Our star, tools for studying stars, and bright and dim
Week 28 · Theme 4 Change and Stability, Unit 4 Patterns in the Sky, Concept 4.3 Sun and Star Brightness (A) · 3 lessons
3
Near is bright, far is dim
Discovery Education Science P5: Term 2, Unit 4, pp. 169–172
GoalUse a flashlight experiment to find how brightness changes with distance, and say what else affects a star’s brightness.
New words
apparent brightness · how bright a star looks from Earthdistance · how far away something isrelationship · the way two things change together
Explanation
Apparent brightness means how bright something looks from where you are. A torch is a good model for a star. Point it at a white card in a dim room and measure the brightness of the spot with a light meter app, or compare it by eye, from different distances. As the distance goes up, the brightness goes down. In our example the readings are 400 units at 1 m, 100 units at 2 m and 25 units at 4 m. When the distance doubles, the brightness drops to a quarter. So a star that is twice as far away looks four times fainter. But distance is not the only factor. A star can look bright because it is near, or because it gives out a huge amount of light even from far away. A torch is like a star because both give out light that spreads out. It is different because a torch is small and cool and a star is a giant hot ball of gas.
Worked examples
Distance
Brightness (units)
1 m
400
2 m
100
3 m
44
4 m
25
Hana’s readings in units: 1 m, 400; 2 m, 100; 3 m, 44; 4 m, 25. From 1 m to 2 m the brightness dropped by 400 − 100 = 300 units. From 2 m to 4 m it dropped by only 100 − 25 = 75 units. The drop is biggest when the torch is near.
Changed
Kept the same
distance
torch, card, room, batteries
Do not forget the fair test. Hana keeps the same torch, the same card, the same dim room and the same batteries, and changes only the distance. If a new battery is used for the last reading, the result would be unfair.
Star
Light it gives out
Distance
A
a lot
far
B
a little
near
Is a very bright star always the closest star? No. A faraway star that gives out a great amount of light can look brighter than a nearby star that gives out little. So brightness in the sky depends on both distance and how much light the star gives out.
Class activity
Brightness and distance (a grown-up helps; do not look into the torch). In a dim room point a torch at a white card on a table. Stand the card at 1 m, 2 m and 4 m from the torch (or use a ruler to measure). Use a phone light-meter app, or compare the brightness of the spot by eye, and write each reading in a table. Change only the distance. Draw a bar graph and say what pattern you see.
Practice
1
What does apparent brightness mean?
How bright a star looks from Earth.
2
When the distance from a torch doubles, what happens to the brightness in our example?
It falls to a quarter: 400 units at 1 m becomes 100 at 2 m.
3
At 3 m the reading is 44 units and at 4 m it is 25 units. How many units fewer at 4 m?
44 − 25 = 19 units fewer.
4
Give one way a torch is like a star and one way it is different.
Like: both give out light that spreads out and looks dimmer far away. Different: a torch is small and cool; a star is a huge hot ball of gas.
