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Thomas Durkin

Engaging in Science: Astronomy for Teachers

Barbra Shaw Ph. D.

Portland State University

March 19, 2010

Laboratory: Stellar Color Analyzer

I. Introduction: This computer laboratory activity is an extension of an astronomy unit. This unit includes a power point presentation that is based on the resource Astronomy Picture of the Day (APOD). Using APOD, students determine the color spectrum of their chosen star, providing data and charts of the color. Students use easily available tools to measure colors and record their results.

II. Concepts Addressed: Stellar color spectrums can be used to determine the surface temperature of a star. From this temperature, the elements present in the star can be inferred.

III. Lab Goals: Students will determine the average color of various stars and correlate these colors to star temperatures

IV. Benchmarks Addressed:

A. Science Grade 7

7.3 Scientific Inquiry: Scientific inquiry is the investigation of the natural world based on observation and science principles that includes proposing questions or hypotheses, designing procedures for questioning, collecting, analyzing, and interpreting multiple forms of accurate and relevant data to produce justifiable evidence-based explanations.

7.3S.1 Based on observations and science principles propose questions or hypotheses that can be examined through scientific investigation. Design and conduct a scientific investigation that uses appropriate tools and techniques to collect relevant data.

7.3S.2 Organize, display, and analyze relevant data, construct an evidence-based explanation of the results of an investigation, and communicate the conclusions including possible sources of error.

7.3S.3 Evaluate the validity of scientific explanations and conclusions based on the amount and quality of the evidence cited.

B. Science Grade 6

6.1E.2 Describe the properties of objects in the solar system. Describe and compare the position of the sun within the solar system, galaxy, and universe.

C. Science Grade 8

Interaction and Change: Systems interact with other systems. 8.2E.1 Explain how gravity is the force that keeps objects in the solar system in regular and predictable motion and describe the resulting phenomena. Explain the interactions that result in Earth’s seasons.

D. Mathematics Grade 7

7.2 Number and Operations, Algebra and Geometry: Develop an understanding of and apply proportionality, including similarity.

7.2.1 Represent proportional relationships with coordinate graphs and tables, and identify unit rate as the slope of the related line.

7.2.3 Use coordinate graphs, tables, and equations to distinguish proportional relationships from other relationships, including inverse proportionality.

V. NASA Big Questions: Astrophysics Questions

A. When and how did the elements of life in the Universe arise?



1. Following the Big Bang and the gradual cooling of the Universe, the primary constituents of the cosmos were the elements hydrogen and helium. Even today, these two elements make up 98% of the visible matter in the Universe. Nevertheless, our world and everything it contains—even life itself—is possible only because of the existence of heavier elements such as carbon, nitrogen, oxygen, silicon, iron, and many, many others. How long did it take the first generations of stars to seed our Universe with the heavy elements we see on Earth today? When in the history of the Universe was there a sufficient supply of heavy elements to allow the formation of prebiotic molecules and terrestrial-like planets upon which those molecules might combine to form life?

All elements heavier than hydrogen and helium are manufactured inside stars, or produced when a star’s life ends as a supernova. NASA seeks to study the way stars evolve over time, and how they end their lives in order to learn about how the Universe has transformed itself from a place of only two simple elements to one with a molecular complexity sufficient to support life.

Understanding how stars form, how they evolve over their lives, and how they die, is key to understanding the history of heavy elements in the Universe. By studying stellar nurseries in which stars are born as well as the supernovae, debris shells, white dwarves, and neutron stars left behind when they die will reveal how the Universe has created and disseminated complex elements throughout its history. This will permit us to predict when there was a sufficient quantity of these elements such that life creation would have been possible. Knowing when the Universe was capable of supporting life will help us evaluate the likelihood that other life exists in the cosmos.

VI. Sources

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VII. Introduction

Color and temperature

A. Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, and other fields. The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that light source. The temperature is conventionally stated in units of absolute temperature, Kelvin (K). Color temperature is related to Planck's law and to Wien's displacement law. Higher color temperatures (5,000 K or more) are cool (blueish white) colors; lower color temperatures (2,700–3,000 K) are warm (yellowish white through red) colors.

B. Stars great and small, and their life cycles

A star’s color is critical in identifying the star, because it tells us the star’s surface temperature in the black body radiation scale. The sun has a surface temperature of 5,500 K, typical for a yellow star. Red stars are cooler than the sun, with surface temperatures of 3,500 K for a bright red star and 2,500 K for a dark red star. The hottest stars are blue, with their surface temperatures falling anywhere between 10,000 K and 50,000 K.

Stars are fuelled by the nuclear fusion reactions at their core. There is a dynamic equilibrium maintained throughout the star’s life between the expanding heat of the reactive core and gravitational forces holding the star together. Fusion produces extremely high energy. Fusion releases some of the energy that binds the particles of the nucleus together, unleashing remarkable power.

