How Far Away Are Our Next Stars?

Perhaps one evening you are sitting outside, gazing up at the stars. But how far away are the stars you see? Wolf 359, Alpha Centauri and Sirius are the ones closest to us. Let's start with Wolf 359. Most people have never consciously seen this star. He is located in the constellation Leo. It is not a spectacular sight, nor does it stand out in the night sky. You need a large telescope, otherwise you simply won't see anything. It is a red dwarf, tiny and faint, with just one-tenth the mass of the sun and a diameter barely larger than Jupiter's. Yet, it ranks among our nearest stellar neighbours. Wolf 359 lies 7.86 light-years away from us. That translates to roughly 74 trillion kilometres. A figure almost impossible to visualise. So, let's try comparing it to the Voyager probe. Voyager travels at 61,200 kilometres per hour. That's 17 kilometres per second. If it were heading directly toward Wolf 359, the probe would take about 138,000 years to get there. He is young, barely a billion years old, and quite volatile for such a small star. It frequently unleashes violent flares, bursts of radiation, that cause its brightness to spike significantly for a few minutes. And he isn't even our closest star. There is a more famous one, one that many of you likely already know, Alpha Centauri. The Alpha Centauri system is located in the constellation Centaurus. It is not a single star, but rather a group of stars orbiting one another. Alpha Centauri A and B, both quite similar to the sun. And, a bit further out, Proxima Centauri. A small red dwarf that is actually our closest star of all. 4.25 light-years. That translates to just under 40 trillion kilometres. It would take Voyager 1 roughly 75,000 years to cover that same distance. Launched in 1977, the probe wouldn't arrive there until sometime deep into the year 77,000. 4 light-years. Compared to the size of our galaxy, that is practically nothing. Yet for us, with our current technology, it represents a barrier against which every spaceflight concept devised so far has founded. A planet, Proxima Centauri B, orbits Proxima Centauri. It has a mass roughly similar to Earth's, and lies in a region around its star where liquid water could exist under the right conditions. Whether water is actually present there, or whether radiation from the nearby active red dwarf has long since sterilised the surface, remains unknown to this day. Sirius. Sirius is the brightest star in the entire night sky, located in the constellation Canis Major. Virtually everyone has seen it at some point, without consciously realising what they were looking at. It is 8.6 light-years away, roughly 81 trillion kilometres. The light reaching our eyes right now set out over 8 years ago. So, we aren't seeing Sirius as it looks at this very moment, but rather as it was 8 years ago. It would take Voyager about 152,000 years to cover that distance. Incidentally, Sirius isn't travelling alone. It has a companion, Sirius B, a white dwarf. This is an object with roughly the mass of our sun, compressed to about the size of Earth. A single teaspoon of this material would weigh several tons on Earth's surface. Lumen 16. Lumen 16, located in the constellation Vela, the sails, isn't strictly a real star. It consists of two brown dwarfs orbiting each other. They are too large and massive to be planets, yet not massive enough to sustain hydrogen fusion like a true star. Essentially, they are two failed stars orbiting one another every 27 years. It lies 6.5 light-years away, a good 62 trillion kilometres. Voyager 1 would take about 115,000 years to get there. This makes Lumen 16, after Alpha Centauri, one of the closest systems known to us, yet it is so faint that it wasn't discovered until 2013. Those were the stars closest to us, but how far away are the most distant stars visible in the night sky? Deneb. At the other end of the scale lies Deneb, in the constellation Cygnus, the swan. It is easy to spot with the naked eye, forms part of the summer triangle, and is one of the brightest points in the entire night sky. And yet, it is one of the most distant bright stars we can see at all. It is about 2,600 light-years away, though the exact figure remains a subject of debate. Even at the lower end of the estimate, roughly 25 quadrillion kilometres. At its current speed, it would take Voyager 1 about 46 million years to cover that distance. The reason Deneb appears so bright is solely due to its luminosity. It is a blue-white supergiant, huge, young, and short-lived for its size, shining around 200,000 times more brightly than the sun. Without such luminosity, it would be completely invisible from Earth. Rho Cassiopeiae. Further out still, in the constellation Cassiopeia, lies Rho Cassiopeia. It is a yellow hypergiant, one of the rarest types of stars in existence, with only a handful known across the entire Milky Way. Estimates of its distance vary by source, ranging from about 3,400 to over 8,000 light-years. The star's extreme luminosity makes precise measurement difficult. At a distance of 8,000 light-years, that spans more than 75 quadrillion kilometres. It would take Voyager 1 roughly 140 million years to cover that distance. Rho Cassiopeia is more than 500,000 times brighter than the sun, and becomes unstable enough every few decades to simply shed large portions of its own atmosphere. It is precisely this instability that makes it visible to us at all. It is a star situated at the very limit of what the human eye can perceive. Alpha Centauri, Sirius, Lumen 16. A few light-years, tens of trillions of kilometres. A hundred thousand years of flight time for Voyager. Even these figures feel like concepts beyond our grasp. And yet, these are our neighbours. Our closest ones of all. Then come Deneb and Rho Cassiopeia. Not just light-years away, but millennia. Not a hundred thousand years of flight time, but millions. There is a factor of more than a thousand between the nearest stars and the most distant ones visible to the naked eye. What remains constant in both cases is this. Voyager 1, one of the most travelled objects we have ever built, would not cover any of these distances within a time frame meaningful to humans. For the nearest stars, we are talking about a hundred thousand years. For the most distant visible stars, tens of millions. Both are spans of time that lie far beyond the duration of any single human life, any civilisation, or even any entire species. And yet, we see them. Every clear night. Without a telescope. Without equipment. Simply by looking up. The light from Wolf 359 has been travelling for a few years before reaching us. The light from Rho Cassiopeiae has travelled for several thousand years, perhaps longer than all of recorded human history. So, we never see the sky exactly as it looks right now. We see a patchwork of past moments, each from a different point in time, layered together into a single image. I also find it fascinating to think that when you look up, you see many of the same stars our ancestors saw. We all share essentially the same starry sky, and you now realise the absurd distances involved, which, incidentally, is truly remarkable. Most people who have ever lived simply had no idea. I hope you enjoyed the video, and I'll see you next time.