As we look into the sky each night one may wonder, “Just how many stars are there in the sky?” As a child you may even have attempted to count them and again recount them as they begin to blur together. Most stars can’t be seen by the naked eye (you can see down to 5th or 6th magnitude without optical aid), so even if you were to sit down and attempt to count the stars, your count would be much too low. If you spend any time researching this intriguing question, you’ll find that we really don’t know the answer exactly. Why? Well, it is physically impossible to count the number of stars in our galaxy directly and the same holds true for counting the number of galaxies in our universe. Which, by the way, is the formula we’ll use for the calculation. That is:

Stars In Universe = Number of Stars In Each Galaxy x Number of Galaxies In Universe

It is easy to become caught up in our galaxy alone, because we see that every night. So the first question to answer is “How many galaxies are there?” and then we can begin to take into account the number of stars per galaxy.

It is estimated that there are between 10×10^11 galaxies and 50×10^11 galaxies in the universe. Let’s look at the specifics of one way to estimate this number. In 2004, the Space Telescope Science Institute (the guys who run the Hubble Space Telescope), released an image they called the Hubble Ultra Deep Field (HUDF). They called it “ultra” because they had already done the Hubble Deep Field image years earlier. In this ground breaking picture taken over the span of 11 days (400 orbits) they imaged 10,000 individual galaxies. This isn’t big news on its own. It’s only when you consider that they saw these 10,000 galaxies in the space of the sky taken up by the size of a large crater on the Moon that you get any sense for the scale of the image. Put another way, it would take 50 of these images to make up a part of the sky equal to the size of the full Moon. Or, taken even further, the entire sky takes up an area 12.7 million times the size of the image. So, here’s where we can start doing some math. If we extrapolate the HUDF across the entire sky, there would be 12,700,000 x 10,000 galaxies or 127 billion galaxies. Now that’s a lot of galaxies. But, we’re not done yet. If an observer were to look at our local group of galaxies from a billion light years away with the Hubble Space Telescope, they would only see 3 or 4 of the largest galaxies (yes, the Milky Way is one of them). However, we know that there are at least another 20 galaxies in our local group, mostly made up of dwarf galaxies. Using our own local group as a standard there could be 5 or 6 times the estimate above if we were to include the more common dwarf galaxies!

Now on to figuring out how many stars are in a typical galaxy. For this estimate, let’s use our own Milky Way since it is the galaxy we’re all the most familiar with. As I mentioned above, it would be pretty difficult to physically count all of the stars in the Milky Way. We’ll need another method…which we have. Our estimate will be based on the mass of the Milky Way which can be determined by simple physics. If we know an objects velocity (say dwarf galaxies around the Milky Way) and their distance from the center of rotation (that is, the center of the galaxy in this case), we can calculate the system’s mass using the formula M = v2D/G. M is the mass, V is velocity, D is distance to the center of the galaxy, and G is the gravitational constant. By plugging in all of the known values for these variables, we arrive at a total galactic mass of about 5.8×10^11 M☉ (M☉ is the symbol for solar masses). Now, we have to account for things like dark matter and dark energy, which make up approximately 90% of the known mass of our galaxy. Meaning only 10% of the Milky Way’s mass is made up of visible stars…a whole other interesting topic. This would leave us with 58 billion stars if all the stars were the same size as our Sun. But there’s a catch. We also know that most stars are smaller than the Sun. By most estimates they average about .3 solar masses. Taking that into consideration, we arrive at a total number of stars in the Milky Way of 193 billion. Quite an impressive number to be sure.

So now that we have an estimate for the number of galaxies…127 billion at the low end…and an estimate for the number of stars in a “typical” galaxy…we’ll round up to 200 billion…we can figure out the total number of stars in the Universe. And the answer is…2.54×10^22 stars. How would you even say that number? Glad you asked. You would say the Universe has over 25 sextillion stars.

Counting the grains of sand or drops of rain is much like counting the stars…a difficult task indeed. With continued research and advancements in technology, our counts may become more definite. The number of stars that exist is truly an awesome concept and an even more beautiful sight.

~ Words by Steve Cullen and Sarah Stecher
~ Images by Space Telescope Science Institute
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