OVER the past decade, spacecraft have beamed back reports from some truly awe-inspiring destinations within our solar system. Rovers have revealed vast river channels carved by ancient floods on Mars. Probes have zipped through the turbulent geysers erupting from Saturn鈥檚 icy moon Enceladus. We have even flown past the frigid mountains of Pluto. But what if we could extend our horizons further 鈥� much further?
Let鈥檚 imagine that we could roam across the Milky Way on a sightseeing trip like no other. Where would I take you? Well, supermassive black holes, strange, pulsating stars and exotic Earth-like worlds are on the itinerary for starters. Aside from the sheer spectacles we鈥檒l witness, we鈥檒l discover how visiting these space oddities might offer clues to some big mysteries 鈥� from how the first stars formed to what is producing weird radio signals from outer space and whether there is life beyond Earth. Here, then, is my guide for the discerning galactic tourist.
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S1 stream
Watch a passing parade of stars bursting with dark matter
To begin our journey, we鈥檒l jump into a river of stars that loops right past our front door and across the entire Milky Way. To truly appreciate the S1 stream, we have to recalibrate our sense of scale, because its length and breadth are counted in thousands of light years. Our solar system, sat in the middle of the stream, is akin to a grain of sand in the Amazon river.
To find the source of the S1 stream, we need to turn back the clock nearly 9 billion years to our galaxy鈥檚 turbulent youth. Seen from the outside, the Milky Way is surrounded by a swarm of smaller 鈥渄warf galaxies鈥� that curve and arc in orbits around it. But when one orbited too close, it fell victim to an act of galactic cannibalism.
This unfortunate dwarf galaxy was obliterated, but its individual stars kept ploughing through the Milky Way鈥檚 interstellar medium 鈥� which is mostly empty space. Today, this extended stream of stars is still distinct. In 2017, using the Gaia spacecraft, astronomers . To confirm their suspicion, they measured the chemical composition of these objects and found that they had lower levels of heavier elements, such as iron, compared with native Milky Way stars. The evidence was undeniable: the S1 stream is the stripped remains of an ancient dwarf galaxy.
It is tempting to think that travelling down the S1 stream would give us the chance to solve one of the most stubborn mysteries in physics. Dark matter was hypothesised decades ago when we observed galaxies moving faster than they should, leading us to conclude that more matter must be present, even if it doesn鈥檛 interact with light. Similar measurements of star motions tell us that dwarf galaxies are uniquely rich in dark matter. So the S1 stream is primarily a river of dark matter, which happens to be hitting the solar system square in the face. But while studies show that a flyby with our best detectors would improve our prospects of identifying dark matter if it comes in the form of particles call axions, the truth is we can probably do that just as well from Earth.
Proxima Centauri b
Drop in on the habitable world next door
So far, we have discovered more than 5000 exoplanets orbiting other stars in our galaxy. But there are hundreds of billions more worlds still to find 鈥� an average of roughly one per star in the Milky Way.
If we could visit any of these, we would surely want to choose an exoplanet that has a good chance of harbouring life. The trouble is that we don鈥檛 know whether life can emerge in conditions beyond those on Earth, nor the spectrum of environments that can arise when planetary systems form. So the safest bet is a planet that looks something like ours.
Over the past year, the James Webb Space Telescope (JWST) has found tantalising signs of , and in far-off exoplanet atmospheres. But it is a planet much closer to home that I would plump for: Proxima Centauri b, which orbits the closest star to the sun.
We still don鈥檛 know what the conditions are like on the surface of this world. Its diminutive parent star is a red dwarf that has a mass only an eighth of the sun鈥檚 and is nearly a thousand times dimmer. This is fortunate, as a sun-like star would roast this closely orbiting planet to a crisp. But it is possible that Proxima Centauri b has the perfect conditions for life.
