Determining the Composition of a Star
Scientists cannot directly investigate the composition of stars, due to their extreme distances from Earth and their extreme temperatures. Fortunately, scientists have developed a method for investigating the composition of stars indirectly.
Stars are large balls of hot gases held together by their own gravity. The starlight that you see comes from gases on a star's outer surface. When exothermic nuclear reactions occur within a star, electrons that are found within the gaseous atoms of a star are promoted to higher energy levels. When these electrons return to their ground state, they emit the absorbed energy in the form of light radiation.
Astronomers use telescopes with diffraction gratings to separate starlight into its component wavelengths. They can identify gases within a star by comparing the star's spectrum to the spectra of various gases. Using this method, astronomers have discovered that most stars are mainly composed of hydrogen and helium. In fact, approximately 90% of the atoms in a star are hydrogen, making hydrogen the most abundant element in the universe!
The sun is our closest star. From its spectrum, scientists know that over 99.9% of atoms in the Sun are either hydrogen or helium. The remaining 0.1% include 67 other elements, such as oxygen, carbon, and nitrogen.
Scientists may not have discovered all the elements in the Sun. Since a star consists of many layers, light that is released from the inner layers is not necessarily detected because it cannot escape from the star. The challenge of the designers of modern astronomical instruments is to design instruments that can precisely measure the spectra of extremely faint stars.
Questions
1. Describe the method that astronomers use to determine the composition of a star (2 marks)
2. When observing a star's spectrum, you notice that the lines of certain elements are not present. Can you conclude that this element is not present in the star? Why or why not? (2 marks)
3. How do stars emit light? (2 marks)
4. In what ways can determining the composition of a star be considered qualitative analysis? (1 mark)