Monster Galaxies and Giant Voids: These Are the Universe’s Largest Known Structures
From a galaxy larger than the Local Group to a galaxy filament that takes up 11% of the observable universe — these structures are so big they don’t compare to anything.
In the nightsky, the universe appears small; the most distant object to be observed with the unaided eye is the Andromeda Galaxy at a distance of two million light years — everything else is just stars, most of them just a few tens of light years away.
In the nightsky, it’s never apparent that the observable universe has a diameter of 93 billion light years. It’s thanks to technological advances like the Hubble Space Telescope and the James Webb Space Telescope that astronomers learned the true scale of what’s out there. For example, the famous Hubble Ultra-Deep Field contains more than 10’000 galaxies in a tiny patch of the nightsky.
But even these 10’000 galaxies are just part of something much bigger. It’s estimated that there are hundreds of billions of galaxies in the observable universe. Within each galaxy, there are hundreds of billions of stars, and each of them has planets…
In other words, there’s just so much to discover. This big universe is filled with structures so large we can’t even fathom their size — today, let’s take a look at some of the universe’s biggest things.
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| Computer simulation of the large-scale structure of the universe; Source: Wikipedia |
10 billion light years: the Hercules-Corona Borealis Great Wall
If we look at the universe’s very large scale structure, it takes on a sponge appearance; bright lanes of galaxies cluster together into walls and filaments.
These filaments are already the largest structures of the universe, but the title of the biggest definitely goes to the Hercules-Corona Borealis Great Wall; a structure so big it takes up a tenth of the observable universe and stretches over five constellations.
The Hercules-Corona Borealis Great Wall was discovered in 2013 after years of careful mapping of gamma-ray bursts (GRBs), powerful explosions observed in distant galaxies. These GRBs showed clustering in certain regions, pointing to the existence of the Great Wall.
At 15 to 17 billion light years comoving distance, the Great Wall measures 10 billion light years in diameter!
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| Visualization of a large quasar group; Source: Wikipedia |
4 billion light years: the Huge Large Quasar Group
Quasars are active supermassive black holes (SMBHs) sitting at the centers of galaxies; they were especially present in the young universe. Most SMBHs in the local universe are dormant, just like the one sitting at the center of the Milky Way. Luckily so because quasars are extremely energetic — even from a distance, they’re very bright.
But quasars form something even more interesting: large quasar groups (LQGs), thought to be the precursors of the massive galaxy filaments such as the Hercules-Corona Borealis Great Wall.
In fact, just months before the Great Wall’s discovery, the Huge-LQG was deemed the biggest structure of the universe. 73 quasars are located within the Huge-LQG, which has a maximal extent of 4 billion light years.
3.3 billion light years: the Giant Arc
Nine billion light years away sits an arc so big it would take up the space of 20 moons if it were visible in the nightsky. This megastructure, discovered via quasar mapping, received the name the Giant Arc, measuring 3.3 billion light years across.
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| The CMB Cold Spot and potential Eridanus Supervoid; Source: Wikipedia |
1.8 billion light years: the Eridanus Supervoid
The cosmic microwave background (CMB) essentially is the imprint of the earliest time of the universe, from just 380’000 years after the Big Bang. The CMB is mostly uniform in temperature, with just tiny fluctuations that point to density differences in the universe as a large scale.
However, there’s one spot in the CMB that stands out. A giant blob that is much colder than expected — where the temperature is 0.00007 K colder than the CMB on average. Now, this may seem like an insignificantly small number; but it’s a bigger fluctuation than the average temperature variations.
A number of explanations have been proposed to explain the “CMB Cold Spot,” including that perhaps a parallel universe bounced against ours at some time. Another explanation suggests that the Cold Spot is a giant void; a supervoid.
Voids lie in between the massive galaxy filaments and are relatively common. Where there is a supercluster there will also be a void nearby. One of the most famous voids is the Boötes Void, aka the Great Nothing, with a diameter of 330 million light years. This size already makes it one of the biggest voids — but the CMB Cold Spot void could be much bigger.
A study in 2015 suggested that this so-called Eridanus Supervoid is aligned with the CMB Cold Spot and has a diameter of 1.8 billion light years! Evidence points to the existence of the supervoid, but cosmologists are uncertain whether the supervoid could actually be aligned with the Cold Spot.
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| The sheer size of the Laniakea Supercluster, where the Milky Way resides; Source: Physics World |
1 billion light years: the Pisces-Cetus Supercluster Complex, home to the Milky Way
The Milky Way is part of something big — it’s part of the Local Group, which in turn belongs to the Virgo Supercluster, and the Virgo Supercluster resides within the Laniakea Supercluster… and the Laniakea Supercluster is part of the Pisces-Cetus Supercluster Complex.
Within this large area, the Virgo Supercluster accounts for just 0.1% of the total mass! The Laniakea Supercluster contains 100’000 galaxies, one of which is our Milky Way.
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| NGC 6503, located at the edge of the Local Void; Source: Wikipedia |
200 million light years: the Local Void
While the Eridanus Supervoid and the Boötes Void are extremely big, there’s a void much closer to home — the Local Void, which extends at the edge of the Local Group. It measures about 200 million light years in diameter and despite being a void, contains a number of galaxies.
The most prominent of them is NGC 6503, a dwarf spiral galaxy 13 million light years away.
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| The giant radio galaxy Alcyoneus; Source: Wikipedia |
16 million light years: Alcyoneus
The Local Group contains about 50 galaxies, the biggest members being the Andromeda Galaxy, the Milky Way, and the Triangulum Galaxy. Even the Milky Way’s satellite galaxies, the Large and Small Magellanic Clouds, are amongst the six biggest of the group!
The Local Group stretches over 10 million light years — a very big space. But three billion light years from Earth resides a single galaxy that takes up more space than the entire Local Group. Named Alcyoneus, it’s the largest radio galaxy discovered to date, measuring 16 million light years in diameter.
What exactly are radio galaxies? These are galaxies with active galactic nuclei (active SMBHs) that release two giant radio jets into space which appear as lobes of radiation from the center of the galaxies.
Nobody knows how Alcyoneus grew such big radio jets, as the galaxy in itself appears to be relatively normal — the galaxy itself measures 240’000 light years, thrice the diameter of the Milky Way!
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| Size comparison between the Sun and WOH G64; Source: Wikipedia |
The biggest star of the universe: WOH G64
When it comes to measuring the largest star in the universe, you’d want to look for red supergiants — stars that already passed the main sequence and ballooned to extreme sizes before dying in brilliant supernovae.
One star that is often named as the biggest star astronomers know of is UY Scuti, 6000 light years away. However, when it comes to physical size, there are a number of bigger stars. For UY Scuti, the star could extend up to the orbit of the asteroid belt if it were placed in the Solar System, measuring about 1000 solar radii.
A star bigger than UY Scuti is VY Canis Majoris, around 4000 light years away. It has a radius of 1400 Suns! Even bigger is WOH G64 in the Large Magellanic Cloud, whose size lies between 1500 and 2000 Suns.
WOH G64 had a short lifetime in comparison to our Sun. It’s just five million years old and already at the end of its life. One day, the star will go up in a massive supernova, leaving nothing behind of the biggest star of the universe.
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