Reading the Rainbow

EXPLAIN
The instructor will show students a Hubble Space Telescope photo of NGC 6397 (or similar star cluster) and asked what they can tell about it just from the photo.

Image: Hubble's view of globular cluster NGC 6397

Other globular clusters can be found on the Hubble Space Telescope website

It is important that the cluster shown has stars of distinctly different colours. Omega centauri is another good one – it has blue, white, orange, reddish stars.

Your instructor has shown you a Hubble Space Telescope photo of a patch of sky. What are FIVE things that you can tell about the object in the photo just from looking at the photograph ?

1. 2. 3. 4. 5.

The students will most likely note that the stars in the photo are different colours, appear different brightnesses, seem to cluster near the middle, and that the orange-ish stars seem more evenly spaced out than the white-ish ones. They will note – probably in jest – that there is darkness in between the stars (this is actually an incredibly profound observation, one that gets to the heart of cosmology and the behavior of the observable universe). They will likely suggest that the orange-ish ones are closer, although they have no evidence other than the fact that they appear brighter. However, their experience with the world has shown them that ‘bigger/brighter = closer,’ so this is a great place to ask what evidence they have to support their “observation” that the orange ones are closer.

Similarly, they will often say the orange ones are bigger because they appear brighter, and because the images actually seem larger. This is a good place to explain that with very few exceptions, stars – even when viewed through large telescopes – look like pinpoints. For Hubble to be able to capture the faintest stars in this image, the brighter ones essentially became overexposed and bled into surrounding pixels, much like you have probably seen when taking cell phone pictures of bright objects.

EXPLORE

At this point, the instructor will explain that sometimes our experience leads to the correct answer (stars appear different colours) and sometimes it makes us leap prematurely to a conclusion that might not be valid (the orange stars are closer. While they might be, the photo itself doesn’t tell us that).

You will now get to experience a more familiar light source to see if you can determine anything new about the stars in the photograph. Your instructor should have set up an incandescent light bulb on a dimmer switch. You will observe the properties of the light bulb and apply what you know to the stars in the photo.

Procedure:
1. Set the dimmer knob on low. Observe the COLOUR and the BRIGHTNESS of the bulb. Under the guidance of your instructor, hold your hand near the bulb and gauge the TEMPERATURE. BE CAREFUL NOT TO TOUCH THE BULB!

1. Set the dimmer knob on medium. Observe the COLOUR and the BRIGHTNESS of the bulb. Under the guidance of your instructor, hold your hand near the bulb and gauge the TEMPERATURE. BE CAREFUL NOT TO TOUCH THE BULB!
   
2. Set the dimmer knob on high. Observe the COLOUR and the BRIGHTNESS of the bulb. Under the guidance of your instructor, hold your hand near the bulb and gauge the TEMPERATURE. BE CAREFUL NOT TO TOUCH THE BULB!
   
3. Fill in your results in the table below:

	Colour	Temperature	Brightness
Low	Orangish	Cool/Warm	Dim
Medium	Yellow-ish (Beige/pale copper)	Medium/Warmer	Brighter
High	White	Hot	Bright

Using what you have learned from INCANDESCENT LIGHT BULBS, answer the next few questions.

Two stars are exactly the same size and distance from us. One appears orange. The other appears white. assuming that stars have different colours for the same reason that light bulb filaments have different colours, which one do you THINK will appear brighter?
A. The orange star
B. **The white star
C. They will both appear the same brightness
D. There is not enough information to answer this question

Explain your choice: with the light bulb example, the white bulb = hottest = brightest. If all other factors are the same (distance, size), one expects white to be brighter.
Your instructor will now show you the photo again.

In your experience with the light bulb, which color corresponded to the dimmest light? ORANGE Brightest? WHITE

In the photo of NGC 6397, which color star appears brightest? ORANGE

Is this observation consistent with the light bulb observation? NO

If not, can you come up with three possible explanations for the difference?

