# Identifying Elements Within Stars Using Spectral Lines ## How Light from Stars Reveals Their Composition - Photons produced by fusion reactions in a star's core travel outward through the layers of gas in the outer atmosphere - These photons initially contain all frequencies of the electromagnetic spectrum - known as a **continuous spectrum** - Gas atoms in the outer atmosphere absorb photons at specific energies, causing electrons to become excited - After excitation, atoms re-emit photons of various frequencies, but in random directions rather than the original path ## Spectroscopy and Stellar Analysis - The light from a star can be analysed using **spectroscopy** - Star atmospheres are not hot enough to produce an emission line spectrum - Instead, stars emit an **absorption line spectrum**

An absorption line spectrum consists of dark lines on top of a continuous spectrum, representing frequencies absorbed by the stellar atmosphere

## Formation of Absorption Spectra - An absorption line appears in a spectrum when an absorbing material is placed between a source and the observer - Think of it like a filter removing specific colours from white light as it passes through - The dark lines represent the frequencies or wavelengths absorbed by the gas medium - This is the equivalent of an emission line spectrum, but with dark lines instead of bright ones on a continuous background ## The Fingerprint of Elements - Each gas produces a **unique pattern of spectral lines** - This uniqueness arises because of specific transitions between an element's discrete energy levels - No two elements have identical energy level structures, so their spectral patterns are always distinct - The presence of spectral lines in a star's absorption spectrum acts as **fingerprints** - These fingerprints can be used to determine the presence of specific elements within the star

Even extremely distant stars can have their chemical composition investigated through spectral line analysis

## Identifying Elements in Stellar Atmospheres - If an element is present in the star, its characteristic pattern of spectral lines will appear as dark lines in the absorption spectrum - By comparing the observed absorption lines to known emission line spectra of elements, astronomers can identify which elements are present - Each element has been catalogued with its unique spectral signature in laboratories on Earth ## Example: The Sun's Composition - The Sun is predominantly made up of hydrogen and helium gas - The chemical composition can be verified by comparing emission line spectra of these gases with the Sun's absorption spectrum ### Hydrogen Spectral Lines The hydrogen emission line spectrum includes prominent lines at: - 410 nm - 434 nm - 486.3 nm - 656.5 nm ### Helium Spectral Lines The helium spectrum includes lines at: - 447 nm - 471 nm - 587.7 nm ### Visualizing the Unique Patterns The chart below shows the wavelength positions of spectral lines for hydrogen and helium. Notice how each element has a distinct pattern - this is the "fingerprint" that allows astronomers to identify elements in stars:

Hydrogen lines (upper row, y = 2) | Helium lines (lower row, y = 1)

## Practical Application - By matching the wavelengths of dark absorption lines in a star's spectrum to known emission wavelengths of elements - Astronomers can create a complete inventory of elements present in the star's atmosphere - This technique works regardless of the star's distance from Earth - The method relies only on the light received, not on physical proximity to the star

Pay attention to the spacing between spectral lines when comparing spectra, as this is key to correctly identifying similar elements

## Summary Table: Spectrum Types | Spectrum Type | Appearance | Source | |--------------|------------|--------| | Continuous | All wavelengths present, unbroken band of colour | Hot, dense sources (star cores) | | Emission Line | Bright coloured lines on dark background | Hot, low-pressure gases | | Absorption Line | Dark lines on continuous spectrum | Light from hot source passing through cool gas | ## Key Points - Stars produce absorption line spectra because photons from the hot core pass through cooler outer atmospheric gases - Atoms of different elements absorb energy only at particular wavelengths corresponding to their energy level differences - These particular frequencies appear as dark lines in the otherwise continuous spectrum - The unique pattern of dark lines serves as an elemental fingerprint - Comparing observed absorption spectra to laboratory emission spectra allows identification of stellar composition - This technique is fundamental to understanding what distant stars are made of without direct sampling