Seeing the stars: Observational astronomy in practice
How do astronomers observe the stars? This article provides an overview of some of the tools that astronomers use to understand more about the Universe and make scientific discoveries. First, I'll provide a background about what light is and how telescopes allow us to measure it. Then, we will take a look at some of the observational techniques that have provided breakthroughs in different fields.
Light and telescopes
The physics of light
Stars typically fuse hydrogen into helium in their cores. This process results in the generation of large amounts of energy that is eventually emitted from the surface of the star as light. Light can be thought of as small, individual packets of energy, which we call photons. Photons are particles with zero mass, that can travel enormous distances throughout the Universe at a finite speed – the speed of light – which effectively is the speed limit of the Universe.
Photons are elemental particles, which means that they have no parts to be broken up into and the energy of a photon is determined by its wavelength. Shorter wavelength photons have higher energies.
The electromagnetic spectrum of light ranging from highest energy gamma ray photons to the lowest energy radio photons. The energy that a photon has is determined by its wavelength so gamma rays have very short wavelengths and radio waves have very long wavelengths. Our eyes can detect photons in the visible range from the higher energy blue photons to lower energy red photons.
The role of telescopes
When a photon emitted from the surface of a star makes the journey through the Universe and reaches the Earth, we have various tools with which to observe it. Our eyes allow us to see some of the photons from the closest and brightest stars, but there is a lot more that our eyes can't see.
Telescopes increase the sensitivity and resolution of an image compared with what can be achieved with our eyes. Sensitivity refers to how many photons can be detected, and resolution is the sharpness of what can be detected. To do this, telescopes use a system of optical elements to focus light into our eyes or on to a detector.
An example of the light path of a reflecting telescope. The primary optical element is a mirror (shaded in grey on the right). In this design, an angled secondary mirror (in the centre marked with a hashed line) is used to direct light from the primary mirror into a lens where it can be observed in focus.
The number of photons a star emits is quantifiable. Clearly, with our eyes, we are unable to directly count the number of photons we detect from a given object and we are unable to provide an accurate record of what we have detected. This is the job of astronomical instrumentation and, in particular, detectors.
When photons interact with matter, a specific amount of energy is transferred, which can be measured. Detectors in astronomical instrumentation are typically semi-conducting diodes, where an incoming photon, with enough energy, excites an electron from the semi-conductor to produce a detectable current. This current is then measured – the intensity of the current is directly related to the number of incoming photons.
The colours of stars
When astronomers take images of the sky, typically one of a number of colour filters is selected, which effectively pre-selects the photon wavelengths that are allowed to reach the detector. This is called photometry or imaging and results in the fantastic pictures of astronomical objects that we so regularly see.
Stars emit a continuum of photons at different wavelengths; the amount of photons emitted at different wavelengths allows us to determine the temperature of the surface of the star. If a star appears bluer, this means it emits more blue photons (“bluer” photons = higher energy) and has a high surface temperature whereas if a star emits more red photons, it appears redder and has a lower surface temperature.
Stars are observed to have a range of different colours and brightnesses, depending on their mass and their stage of life. Looking at the constellation Orion and comparing the colours of the stars either side of Orion’s belt (Betelgeuse towards the head and Rigel towards the foot) is a great way to get acquainted with the different colours of stars.
We can tell a lot about a star by observing the presence or lack of photons at different wavelengths. By dispersing the light from a star, using an optical element such as a prism, we can split the light into different wavelengths and observe their intensities. This is an extremely powerful technique, which is called spectroscopy.
When starlight is dispersed into its constituent parts, a series of dark lines are observed on top of a continuum (this continuum is what we see as rainbows). These lines represent the presence of different chemical elements in the atmospheres of the star and together they act like a stellar barcode. Each element in the periodic table has a unique set of lines and, by interpreting these codes, we can determine many fundamental properties of stars, galaxies or planets.
The barcode of the Sun. When the light from a star is passed through a spectrograph, we observe a range of dark absorption lines that allow us to identify different chemical elements present on the surface of the star. Credit: NOAO/AURA/NSF.
In general, stars rotate around the centre of a galaxy. Just like planets orbiting the Sun, the Sun orbits the centre of our Galaxy every ~250 million years. On Earth we observe the stars moving from east to west during the course of the night because of the rotation of the Earth, but their movements relative to each other are much more complicated.
By observing what is known as a ‘Doppler shift’ in the spectroscopic barcodes, which is like hearing a siren change frequency as it moves, we can determine how fast an object is moving towards or away from us. Measuring these radial velocities is a widely used technique that is used to study a huge variety of interesting objects from discovering planets around other stars to measuring the acceleration of the expansion of the Universe.
In addition to stars moving directly towards or away from us, we can also use photometry to observe the positions of stars changing in the sky with respect to background stars. By observing the tiny changes in the precise positions of stars over the course of the Earth’s cycle around the Sun, we can determine in which direction stars are moving.
By combining these two types of motion – which is to say, combining photometric and spectroscopic observations – we can build a three-dimensional picture of how stars move, which is exactly the technique used to study stars orbiting the supermassive black hole at the centre of our Galaxy, a key part in the 2020 Nobel prize for physics.
A simulation of the motion of stars around the supermassive black hole at the centre of our Galaxy. Credit: ESO/L. Calçada/spaceengine.org
Gravitational waves: seeing without light
Until recently, studying the light emitted from astrophysical objects was the only tool available to astronomers. The recent direct discoveries of gravitational waves marks an entirely new phase in the field of astrophysics, which does not rely on detecting light. Gravitational waves are produced when two compact objects (such as black holes or neutron stars) merge together, resulting in a ripple in the fabric of the Universe. These detections are remarkable, as they represents an entirely different form of observations, which means that we learn about physical processes that we cannot observe with light.
The experimental setup used to detect gravitational waves, which uses a laser to detect tiny changes in the distances of the two arms of the detector. Credit: ©Johan Jarnestad/The Royal Swedish Academy of Sciences.
Piecing it all together
By using the available observational tools to gather information, like a detective gathering clues, we can begin to understand the physical processes that produce the dizzying array of astrophysical objects observed in the sky. All different astronomical discoveries – such as tracking the motion of stars orbiting black holes, measuring the acceleration of the expansion of the Universe and detecting planets around different stars – rely on our ability to accurately detect and measure photons using variations on the basic techniques of photometry and spectroscopy, or a combination of both. These observations provide us with the tools required to make fascinating discoveries and the direct detection of gravitational waves represent an entirely new type of observations that can be used to study the Universe.
Dedicated to the memory of Lance corporal Dean Ashworth, a great friend that was taken too soon.
Edited by Tide Services.