Sky, Celestial Sphere and Constellations（天空,天球和星座）.pdfVIP

Sky, Celestial Sphere and Constellations Last lecture • Galaxies are the main building blocks of the universe. – Consists of few billions to hundreds of billions of stars , gas clouds (nebulae), star clusters, debris from dead stars … – Few hundreds of thousands of light years in size – Shapes spiral, elliptic, irregular • Our galaxy: Milky way a spiral galaxy 100,000 LY in size, 100 billion stars with many satellite galaxies. • Galaxies group together, local clusters, clusters, super clusters of galaxies • Largest scale they from a filamentary/wall like structure: cosmic web – Largest scales universe homogeneous , isotropic • Universe is 13.8 years old, first stars/galaxies formed 400M years after that. • So oldest galaxies 13.4 billion years old, at a distance 13.4 billion LY away. • Regions beyond that are not visible to us, light from those regions has not reached us yet. Earth and the Sky this part of the sky visible here sunlight this part of the sky visible here • The side of the Earth facing sun at a give time gets sunlight and it is daytime. • The other side facing away for the sun is experiencing nighttime. • During the day, sky looks blue, at night sky is dark and we can see stars and other celestial objects in the sky. • At a given time and a given location only half of the sky is visible. • On a typical moonless clear night about 3000 stars are visible. Blue Sky during the day Sun more