Its natural 鈥渆quilibrium temperature鈥� is -39掳C, based on the energy output of its sun. If it has the right kind of atmosphere, however, a greenhouse effect could raise this to something friendlier to life. Stopping here, we find out for sure with a close-up look at its atmosphere. Depending on the composition of its air, Proxima Centauri b could be anything from a warm ocean world to a frozen ball of ice. (And if you are wondering about the power of an atmosphere to shift planetary conditions, Earth鈥檚 equilibrium temperature is around -20掳C (-1掳F), and it is perfectly habitable thanks to the gases that blanket it 鈥� for now, at least.)
Even a glimpse of clouds or oceans would suggest the possibility of life on this alien world. But even if it is barren, I can at least promise you the Milky Way鈥檚 most spectacular sunset. Proxima Centauri b orbits its parent star every 11 days and is tidally locked, meaning that the same side of this world always faces inwards 鈥� much like one side of the moon always faces Earth. One side of the planet experiences perpetual daytime, while the opposite side is trapped in eternal night. Between the two is its twilight strip, known as the 鈥渢erminator鈥� zone. Here, fierce winds rush from the searing hot to freezing cold areas. If we can brave it, we will see a truly alien spectacle: a sunset hanging forever in the sky.
Betelgeuse
For a spot of supernova gazing, if we鈥檙e lucky
Our next destination is an enormous star, swollen with age and 100,000 times more luminous than the sun. Many of us will have seen it at one time or another, glowering deep red in the cold winter sky, sitting in the armpit of the constellation Orion. And it happens to be intriguingly unstable, which suggests it could explode as a supernova at any moment.
This stellar titan, known as Betelgeuse, is so massive you could fit our sun inside it hundreds of millions of times over. If it were at the centre of our solar system, it would swallow Mercury, Venus, Earth and Mars, and would come close to engulfing Jupiter too. It is so big partly because it was born large, but mostly because it has reached the final stages of its life and has expanded to become a red supergiant. Having run out of the hydrogen that sustains the nuclear fire in our sun, Betelgeuse is burning helium. The fusing of helium nuclei releases much more energy than hydrogen nuclei fusing, puffing the star up from the inside.
This is what suggests the end is nigh. There is typically only enough helium to support a star for a couple of million years. Once it is gone, the star frantically burns through whatever heavier elements are left, like a desperate ship鈥檚 captain tearing up the floorboards to stoke the furnace. The star can burn carbon for a few hundred years, then oxygen for six months. It will spend one final day burning silicon into iron before fusion stops altogether.
At that point, the weight of the star collapses inwards under gravity. As the outer matter plummets towards the centre, it accelerates to a decent fraction of the speed of light. Then it crashes into the dense star core, bouncing back as a shock wave that outshines a billion stars. This is a supernova explosion.
For years, astronomers have kept a close eye on Betelgeuse, hoping to find out how far it has progressed along this path 鈥� and how long we will have to wait for its explosive end. Earlier this year, astronomers , becoming visibly dimmer and brighter in the sky. From this, they reckon the star is in the late stages of carbon burning, meaning the explosion is due within centuries. Other astronomers argue that the dimming is caused by ejected by the dying star. In that case, Betelgeuse may still have hundreds of millennia left.
A flyby would tell us what is really going on. We may even land ourselves a front-row seat for one of the most spectacular firework displays in the cosmos, with waves of colour rolling off the star鈥檚 outer layers in anticipation of the searing final display. Just make sure your travel insurance is up to date, as supernova explosions are incredibly violent. Even if we were to watch from Earth, when Betelgeuse finally lets loose, it will shine almost as bright as the full moon for weeks on end.
Magnetar SGR 1935+2154
See the source of strange signals from deep space
Fast radio bursts (FRBs) have long baffled astronomers. In 2007, an extraordinarily quick blip of radio waves, 10 times shorter than a lightning strike, was in Australia. The signal showed signs of a long journey too. The way the wave frequencies were dispersed suggested this burst had travelled more than 3 billion light years before reaching Earth.
