Before the invention of photography, astronomers observed and recorded their observations by drawing. Galileo, for example, recorded the positions of Jupiter and its four major moons on paper. However, drawing records were inevitably slow and prone to errors. After the invention of photography, astronomers finally had a tool to record observation targets in real time. By analyzing photographs, astronomers could study the appearance of stars in detail, their temperatures, velocities, and even their ages and the composition of their chemical elements. To this end, astronomers designed various specialized cameras, whose photos could be divided into three main categories: imaging of celestial objects, spectroscopy of celestial objects, and tracking the changes of celestial objects over time (light variation). Through these photos, we can see the diverse shapes of stars, nebulae, and galaxies, study their structures, such as the central structures of spiral galaxies, spiral arms, and surrounding halos; understand their chemical compositions and physical states, such as what elements they contain and their temperatures; and, by comparing photos taken at different times, observe their brightness variations.
Astronomers' photos of stars are often not directly usable. They need to undergo complex processing and analysis before reliable conclusions can be drawn. The basic theoretical basis for astronomers to analyze celestial photos is the radiation mechanism of celestial bodies. So-called radiation mainly refers to the electromagnetic waves emitted by celestial bodies. These waves are "light" emitted by atoms and molecules that make up celestial bodies under different physical conditions. Different lights correspond to different physical mechanisms and chemical compositions. For example, astronomers determine the temperature, pressure, and chemical composition of the Sun's surface by photographing its spectrum and then using models of its surface radiation and atmospheric structure.
You might soon ask why not use video recording instead of photography. This is because, except for special celestial bodies such as the Sun, most celestial bodies' light reaching Earth is extremely weak. With our current technology, we still cannot achieve clear video recording. Astronomers often need to expose a photo for 1 hour or longer to accumulate enough photons and provide sufficient information for scientific analysis. It can be said that the development of modern astronomy relies on a large number of star photos. From photographic glass plates costing hundreds of yuan each to current CCD cameras priced at hundreds of thousands of yuan; from data of tens of megabytes per night to photos containing hundreds of megabytes of data. Behind these star photos lies the integrated development of astronomy with other disciplines.