 Greetings and welcome to the Introduction to Astronomy. One of the things that I like to do in each of my introductory astronomy classes is to begin the class with the astronomy picture of the day. From the NASA website that is apod.nasa.gov slash apod. At today's picture, 4 September 4th of 2023, well, it is titled Cygnus Bubble and Crescent. So, what do we see here? Well, here we see looking toward the constellation of Cygnus the swan and we see a couple of different nebulae here. We see in one case the soap bubble nebula down towards the lower left-hand side and you might guess why that gets its name for its distinct appearance. And to the upper right we see the crescent nebula. Now, both of these are different types of nebulae associated with stars. Now, the soap bubble nebula is an example of a planetary nebula. That's what happens at the end state of a star, much like our own sun. As it expands outward, the outer layers are expelled out into space and the inner layers compact down to become a compact white dwarf star. So, this is something that our sun has to look forward to in many billions of years when it finally uses up all of the hydrogen fuel within its core and begins to expand. So, it will become what is called a red giant star, then eventually a red super giant star, and then those outer layers not being tightly held will be expelled out into space and give it the view to a distant astronomer at that time of something like perhaps what we see is the soap bubble nebulae here. Now, the other that we see is what we call the crescent nebula, and that is the nebula around a very hot massive star called a wolf-ray star. Now, a wolf-ray star is an extremely hot star, some of the most massive stars, and they are at the very end of their lives. How do we know they're at the end of their lives? Well, guess what? They don't live very long. They have extremely short lives and may not even live for more than a million years. So, they have very, very short lives, meaning that they will eventually become a supernova. So, that's what happens to a very massive star at the end of its life. It will become unstable and eventually rip itself apart in a massive explosion, and that's what we have to look forward to with the crescent nebula at some point in the future. Now, as we look at this here, we see the material that this is already pulsing out some material around that wolf-ray star, simply because of the immense amount of energy that it is emitting and pushing out outer layers. However, at some point, this will be gone, and it will actually become a supernova, the whole thing having exploded, and that within the next few million years or so, it will be gone, and we will not see either of these nebulae as they continue to expand outward into space. So, nebulae are a short-lived phenomena in the galaxy. Some will last for a longer time than others, but still relatively short relative to the entire age of the galaxy. So, we only get to see them when we catch them at that little bit of time when they happen to be visible. So, that was our picture of the day for September 4th of 2023. It was titled, Cygnus Bubble and Crescent. We'll be back again tomorrow for the next picture. So, until then, have a great day, everyone, and I will see you in class.