## 23.4 Spectra of Light This section explores how astronomers analyze light from celestial objects to determine their composition and motion, focusing on absorption and emission spectra, redshift, and its implications for an expanding universe. ### Astronomical Investigation through Light Astronomers investigate celestial objects primarily by analyzing the light they emit. This light carries information about the object's composition, temperature, and motion. ### Spectral Signatures Every element possesses a unique set of absorption and emission lines, known as a **spectral signature**. When light from stars passes through gases, elements in those stars (primarily hydrogen and helium) absorb specific wavelengths, creating characteristic lines in the spectrum. Astronomers compare these signatures with known elements to determine the composition of celestial objects. The absorption and emission spectra of an element are inverses of each other, meaning their lines occur at the same wavelengths. ### Absorption Spectra Absorption spectra occur when light passes through a cool gas. Atoms and molecules in the gas absorb certain colors (wavelengths), resulting in a rainbow with dark absorption lines against the continuous spectrum. ### Emission Spectra Emission spectra occur when a hot low-density gas glows. The gas gives off very specific colors, producing an emission spectrum with bright emission lines on a dark background. ### Spectra of Light from Stars and Galaxies Light from distant galaxies shows differences in spectral lines compared to light from the Sun. These lines appear shifted toward the red end of the spectrum, indicating an increase in wavelength, while maintaining the same characteristic pattern. This phenomenon is known as **redshift**. ## Redshift and Speed ### Redshift Redshift is observed in almost all galaxies, where spectral lines are shifted toward longer (redder) wavelengths. The cause of redshift is the expansion of space between Earth and the galaxies, which stretches out the light waves traveling through space. ### Blueshift If an object is moving toward Earth, the spectral lines shift toward shorter (bluer) wavelengths. This is known as **blueshift**. ### Doppler Effect The Doppler Effect refers to the apparent change in wavelength or frequency of radiation due to the relative motion of the source toward or away from the observer. For light: - Redshift occurs if the object is moving away from Earth, resulting in an increase in wavelength. - Blueshift occurs if the object is moving toward Earth, resulting in a decrease in wavelength. ### Redshift Formula The relationship between redshift and speed of recession is given by: $$\frac{\Delta \lambda}{\lambda} = \frac{v}{c}$$ Where: - $\Delta \lambda$ is the shift in wavelength - $\lambda$ is the wavelength emitted from the source - $v$ is the speed of recession - $c$ is the speed of light in a vacuum ## Expanding Universe The widespread observation of Doppler redshift in almost all galaxies led to the conclusion that the universe is expanding. The expansion of space stretches light waves, causing redshift. The faster a galaxy recedes (indicated by greater redshift), the faster it is moving away from Earth. ### Balloon Analogy The expansion of the universe can be visualized using dots marked on the surface of an inflating balloon. As the balloon inflates, each dot moves away from every other dot. Similarly, galaxies move apart in an expanding universe. | Concept | Description | Formula | | :--- | :--- | :--- | | **Redshift** | Fractional increase in wavelength due to recession | $\frac{\Delta \lambda}{\lambda} = \frac{v}{c}$ | | **Doppler Effect** | Apparent change in wavelength/frequency due to relative motion | | The analysis of light spectra and the phenomenon of redshift are fundamental tools in astrophysics, enabling scientists to understand the composition, motion, and evolution of the universe.