The fact that the temperature and energy generation of an object can be determined by its Planck curve made the study of spectrum the key component to stellar astronomy. The amount of energy emitted from stars is determined by measuring their brightness or the amount of light they emit. This is called photometry. However, two major developments expanded our understanding of the chemical make-up of stars. They were:
The invention of the spectroscope, a device that separates white light into component colors called a spectrum. And the discovery that elements emit a unique spectrum, i.e. produce a unique chemical fingerprint in the spectrum.
The two discoveries combined to produce a new field called spectroscopy, and allowed astronomers to measure the chemical composition of stars for the first time.
The discovery of spectra lines was made by Fraunhofer who, in the early 1800’s, magnified the Sun’s spectrum and discovered dark lines which could be identified with particular elements (based on spectra in the laboratories).
English astronomer Lockyer, in the late-1800’s, discovered an unknown element in the Sun, i.e. a set of spectral lines which did not correspond to elements in the lab. He named this element helium (Latin for Sun element).
By the 20th century, spectral lines for all the elements in the periodic table have been classified and astronomers could examine the chemical composition of stars and planets. What was missing was an explanation for why these lines should exist. Why did particular atoms absorb photons at particular wavelengths?