Spectral types classify stars based on their temperatures and the characteristics of their light. The main categories are O, B, A, F, G, K, and M, with O stars being the hottest and M stars the coolest. Each type is further divided into subclasses using numbers, allowing for a more precise classification. The classification is determined by analyzing the star's spectrum, which reveals the presence of specific elements and molecules. For example, G type stars, like our Sun, have a temperature range of about 5,300 to 6,000 degrees Celsius and show strong absorption lines of hydrogen and helium.