The Week in Space and Physics #9
Cosmologists have a pretty good idea how galaxies change over time. Telescopes pick out millions of them, covering every possible shape, colour and phase of life. By examining these galaxies, and peering deep into space and far back in time, cosmologists are able to trace out the standard lifecycle of galaxies, and project what might happen in the future.
This picture, however, is a general one. We can say what has, and what will, happen to a typical galaxy; but we cannot say in detail what has happened to any individual one. Each galaxy faces a unique situation. Its path through time is moulded by the stars and gas it holds, by the galaxies around it, and by the interactions it experiences over time.
Our own galaxy, for example, has absorbed a dozen others over its long history. Each encounter has shaped it in some way, giving it a unique fingerprint and, eventually, giving rise to our Sun, our planet and our civilization. How can we trace this fine history? One option is to look at old stars; those which have stuck with our galaxy since its earliest days.
Each star contains a particular ratio of chemicals, reflecting the makeup of its surroundings at the moment it formed. By studying large numbers of stars of different ages astronomers can trace how the chemical makeup of the galaxy has changed over time. That can reveal details about what was happening in the Milky Way, and give us an idea, at least vaguely, of its history.
In the hope of doing this two researchers, Maosheng Xiang and Hans-Walter Rix, analysed a quarter of a million stars in our galaxy. They traced the ages, orbits and chemical makeup of each, giving them a vast dataset with which to analyse our galactic history.
Their study revealed the enormous age of the galaxy: its disk started to form roughly thirteen billion years ago – less than a billion years after the Big Bang. Shortly after that – roughly eleven billion years ago – the galaxy experienced a sudden burst of star birth. Such events are often triggered by collisions between galaxies. In this case the encounter probably involved the Gaia-Enceladus galaxy, sometimes known as the Gaia Sausage.
Then, roughly five billion years after the galaxy was born, a rather abrupt shift appears in their data. Around that time the Milky Way’s original supply of hydrogen seems to have been exhausted. Star birth slowed, and the galaxy entered a long quiet phase of existence. In some galaxies these quiet periods can be interrupted if a big collision takes place – but nothing like that seems to have happened to the Milky Way.
Our galaxy probably has another few billion years of quiet life ahead. Eventually, however, a big smash up will come. The Milky Way seems to be on a collision course with Andromeda, a large galaxy currently lying two million light years away. That, when it comes, will likely tear both galaxies apart, result in a burst of star formation and eventually – after a few more billion years – in a new, larger galaxy.
Unmasking the ORCs
A few years ago a radio telescope in Australia picked up odd signals coming from deep space. They appeared as circular objects floating alone in space. That, though, was almost all astronomers could say. Even fairly basic things – like how big they are, or how far away they lie – were impossible to say with certainty.
Astronomers have now managed to take a closer look, and are starting to find answers to some of these questions. The patterns, nicknamed odd radio signals, or ORCs for short, seem to be massive: stretching roughly a million light years across. At least three of the five known ORCs envelop galaxies – something that hints both at their vast extent and their possible cause.
Since they look rather like the aftermath of powerful supernova, some wondered if they are the lingering remains of massive explosions. They could, perhaps, come from colliding supermassive black holes – surely a cataclysmic event – or from galaxies smashing into each other. More outlandish ideas have been proposed: that they are signs of distant wormholes or other disruptions to the fabric of space and time.
New images from the MeerKAT radio telescope in South Africa have given us the clearest view yet of these mysterious objects. They show, for the first time, details of the interior of an ORC; revealing a series of rings rather like a Fabergé Egg. At the centre of the ORC lies a faint galaxy, one that is now quiet but probably, a few billion years ago, underwent a period of sudden star formation.
Such bursts of activity often occur when galaxies collide. A sudden inflow of fresh material triggers star formation. Clouds of gas falling into the central black hole can result in powerful jets of radio waves streaming out of the galaxy. That, over long periods of time, could result in vast radio bubbles that look rather like the ORCs.
Astronomers cannot yet be sure how accurate this story is. Only a handful of ORCs – five or six – are known, and detailed observations have been scarce. As new radio telescopes switch on over the next few years – including the Square Kilometre Array in Australia and South Africa – expect them to turn towards the ORCs, probing their secrets.
The Background Glow: Brighter Than We Thought
Astronomers call it the Cosmic Optical Background: a glow of light from far away stars, galaxies and dust clouds that fills the night sky. The objects that create this light are too distant and ancient to pick out individually; only see their combined output can be seen. Nevertheless, measuring this light can give us an idea of the cosmos on its largest scale.
Accurately measuring it, however, is difficult. Astronomers must first subtract the light from everything we can see: all the known planets, stars and galaxies. From Earth this is almost impossible. The inner solar system is filled with dust that scatters sunlight, overwhelming the faint background glow.
Levels of dust fall as you head further from the Sun. For a probe like New Horizons – flying far beyond Pluto – picking out the Cosmic Optical Background should be possible. To try to measure it, then, operators recently directed its cameras towards an empty region of space; one free of nearby stars and galaxies. Further processing removed all known sources of light: those fainter, further away galaxies, dust clouds and even the heat of the probe itself.
The result was intriguing. The Optical Background seems to be twice as bright as predicted – hinting that there is far more out there than we thought. That could be faint but nearby objects. Large numbers of stars floating in intergalactic space, for example, or possibly a new type of small galaxy we haven’t been able to see before.
Lucy in the Sky with Mercury?
The surface of Mercury may, according to a recent study, be littered with diamonds. Scientists have long known that the small planet is unusually dark, a phenomenon chalked up to a high level of carbon coating the crust.
That carbon, measurements suggest, exists in the form of graphite. Layers of this mineral may once have covered much of the planet, later getting churned up by countless meteorite impacts and volcanic eruptions. Those impacts, however, should also have been powerful enough to convert the graphite into a denser form of carbon: diamond.
Calculations by Kevin Cannon, a planetary scientist, suggest the impacts could have created trillions of tonnes of diamonds. These, though, may not be as pretty as the diamonds found on Earth. They are likely to be small and cloudy diamonds, better suited for industrial purposes.
ESA currently has a probe en route to Mercury: BepiColombo. Once it arrives in late 2025, the instruments onboard should be able to detect the presence of these diamonds, if they exist.