How do we use light from distant stars and galaxies to learn about the universe and its history?
Cosmology: using the electromagnetic spectrum and telescopes to study the universe, line spectra as a fingerprint of elements, and redshift as evidence that the universe is expanding.
An SQA National 5 Physics answer on cosmology, covering how the different bands of the electromagnetic spectrum and the telescopes that detect them are used to study the universe, how line spectra act as a fingerprint to identify elements in stars, and how the redshift of light from distant galaxies is evidence that the universe is expanding.
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What this key area is asking
The SQA wants you to describe how the electromagnetic spectrum and telescopes are used to study the universe, explain how line spectra identify the elements in stars, and explain how the redshift of distant galaxies is evidence that the universe is expanding.
Studying the universe with the whole spectrum
For example, cool dust clouds glow in the infrared, very hot gas around black holes emits X-rays, and the cosmic microwave background (a faint glow left over from the early universe) is detected as microwaves. Many telescopes for these bands are placed on satellites above the atmosphere, which links back to space exploration.
Line spectra: a fingerprint of the elements
To find out what a star is made of, astronomers pass its light through a prism or grating to spread it into a spectrum, then compare the pattern of lines with the line spectra of elements measured in the laboratory. A match means that element is present in the star. This is how we know that hydrogen and helium are the most common elements in stars, even though we can never visit one.
Redshift and the expanding universe
The size of the redshift can be linked to how fast a galaxy is moving away, using the simple relationship below, which compares the change in wavelength with the original wavelength.
Because more distant galaxies are moving away faster, running the expansion backwards suggests everything was once together. This is the basis of the Big Bang model of the origin of the universe.
Try this
Q1. Name two bands of the electromagnetic spectrum, other than visible light, used to study the universe. [1 mark]
- Cue. Any two of radio, microwave, infrared, ultraviolet, X-ray, gamma.
Q2. State what a line spectrum tells astronomers about a star. [1 mark]
- Cue. Which elements are present, because each element has a unique pattern of lines.
Q3. A line at rest wavelength is observed at . Calculate the redshift. [2 marks]
- Cue. .
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksExplain how line spectra can be used to identify the elements present in a distant star.Show worked answer →
Each element gives out (and absorbs) light at its own particular set of wavelengths, producing a unique line spectrum that acts like a fingerprint.
Astronomers split the light from a star into a spectrum and compare the pattern of lines with the known line spectra of elements measured in the laboratory.
When the pattern of lines matches that of a known element, that element must be present in the star. Markers reward the idea of a unique set of wavelengths per element, comparing with laboratory spectra, and matching the pattern to identify the element.
SQA N5 style3 marksThe light from distant galaxies is found to be redshifted. State what redshift means and explain what it tells us about the universe.Show worked answer →
Redshift means the light from the galaxy is shifted towards the red (longer wavelength) end of the spectrum compared with the same light measured on Earth.
This happens because the galaxy is moving away from us, which stretches the wavelength of the light it emits.
Because nearly all distant galaxies show redshift, and more distant ones are more redshifted, this is evidence that the universe is expanding. Markers reward defining redshift as a shift to longer wavelength, linking it to galaxies moving away, and concluding that the universe is expanding.
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Sources & how we know this
- SQA National 5 Physics Course Specification — SQA (2019)