Our universe is big, empty, and dark. It isn’t even cold per se, because only an object can can have a temperature, and space is by definition the absence of one. When physicists and astronomers talk about the temperature, they don’t refer to the universe itself; instead they talk about the temperature of those pitiful things we’ve found in it.
Here is spot of lazy math: Stars come in all sizes. There are gargantuan hypergiants so diffuse that they are closer to convective hot nebula than the incandescent ball of plasma that lights our own skies. If you dropped one into where the sun sits now, its periphery would be way out in space halfway between the orbits of Jupiter and Saturn.
Opposite the hypergiants are petite neutron stars. These zombie star pack the mass Sol into a volume scarcely twenty kilometres across. If a neutron star popped into existence above Dublin, the locals would marvel at its tiny size in the nanoseconds before their utter obliteration.
There is a massive range of stars that lie between these extremes of volume: The red dwarfs and giants (no relation), the luminous blue giants and dark brown dwarfs, and yes, our sun. There are as many subtle variations as there are people on Earth. Every star has a personality, a face, and a fingerprint that is wholly unique.
- Stars are people, and as with people you can take averages: Sum the total, then divide by the quantity.
- Sol is an archetypical star. It’s remarkably close to the average, with a diameter of 1,391,684 kilometres.
- There are an estimated 100,000,000,000 stars in our wholly average. Thank the isotropic principle for that.
- And there are an estimated 10,000,000,000 galaxies in the visible universe.
- One hundred billion stars each in ten billion galaxies. Each is about the same width as Sol.
- That gives us in the neighborhood of 1,000,000,000,000,000,000,000 stars in visible universe. And finally, there are 9,460,528,400,000 kilometres in one light year.
9460528400000 / 1391584 = 6797899
Set touching end-to-end you can fit 6,797,899 stars average stars in a light year-long conga line. Now cube this conga line-multiply the length by the width by the height.
6797899 * 6797899 * 6797899 = 314140639320426163699
A cubic volume one light year on a side could hold 314,140,639,320,426,163,699 stars.
314140639320426163699 / 1000000000000000000000 = 0.31414 0.31414 * 100 = 31.41400 31.41%
That’s one-third of all stars in our averaged visible universe. Repeat until you run out of stars.
You can fit every single star in the universe into the space between Sol and its nearest neighbor, Proxima Centauri. And the rest of the baryonic matter in the universe would fit neatly into the gaps between the stars.
Dive into the hearts of a galaxy, or a globular cluster, or even our own asteroid belt. You’ll find empty with a low chance of excitement.