"The story of elements in nature, of why we are here, of our cosmic roots, is strongly tied to the story of stars in our galaxy and universe. And that means exploring the lives of stars, how stars come to be, what happens during their lifetimes, and how they, too - like humans - eventually die. We may not all know it, but we are part of the biggest recycling program that exists - the birth, life, and death of stars. Stars are born in great clouds of gas called nebulae. The word nebula comes from the Latin, and means "cloud" or "fog." These clouds exist between the stars, and consist of mostly hydrogen, helium, and other gases, are they are typically ionized. That means they are excited, energized, from hot stars inside and nearby them, and that causes them to glow. Thankfully, from this process, we can see them from very large distances, across our galaxy, or even in other nearby galaxies. These so-called interstellar clouds, nebulae, also contain various amounts of dust. Astronomers believe the veils of dust in the universe were formed in supernova explosions. Recent observations of the universe have shown that not only is the cosmos expanding, as we have known for more than a century, but that the universal expansion is accelerating over time. On large scales, everything is moving away from everything else, and the universe is getting bigger. But various forces are at work in the universe. One of the most important, the very force that keeps us on Earth's surface, is gravity. The attraction of gravity means that even though the universe is expanding, things that are near each other are drawn together because of their mass. This means that galaxies close to each other can merge together as one. It also means that new stars can be born, as gravity causes the gas and dust in nebulae to be condensed down into smaller volumes. As this process occurs, eventually enough mass comes together to form a new star - enough hydrogen, helium, and other elements are compressed so that a critical mass is reached, and a new nuclear fusion reactor - a star - is born. So, the majority of nebulae we see scattered across our sky, almost all belonging to our Milky Way Galaxy, are cauldrons that make possible the births of new stars. They are stellar nurseries, and that's why they are frequently intermingled with clusters of young stars. By observing nebulae, we are peering into the world of infant suns, seeing a process that for our own Sun took place some 4.6 billion years ago. This is an important story to understand, and appreciate, because it truly allows us to see where we came from, and why we're here on a planet orbiting one rather ordinary star in the Milky Way"--