Intrigued, astronomers sought out more of these mysterious events. By trawling through old radio telescope records, and by looking in the right place at the right time, they detected dozens of new signals. But no one could make sense of what was causing them. One burst, in 2012, went off 10 times in two months before falling silent. Then, in 2018, it chirped 21 times in a single hour. Some FRBs cried out once then disappeared forever.
All FRBs seemed to come from deepest space, far beyond the Milky Way, which made them particularly tricky to study. But in 2020, astronomers . Excitingly, the burst came from a known object: magnetar SGR 1935+2154, which is the next stop on our tour.
Astronomers are still getting to grips with magnetars. We know these extreme objects start as the remnants of supernovae explosions: as a giant star collapses and goes supernova, it crushes particles together into an incredibly dense, spinning ball just a few kilometres across, called a neutron star. These are so dense that a teaspoonful of neutron star matter would weigh roughly 10 million tonnes.
Neutron stars become magnetars when their interior somehow whips up the strongest magnetic fields in the universe 鈥� a million billion times more powerful than Earth鈥檚 field. So tourists should be wary: even a thousand kilometres away from SGR 1935+2154, the intense magnetism would shred your atoms into a fine mist. From a safe distance, though, we will be treated to strange, shimmering patterns of light that coruscate across the surface. The magnetar warps space-time around it so that light travelling nearby will be refracted into a great cosmic hall of mirrors.
A closer look might even help us figure out how magnetars produce FRBs. In January, SGR 1935+2154 suddenly , known as a 鈥渟pin-down glitch鈥�, before releasing three new radio bursts. One idea posits that winds of charged particles flood out of the magnetar鈥檚 north and south poles, smashing into the magnetic fields and triggering emissions. Another idea is that FRBs result from gargantuan tremors in the magnetar鈥檚 crust called 鈥渟迟补谤辩耻补办别蝉鈥�. If we got very lucky with the timing, we might be able to see for ourselves.
Sagittarius A*
Gawp at a monstrous black hole
No tour of the galaxy would be complete without a visit to the supermassive black hole at the heart of the Milky Way, where we are guaranteed a mesmerising light show 鈥� and we might even spot crucial hints about the true nature of space-time.
Radio engineer Karl Jansky picked up the first hints of Sagittarius A*, as it is known, almost a century ago while he was investigating the static that plagued early radio communications. But astronomers only in 1998. By precisely observing how stars orbit the galactic centre, they proved the existence of a body with a mass some 4 million times that of the sun. Telescope images show stars locked in orbit around what appears to be nothing at all.
Last year, astronomers even captured an image of this black hole. The team behind the Event Horizon Telescope connected observatories all around Earth to build what is in essence a telescope the size of the planet. The resulting fuzzy orange doughnut image, which made headlines all over the world, reveals the accretion disc around Sagittarius A*: a ring of doomed gas and dust, with a diameter smaller than the orbit of Mercury, swirling around a cosmic plughole.
The telescope has the highest resolution ever achieved by astronomers. Observing Sagittarius A* is like reading the lettering on a coin in New York using a telescope in London. But there are some questions physicists will only be able to answer by getting a closer look.
The extreme environments in and around black holes have long been a playground for theorists testing our best understanding of fundamental physics. Currently, we have two incompatible theories that explain how the universe works. General relativity works on large scales and with large masses, describing gravity鈥檚 effects in the cosmos. Quantum mechanics, on the other hand, is the science of the very small. To find out what happens when these theories collide, and to try to unify them, you need something that is both very heavy and small. Nothing like this exists in the lab. A black hole, though, is perfect.
The edge of a black hole, beyond which not even light can escape, is called the 鈥渆vent horizon鈥�. Careful experiments observing how matter accelerates at this cosmic precipice could provide evidence for various theories of quantum gravity, each of which says that space-time can be broken down into tiny quantum packets in different ways. Studying the surrounding accretion disc, meanwhile, would teach us a lot about black hole behaviour. When gas spirals inwards, it can ignite fountains of radiation upwards and outwards at close to the speed of light. But as the cores of black holes are usually shrouded in dust, we have little idea how this happens.
