OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Cosmology: (pc)
The abbreviation (pc) stands for the parsec, the distance unit used throughout OCR A Level Physics A cosmology.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 3 of 17
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The abbreviation (pc) stands for the parsec, the distance unit used throughout OCR A Level Physics A cosmology. One parsec is defined as the distance at which one astronomical unit subtends an angle of one second of arc, so 1 pc ≈ 3.09 × 10¹⁶ m, about 3.26 light-years. Because the definition is built from a parallax angle, the parsec links directly to stellar parallax and to the equation d = 1/p. In multiple-choice questions you may be asked to identify the parsec from its definition, to convert between parsecs, metres and light-years, or to choose the correct order of magnitude for a stellar or galactic distance. Always check whether the value given is in pc, kpc or Mpc before selecting an answer.
What earns the marks
- States that pc is the symbol for the parsec, a unit of astronomical distance.
- Defines 1 pc as the distance at which 1 au subtends an angle of 1 second of arc.
- Recalls 1 pc ≈ 3.09 × 10¹⁶ m, equivalently about 3.26 light-years.
- Links the parsec to parallax measurement and to the equation d = 1/p.
- Applies the correct prefix when converting between pc, kpc and Mpc.
Common mistakes and how to put them right
Confusing the parsec with the light-year.
Put it right: The parsec is defined from a parallax angle, whereas the light-year is the distance light travels in one year; 1 pc ≈ 3.26 ly.
Treating 1 pc as the distance to a star with parallax 1 second of arc measured in degrees.
Put it right: The parallax must be in seconds of arc, so a parallax of 1° would give a far smaller distance.
Using the wrong power of ten when converting parsecs to metres.
Put it right: The correct value is about 3.09 × 10¹⁶ m, not 10¹³ m or 10¹⁹ m.
Assuming kpc and Mpc are smaller than a parsec.
Put it right: The prefixes kilo- and mega- make them larger by 10³ and 10⁶ respectively.
Exam tips
- Learn the definition of the parsec in words as well as the numerical conversion, because multiple-choice distractors often test the definition rather than the number.
- Write the conversion factor 1 pc ≈ 3.09 × 10¹⁶ m on your rough working before converting, so you do not lose a power of ten under time pressure.
- Check the prefix in the question stem: a distance in kpc is 10³ pc and a distance in Mpc is 10⁶ pc, so convert to pc before comparing options.
- Eliminate options by order of magnitude first; stellar distances are typically a few pc, while galactic distances are kpc and intergalactic distances are Mpc.
Try the lesson
The start of the Cosmology: (pc) lesson, made from checked teaching notes for this specification point.
Listen
The abbreviation (pc) stands for the parsec, the distance unit used throughout OCR A Level Physics A cosmology. One parsec is defined as the distance at which one astronomical unit subtends an angle of one second of arc, so 1 pc ≈ 3.09 × 10¹⁶ m, about 3.26 light-years. Because the definition is built from a parallax angle, the parsec links directly to stellar parallax and to the equation d = 1/p. In multiple-choice questions you may be asked to identify the parsec from its definition, to convert between parsecs, metres and light-years, or to choose the correct order of magnitude for a stellar or galactic distance. Always check whether the value given is in pc, kpc or Mpc before selecting an answer. A strong answer earns marks for these points. States that pc is the symbol for the parsec, a unit of astronomical distance. Defines 1 pc as the distance at which 1 au subtends an angle of 1 second of arc. Recalls 1 pc ≈ 3.09 × 10¹⁶ m, equivalently about 3.26 light-years. Links the parsec to parallax measurement and to the equation d = 1/p. Exam tip: Learn the definition of the parsec in words as well as the numerical conversion, because multiple-choice distractors often test the definition rather than the number.
See it
Key words
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Astronomical distance units
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Definition of the parsec
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Parallax and distance measurement
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Powers of ten and unit prefixes
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See every definition
- Astronomical distance units
- The parsec is the distance unit used throughout OCR A Level Physics A cosmology, and it can be converted between parsecs, metres and light-years.
- Definition of the parsec
- One parsec is defined as the distance at which one astronomical unit subtends an angle of one second of arc, and 1 pc ≈ 3.09 × 10¹⁶ m, about 3.26 light-years.
- Parallax and distance measurement
- Because the definition is built from a parallax angle, the parsec links directly to stellar parallax and to the equation d = 1/p.
- Powers of ten and unit prefixes
- The prefixes kilo- and mega- make kpc and Mpc larger than a parsec by 10³ and 10⁶ respectively, so the prefix must be checked when converting.
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