Stars begin as a mass of dust and gas dense enough to start collapsing inwards under the pressure of its own gravity. If this protostar is massive enough, it will eventually initiate a nuclear reaction in its hot, dense core. This initiates the main sequence of a star’s life cycle, when hydrogen forms helium at the star’s core through the process of nuclear fusion. Heat from the star’s core radiates outwards through the layers of the star to the photosphere, the visible surface, which emits electromagnetic energy and charged particles as a solar wind.

A star does not stay the same color throughout its lifecycle, since the surface temperature alters depending on the type of fusion reaction fuelling the star at the time. Depending on the initial mass of the star, it will evolve along the lines of one of three main star types: low-mass stars, intermediate-mass stars (like our sun) and high-mass stars.

C. If you look carefully at the stars, even without binoculars or a telescope, you will see a range of color from reddish to yellowish to bluish. For example, Betelgeuse looks reddish, Pollux -- like the sun -- is yellowish, and Rigel looks bluish.

A star's color depends on its surface temperature. Astronomers measure star temperatures in a metric unit known as the Kelvin. One Kelvin equals exactly 1 Celsius degree (1.8 Fahrenheit degree), but the Kelvin and Celsius scales start at different points. The Kelvin scale starts at -273.15 degrees C. Therefore, a temperature of 0 K equals -273.15 degrees C, or -459.67 degrees F. A temperature of 0 degrees C (32 degrees F) equals 273.15 K.

Dark red stars have surface temperatures of about 2500 K. The surface temperature of a bright red star is approximately 3500 K; that of the sun and other yellow stars, roughly 5500 K. Blue stars range from about 10,000 to 50,000 K in surface temperature.

Although a star appears to the unaided eye to have a single color, it actually emits a broad spectrum (band) of colors. You can see that starlight consists of many colors by using a prism to separate and spread the colors of the light of the sun, a yellow star. The visible spectrum includes all the colors of the rainbow. These colors range from red, produced by the photons (particles of light) with the least energy; to violet, produced by the most energetic photons.

Visible light is one of six bands of electromagnetic radiation. Ranging from the least energetic to the most energetic, they are: radio waves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays. All six bands are emitted by stars, but most individual stars do not emit all of them. The combined range of all six bands is known as the electromagnetic spectrum.

Astronomers study a star's spectrum by separating it, spreading it out, and displaying it. The display itself is also known as a spectrum. The scientists study thin gaps in the spectrum. When the spectrum is spread out from left to right, the gaps appear as vertical lines. The spectra of stars have dark absorption lines where radiation of specific energies is weak. In a few special cases in the visible spectrum, stars have bright emission lines where radiation of specific energies is especially strong.

An absorption line appears when a chemical element or compound absorbs radiation that has the amount of energy corresponding to the line. For example, the spectrum of the visible light coming from the sun has a group of absorption lines in the green part of the spectrum. Calcium in an outer layer of the sun absorbs light rays that would have produced the corresponding green colors.

Although all stars have absorption lines in the visible band of the electromagnetic spectrum, emission lines are more common in other parts of the spectrum. For instance, nitrogen in the sun's atmosphere emits powerful radiation that produces emission lines in the ultraviolet part of the spectrum.

There is a precise relationship between the temperature of a body and its color, which comes from the fact that a heated surface does not emit the same amount of energy at all possible electromagnetic wavelengths. In fact, the light follows a unique curve deduced by physicist Maxwell Planck. We call it the 'Planck Blackbody Curve' because all bodies, from iron bars, to the distant stars, follow this same curve as the emit light at a specific temperature. Figure 2 shows a few of these curves for different temperatures. Notice that the wavelength of the peak of the curve where most of the light is emitted, shifts from short wavelengths (bluish color) near 500 nm to longer (reddish color) wavelengths near 800 nm as the temperature decreases. This is why a cooling iron bar first appears 'white hot' then changes to yellow to orange to red as it cools.

D. By measuring the exact amount of light produced by a star at specific wavelengths from the ultraviolet to the infrared, astronomers can 'fit' a unique Planck Curve to this data and deduce the star's surface temperature - one of the most fundamental things you can ever hope to learn about a star without actually visiting it!

Another thing that goes along with the temperature of a gas is the pattern of ‘fingerprint’ lines its constituent atoms produce. As atoms collide, their electrons make jumps from one energy to another in specific steps. Each step downwards in energy causes a photon of light to be emitted. The particular pattern of lines for an element is unique to that element, so astronomers can tell which elements are present in a star’s atmosphere by carefully studying these ‘spectral lines’. A cool star has elements that produce many of these lines, while a hot star can be so hot that electrons are stripped out of the atoms, thereby reducing the number of lines that can be produced in the atoms (or ions!) fingerprint. By carefully studying the spectra from stars, astronomers can order them from hottest to coolest.

E. The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosity versus their spectral types or classifications and effective temperatures. Hertzsprung-Russell diagrams are not pictures or maps of the locations of the stars. Rather, they plot each star on a graph measuring the star's absolute magnitude or brightness against its temperature and color.

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The lab is divided into two parts, the Excel session and the GIMP sessions.

