The Big Dipper, the brightest part of the constellation Ursa Major the Great Bear, is followed down into the northwestern sky by twinkling orange Arcturus and the constellation Bootes, the Bear Guard. Stellarium
Most of us would consider Bootes the Bear Guard a spring constellation. That’s when it first appears in the eastern sky, following the tail of Ursa Major the Great Bear as the snow drifts recede. Come fall, Bootes (Boh-OH-tease) and its bright luminary Arcturus recline in the west within spitting distance of the Dipper.
Every evening I look up to check on the condition of the sky. Arcturus is either flashing happily or gone missing, hidden by clouds. Gazing up from Arcturus, it’s easy to trace out the remaining points of starlight that form the kite-like figure of Bootes. Like a kite let go, Bootes drifts away to the west as the night deepens.
The next clear night, follow the arc of the Dipper’s Handle to Arcturus and then work your way up and around to pick out the constellation’s fainter stars. If you now direct your gaze to a blank spot between Bootes and the end of the Dipper’s handle, you’ll be staring at the center of a remarkable nothingness, the Bootes Void.
Map of the Bootes Void showing it alongside other dense superclusters of galaxies. Credit: Richard Powell
Normally we talk about the presence of something in the sky in this blog, but today we’ll focus on absence. The Void, a roughly spherical realm of space 250 million light years in diameter, is virtually empty. Space is already empty enough. If the sun were shrunk down to the size of a grapefruit, the nearest star system, Alpha Centauri, would be 2,000 miles away. From there, it’s another 1,000 miles to the next closest, Barnard’s Star.
American astronomer Robert Kirschner discovered the void in 1981 as part of a survey to measure how fast distant galaxies were fleeing from one another as the fabric of space expands in the ever-widening wake of the Big Bang. Six years later, Kirschner and team turned up 8 galaxies in this vast volume of space centered 700 million light years from Earth. By the late ’90s only 60 galaxies were known, making the Void not as devoid of galaxies as originally thought.
Still, the wind blows through it like a ghost town. Considering that the average distance between galaxies in typically a few million light years, the Void should contain some 10,000 inhabitants. Where have they fled?
The Millenium Simulation will give you a feel for the large-scale structure of the universe. Enjoy the ride!
Galaxies are vast assemblages of stars, clusters, gas clouds and planets thousands of light years across. Their mass gives them considerable gravitational might, so they’re attracted to one another. Over the lifetime of the universe, galaxies congregate into strands, clumps and clusters. The surrounding space empties out like a parking lot at closing time and becomes a void.
The Bootes Void is no ordinary emptiness. It’s HUGE. Too big to have formed with the current lifetime of the universe say astronomers. That’s why it’s thought to have agglomerated from smaller voids that merged together to form one of the largest voids in the known universe, a so-called supervoid.
Millenium Simulation of the large-scale structure of the universe shows a sponge-like texture of filaments of galaxies threading empty voids of space. The galaxies – each too tiny to see individually at this huge scale – clump around invisible dark matter and each other. Credit: Millenium Simulation
Galaxy clumping has amazing consequences for the large-scale structure of the universe. Astronomers think the visible matter of the universe clustered around clumps of dark matter, which makes up 73% of all the ‘stuff’ out there, shortly after the Big Bang. Once galaxies formed, they continued their clustering ways up to the present day. Instead of a random distribution of matter across space, the universe looks like a sponge where hundreds of billions of galaxies swirl in filaments and nodes around the comparatively empty voids.
Funny how the biggest things in the universe can be so surprisingly close to home. Look up toward the Big Dipper and Arcturus in the western sky after sundown and think of where you are.