Starlight: Magnitude, Spectra and the HR Diagram
A star is too far to visit, but its light gives it away. Decode brightness with magnitude, temperature and chemistry with spectra, and the whole family of stars with the Hertzsprung-Russell diagram.
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Reading the light of the stars ⭐
A star is far too distant to visit, yet its **light** tells us a great deal: how **bright** it truly is, how **hot**, what it is **made of**, and even how it is **moving**. This module covers **magnitude** (brightness), **spectra** (starlight split into colours), and the **Hertzsprung-Russell diagram** that maps the whole family of stars.
Measuring brightness 🔑
Four ideas run through the study of starlight:
Which star is brighter? 🔆
On the magnitude scale, star A has magnitude 1 and star B has magnitude 6. Which star is brighter?
- Star A, because a smaller magnitude means a brighter star
- Star B, because a bigger number means brighter
- They are equally bright
- Magnitude does not tell you brightness
The scale runs backwards 📏
Two things trip people up. First, the **magnitude scale is inverted**: the **smaller** (or more negative) the number, the **brighter** the star. Second, **apparent** and **absolute** magnitude are different. Apparent is how bright a star **looks** from Earth; absolute is how bright it **would look** if every star sat at the same distance of **10 parsecs**. Absolute magnitude lets us compare the true brightness of stars fairly.
Match each term 🔗
- Apparent magnitude
- Absolute magnitude
- Parallax
- Stellar spectrum
- How bright a star looks from Earth
- How bright a star would look at 10 parsecs
- A star's yearly shift, used to find its distance
- A star's light split into colours, revealing its secrets
Work out the absolute magnitude 🔢
An interactive activity.
What does a spectrum tell us? 🌈
A star's light is split into a spectrum. Select the TWO things it can reveal.
- What chemical elements the star is made of
- How hot the surface of the star is
- The name the star was given by astronomers
- Exactly how many planets orbit the star
The family of stars 📊
The Hertzsprung-Russell (HR) diagram plots stars by brightness and temperature. Four groups stand out:
Order the star classes 🪜
An interactive activity.
Hot and cool stars 🔥
A star colour is a direct clue to its temperature.
Complete the picture 🧱
How bright a star looks from Earth is its _____ magnitude, while its brightness at a standard distance of 10 parsecs is its _____ magnitude. On this scale a _____ number means a brighter star. Splitting starlight into a _____ reveals its temperature and composition. Most stars, including the Sun, lie on the _____ sequence of the HR diagram.
True about the HR diagram? 🖍️
An interactive activity.
Read the starlight 🔀
An interactive activity.
Explain the HR diagram ✍️
An interactive activity.