Regardless of what we might learn, we would still have the light show. Anything crossing the event horizon of Sagittarius A* is swallowed by the black hole, but just outside of this perimeter, the extreme warping of space-time means light can be pulled into endless circles. Light coming from all the stars in the Milky Way and beyond is trapped in this layer, known as the 鈥減hoton sphere鈥�. Here, then, is our chance to see a truly infinite cosmic movie.
PSR J1719-1438 b
Goggle at a diamond planet
When is a planet not a planet? The answer to this riddle lies around 4000 light years from Earth, in one of the strangest solar systems we know of. At the centre sits a rapidly spinning neutron star called a pulsar, which emits powerful beams of radiation like a lighthouse. Around this star orbits an ambiguous planet, J1719-1438 b, which itself was once a star.
By observing the dance of this odd couple, astronomers 鈥� four times denser than Earth, which is the densest planet in our solar system. The only way to explain this is if J1719-1438 b is mostly composed of crystalline carbon. In other words, what we have here is a giant diamond world.
Astronomers suspect it was made by stripping a star. Small stars like the sun end their life as compact spheres of carbon and oxygen called white dwarfs. The pulsar鈥檚 radiation beams may have peeled away the outer layers of this stellar remnant, leaving the crystal core glinting in the vacuum of space.
Now coated in interstellar dust and baked by the pulsar鈥檚 radiation, the surface of this cosmic jewel may well be blackened. We can only know for sure by heading there. We may even find that PSR J1719-1438 b isn鈥檛 a diamond planet at all, but a lump of exotic 鈥渜uark matter鈥�. This weird phase of matter could occur when particles are forced even closer together than those in a neutron star. The stew of fundamental quarks that results is still hypothetical, but some theorists reckon it sloshes about like a fluid with zero viscosity.
Sculptor dwarf galaxy
Witness a proxy for the early universe
The Sculptor dwarf galaxy appears as a gossamer of galactic fluff on the outskirts of the Milky Way 鈥� taking us to the furthest and final stop on our tour. It is well worth the 300,000-light year trip, as our galactic neighbour is a time capsule from the universe鈥檚 deep past.
Thirteen and a half billion years ago, after the fireball of the big bang, the universe was dull, featureless and devoid of light. The dark ages, as they are known, lasted hundreds of millions of years until, out of the primordial gas, the first stars sparked into life.
These 鈥淧opulation III鈥� stars are very different to those in our galaxy today. Since the dark ages, many generations of stars have lived and died. The heavier elements forged in the cores of stars 鈥� such as oxygen, carbon and iron 鈥� became part of the next generation, forever altering their chemistry and characteristics. But the first stars contained none of these contaminants. Pristine Population III stars were made out of the elements created in the moments after the big bang: hydrogen, helium and a sprinkling of lithium. As a result, these first stars were far bigger and hotter.
Population III stars are pivotal to the evolution of the universe. Exactly how big, and how abundant, these were can reveal clues about how the first galaxies formed and grew 鈥� so astronomers understandably want to find them. JWST has found . But looking that far back in time also means looking far away, which makes for low-resolution images.
This is where the Sculptor dwarf galaxy comes in. Most of its slow-burning stars are ancient, around 12 billion years old, which isn鈥檛 far off the age of the first stars. This makes them emissaries from the early universe. And in 2021, astronomers found one particular star, called AS0039, so deficient in heavier elements that it seems to be , born from the material produced when that primordial star died. Taking a good telescope and studying AS0039 up close would help us to fathom these primeval cosmic furnaces.
The Sculptor dwarf galaxy also happens to be the perfect place to finish our tour. Even with the naked eye, its position would afford you a unique vista of the great spiral of the Milky Way splashed across the sky. Where holiday snaps are concerned, it doesn鈥檛 get much better than that.
Matthew Bothwell is the public astronomer at the University of聽Cambridge, UK