You should now engage the class in a discussion of how the brighter stars could be the orange ones. The most reasonable possibilities are:

1. The orange stars stand out because they’re closer
2. The orange stars stand out because they’re bigger
3. The colour of the stars is not representative of their temperatures, like the light bulb’s colour. Perhaps an entirely different mechanism gives them their colour/brightness.
4. Occasionally a student will have an intriguing idea that deserves discussion, e.g. “maybe the hubble camera is more sensitive to orange.” [assure them that astronomers go through quite a bit of work to make sure the images are as accurate as possible]

ANOTHER LIGHT SOURCE

There is another common type of light source, though. Commonly called “discharge” tubes or “spectrum tubes,” these contain very low density gases that are energized by a high-voltage power supply.

Your instructor should have a few examples for you to observe. Once again, you should observe the colour and brightness. You should also gauge the temperature by holding your hand near the tube.

When observing the discharge tubes, do you find that there is a clear relationship between colour, temperature, and brightness? NO If so, what relationship do you note?

There should be none. If plugged in the same amount of time, they don’t feel any different from each other. Also the brightness/colour aren’t related like they are with the regular incandescent light bulb.

Please note that the spectrum tubes can get extremely hot. Either turn them on for a brief time or allow sufficient cool-down time before changing them.

Look back at the image of NGC 6397. Which spectrum tube do the orange-ish stars most closely resemble? HELIUM

Is there a setting for the light bulb that gives approximately the same colour? YES Which one? MEDIUM-ISH (THIS WILL DEPEND ON YOUR EXACT EQUIPMENT)

If you had to rely only on your visual observations of these objects, could you tell whether the mechanism producing the colours in the stars was more likely the same as the light bulbs or the spectrum tubes? __NO___

EXPLAIN

For some star colours, you can mimic the colour with either the right setting on the dimmer switch for the light bulb or the right type of spectrum tube. Just eyeballing the colour can’t tell you which is right.

GUESS: Do you think that the colours of stars come from their temperatures (like the light bulb colour) or from their compositions (like the colours of the spectrum tubes)? do not worry about guessing incorrectly at this time. You will return to this question.

This is a guess – you will likely find that over half of your class will attribute a star’s colour to its composition, but at this point, there is no need to correct them. They will explore more about light in the next activity and ideally deduce that a star’s colour is related to the temperature.

Your instructor will now give you a special tool for exploring the light more thoroughly. This is a diffraction grating, and it splits the different colours of light up like a prism. Observe both the light bulb and the spectrum tube through your diffraction grating.

Now do you think there might be a way to figure out which mechanism is responsible for the colours of stars? ______

EXPLORE

There are two main reasons behind the colour of a light-emitting object: temperature (as in the case of the incandescent light) and composition (as in the case of the spectrum tubes). To figure out which is the reason behind the colours of stars, you must explore the light more thoroughly and go beyond simply the visual colour. To do this, you will need a device to split the light up.

Your instructor has provided your group with a diffraction grating or spectroscope. CDs and DVDs do the same job.

With the brightness of the lamp turned up, look towards the lamp through your diffraction grating and look for the spectrum of the lamp (you will need to look off to the side). Once you’ve found the spectrum, draw it using your coloured pencils.

This shouls look like a rainbow

Your instructor will adjust the brightness of the lamp. Describe how the spectrum of the lamp CHANGES when you view the DIMMING lamp through your diffraction grating. You might need to repeat the brightening/dimming process a few times to see the changes. Pay special attention to the intensity of the colours.

What they should note is that the purples and blues become much harder to see. When dim, the light bulb gives off less of every colour, but the purple seems to disappear.

Are there any colours that are present when it is bright that seem not to be present when it is dim? _YES_ If so, which ones? __PURPLE/BLUE___Are there any colours that are present when it is dim that don’t appear to be present when it is bright? __NO____ If so, which ones? ___________

Recall the first part of this exercise. Is the light bulb’s colour related to its temperature or its composition? _TEMPERATURE____

Your instructor will now set up a number of spectrum tubes. Note: These lamps require very high voltages, and the power supplies are very dangerous (a shock from one of these could be fatal). Please be very careful when using this equipment.

