The fundamental forces, the first stars, heavier elements and the first molecules were all in existence before the universe was a few hundred million years old. The universe formed 13.77 billion years ago, yet our solar system formed only 4.6 billion years ago. What happened in the intervening billions of years?
Galaxies formed about 13.6 billion years ago, and they exist in groups and clusters across the heavens. The Milky Way is one of at least fifty-four galaxies in a cluster called the Local Group, which in turn is one of a hundred or so groups that form the Virgo supercluster of galaxies, which itself is a neighbourhood of the Laniakea supercluster of 100,000 galaxies. The smallest of these galaxies contain only a thousand stars or so, while the largest contain 100 trillion or more.
Galaxies move. All galaxies spin around a central point because of the way they formed from vast clouds of hydrogen gas being pulled together by gravity. On average, galaxies are also moving apart as the universe expands. However, galaxies within clusters like the Local Group can move towards one another, and they can collide. Astronomers have discovered 30,000 or so stars in the Milky Way with orbits that go in the opposite direction to that of most of its stars, and they have identified these as being part of another galaxy which crashed into the Milky Way many eons ago. Calculations of the trajectories of the Milky Way and the Andromeda Galaxy, a close neighbour only 2.5 million light years away, suggest they will collide in 4.5 billion years’ time.
Although all galaxies began to form at about the same time, they don’t all die at the same age. The Hubble Space Telescope, a remarkable device orbiting Earth that has provided many insights into our universe, has identified six galaxies that died by the time the universe was 3 billion years old. For reasons unknown, they had used up all their hydrogen and could no longer form new stars. In contrast, most galaxies are still producing stars, although the rate at which they do varies. Some galaxies contain lots of heavy elements like iron, having burned through many generations of stars, while others contain few heavy elements, being on star generation one or two. Within the Milky Way, some areas are more metal and oxygen rich than others, revealing that the history of different parts within a galaxy can also vary.
Most of the first stars in the Milky Way have died, but many new stars continue to be born. Parts of our galaxy consist of dust and molecules from the death of past stars, along with vast amounts of hydrogen and helium created in the early universe but not yet captured by stars. Sometimes these vast clouds are shaken into action by the explosion of stars several times more massive than our sun. Such supernovae are some of the most violent events in our universe, and they can result in dust and gas clouds forming nebulae, the birthplaces of new stars. The Helix Nebula is Earth’s closest star-forming nursery, with light from it taking seven centuries to reach us. In nebulae such as these, gravity causes cold gas and dust to start to clump together, becoming warmer and eventually forming new stars. These processes occur throughout the galaxy where there is sufficient hydrogen, with small, medium and large stars being produced, many with planets, but a few without.
Averaging across the star systems studied to date, each star has between one and five planets. Each of the star systems and each of its planets is unique. Although their formation has been driven by gravity, variation in the density and composition of the dust in the nebula in which they were born, and the strength of the star’s gravitational pull at the location where each planet formed, determine the planet’s size and composition. Each star and each planet has its own history, so now to turn to the history of the solar system.
Between 4.9 and 5 billion years ago a supernova triggered the formation of our solar system. If the nebula in which our sun formed was like those we observe elsewhere in the galaxy, many other stars and solar systems formed within it. Once young stars and solar systems form in nebulae, the spin of the Milky Way sets them off on their first orbit around the centre of our galaxy. At the centre of the Milky Way is an enormous black hole weighing about 4 million times the mass of our sun. We call the black hole Sagittarius A*, and the stars and solar systems of our galaxy orbit this black hole, at least until they are sucked into it, or they burn out, having used up all their hydrogen fuel. Although large, there are some black holes that are an order of magnitude larger than Sagittarius A*: in early 2023 astronomers estimated that a black hole called TON618 is ten times larger.
Our solar system is 25,640 light years from the centre of the galaxy. Two hundred and fifty-six centuries ago, humans built the first settlement, consisting of huts built of rocks and mammoth bones. Light that left the sun at the time of these innovative ancestors of ours will only now be arriving at the centre of the Milky Way. Given how far it is to the centre of the galaxy, it is not surprising that it takes a long time for our solar system to make a complete orbit of the galaxy. So long that scientists got fed up with dealing with such large numbers and instead defined a new measure of time to describe the orbit: a galactic year. Each galactic year is between 220 and 230 million years (it’s hard to measure), and the Earth is about twenty galactic years old. The first life evolved about 16.9 galactic years ago, while the first humans made their appearance approximately a galactic fortnight ago.
The Sun’s Position in the Milky Way
By 4.6 billion years, or 20.4 galactic years ago, our sun had formed, with huge amounts of hydrogen being pulled together by gravity until nuclear fusion began. The sun is by far the biggest in the solar system and constitutes 99 per cent of its mass. Although tiny compared to some of the objects I’ve discussed in this chapter such as massive black holes, it is, to us, huge. It has a diameter that is eleven times larger than Jupiter’s, the next-largest object in the solar system, and 109 times larger than that of Earth’s.
Other stars that formed in the same nebula as the sun set off on different orbits around the galaxy, with some moving faster and others more slowly. The closest stars to us did not form in the same nebula as Earth, and we know this because they vary significantly in age. For example, Barnard’s Star, the second-closest star system to Earth, is about twice as old as our sun, while the Luhman 16 system, the next-closest system, is only 600–800 million years old. Because different star systems orbit the galaxy at different rates, Alpha Centauri, the closest star system to us today, will likely not be our closest neighbour a billion years hence.
Much as the trajectory of galaxies within galaxy clusters reveals each galaxy pursuing its own journey across the universe, each star system within our galaxy follows its own orbit around Sagittarius A*. Some of these orbits mean that some stars repeatedly pass close by one another. Every galactic year or so, our sun will fly by star systems it has encountered before. Some scientists have argued that such close encounters may result in meteoroids and asteroids being displaced from their orbits in the outer reaches of our solar system, putting them on new trajectories that take them closer to the sun, potentially putting them on a collision course with Earth. There is evidence that asteroid and meteorite impacts peak and trough on galactic year timescales, although not all scientists are persuaded. Collecting more data to confirm this happens will take a very long time.
The terminology of space rocks can be confusing. I have mentioned asteroids, meteoroids, meteors and meteorites, but how do they differ? An asteroid is a rocky object that has a diameter of greater than a metre and that is smaller than a planet. A meteoroid is also rocky but is less than a metre in diameter. When a meteoroid enters the Earth’s atmosphere it burns up, producing a meteor that appears as a shooting star across the night sky. If the meteor hits the ground it is classified as a meteorite. As an asteroid approaches a planet, it often breaks up into meteoroids, which become visible in the night sky as a meteor storm. The impact that killed the dinosaurs was an asteroid that didn’t disintegrate completely into meteoroids, and which resulted in a large lump of rock crashing into the planet. In contrast to an asteroid, meteoroid, meteor, meteorite or space rock, a comet is made of ice and dust rather than rock. As comets in our solar system near the sun, they partially melt, leaving a trail of water and dust. Light reflecting off the melting comet makes it appear as a bright light travelling across the heavens.