HAVE you ever looked at the stars and thought "which one's the biggest?"
Playing 'big, bigger, biggest' in space is truly spectacular, with the sheer size of planets, stars, and galaxies being beyond our ability to imagine properly.
Even then, there are 'things' larger than galaxies out there.
What's even more impressive for those of us curious about the titans of the sky is that we're always finding new planets, new stars, new structures that force us to rethink what we know about the universe.
Keep in mind that this list is based on readily published data at the time of writing. Tomorrow we may find a planet twice the size of WASP-17b or discover that UY_Scuti is a little smaller than we thought.
We've defined 'biggest' as the longest physical dimension of the object according to scientist's method of measuring them.
Biggest Planet - WASP-17b (1.991 x Jupiter)
WASP-17b is a funny planet. It is truly enormous at nearly twice the size of Jupiter but has only half of its mass.
Planets like WASP-17b are commonly referred to as 'fluffy planets' because of how light and diffuse they are. If you had a big enough bath, WASP-17b would float in it because the water is more dense.
WASP-17b is also a little odd because it orbits its sun the wrong way. Earth and all of the planets in our solar system orbit in the same direction as our sun rotates. Not WASP-17b, it has to be special.
It would take two years to drive around WASP-17b at 60kmh, though you'd have to work out how to not sink. You'd take a lot longer to drive around the largest star ...
Biggest Star - UY Scuti (1,708 solar radii)
UY Scuti isn't a big star, or a giant star, it's a supergiant star. Astronomers aren't so creative when naming things, but the reality of this star's size means there are few words strong enough to describe it.
UY Scuti is 1,708 times the size of our own sun. If it were in the sun's place it surface would reach half way between the orbits of Jupiter and Saturn. It takes light over an hour to cover that distance.
If that's not enough to impress, it also means UY Scuti's volume is five billion times our own sun's.
This star is so big and so bright you can see it from 9,500 light-years away with binoculars.
Then we start running into trouble actually measuring the size of these things ...
Biggest Galaxy - IC 1101 (2 million light-years)
If you look at IC 1101 you're seeing how it appeared more than a billion years ago thanks to how far away it is. You're also looking at a monster collection of stars 2 million light-years across. Sort of.
The usual galactic structure says IC 1101 is at least 212 thousand light-years across - but this galaxy then ignores that structure and continues emitting light out to 200 million light-years.
The difference is large enough for there to be debate on how to measure it at all. Either way there's plenty of room for the 100 trillion stars calling it home.
Think of this: if IC 1101 replaced Andromeda as our closest galaxy, instead of it taking up 3 degrees of the sky (the full moon is 0.5 degrees), it would take up (very roughly) more than 117 degrees.
To go any bigger, the usual methods of measurement aren't enough ...
Biggest Void - Giant Void (1 billion light-years)
Even though we can see plenty of what's going on in the sky, space is aptly named - most of it is cold, empty, and stark.
Some regions take this too far. The Giant Void is a region of space 1 to 1.3 billion light-years in diameter where even the usual standards of nothingness stand aside.
Do note, void doesn't mean 'absolutely nothing there' - it means an area of space with less than one-tenth the expected density of the observable universe.
Still, the Giant void is so large it has 17 galaxy clusters floating around inside it. These clusters are still so distant from each other they don't have any realistic gravitational interaction.
Still, this isn't the largest coherent pattern we can see ...
Biggest Structure - NQ2-NQ4 GRB Overdensity (6-10 billion light-years)
For this we'll need to think of 'things' in a slightly different way. Planets and stars are objects because the matter they're made of is in contact.
Galaxies and Nebulas are objects because they're regions filled with similar objects or matter all interacting with each other. The same goes for galaxy clusters and superclusters - they're collections of objects able to influence how the rest behave.
Structures larger than galactic superclusters exist. The Coma Wall is a "giant wall of galaxies" (filament) 500-700 million light-years long.
Obviously, this doesn't mean the galaxies are packed tight like bricks. We consider them a wall because when viewed in comparison to the space around it, we see what looks like a big wall.
For a long time we thought the laws of physics put a limit to the size of these structures. This cosmological principle says "zoom out far enough and everything looks smooth". So we would expect, when able to see the whole universe, no real patterns would emerge, so nothing larger than 1.2 billion light-years could exist.
Surprisingly, we've found five of these law-breakers, the largest of which is the horribly named NQ2-NQ4 GRB Overdensity (the Overdensity).
The Overdensity is so large (6-10 billion light-years) we didn't even see it the normal way. A broad survey of the sky began to notice gamma ray bursts (GRBs) oddly close to each other in a patch of sky too big for it to make sense as an accident.
Astronomers are trying to prove this structure doesn't exist, but the current measurements make the Overdensity somewhere from (roughly) 1/15th to 1/9th the size of the observable universe.
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A note on sizes
All the above values are rough. The distances to these objects and resolution of the telescopes used to measure them mean we can't nail down an exact size.
While I've given a single number for most of the objects above, they're usually expressed as being of a certain size, give or take a certain amount of possible error.
Astronomers are finding new objects every day, and we've not come close to actually observing the entire observable universe.
You can blow your mind fairly easily when learning about space, but the most exciting part is that you don't have to be an academic at an observatory to join in. There should be an amateur astronomy club near you, check them out and you could be the one to find the next 'largest star'.
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