☰ Year plan · Science

Why stars differ in brightness, space telescopes and a human sundial

Week 29 · Theme 4 Change and Stability, Unit 4 Patterns in the Sky: Concept 4.3 Sun and Star Brightness (B), Unit Project Human Sundial · 3 lessons
1

Why do some stars look brighter?

Discovery Education Science P5: Term 2, Unit 4, pp. 173–175
GoalWrite a claim, evidence and reasoning about why stars differ in apparent brightness, and judge four ideas about the Sun and stars.
New words
twinkle · to shine with a light that seems to flicker, as stars do because moving air bends the lightevidence · facts or measurements that support a claimreasoning · the explanation that links the evidence to the claim
Explanation

Our question is: why do some stars, including the Sun, look brighter than others? A scientific explanation has three parts. The claim is a one-sentence answer. The evidence is what we found out. The reasoning links the two. Our claim: a star looks brighter when it is closer or when it gives out more light. Our evidence: with a torch, readings fell as the distance grew, from 400 units at 1 m to 100 units at 2 m. Our reasoning: light spreads out as it travels, so the farther away a star is, the less of its light reaches our eyes. The Sun looks the brightest because it is by far the nearest star. Four ideas are often heard. “It is the biggest star” is wrong; many stars are bigger. “It is the hottest star” is wrong too; many are hotter. “Stars twinkle because they explode” is wrong: twinkling is caused by moving air. “The closest star looks brightest” is the idea our evidence supports.

IdeaIs it supported?
The Sun is the biggest star.no
The Sun is the hottest star.no
Stars twinkle because they explode.no: moving air
The closest star looks brightest.yes
Worked examples
Write the explanation step by step. Claim: the Sun looks brighter than other stars because it is much closer. Evidence: the torch’s brightness fell from 400 units to 100 units when the distance doubled. Reasoning: light spreads out, so less reaches us from far away.
TorchThe Sun
small, coolhuge, extremely hot
about 1 to 4 m awayabout 8 light-minutes away
Where is the flashlight like the Sun? Both give out light that spreads out and both look dimmer from farther away. Where is it different? A flashlight is small, cool and battery-powered. The Sun is a huge, extremely hot ball of gas, and it is also very much farther away than our test distances.
Twinkling: stars twinkle because their light passes through moving layers of air, which bend it a little in different directions. Planets look steadier. Twinkling has nothing to do with explosions.
Class activity

Judge the claims. Four children say: A, “The Sun is the brightest star because it is the largest.” B, “The Sun is the brightest star because it is the closest.” C, “The Sun is brightest because it is the hottest.” D, “Stars twinkle because they are exploding.” With a grown-up, decide which one you agree with most and write a claim, evidence and reasoning to defend your choice. Use the numbers from the torch test.

Practice
1
Name the three parts of a scientific explanation.
2
Why do stars twinkle?
3
In the test, a torch reads 360 units at 1 m and 90 units at 2 m. Is that evidence that farther means dimmer? How many times dimmer?
DistanceUnits
1 m360
2 m90
4
A friend says, “The brightest star in the sky must be the biggest.” Give a reason to doubt this.