• Draw the spectrum of the first lamp. Which substance is contained in the tube?
• Draw the spectrum of the second lamp. Which substance is contained in the tube?
• Draw the spectrum of the third lamp. Which substance is contained in the tube?

This will depend on the lamps, but a few common substances and their spectra are given below. An internet search on “emission spectrum (element name)” will yield plenty of images.

Is the colour of a spectrum tube related to its temperature or its composition? COMPOSITION

Explain:

Your instructor will now explain a bit more about the types of spectra you have just seen, but the basic rules of objects emitting their own light are as follows:

When you see a complete rainbow, or CONTINUOUS SPECTRUM, the colour of the object is related to the TEMPERATURE.

When you see instead a series of bright lines, or EMISSION SPECTRUM, the colour of the object is related to the COMPOSITION.

There is an object in space called Eta Carinae whose spectrum looks like this:

Do you think the colour of this object is related to its TEMPERATURE or to its COMPOSITION? STUDENTS WILL LIKELY SAY COMPOSITION
Explain your answer. It shows the bright emission lines.

However, if you look at it closely, you see a variety of behaviors depending on which part of the object you’re looking at. Cutting across the middle, it appears continous-ish, so for that part of the object, perhaps it’s denser. Then you can see the emission lines beginning to dominate when you go ‘up’ and ‘down’ from the middle.

Which astronomical object is this spectrum from? THE SUN!

Do you think the colour of this object is related to its TEMPERATURE or to its COMPOSITION? TEMPERATURE

Explain your answer. it’s a continuous spectrum, and we have found out that objects that give a continuous spectrum get their colours from their temperatures.

Now look back at the image of NGC 6397. What type of astronomical objects are represented in the photo?
STARS

What type of spectrum do you think they have – one more like the light bulb’s, or one more like the spectrum tube’s? MORE LIKE THE LIGHT BULB’S

Explain.
The sun is a star, and the sun gives a continuous spectrum (rainbow). So if the objects in ngc 6397 are also stars, they probably give the same type of spectrum, which is like the light bulb’s. Interestingly, though, students have very little evidence at this point that the sun is a star, except for the word of all their teachers.

Do you think their colours are related to their composition or their temperature?
TEMPERATURE – LIKE THE LIGHT BULB.

Is this the same answer you gave at the end of the previous exercise? _____ If not, what information has changed your mind?

Probably not, but it might be. The information that should have changed their mind is seeing the spectra of objects whose colour comes from their temperature and the spectra of objects whose colour comes from their composition. Interestingly, even though they have just seen the sun’s spectrum and the light bulb’s, there will still be a fair number of students who hold firmly to the alternative conception that the colour of the sun and stars comes from their temperature. Some additional personal response questions might help them shake this misconception. For example, you could use the following series of questions outlining the logical steps:

A light bulb’s colour is a result of its…
1. composition 2. temperature

A discharge tube [or fluorescent light] (e.g. a long glass tube plugged into that big power supply) gets its colour from its…
1. composition 2. temperature

A discharge tube’s spectrum looks like __.
1. a rainbow
2. a series of different coloured lines
3. a single streak of colour
4. a light bulb’s spectrum

A light bulb’s spectrum looks like ___.
1. a rainbow
2. a series of different coloured lines
3. a single white streak
4. a discharge tube’s spectrum

A star’s spectrum (think of the Sun) looks like…
1. a rainbow
2. a series of different coloured lines

Therefore a star’s colour must result from the same mechanism as a _____’s colour.
1. light bulb 2. discharge tube

This means a star’s colour is related to its…
1. temperature 2. composition

Now consider the image of the familiar constellation Orion. The coolest star is BETELGEUSE, WHICH APPEARS DISTINCTLY ORANGE-ISH IN COLOR.