Astronomy
The Use of Spectroscopy to Determine Chemical Composition of Stars
Quick fact
The element helium was discovered in the Sun's spectrum before it was found on Earth—its name comes from the Greek word for the Sun, Helios.
Why this is interesting
You can never visit a star, yet astronomers know it's made of hydrogen, helium, and a pinch of iron. How? By borrowing a trick from a simple glass prism that splits sunlight into a rainbow—with hidden barcodes.
Read the full explanation
Understanding The Use of Spectroscopy to Determine Chemical Composition of Stars
Imagine shining a flashlight through a prism; white light spreads into a continuous rainbow of colors. Now, if you place a thin gas between the light and the prism, you might see dark lines interrupting the rainbow—like a barcode. These are absorption lines. Every chemical element (hydrogen, helium, iron, etc.) absorbs light at a unique set of wavelengths, creating a distinct pattern of lines. This is the element's fingerprint. When we point a spectroscope at a star, we capture its light and spread it into a spectrum. The dark lines in the star's spectrum are 'missing colors' that were absorbed by atoms in the star's outer layers. By matching these line patterns to known elements in the lab, astronomers can identify which elements are present in the star. The process is like decoding a cosmic barcode.
A deeper explanation
The mechanism behind spectroscopy lies in quantum physics. Electrons in atoms occupy specific energy levels. When a photon of light with energy exactly matching the gap between two levels hits an atom, it can be absorbed, jumping the electron to a higher level. This removes that precise wavelength from the light we observe, creating a dark line. Conversely, when an excited electron falls back, it emits a photon of the same wavelength, producing a bright emission line. In a star's atmosphere, a continuous spectrum from the hot interior passes through cooler outer gases, producing absorption lines. The pattern of lines is unique to each element because no two elements have identical electron energy-level structures. By analyzing the wavelengths and intensities of these lines, astronomers determine composition, temperature, density, and even motion (via Doppler shifts) of stars. This technique is fundamental to our understanding of stellar chemistry and the origin of elements in the cosmos.