1. Students will create the Excel spreadsheet, Color_Analyzer, during the first computer lab session. Look at the screen shots of Excel for more detailed instructions. I found this preparation to be extremely helpful during the GIMP Color Picker sessions.

2. Students analyze saved photograph files of two stars using GIMP Color Picker functions. The instructions for this are detailed in step 3.

VIII. Procedure:

1: Create the Excel document Color_Analyzer. See example below.

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2: Download four pictures:

|APOD Image |Saved as: |

|January 8, 2010: The Mystery of the Fading Star |EpsilonAurigae_wong900 |

|January 6, 2010: The Spotty Surface of Betelgeuse |Betel_haubois800 |

|December 21, 2009: Star Cluster R136 Bursts Out |30dor_hst |

|August 5, 2009: Betelgeuse Resolved |Betelgeuze_eso |

3: Open Gimp.

● File menu: “Open” the December 21st picture Star Cluster R136 Bursts Out saved as 30dor_hst, and abbreviated as “Hot” in these directions. I selected this to set my “hot star” point in my data.

● Find the brightest area. Using the rectangle select tool, highlight a large portion of that area.

● View menu: under “Zoom” enlarge to 800%.

● Again find the brightest area. Using the rectangle select tool, highlight a large portion of that area.

● View menu: under “Zoom” enlarge to 1600%

● Because you selected the area for the “brightest”, use the rectangle upper left corner as the fixed spot.

● Select the rectangle tool again, and re-adjust the size of your rectangle to 5 pixels across and 5 pixels down, with the upper left corner as your starting point. This will give us a 25 point sample of the brightest area.

4: Open Excel and record the location of your 5x5 grid (on the picture below, I used the following abbreviations:

o U = upper

o L = lower

o L = left

o R = right

o H = horizontal

o V = vertical

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● On Column A, record the grid location on your picture as Grid 1.1 for row 1 column 1, 1.2 for row 1, column 2, etc.

● Back on the Gimp menu, select the “Color Picker” tool.

● Using that tool, point on the pixel 1.1 and click on that color.

● The color will appear in this box (foreground color) in your Gimp menu.

o If that color appears in the lower box, you need to switch to foreground. Click this button.

● Click on that box, and this menu (Change Foreground Color) will appear.

● Note the numbers on the right side? They are values for:

o H = hue

o S = saturation

o V = value

o R = red

o G = green

o B = blue

● Use those titles for columns B-G in Excel.

● Record each value in the appropriate column for Grid 1.1.

● Point the “Color Picker” on the next pixel, row 1, column 2 (Grid 1.2) and click.

● Move your “Color Picker” tool to the box in the Gimp menu and click.

● That will give you the values for this next color in the Change Foreground Color menu.

● Record those values for Grid 1.2.

● Be sure that you always use the foreground color, rather than trying to swap between foreground and background colors. Simplifies the data collection procedures.

● Using the “Color Picker” tool, point to pixel Grid 1.3 (1st row, 3rd column) and click.

● Move the “Color Picker” tool to the foreground color box on the Gimp menu and click.

● Record those data from the Change Foreground Color menu.

● Repeat for each pixel in your 5x5 square. You should end up with 25 rows of data, 6 columns deep.

● Find the average of each column, H, S, V, R, G, and B. You now have the average color for the hot stars.

● Betelgeuse is a cool star, and we use that to set the cool end of stars. Repeat all these steps using the August 5, 2009: Betelgeuse Resolved picture.

● Students create a line chart using these two sets of data. They compare and contrast the data

● Do not use the Spotty Surface of Betelgeuse. Students might ask questions about Spotty color and the Resolved pictures. A line of inquiry might be: Does their data support the average temperature for Betelgeuse in both pictures? Does that support the Spotty Surface data?

5: Students will need to spend some time researching temperatures and stars, and once they get them set with a few known stars, they can analyze their colors, and then see if color is a good predictor of temperature.

IX. Materials and Costs:

A. Materials

1. Access to the internet, specifically APOD site

B. List the equipment and non-consumable material and estimated cost of each

Item: Computer(s) with software installed:

Web Browser, Microsoft Word, Microsoft Excel, GIMP2 (GNU Image Manipulation Program), or other paint program,

Internet access (preferably DSL or High Speed)

Estimated total, one-time, start-up cost: $0.00

1.

C. List the consumable supplies and estimated cost for presenting to a class of 30 students

Estimated total, one-time, start-up cost: $0.00

D. Time:

|Preparation time: |1 hr. - Lab to set up the student Excel Color Analyzer spread sheet |

|Instruction time: |1 hr - Lab to run Color Picker and complete Color Analyzer spreadsheet |

| |1 hr - Lab to run Color Analyzer spreadsheet and graphing |

|Clean-up time: |none |

X. Assessment (include all assessment materials):

Students will be assessed through observation of their efforts in the lab, and their finished products, Color Analyzer spreadsheet and chart

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Rectangle Select Tool

Selected area anchor point

Area adjusted to 5 across and 5 down

1600% magnification

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