HomeSkyExploring the Lonely Realm of Intergalactic Space Charles April 3, 2017 Sky Intergalactic space may be dark, cold and forever beyond our reach, but there’s more to these lonely regions of the Universe than you may think. The Andromeda Galaxy lies some 2.5 million light years away. Appearing as a small grey smudge, our nearest galactic neighbour is also the most distant object we can see with the unaided eye. Now just imagine you’re in the region between the galaxies or perhaps even in one of the colossal voids between the clusters and groups of galaxies that make up an estimated ten percent of the Universe. In some areas of regions such as the Boötes or Eridanus Voids, it might even be that there’s no night sky at all – not a single star nor galaxy within view. Intergalactic space is certainly desolate, being home to the purest vacuum in existence. At sea level, the air we breathe contains 2.5×1025 (250 trillion trillion) molecules per cubic meter. The same amount of space in the intergalactic medium contains just a few hydrogen atoms, probably less than you could count on the fingers of one hand. Nonetheless, despite the barrenness of these intergalactic voids, some surprises do await. Satellite Galaxies with Splendid Views ESA, Hubble & NASAThe Large Magellanic Cloud is one of the largest satellite galaxies of the Milky Way. In just the same way that planets have moons and stars have planets, galaxies also have satellites of their own that are gravitationally bound to a host. Our own Milky Way has at least 50 of these diminutive galaxies, although some of them may be in orbit of each other rather than bound to the Milky Way directly. The Small and Large Magellanic Clouds are among the largest and, known since prehistoric times, are also visible with the naked eye. The LMC lies about 163,000 light years away from the Milky Way. Crossing this void, the sky will get emptier and emptier. However, just as a lack of city lights makes for better skywatching, more galaxies and other objects will be visible without the overwhelming brightness of the Milky Way to interfere with your view. At 163,000 light years from home, the night sky would be a spectacular site, provided the Milky Way is in view. The apparent diameter of our galaxy would span around 36° of the sky, which is approximate 72 times more than the full moon. Of course, however, if you’re in an orbiting satellite galaxy, there will also be plenty of other stars to complement the celestial backdrop. Nonetheless, the view would still be quite otherworldly. Rogue Stars Hurtling through the Endless Voids SA,Hubble, NASA, S. GeierThe fastest-moving star so far discovered, US 708 is speeding away from our galaxy at over 7,000 miles (11,000 km) per second. Our galaxy is home to some 100 to 400 million stars, and Andromeda is home to around three times that. However, research published back in 2015 revealed that as many as half of all stars in the Universe may reside outside galactic borders. These rogue stars are gravitationally free, instead moving through the millions of light years between galaxies along their own paths. Until recently, it was assumed that all stars resided within the confines of galaxies. Nonetheless, the discovery of several unbound stars over the past couple of decades has led astronomers to believe that there might be as many as a trillion of these intergalactic stars in the Virgo cluster alone. No one is quite sure how these stars end up on their eternal, lonely journeys, but there are a couple of theories. Perhaps one of the most splendid examples of how stars can end up going rogue is US 708, a tiny subdwarf star composed almost entirely of helium. Currently around 62,000 light years away in the galactic halo surrounding the Milky Way, US 708 is hurtling towards intergalactic space at a staggering 26 million miles (43 million km) per hour. This is far greater than the galaxy’s escape velocity, meaning that the Milky Way can no longer keep it within its grasp. Quite what caused this little star to leave in such a rush remains uncertain, but a possible explanation is that a supernova event booted it out of the galaxy. Stars may also end up being ejected from their host galaxies by black holes. Although, ordinarily, a black hole consumes anything that passes its event horizon, things get a bit more complicated if it encounters a triple star system. Simulations show that one of the three stars will end up being devoured by the black hole, effectively ceasing to exist. However, the remaining pair should recoil, hurtling away faster than the galaxy’s escape velocity, thus becoming a hypervelocity binary star system that eventually leaves the confines of its host galaxy behind. Since there is literally nothing to slow it down in the near complete emptiness of intergalactic space, a rogue star will, effectively, continue its journey forever. As such, it’s quite possible that even the emptiest regions of the Universe have a few. Dark Worlds Floating Aimlessly through the Intergalactic Medium NASA, JPL-CaltechRogue planets are worlds with no suns, bound only to the galactic core itself. Nonetheless, perhaps planets also exist outside of galaxies as well? Rogue planets may number in the billions in the Milky Way galaxy alone. However, even these sunless worlds are still bound to the galactic core. No planets have ever been discovered in the intergalactic medium. With current technology, it is practically impossible to find a planet-sized object at such a distance, particularly if it doesn’t have a host star to make transiting photometry possible. Expelling a planet out of a galaxy is somewhat more complicated than expelling a star. Planets are not massive enough for three-body interactions with black holes to work, and a supernova event is far more likely to evaporate any nearby planet than get it on the move. Nonetheless, gravitational interactions between galaxies might just do the job. For example, the ongoing collision between two galaxies collectively dubbed NGC 4676 or, informally, ‘The Mice’, may be causing stars and even entire solar systems and rogue planets to miss out on the merger, instead leaving them behind entirely. NASANGC 4676 is the product of two galaxies colliding, an event that could leave countless stars and planetary systems floating freely through the Universe. Another possibility is that intergalactic planets could exist as part of intergalactic solar systems from the times when they were still bound to galaxies. Nonetheless, the immensely destructive forces that usually cause a star to get kicked out of its host galaxy in the first place would normally obliterate any planets. • Hunting stars in the voids between the galaxies is exponentially more challenging than exploring our own little region of the Universe. However, perhaps one day we’ll find out more about these remote and unusual places. What do you think lies in the silent emptiness between the galaxies? Let us know in the comments below! 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