What is the difference between extinction and reddening?

In one line

The rule

Extinction is the dimming of light from a star or object as it passes through dust and gas. Reddening is the change in colour of that light, making it appear redder, because dust scatters blue light more than red.

There is no single formula, but astronomers often use: A = m_observed − m_true (for extinction, where A is extinction, m_observed is the observed brightness, m_true is the true brightness).

Let's understand it simply

Imagine you are looking at a streetlight through thick fog. The light looks dimmer because the fog blocks some of it. This is extinction. Now, if you look closely, the light may also look more orange or red than usual. This is because the fog scatters blue light away, letting more red light reach your eyes. This is reddening.

The same thing happens with starlight. When light from a star travels through space, it passes through clouds of dust and gas. The dust blocks some light (extinction) and makes the star look redder (reddening).

How it works

1. Light from a star travels through space.
2. It enters a cloud of dust and gas (interstellar medium).
3. Dust particles scatter and absorb some of the light.
4. Blue light is scattered more than red light.
5. Less light reaches us (extinction), and the light that does is redder (reddening).

The story behind it

Astronomers noticed that some stars looked dimmer and redder than expected. In the early 20th century, they realized this was not because the stars were different, but because dust between us and the stars was changing the light. This helped scientists understand our galaxy better and correct their measurements.

Where you see it in real life

• Sunset: The sun looks redder and dimmer at sunset because its light passes through more air and dust (reddening and extinction).
• Distant stars: Stars near the Milky Way's center look redder and fainter due to dust (both effects).
• Headlights in fog: Headlights look dim and yellowish in thick fog (extinction and reddening).
• Diwali rockets: Fireworks seen through smoke look less bright and more reddish (same reason).

Why it matters

• Astronomers must correct for extinction and reddening to measure true star brightness and colour.
• Helps us map dust clouds in our galaxy.
• Important for understanding distances in space (distance measurements can be wrong if extinction is ignored).
• Used in studying the life and age of stars.

Common mistakes

• Mixing up extinction and reddening: Extinction is about brightness, reddening is about colour.
• Thinking only blue light is blocked: All light is blocked, but blue is blocked more.
• Forgetting that both effects happen together: If you see reddening, extinction is also happening.

Remember it

Extinction dims, reddening reddens — both caused by dust, but one is about brightness, the other about colour.