AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Dimmest visible stars have a magnitude of 6.
Full specification wording
Classification by luminosity (A-level only): Dimmest visible stars have a magnitude of 6.
On the Hipparcos scale, the dimmest stars visible to the unaided eye under good dark-sky conditions are assigned apparent magnitude 6.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
On the Hipparcos scale, the dimmest stars visible to the unaided eye under good dark-sky conditions are assigned apparent magnitude 6. This value marks the faint limit of naked-eye visibility, while brighter stars have smaller magnitudes, including zero and negative values. The scale is logarithmic: a difference of 5 magnitudes corresponds to a brightness ratio of 100, so a magnitude 1 star is 100 times brighter than a magnitude 6 star. A magnitude 6 star is therefore much fainter than a first-magnitude star. Modern telescopes detect objects far fainter than magnitude 6, and the scale extends to larger magnitudes for these. When comparing stars, remember that magnitude 6 is the faintest typically visible without optical aid, not a fixed physical property of the star itself.
What earns the marks
- State that magnitude 6 represents the faintest apparent magnitude typically visible to the unaided eye under good observing conditions.
- Explain that the scale is inverted, so magnitude 6 is fainter than magnitude 1, and brighter objects have smaller or negative magnitudes.
- Apply the logarithmic relationship that a difference of 5 magnitudes corresponds to a brightness ratio of 100, so a magnitude 1 star is 100 times brighter than a magnitude 6 star.
- Recognise that magnitude 6 is a limit of human vision, not an intrinsic property, and that telescopes can detect objects with much larger magnitudes.
Common mistakes and how to put them right
Believing magnitude 6 is the brightest possible value.
Put it right by stating that smaller and negative magnitudes represent brighter objects.
Assuming a magnitude 6 star is six times fainter than a magnitude 1 star.
Put it right by using the logarithmic scale, where 5 magnitudes correspond to a factor of 100 in brightness.
Treating magnitude 6 as an absolute magnitude.
Put it right by identifying it as an apparent magnitude limit for naked-eye visibility.
Exam tips
- Quote magnitude 6 as the approximate naked-eye limit and contrast it with brighter objects that have smaller or negative magnitudes.
- Use the 5-magnitude equals 100-times-brightness relationship to compare a magnitude 6 star with a magnitude 1 star.
- Specify that the limit depends on observing conditions and that optical instruments allow much fainter objects to be seen.
Try the lesson
The start of the Classification by luminosity (A-level only): Dimmest visible stars have a magnitude of 6. lesson, made from checked teaching notes for this specification point.
Have a go first
According to the Hipparcos scale, what is the apparent magnitude of the dimmest stars visible to the unaided eye under ideal dark-sky conditions?
Listen
On the Hipparcos scale, the dimmest stars visible to the unaided eye under good dark-sky conditions are assigned apparent magnitude 6. This value marks the faint limit of naked-eye visibility, while brighter stars have smaller magnitudes, including zero and negative values. The scale is logarithmic: a difference of 5 magnitudes corresponds to a brightness ratio of 100, so a magnitude 1 star is 100 times brighter than a magnitude 6 star. A magnitude 6 star is therefore much fainter than a first-magnitude star. Modern telescopes detect objects far fainter than magnitude 6, and the scale extends to larger magnitudes for these. When comparing stars, remember that magnitude 6 is the faintest typically visible without optical aid, not a fixed physical property of the star itself. A strong answer earns marks for these points. State that magnitude 6 represents the faintest apparent magnitude typically visible to the unaided eye under good observing conditions. Explain that the scale is inverted, so magnitude 6 is fainter than magnitude 1, and brighter objects have smaller or negative magnitudes. Apply the logarithmic relationship that a difference of 5 magnitudes corresponds to a brightness ratio of 100, so a magnitude 1 star is 100 times brighter than a magnitude 6 star. Recognise that magnitude 6 is a limit of human vision, not an intrinsic property, and that telescopes can detect objects with much larger magnitudes. A common slip: Believing magnitude 6 is the brightest possible value. Instead, stating that smaller and negative magnitudes represent brighter objects. Exam tip: Quote magnitude 6 as the approximate naked-eye limit and contrast it with brighter objects that have smaller or negative magnitudes.
Key words
Say what each word means before you turn the card.
Naked-eye visibility limit
Try answering aloud or in your head before turning the card.
Inverted magnitude scale
Try answering aloud or in your head before turning the card.
Logarithmic brightness ratio
Try answering aloud or in your head before turning the card.
Apparent magnitude
Try answering aloud or in your head before turning the card.
See every definition
- Naked-eye visibility limit
- The faint limit of naked-eye visibility is apparent magnitude 6, the dimmest stars visible to the unaided eye under good dark-sky conditions.
- Inverted magnitude scale
- Brighter stars have smaller magnitudes, including zero and negative values, so magnitude 6 is fainter than magnitude 1.
- Logarithmic brightness ratio
- A difference of 5 magnitudes corresponds to a brightness ratio of 100, so a magnitude 1 star is 100 times brighter than a magnitude 6 star.
- Apparent magnitude
- Magnitude 6 is the faintest typically visible without optical aid, a limit of human vision rather than a fixed physical property of the star itself.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Classification by luminosity (A-level only): Dimmest visible stars have a magnitude of 6.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge