02 Observing the Universe

Light as evidence

Astronomers usually cannot visit the objects they investigate, so they study the radiation that reaches Earth or a spacecraft. The radiation's brightness, direction, wavelength, and changes over time provide evidence about an object's composition, temperature, and motion.

Light is electromagnetic radiation that behaves both as waves and as particles called photons. A wave's wavelength $\lambda$ and frequency $\nu$ are related by

where $c$ is the speed of light. A photon's energy is

where $h$ is Planck's constant. Therefore, shorter-wavelength light has higher frequency and more energetic photons.

Visible light is only a small part of the electromagnetic spectrum. From longest wavelength and lowest photon energy to shortest wavelength and highest energy, its main regions are radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma-ray. Different regions reveal different processes: infrared can show relatively cool objects and radiation from dust, ultraviolet can trace hot stars and gas, X-rays often reveal extremely hot gas, and radio observations can detect emissions from atoms, molecules, and energetic particles.

Takeaway: No single wavelength gives a complete picture of an astronomical object.

Telescopes collect and focus light

A telescope gathers more light than the unaided eye and directs it toward an instrument. A larger collecting area captures more photons, which helps astronomers study faint objects. Telescope size also affects angular resolution: the ability to distinguish two nearby points as separate.

For an ideal circular aperture, the diffraction limit is approximately

Here, $\theta$ is the angular resolution, $\lambda$ is the observed wavelength, and $D$ is the aperture diameter. A longer wavelength or a smaller aperture generally produces lower resolution. Interferometry combines observations from separated telescopes to improve resolution, but it does not provide the collecting area of a single dish spanning the same distance.

Telescope design depends on the radiation being observed. Optical and infrared telescopes commonly use mirrors to focus light. Radio telescopes typically use large dishes to collect radio waves and direct them to sensitive receivers. X-ray telescopes use special optics that guide X-rays at shallow angles, while gamma-ray instruments generally record interactions rather than focusing the rays with ordinary mirrors.

Takeaway: Collecting area helps reveal faint objects, while aperture and wavelength affect the detail a telescope can distinguish.

From incoming light to data

Detectors turn incoming radiation into measurements. The eye detects visible light, but astronomical instruments can measure wavelengths the eye cannot see and signals too faint for direct viewing.

A camera sensor divides an image into pixels. In a charge-coupled device (CCD), incoming photons create electrical charges in the pixels. Electronics read the charges and convert them into digital measurements. Other wavelength bands require detectors and receivers suited to their radiation. Some infrared instruments are cooled to limit unwanted heat signals.

Astronomers can also record how a source's brightness changes over time. This record is called a light curve. For example, a repeating dip in a star's brightness can be evidence that an orbiting planet passes in front of the star. Images, spectra, and light curves are different ways to record and study incoming light.

Takeaway: Detectors make signals measurable, and the form of the data depends on what astronomers want to observe.

Reading spectra

A spectrum shows how much light an object emits or absorbs at different wavelengths. A spectrograph separates incoming light, using components such as a prism or diffraction grating, and records the resulting pattern with a detector.

Three common patterns are:

• A continuous spectrum spreads across a broad range of wavelengths, as light from a hot, dense source can do.

• An emission spectrum has bright lines at particular wavelengths, produced when atoms or molecules emit light.

• An absorption spectrum has dark lines where material between the source and observer has absorbed light.

Each element and molecule produces a characteristic pattern of spectral lines, allowing astronomers to identify chemical composition. The overall spectrum also provides clues to temperature and physical conditions. When spectral lines shift toward longer or shorter wavelengths, the Doppler effect can reveal motion along the line of sight.

Takeaway: Spectral patterns reveal what an object is made of and can provide clues about its conditions and motion.

Observing across wavelengths

Earth's atmosphere protects life but also blocks or absorbs much of the radiation arriving from space. Most visible light and some radio waves reach the ground; many ultraviolet, X-ray, and gamma-ray wavelengths do not. Some infrared wavelengths are also strongly absorbed.

Astronomers place telescopes on high mountains, in balloons, or in space depending on the wavelength and observation needed. Observations in different wavelength bands complement one another. Visible light can show stars and glowing gas in a nebula, while infrared observations can reveal cooler material and regions obscured by dust. Combining these views is called multiwavelength astronomy, and it helps build a fuller account of the same object.

Takeaway: Specialized observing locations and instruments are needed because the atmosphere transmits some wavelengths and blocks others. Combining wavelength bands reveals features that one view alone may miss.