Good morning, John.  Now, okay, what is this?  Why am I doing this?  Now, why am I showing you this very bad image?  You know, just because it's objectively one of the best images ever taken in all of human history but first...when I was a kid, I was really frustrated by something.  We knew where we lived in our galaxy.  We knew how big the galaxy was, how far away other galaxies were, but there was a pretty simple question that no one could answer for me: How many stars are in the sky?

Now, there's a bunch of reasons why this is a hard question to answer.  First, that number is very different depending on conditions.  Like, if it's cloudy, the number is zero.  If you're in the middle of the wilderness on a clear day, it is much higher and also it's very different depending on the equipment you're using.  Like, if you're using your eyes or binoculars or a space telescope, and once you start thinking about equipment, then you're not really asking how many stars are in the sky, you're asking how many there are in the Milky Way galaxy and now you got a new problem.  

Now, you can't just count the stars in the Milky Way for two reasons.  One: if you take a picture of it, you can't see a lot of the stars because there are these giant clouds of gas blocking out huge sections of the galaxy.  Second, and more importantly, there are just too many stars.  

John, back to my image.  This photo is actually two million photos, taken over ten years by the Spitzer Space Telescope which ended its mission this week, and let's actually move over to some software that will allow me to zoom in on it without crashing my computer.  Spitzer is an infrared telescope, which means it can see the heat of stars even through all of those gas clouds and so we can actually see a more unoccluded galactic core, and still, we can't see all the stars because of all the stars that are in the way.  It's like a soup of stars, like bioluminescent plankton in the waves, except that every point of light, every microorganism instead is a solar system. 

This view is looking through almost all of the galaxy, and because we are taking that picture from the edge of the Milky Way where our Sun is, there's a lot of galaxy to look through.  Instead, we can also look outward, through the thin band of the galaxy that is between us and the intergalactic void and there, beautifully and upsettingly, we have a thinner soup but still a soup. 

John.  There are a lot of stars and if you ask an astronomer today how many stars there are in the Milky Way, they will tell you, somewhere between 100 and 400 billion and that just seems so wrong.  That's a huge range, but we cannot count so we estimate based on the galaxy's mass and what we think the average mass of a star is, but both of those numbers have significant wiggle room right now and so, dear child Hank of the 1980s, it is 2020 now and we still don't know how many stars there are in the Milky Way galaxy, but you can get an idea of what hundreds of billions of stars look like from this image that took decades of work to create and that's pretty cool.  John, I'll see you on Tuesday.  

Thank you to all the people who worked on the Spitzer Space Telescope project over the years.  This galactic map was, of course, only one of the amazing achievements of that project.  It was only 100 years ago that we were arguing about whether or not the Milky Way was the entire universe or whether there were other galaxies out there.  The person who finally proved those other galaxies were there and did exist, his name was Edwin Hubble and the guy who first conceived of the idea of putting telescopes in outer space and was a huge champion of the Hubble Space Telescope, well, his name was Lyman Strong Spritzer.