Analyzing a star's light spectrum to calculate distance is primarily done through a method called spectroscopic parallax (also known as spectroscopic distance). Despite its name, this method does not involve measuring tiny stellar shifts (geometric parallax), but rather uses the spectrum to determine a star's intrinsic brightness. By comparing this known intrinsic brightness to how dim the star appears from Earth, astronomers can calculate its distance using the inverse square law of light (more below on this). 

​

1. The Spectrum is anaylised (Stellar Classification)

Light is collected from a star and passed through a spectrograph which splits it into its component colours (spectrum). This spectrum shows up the absorption lines, which is essentially a fingerprint of the star's atmosphere, this reveals its temperature and chemical composition. 

 

• Spectral Type: The spectrum tells us the star's surface temperature, classifying it into types (O, B, A, F, G, K, M).

• Luminosity Class: The widths and details of these spectral lines also reveal the star's density and size (dwarf, giant, supergiant). 
  
  
  
  
  
   

2. Determine Absolute Luminosity

Using the spectral type (temperature) and luminosity class (density), astronomers place the star on a Hertzsprung-Russell (H-R) diagram. 

 

Lumen Learning

• The H-R diagram shows that stars of certain types and sizes have a specific, known absolute magnitude (true intrinsic brightness). For instance, a G2V star (explained further below) like our Sun is assumed to have the same luminosity as the sun. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
   

3. Measure Apparent Brightness

Next, astronomers measure the apparent brightness (how bright the star appears from Earth—using a telescope). 
 

4. Calculate Distance

With both the intrinsic brightness (luminosity) and the apparent brightness known, the distance is calculated using the inverse square law of light:










 

The inverse square law of light states that the intensity or brightness of light decreases rapidly as distance from the source increases. Specifically, if you double the distance, the light becomes one-quarter as bright, not half. It works because light spreads spherically; the same energy must cover a larger area as it travels. 

 

• By comparing the observed brightness to the known absolute luminosity, astronomers can determine how much the light has dimmed, which corresponds to its distance.
   

• Summary of Key Concepts

• Spectroscopic Parallax: The specific method of using spectral lines to infer luminosity to find distance.

• Main Sequence Fitting: A sub-method that involves fitting a group of stars (like a cluster) to the "main sequence" on an H-R diagram to find their average distance.

• Range Limitations: While this method can measure stars out to thousands of parsecs—much further than geometric parallax—it is less precise than direct geometric measurements and must be calibrated using closer, well-measured stars. 
   

This technique acts as a vital step on the "cosmic distance ladder," allowing astronomers to measure distances to stars in the Milky Way that are too far away for geometric parallax to work. 



G2V Star Classification

A G2V star is a classification used in the stellar spectral classification system to describe a star’s temperature and luminosity.

The label has two parts:
 

1. Spectral Type: G2

This describes the surface temperature and color of the star.

• G = A yellow star with a surface temperature of about 5,300–6,000 K
   

• 2 = A subdivision within the G class indicating a specific temperature within that range
   

Characteristics of G-type stars:

• Color: yellow
   

• Surface temperature: about 5,500–6,000 K
   

• Spectra show strong absorption lines from metals (like iron and calcium)
   

2. Luminosity Class: V

The Roman numeral describes the star’s size and evolutionary stage.

• V = Main-sequence star (a dwarf star)
   

• These stars generate energy by fusing hydrogen into helium in their cores
   

Luminosity classes include:

• I – Supergiants
   

• III – Giants
   

• V – Main-sequence (dwarf) stars
   

Example

The Sun is classified as a G2V star.

This means:

• It is a yellow star
   

• Surface temperature ≈ 5778 K
   

• It is a main-sequence star currently fusing hydrogen
   

Simple interpretation

A G2V star is essentially a Sun-like star:
a yellow, medium-temperature main-sequence star.

​