Spectroscopic Parallax: Definition & Uses

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  • 0:02 Spectroscopic Parallax
  • 1:09 Betelgeuse
  • 1:43 Apparent & Absolute Magnitude
  • 2:51 The Magnitude Distance Formula
  • 4:47 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
How can you find the distance to a star? One way is to use the spectroscopic parallax to approximate the distance to a star using two key concepts in astronomy: apparent and absolute magnitude.

Spectroscopic Parallax

Little Timmy was walking in the woods near his home one day, a home located close to a secret military base. He saw a piece of paper sticking up out of the ground. It looked really old and ragged, so he carefully dug it up out of the ground.

On the piece of paper it read:

Dec. 1998
Beetlejuice is actually brighter than it looks today.
Don't let it fool you.
Use HRD to find out how far away it is so we know how much fuel to borrow from these pesky people.

Little Timmy stopped for a moment; after all, the paper told him to do so many times. He just stood there. He was unsure of what all this meant. But his Dad, a certain Jesse Marcel, was an astronomer and had a hunch as to what all of this meant. Jesse thought this was a vague statement having something to do with spectroscopic parallax, a method that helps approximate the distance to a star using apparent and absolute magnitudes.


First, Jesse explained to his son that Beetlejuice was likely a code name for Betelgeuse, a distant red supergiant, a kind of really big star.

'Why is Beetlejuice the code name for it?' asked little Timmy.
'Because it was the name of a popular movie the year the note was written. I actually have a VHS tape of it,' said his Dad.
'What's a VHS tape?' asked little Timmy.
'Never mind,' said his Dad.

Apparent and Absolute Magnitude

Jesse went on to explain that Betelgeuse is actually one of the brightest stars in the night sky. However, the brightness of a star as it appears to us on Earth, its apparent magnitude, is different than the true brightness of a star, its absolute magnitude. That's why the code says that 'Beetlejuice' is brighter than it appears to us because starlight dims with distance.

We can record how bright a star appears to us using precise instruments and then astronomers can use a treasure map of the sky, called the Hertzsprung-Russell Diagram (HRD) to help figure out the star's absolute magnitude. From there, we can approximate the distance to the star in question and whoever wrote the note would then be able to figure out how much fuel they need to reach the real Betelgeuse.

'Ready to figure it out with me, Son?'
'No STOP! Please STOP!' cried little Timmy.
'I think we should, Timmy boy.'

The Magnitude Distance Formula

First, they take a star's spectrum, like a fingerprint of a star, to determine its spectral class. On this treasure map, the spectral class tells us the star's horizontal location on the H-R diagram. Based on this, we know Betelgeuse is a class M2 star.

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