 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy we are going to talk about an asterism and see what an asterism means in astronomy. So what is an asterism? Well an asterism is a grouping of stars. Now you may think a grouping of stars, well that's a constellation, however the two things are quite different. And an asterism is not a constellation. A constellation is a specific region of the sky. Now here we see for example the part of Sagittarius. This is a part of Sagittarius known as the teapot. And you might be able to see that from the sketch in there of the shapes connecting these stars in the shape of a little teapot here. However the constellation of Sagittarius is actually a much larger region. And you can see here is that teapot of Sagittarius. That is still located here exactly the same as we saw previously. However the actual constellation is a region of the sky and it has very specific boundaries which in this case are the whole lighter area here and go well beyond the bounds of just the teapot that we looked at previously. So constellations, there are 88 of them that will make up the entire sky. An asterism is just a portion of a constellation or it can actually span multiple constellations. So another example would be what is called the big dipper. So here we see the big dipper down below and the seven bright stars that make up that and we also see the little dipper as well. Again these are two other asterisms. These are asterisms which are located within a single constellation. So the teapot is completely within Sagittarius. The big dipper is completely within Ursa Major. The little dipper within Ursa Minor. They are part of a constellation. However that is not a requirement. We also can have asterisms which span multiple constellations. It is just a pattern of stars that we see in the sky. Here we see what is known as the summer triangle. There are three bright stars here. There's the star Vega down here, Deneb up to the top, and Altair off to the left. And these form a large triangle in the sky, often visible, at least in different parts of the world, visible in late summer to early fall, giving it its name as the summer triangle. So each of these bright stars that make up the triangle is the brightest star in its own constellation. Now we can have another one of these. How about one for the winter sky? Well we have the winter hexagon. And here we can see that. Now do you see the hexagon here in a pattern of bright stars? It doesn't necessarily jump out at you, but if you know what to look for, you can actually connect some of these stars from, for example, here's one side of the hexagon. Here is a second side. Here is a third, a fourth out to here, a fifth down this way, and then a sixth here, making a large hexagon in the winter sky. You may perhaps recognize here is the constellation of Orion. The bright star Rigel is one of the stars in this. Each of these stars within the hexagon, again, is within another constellation itself. So it's not just one constellation, it is actually six stars in six constellations that make a nice pattern within the winter sky in this case. So let's finish up with our summary. And what we've looked at this time is that an asterisk is a grouping of stars that forms a pattern in the sky. It is different from a constellation. Constellations are specifically defined regions of the sky. And an asterism can be a pattern within a constellation or can span multiple constellations. So that concludes this special topic on the idea of an asterism. We'll be back again next week for another special topic in astronomy. So until then, have a great day, everyone, and I will see you in class.