More stars in the universe than grains of sand on earth?
How many grains of sand are there on earth?
Off all places, a group of scientists at the University of Hawaii came up with a reasonable way to get an estimate.
They began with postulating an average size for a grain of sand and by calculating the number of sand grains in a teaspoon. Then the estimated volumes of beaches and deserts in the world were factored in. Combined together, their estimate was a staggering 7.5 x 10^18 grains of sand, 75 followed by 17 zeros. In perhaps simpler, though still incomprehensible terms, that is 7 quintillion, 500 quadrillion grains. Or in terms simpler still: a lot.
In the 1980 bestseller, Cosmos, astronomer Carl Sagan famously wrote:
The total number of stars in the Universe is larger than all the grains of sand on all the beaches of the planet Earth.
How could this possibly be true?
Cosmos was published in 1980, and in the past years many astronomical discoveries and developments (such as the Hubble’s Space Telescope) have been made. So we may ask ourselves: are astronomers today still agreeing with this statement?
How many stars are in the universe?
Well, let’s start by something almost everyone can do: look at the sky with the naked eye. According to a study published in 2011, nowadays more than half of people live in cities, and this number is claimed to rise to 70% in 2050.
It’s important to note that artificial lighting in these areas can greatly impact the number of stars you can see. Artificial light is reflected on airborne particles and reduces the darkness of the skies, and apparently decreases the magnitude of all celestial objects. This light pollution is also called “skyglow”.
But when we don’t take this limiting factor into account, when we would position ourselves at a remote, dark place, the total number of stars seems uncountable, right? And why would we even guess when someone has already done the counting for us? Astronomer Dorrit Hoffleit of Yale University compiled the Yale Bright Star Catalog some decades ago. It contains every star visible from Earth, which is based on a brightness scale used by astronomers, called the magnitude scale.
In this logaritmic scale, a difference of five magnitudes is defined as a brightness ratio of exactly 100 to 1 One magnitude thus corresponds to a brightness difference of exactly the fifth root of 100, or very close to 2.512 — a value known as the Pogson ratio.
While this is a rather interesting scale historically, because the scale extends into negative numbers, the thing relevant for now is the magnitude 6.5, which is the naked eye limit for most of humanity.
So, taking all stars which are magnitude 6.5 or brighter into account, the Yale Bright Star Catalog comprises 9,096 stars visible across the entire sky. Both hemispheres combined. Since we can only see half the celestial sphere at any moment, we should divide that number by two to arrive at 4,548 stars as a rough estimate.
With a good pair of binoculars, you can observe stars down to magnitude 9, meaning that the number of visible stars jumps to more than one hundred thousand stars. A small telescope capable of resolving a magnitude of around 13, will let you count up to millions of stars.
But how many stars are out there? It is just not possible to count stars individually, not even by using a telescope such as the Hubble Space Telescope built by NASA..
One could use a galaxy’s mass, however,. Astronomers estimate this mass by looking at how the galaxy rotates, as well as its spectrum using spectroscopy.
But in a typical galaxy, if you measure its mass, about 90 percent of that is thought to be dark matter.
The fact that much of the remaining “stuff” in the galaxy made up of diffuse gas and dust, combined with luminosity investigations, leads to estimations that about 3 percent of the milky way’s mass is made up of stars.
According to one such calculation, the Milky Way therefore a mass of about 200 billion solar masses, in which a solar mass refers tot the mass of our own star, the sun. So it is easiest to translate that to 200 billion stars. This accounts for the stars that would be bigger or smaller than our sun, and averages them out. Other estimates have said the galaxy has a mass of between 400 billion or even 700 billion solar masses. For simplicity, we will go with the lowest of these estimates here: 200 billion stars in the milky way.
But that’s just our own galaxy. How many galaxies are in the observable universe?
European Space Agency’s infrared space observatory “Herschel” has made an important contribution by ‘counting’ galaxies in the infrared, and measuring their luminosity in this range – something never before attempted.
However, most calculations are in fact based on analysis from pictures made by the Hubble Space Telescope. Astronomers are capable of detecting billions of galaxies.
For example, wherever the Hubble points, in all directions, thousands of never before seen galaxies are seen extending far into the distance.
While estimates among different experts vary, an acceptable range is between 100 billion and 200 billion galaxies in the observable universe. When you take into account smaller galaxies with less stars, however, this number is thought to be 2 septillion. But for the sake of being able to calculate with an average sized galaxy, we’ll go with 100 billion galaxies.
So, if we assume that our Galaxy is more or less typical and use the lower estimate, 200 billion for its total stellar population then multiply that by the number of galaxies within reach of the Hubble Space Telescope, we arrive at 20 sextillion (or 20.000.000.000.000.000.000.000 / a 2 followed by twenty-two zeros / 20 thousand million million million) stars in the observable universe.
But here’s the catch.
When we’re observing the universe, when we look at galaxies billions of lightyears away, we aren’t evaluating the current number of stars, at all. Rather we are looking back in time. Photons left these stars a long time ago and had to travel all those years to finally reach our telescopes and observatories, in the 21st century.
Our images of stars are just as out of date as they are distant from us, and we won’t ever be able to get around that limitation unless we can go visiting them so that the light-delay isn’t so severe.
So, even if we could literally count all stars in the entire universe, no matter how dim and far away, we still wouldn’t know the exact number as it is at the current moment. And who knows what might have happened in the meantime…
But what hurts more? Knowing that we don’t and probably won’t ever have a definitive answer on the number of stars currently out there, or the realization that as humanity started moving to cities, technology developed, air polluted, the nights lost their stars…
Links used in this project (and the project “One, Two, Many”):
https://www.nature.com/news/2004/040816/full/040816-10.html https://mpi-lingweb.shh.mpg.de/numeral/Piraha.htm https://en.wikipedia.org/wiki/Pirah%C3%A3_people https://www.ee.ryerson.ca/~elf/abacus/history.html http://www.newworldencyclopedia.org/entry/Abacus https://www.npr.org/sections/krulwich/2012/09/17/161096233/which-is-greater-the-number-of-sand-grains-on-earth-or-stars-in-the-sky http://news.bbc.co.uk/2/hi/science/nature/3085885.stm https://www.bbc.co.uk/sounds/play/w3cswk2g https://www.youtube.com/watch?v=sJ_tZr0D2pk https://www.statista.com/statistics/274520/global-share-of-people-living-in-cities/ https://www.skyandtelescope.com/astronomy-resources/how-many-stars-night-sky-09172014/ https://www.herschel.caltech.edu/page/about https://www.nasa.gov/mission_pages/hubble/main/index.html