. The Milky Way is part of a cluster of about 40 galaxies known as the Local Group, which is itself associated with the much bigger Virgo Cluster containing more than 2000 galaxies. This and other clusters are bound up superclusters. It is these superclusters that make up the landscape of the universe on the largest scales. Figure 1 shows an artist’s impression of the spiral structure of our galaxy, the Milky Way, based on recent observations. The Sun is just below the ‘o’ in the Local Arm. The image is about 100 000 light years across.

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Figure 1 The Milky Way

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This is a visualisation of the Milky Way with various arms labelled: Sagittarius-Carina arm; Local arm; Perseus arm; Scutum-Centaurus-OSC arm; Norma-outer arm; Near-3kpc arm and Far-3kpc arm.

Figure 1 The Milky Way

Figure 2 depicts the superclusters that make up our neighbourhood in the universe. The Local Group lies just to the left of the Virgo cluster in the centre of the image, which is about 2 billion light years across.

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Figure 2 Superclusters

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This is a visualisation of the universe within 1 billion light years of Earth, showing local superclusters (including: Centaurus Supercluster, Coma Supercluster, Hercules Supercluster, Shapley Supercluster, Leo Supercluster, Ursa Major Supercluster, Corona-Borealis Supercluster, Bootes Supercluster among others). Approximately 63 million galaxies are shown.

Figure 2 Superclusters

Our clear picture of the universe is relatively recent. Barely a century ago the universe was thought to be a much smaller place. The common belief was that the Milky Way was the entire universe and sat in an otherwise empty void. The misty patches now known to be other galaxies were thought to be part of the Milky Way itself. It was only in the early 1920s that astronomers began to realise the enormous scale of the universe, and our knowledge of its extent has grown ever since. Video 2 explores our history of looking into space, and introduces a few of this week’s ideas.

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Transcript: Video 2 Timeline of our understanding

MARCUS DU SAUTOY

No matter how busy life gets, looking up at the night sky always fills me with wonder. And I’m not alone. Humans have been gazing at stars for as long as they’ve walked the Earth. It makes you think about our place in the universe, and it’s inspired many people throughout history to write down their observations to figure out more about our humble planet, and how we fit into the picture – the really big picture.

Early humans enhanced their own lives by gazing at the night sky. They worked out calendars which allowed them to improve their farming, and they could plan practical things, such as the best nights for moonlight if they had a long journey to make.

Around 280BC, Aristarchus of Samos proposed that the Earth moves around the Sun – a notion that was not accepted by his contemporaries. He made the first estimates of the distance to the Sun, and the relative sizes of the Sun and the Moon. Just a few decades later, Eratosthenes of Cyrene makes the first pretty accurate measurements of the size of the Earth, by comparing the lengths of shadows cast at different latitudes.

Around 400 years later, Claudius Ptolemy consolidated the prevailing picture of the universe with the Earth at the centre, and the Sun and planets rotating around it every 24 hours. He also estimated the distance of the Sun as 1210 times the radius of the Earth. This geocentric view remained unchallenged for 1400 years, although Muslim scholars from the 10th to the 14th century criticised the work, al-Haytham correctly pointing out that Ptolemy’s hypothesis regarding the arrangement and motion of the planets was contradictory between two of his own publications.

Many centuries later, Nicolaus Copernicus published his own model of the universe, showing the Sun at the centre, with the Earth and other planets moving around it, and with the stars lying on the outskirts. In 1785, William Herschel published the first map of the Milky Way galaxy, based on counting stars in all directions, and speculated that some of the faint cloud-like formations may be other galaxies beyond our own. In 1838, Friedrich Bessel measured the parallax of 61 Cygni, the first direct measurment of the distance to a star, showing it to lie about 10 light years away. That’s about 95 trillion kilometres.

Having spent most of history thinking that the Earth is the centre of the universe – only to find out that it isn’t – in 1918, Harlow Shapley told us that our Sun isn’t even in the centre of our Milky Way galaxy, but thousands of light years towards the edge. And just a few later, controvery over the scale of the universe was resolved as astronomers, led by Edwin Hubble, came to accept that objects such as the Andromeda galaxy are indeed not part of the Milky Way, but far outside it, and separate galaxies in their own right.

A few decades later, George Abell published the first part of a survey of clusters of galaxies, demonstrating the existence of superclusters as the largest structures in the universe. In 1964, while Arno Penzias and Robert Wilson were trying to trace the source of noise interfering with a radio antenna, they accidentally discovered it was not just static hiss, but the sounds of the universe being born. It was the cosmic microwave background radiation (CMBR), lending support to the Big Bang theory. The CMBR marks the edge of the observable universe. And in 2016, a team using the Hubble Space Telescope discovered the most distant known object, a galaxy called GN-z11, which is now around 32 billion light years away, and seen about 400 million years after the Big Bang.

And that’s only the story so far! With our ability to observe the universe improving all the time, who knows what we will discover over the next few years? But the universe still holds many great secrets. So, will there be things that we simply cannot know?

End transcript: Video 2 Timeline of our understanding

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Video 2 Timeline of our understanding

The distances involved in this topic are staggering when you think about them. The furthest superclusters in Figure 2 lie about a billion light years away. That’s a huge distance – but how much further can we go?