Astronomical spectroscopy

Astronomical spectroscopy is a specialized field of astronomy. It uses the techniques of spectroscopy to measure electromagnetic radiation. This radiation comes from stars and other celestial objects. It includes visible light, ultraviolet, X-rays, infrared, and radio waves.

By analyzing these waves, scientists can discover many hidden properties. They can determine a star's chemical composition and temperature. They can also find its density, mass, distance, and luminosity. Spectroscopy even shows the velocity of motion. It does this by measuring the Doppler shift. This shift tells us if an object moves toward or away from an observer.

To understand spectroscopy, we must look at how light is separated. A spectrum is a map of different wavelengths. Historically, scientists used prisms to split light. Isaac Newton first used a simple prism to observe how light refracts. In the early 1800s, Joseph von Fraunhofer created very pure prisms. He discovered 574 dark lines within a continuous spectrum.

Later, J.S. Plaskett developed high-quality reflection gratings in Canada. These gratings use many tiny, parallel mirrors to focus light. Today, scientists also use holographic gratings. These use a thin film of dichromated gelatin on glass. They use a process called Bragg diffraction to reflect light. These gratings can have up to 6000 lines per millimeter. They are much more efficient at collecting light than older tools.

Different types of radiation require different methods for observation. The Earth's atmosphere acts as a barrier for some waves. Ozone and molecular oxygen absorb light with wavelengths under 300 nm. This means X-ray and ultraviolet spectroscopy require satellites or rockets. Infrared light is absorbed by atmospheric water and carbon dioxide. Therefore, satellites are also needed to record much of the infrared spectrum. Radio signals have much longer wavelengths than optical signals. These require antennas or large radio dishes to capture.

Radio astronomy began in the early 1930s with Karl Jansky. He discovered radio noise coming from the center of the Milky Way.

Spectroscopy reveals the chemical secrets of the stars. In the 1850s, Gustav Kirchhoff and Robert Bunsen explained dark lines. They found that hot gases emit light at specific wavelengths. Hot solid objects surrounded by cooler gases show dark absorption lines. By comparing these lines to known gases, we find a star's makeup. For example, the Sun has over 20,000 absorption lines. In 1868, Norman Lockyer and Pierre Janssen found a new element. They called it Helium, though it was not found on Earth until 1895. Other lines were once thought to be a new element called coronium. We now know those lines are actually caused by highly ionized iron.

We can also calculate the physical scale of celestial bodies. Scientists use Wien's Law to find a star's surface temperature. This law relates temperature to the peak emission wavelength. For instance, a star with a peak wavelength of 502 nm has a temperature of 5772 kelvins.

We can also find a star's radius. We do this by knowing its luminosity and its temperature. The luminosity is the total energy output over time. This calculation uses the Stefan-Boltzmann constant to relate these values. These precise measurements allow us to map the size of the universe.

Spectroscopy helps us categorize different groups of stars. Astronomers use stellar populations to organize them. Population I stars are the youngest stars. They contain a high amount of metal content. The Sun is a Population I star. Population III stars are the oldest stars. They have a very low metal content. This helps scientists understand how the universe has changed over time. It also allows us to study the evolution of galaxies.

Finally, spectroscopy connects us to the largest structures in space. It helps us study galaxies and active galactic nuclei. In 1937, Fritz Zwicky studied galaxy clusters. He found they moved faster than expected based on visible light. He hypothesized the existence of dark matter to explain this.

We also use it to study quasars. These are very bright galaxies from the early universe. They are powered by super-massive black holes. Through spectroscopy, we can see the history of the entire cosmos.