Wonders of the Universe

THE INVISIBLE STRING

Everything in the Cosmos is subject to the force of gravity. From the manmade satellites that rotate around our planet creating the technological infrastructure of the twenty-first century, to the orbit of our only natural satellite – the Moon – which journeys around Earth every 27.3 days, it is gravity that provides the invisible string to guide them on their path. The journey of every planet, moon, ball of rock and mote of dust in our solar system is guided by gravity; from the 365-day trip our planet takes around the Sun to each of the orbits of the seven planets and 166 known moons in our neighbourhood. Beyond our solar system, gravity continues to conduct the flow of the Universe, with everything affected by the gravitational pull of something else, no matter how tiny or how massive.

Our solar system orbits around the centre of the Milky Way Galaxy, a place dominated by a supermassive black hole, the heart of a swirling system of over 200 billion gravitationally bound stars. And even this vast, rotating structure isn’t where the merry-go-round of the Universe ends, because even the galaxies are steered through the vast Universe by the action of gravity.

The Virgo Supercluster of galaxies is a good example of how gravitational pull exerts itself. This cluster of galaxies has a gravitational pull on the Local Group of galaxies that surround our Milky Way Galaxy.

NASA / SCIENCE PHOTO LIBRARY

The supermassive black hole at the centre of the Milky Way Galaxy, Sagittarius A^*, is the heart of a swirling system of over 200 billion stars which are gravitationally bound.

NASA

The elliptical galaxy M87 is located at the centre of the Virgo Cluster. This huge galaxy includes several trillion stars, a supermassive black hole, and a family of 15,000 globular star clusters which may have been graviationally pulled from nearby dwarf galaxies.

Caltech

Beyond our solar system, gravity continues to conduct the flow of the Universe, with everything affected by the gravitational pull of something else, no matter how tiny or how massive.

Our galaxy is part of a collection of galaxies called the Local Group – a cluster of over 30 galaxies named by the American astronomer Edwin Hubble in 1936. Over ten million light years across, this vast dumbbell-shaped structure contains billions and billions of stars, including the trillion stars that make up our giant galactic neighbour, Andromeda. Just as the Moon orbits Earth, Earth orbits the Sun, and the Sun orbits the Milky Way, so the Local Group orbits its common centre of gravity, located somewhere in the 2.5 million light years between the two most massive galaxies in the group: our Milky Way and Andromeda. But even this giant community of galaxies isn’t the largest known gravitationally bound structure. As you sit reading this book, gravity is taking you on an extraordinary ride. Not only are you spinning around as Earth rotates once a day on its axis, not only are you orbiting at just over 100,000 kilometres (62,137 miles) per hour around the Sun, not only are you rotating around the centre of our galaxy at 220 kilometres (136 miles) per second, and not only is the entire Milky Way tearing around the centre of gravity of the Local Group at 600 kilometres (372 miles) per second, but we are also part of even an grander gravitationally driven cycle.

The Local Group is part of a much larger, gravitationally bound family called the Virgo Supercluster – a collection of at least 100 galaxy clusters. Nobody is sure how long it takes our Local Group to journey around the Virgo Supercluster; vast beyond words, stretching over 110 million light years, it is, even so, only one of millions of superclusters in the observable Universe. It is now thought that even superclusters are part of far larger structures bound together by gravity, known as galaxy filaments or great walls. We are part of the Pisces-Cetus Supercluster Complex.

Gravity’s scope is unlimited, its influence all-pervasive at all distance scales throughout the entire history of the Universe. Yet, perhaps surprisingly, given its colossal reach and universal importance, it is the first force that we humans understood in any detail