# Stellar Parallax(OCR A Level Physics)

Author

Katie M

Expertise

Physics

## Stellar Parallax

• The principle of parallax is based on how the position of an object appears to change depending on where it is observed from
• When observing the volume of liquid in a measuring cylinder the parallax principle will result in the observer obtaining different values based on where they viewed the bottom of the meniscus from

• Stellar parallax can be used to measure the distance to nearby stars
• Stellar Parallax is defined as:

The apparent shifting in position of a nearby star against a background of distant stars when viewed from different positions of the Earth, during the Earth’s orbit about the Sun

• It involves observing how the position of a nearby star changes over a period of time against a ﬁxed background of distant stars
• To an observer the position of distant stars does not change with time
• If a nearby star is viewed from the Earth in January and again in July, when the Earth is at a different position in its orbit around the Sun, the star will appear in different positions against a backdrop of distant stars which will appear to not have moved
• This apparent movement of the nearby star is called the stellar parallax

## The Parallax Equation

• Applying trigonometry to the parallax equation:
• 1 AU = radius of Earths orbit around the sun
• = parallax angle from earth to the nearby star
• = distance to the nearby star
• So, tan(p)
• For small angles, expressed in radians, tan(p)p, therefore: p
• If the distance to the nearby star is to be measured in parsec, then it can be shown that the relationship between the distance to a star from Earth and the angle of stellar parallax is given by

• Where:
• p = parallax (")
• d = the distance to the nearby star (pc)
• This equation is accurate for distances of up to 100 pc
• For distances larger than 100 pc the angles involved are so small they are hard to measure accurately

#### Worked example

The nearest star to Earth, Proxima Centauri, has a parallax of 0.768 seconds of arc.

Calculate the distance of Proxima Centauri from Earth

1.  in parsec
2.  in light–years

Part (a)

Step 1: List the known quantities

• Parallax, p = 0.768"

Step 2: State the parallax equation

Step 3: Rearrange and calculate the distance d

Part (b)

Step 1: State the conversion between parsecs and metres

• From the data booklet:

1 parsec ≈ 3.1 × 1016 m

Step 2: Convert 1.30 pc to m

1.30 pc = 1.30 × (3.1 × 1016) = 4.03 × 1016 m

Step 3: State the conversion between light–years and metres

• From the data booklet

1 light–year ≈ 9.5 × 1015 m

Step 4: Convert 4.03 × 1016 m into light–years

= 4.2 ly (to 2 s.f)

#### Exam Tip

Make sure you know the units for arc seconds (") and arc minutes (')

• 1 arcminute is denoted by 1'
• 1 arcsecond is denoted by 1"

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