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.
Get the method right under pressure
Free interactive practice on the steps that lose marks under exam pressure.
Start revising freeWhat you'll cover
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.
Which quantity behaves like this
- Apparent magnitude
- Absolute magnitude
- Parallax
- Stellar spectrum
- Move the star closer and this number changes, though the star has not
- Move the star anywhere you like and this number stays put
- Found by photographing the same star six months apart
- Dark lines in it tell you which elements the star contains
Work out the absolute magnitude
A star has apparent magnitude m = 6 and lies at a distance d = 100 parsecs. Using the distance modulus M = m + 5 - 5 log d, and taking log 100 = 2, what is its absolute magnitude M?
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
Astronomers class stars by temperature using the letters O B A F G K M. Put these classes in order from the HOTTEST to the coolest.
- O: the hottest, blue-white stars
- B: hot, blue stars
- A: white stars
- G: yellow stars, like our Sun
- M: the coolest, red stars
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?
Tap the TWO statements that are TRUE about the Hertzsprung-Russell diagram.
- Most stars, including the Sun, lie on the main sequence.
- On the magnitude scale, bigger numbers always mean brighter stars.
- White dwarfs are hot but faint, in the lower part of the diagram.
- Every star has exactly the same brightness and temperature.
- The diagram only shows the planets of the Solar System.
Read the starlight
Use what you know to interpret each observation.
- A star appears deep red. What does its colour tell you?
- Two stars look equally bright, but one is far more distant. Which has the greater true brightness?
- Astronomers want the distance to a nearby star. Which method fits best?
Explain the HR diagram
Explain what the Hertzsprung-Russell diagram shows, and how a star's magnitude and spectrum are used to study it.
- State what the two axes of the HR diagram represent
- Name two groups of stars shown on the diagram
- Explain the difference between apparent and absolute magnitude
- Explain one thing a stellar spectrum reveals