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When you look up at the night sky, have you ever wondered why some stars are brighter than others? This lesson will teach you how to build a scaled model and visualize the data to understand the relation between brightness(magnitude), distance and luminosity.
Astronomers define star brightness in terms of apparent Magnitude, which depends on:
– Luminosity: how bright it is physically.
– Distance: how far away it is from the earth.
Video reference : https://youtu.be/d7A-_5GFEVo
For more class contents, please visit edu.dfrobot.
1 Wide Cardboard Box
1 Cardboard Sheet
1 Straw
1 Bottle Cap
2 Wires
1 Arduino Uno with Extension Shield
1 LED
220Ω Resisitor
In this model, a LED represents an individual "Star", the brightness and position can be varied. On the "Earth" side, there is an ambient light sensor to get the light intensity, and an ultrasonic sensor to detect the distance. The model connects to a microcontroller then visualize the data in Mind+.
Make the Base:
Make the Slider and Fix the Sensors:
Connect sensors and LED to the Arduino. Then connect the arduino to your computer.
When the star's luminosity is fixed, we can study how the brightness of a star changes with distance by gently moving the LED and seeing the data visualize simultaneously in Mind+.
Download and install Mind+, the programming tool we will use.
Download the sample program 1 and open it, switch to offline mode, and Upload the code to your microcontroller(Arduino Uno). In this program, the LED is set to maximum brightness(255), and the sensor data is constantly read and sent to the serial port.
Switch to Online mode, where we can visualize the received data on the stage.
First, connect the device to start receiving upload(offline) mode data.
Second, click the green flag to draw the data on the stage. Pull or push the straw to change the LED position, here we translate the star distance into cm, at a scale of 1cm = 10 lightyears.
*Please note the distance reading would be unstable within 2cm (ultrasonic measurement range: 2cm～500cm)
This program sets a default max-brightness to be 3000, but if you find the graph is too big or small due to different lighting conditions, you can change this value according to the actual highest reading from the ambient light sensor (when the LED gets to the closest position).
Result:
It is clear to see that brightness decreases with distance, and the graph looks like a curve rather than a straight line, which follows the inverse-square law：
Therefore, we can draw a conclusion that the brightness of a star depends on its distance from us.
The Big Dipper is one of the most famous and recognizable asterisms in the northern sky. It consists of seven stars: Dubhe, Merak, Phecda, Megrez, Alioth, Mizar, and Alkaid. Let's test out our hypothesis that they appear brighter or dimmer as a comprehensive result of distance and luminosity.
We can get the fundamental parameters of given stars from the internet:
Once we get the distance and luminosity data of the stars, we can measure their relative magnitudes with the model.
Download sample program 2 and open it, switch to offline mode, and Upload the code to your microcontroller(Arduino Uno). In this program, we will send the luminosity value from online mode to change the LED brightness.
Switch to Online mode, and connect the device to start receiving upload(offline) mode data. You’ll see the stage with the star names and distances, the dist reading at the top indicates the current LED position.
The distance and luminosity data of seven stars are pre-imported as lists.
Measure each star individually:
Now you can set the star distance and luminosity data and measure their magnitude.
1) Move the LED to match the dist with the star distance/10 rounded down (eg: Merak 79 – dist 7).
2) Press keyboard 1-7 to set the luminosity and get the magnitude for each star
3) Compare the graph
Result:
From the graph, we can find out: Dubhe, Alioth and Alkaid are the three bright stars in The Big Dipper, and Megrez is the least visible one. It is consistent with astronomical observations.
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