Brian Monetti, I <3 astronomy
VY Canis Majoris is one of the wildest stars in our solar system, and it is HUGE! If it were placed in the location of our sun, it would be so wide that it would swallow up Jupiter.
Now, about the atoms. First, we need to think about how massive the star itself is. Because it is so far away, scientists aren't entirely sure how much mass it actually has. Most estimates put it close to 20 times the mass of the sun. Now you might be wondering, how can something which has a radius that is thousands of times larger than the sun have a mass that is only 20 times? It turns out the VY Canis Majoris is an incredibly low density star, with an average density less than earths atmosphere.
So, how many atoms would be in something this star which is 20 times as massive as our sun? First, we should convert to kilograms. It turns out that 1 Solar Mass is just about equal to 2x10^30 kilograms. So how many atoms per kilogram? Well that depends on what element we are considering. For this calculation, lets just assume that the star is made of hydrogen. Now of course there will be lots of other elements mixed in, notably helium, but the percentages are very low, so lets just say it is made entirely of hydrogen.
To make this calculation, we divide the number of grams (1000 for a kilogram) by the molar mass (which for hydrogen is 1.0079g/mole) and then multiply by Avagadro's number, to get a count of the atoms. This tells us that there are 5.975x10^26 atoms per kilogram of hydrogen. This is described here: https://www.noodle.com/learn/details/105521/what-is-avagadros-number-and-molar-mass-chemistry-tips
So now we know how many atoms are in a kilogram of hydrogen, and how many kilograms of hydrogen are in VY Canis Majoris. Multiplying these two numbers will tell us how many atoms will be in this star:
5.97x10^26 x 20 x 2x10^30 = 2.39x10^58 atoms. That is quite a lot of atoms!