Introduction

Most of what astronomers know about stars, galaxies, and nebulae comes not from travelling to them, but from studying the light they emit. By carefully analysing this light, scientists can determine a star's composition, temperature, motion, and many other properties. This powerful technique is called spectroscopy.

Spectroscopy works because every chemical element interacts with light in its own unique way. When light is separated into its different wavelengths, it forms a spectrum. The patterns found in these spectra act like fingerprints, allowing astronomers to identify the elements present in distant objects—even those billions of light-years away.

---

What Is Spectroscopy?

Spectroscopy is the study of how matter interacts with electromagnetic radiation.

In astronomy, spectroscopy is used to analyse the light from:

• Stars.
• Galaxies.
• Nebulae.
• Planets.
• Comets.

By studying a spectrum, astronomers can learn about an object's:

• Chemical composition.
• Temperature.
• Motion.
• Density.
• Magnetic fields.

---

Figure 1. Spectroscopy separates light into its component wavelengths to reveal information about astronomical objects.

---

What Is a Spectrum?

A spectrum is the range of wavelengths or colours that make up light.

When white light passes through a:

• Prism, or
• Diffraction grating,

it spreads into its different wavelengths, producing a spectrum.

Different light sources produce different kinds of spectra.

---

Types of Spectra

There are three main types of spectra:

• Continuous spectrum.
• Emission spectrum.
• Absorption spectrum.

Each provides different information about the source of the light.

---

Continuous Spectrum

A continuous spectrum contains all wavelengths of visible light without interruption.

It is produced by:

• Hot, dense solids.
• Hot liquids.
• Dense gases.

Examples include:

• The glowing filament in an incandescent light bulb.
• The dense interior of a star.

A continuous spectrum appears as a smooth rainbow of colours.

---

Emission Spectrum

An emission spectrum consists of bright coloured lines on a dark background.

It is produced by:

• Hot, low-density gases.

Each bright line corresponds to a specific wavelength emitted by an element.

Emission spectra help identify the elements present in glowing gases, such as those found in nebulae.

---

Absorption Spectrum

An absorption spectrum consists of dark lines crossing a continuous spectrum.

It is produced when:

• Light from a hot, dense source passes through a cooler gas.

The cooler gas absorbs specific wavelengths, leaving dark lines in the spectrum.

Most stars, including the Sun, produce absorption spectra.

---

Figure 2. Continuous, emission, and absorption spectra each reveal different information about light sources.

---

Spectral Lines

The bright or dark lines seen in emission and absorption spectra are called spectral lines.

Each spectral line corresponds to a specific wavelength of light.

Spectral lines form because:

• Electrons in atoms can only occupy certain energy levels.
• Electrons absorb or emit light when moving between these energy levels.

This produces a unique pattern of lines for each element.

---

Why Every Element Has a Unique Spectrum

Each element has:

• A different number of protons.
• A unique arrangement of electrons.
• Different energy levels.

Because the energy levels are unique:

• Each element absorbs and emits different wavelengths of light.

This creates a unique set of spectral lines called its atomic spectrum.

Scientists often describe this as an element's spectral fingerprint.

---

Figure 3. Every element produces a unique pattern of spectral lines that can be used for identification.

---

Identifying Elements

Astronomers compare the observed spectrum of a star with laboratory spectra of known elements.

For example:

• Hydrogen produces one pattern.
• Helium produces another.
• Sodium has its own distinctive lines.
• Calcium and iron also produce unique patterns.

If the patterns match, the element is present in the star.

Using this method, astronomers have identified dozens of elements in stars and galaxies.

---

Spectroscopy and Stars

Spectroscopy allows astronomers to determine:

Chemical Composition

Which elements are present.

---

Temperature

Hotter stars produce different spectra than cooler stars.

---

Motion

The Doppler effect shifts spectral lines.

• Redshift → object moving away.
• Blueshift → object moving toward Earth.

---

Density

The width and shape of spectral lines provide information about the density of the gas.

---

Magnetic Fields

Strong magnetic fields can slightly split spectral lines, allowing astronomers to detect magnetic activity.

---

Figure 4. A star's spectrum reveals its composition, temperature, and motion.

---

Spectroscopy Beyond Stars

Spectroscopy is also used to study:

• Galaxies.
• Nebulae.
• Exoplanet atmospheres.
• Comets.
• Planetary atmospheres.
• Interstellar gas clouds.

By analysing light from these objects, astronomers learn about their composition and physical conditions without visiting them.

---

Why Spectroscopy Is Important

Spectroscopy has helped scientists:

• Discover helium before it was found on Earth.
• Measure the expansion of the universe.
• Detect planets orbiting distant stars.
• Study the atmospheres of exoplanets.
• Investigate the life cycles of stars.

It is one of the most powerful tools in modern astronomy.

---

Figure 5. Spectroscopy allows astronomers to study distant objects by analysing their light.

---

Worked Example

Question

A star's spectrum contains dark absorption lines that exactly match the known spectral lines of hydrogen.

What can astronomers conclude?

Solution

The dark absorption lines show that hydrogen is present in the cooler outer layers of the star.

Because every element has a unique pattern of spectral lines, matching the hydrogen pattern confirms the presence of hydrogen.

---

Real-World Connection

Astronomers use spectroscopy to study the atmospheres of exoplanets as they pass in front of their parent stars. During a transit, a small amount of starlight passes through the planet's atmosphere before reaching Earth. Different gases absorb specific wavelengths of light, producing characteristic absorption lines. By analysing these lines, scientists can detect substances such as water vapour, carbon dioxide, methane, and sodium, helping them investigate whether distant planets could support life.

---

Did You Know?

The element helium was first discovered in 1868 by astronomers studying the Sun's spectrum during a solar eclipse. They observed a yellow spectral line that did not match any known element on Earth. This new element was named helium, after Helios, the Greek god of the Sun. It was only discovered on Earth nearly 30 years later.

---

Key Terms

Absorption spectrum – A spectrum containing dark lines where specific wavelengths have been absorbed by a cooler gas.

Continuous spectrum – A spectrum containing all wavelengths without interruption.

Diffraction grating – An optical device that separates light into its component wavelengths.

Emission spectrum – A spectrum containing bright lines produced by excited atoms or ions.

Spectral line – A bright or dark line at a specific wavelength in a spectrum.

Spectrum – The range of wavelengths produced when light is separated into its component colours.

Spectroscopy – The study of the interaction between matter and electromagnetic radiation, especially through the analysis of spectra.

---

Key Takeaways

• Spectroscopy is the study of light to determine the properties of astronomical objects.
• A continuous spectrum contains all wavelengths, an emission spectrum contains bright lines, and an absorption spectrum contains dark lines.
• Spectral lines form because electrons absorb or emit specific amounts of energy when moving between energy levels.
• Every element has a unique spectrum, allowing astronomers to identify elements by their spectral lines.
• Spectroscopy reveals the composition, temperature, motion, density, and magnetic fields of stars and other astronomical objects.
• By analysing light alone, astronomers can investigate distant galaxies, nebulae, and exoplanets without leaving Earth.