Astronomy Worksheet



Astronomy Lab - Interpreting Stellar Spectra

Introduction: From Colors to Curves

In this lab you will determine the spectral type of a star by doing spectral analysis. In the process you will review how spectra are used to interpret various stellar parameters.

Spectral images look like a series of colored bands. Research-grade spectroscopes display this color information in graphical format. The advantage is that every part of a spectrum is quantified and can be precisely measured.

The vertical or y-axis of a spectral graph measures flux, the amount of energy emitted at each wavelength. Real stellar spectra are a combination of different types of spectra: continuous, emission and absorption. The continuum is represented by the general trend of the graph. Emission lines are spikes above the continuum, while absorption lines are dips or spikes below the continuum. The x-axis displays wavelengths in Angstroms (Å).

Each emission or absorption line is the result of electron transitions between energy levels in atoms or molecules. Whether it is emission or absorption just depends on its state: Hot gases make emission lines, while cool gases produce absorption. Each molecule and element has its own unique set of lines, a “fingerprint” that identifies it.

Procedure

You will be assigned a star to analyze. Use Graphical Analysis to examine the stellar spectra.

Two very useful tools are located in the Analyze menu. Open the spectrum of your star and activate the first one now: It is called Examine. Notice that a black line appears on the graph. Examine allows you to precisely determine the x, y coordinates of any point along the graph by simply dragging the black line to the desired place.

Another useful feature is the zoom tool. To magnify a section of the graph, click and drag, beginning at the upper left to outline it with a rectangle, then click on the magnifying glass.

Directions

1. Use Graphical Analysis. Select File/Open. Change to the directory called Star Spectra. A list of star names will

appear. Open the star’s spectrum. Maximize it according to the directions given in lab, and print it using the

landscape mode. Write your name and the name of the star on the graph.

2. Use Graphical Analysis to identify at least 4 of the major absorption lines. Beside each line, write its exact

chemical identity, such as Ha Or CaII and its wavelength. To identify the line, use the Excel spreadsheet of

identified spectral lines located in the same folder as the star spectra. A good place to start is to see if your star

displays the Balmer lines series or a Calcium break (see the Decoding the Stars section in this lab.) Try the

common spectral lines section first. Most of the common listed lines are in the 4000-5000 Å region of the

spectrum.

Since stars behave as blackbodies, their continuum curves will peak at a certain frequency. Find the peak

wavelength of your spectrum: ___________Angstroms.

Wien’s Law states that the higher the frequency (or, the shorter the wavelength) of the peak, the hotter the star. Temperatures of stars are measured in Kelvins (K). This is a preferred alternative temperature scale to Celsius, since it eliminates the use of 0 in temperature calculations. Wavelengths are measured in Angstroms. 1 Angstrom (Å) = 1 x 10-10m.

Use Wien’s Law (equation on the next page) to calculate the surface temperature of your star. “K• Å” is a derived

unit. Just insert the peak wavelength of the star in the denominator to solve for temperature.

|Class |Color |Temperature |

|O |blue |above 31,000 K |

|B |blue-white |9750-31,000 K |

|A |white |7100-9750 K |

|F |yellowish white |5950-7100 K |

|G |yellow |5250-5950 K |

|K |orange |3800-5250 K |

|M |red |2200-3800 K |

|L |red-infrared |1500-2200 K |

|T |infrared |1000 K |

Wien’s Law equation:

3. Write the calculated star temperature on your graph. Show

the set-up of your calculation.

4. Use the chart at right to estimate the color of your star.

Write the star color on the graph.

In addition to gases around stars absorbing energy, Earth’s atmosphere absorbs some of the light from stars. These telluric absorption bands can be seen in every spectrum taken from ground level on Earth. They most often appear near the red end of the spectrum.

Absorption bands look like broad, V-shaped notches, often spanning 50 Å or more. These are caused by molecular absorption instead of absorption from a single element, so they are also known as molecular bands.

5. A set of telluric bands are listed in the far right column of the spectral lines chart. Locate a telluric band on the

spectrum of your star and label it with a T.

The true spectral class of your star can now be determined by using your estimated spectral class from the chart above and the information in the next section. The estimated spectral class gives you a starting point, but in reality, the chemistry, not the temperature, determines the spectral class. The actual spectral class could be one level above or below the estimate. Begin by reading through the entire section called Decoding the Stars - it will help you greatly! Then take some time to study the spectrum of your star to determine it spectral class.

6. Write the true spectral class on the star spectrum.

7. In summary, write a paragraph on the back of the graph that lists the evidence you used to identify the star.

You must list 3 pieces of evidence that confirms your spectral class. This should be based on the chemistry of

the star, not on your temperature calculation. Discuss what lines or features are present and why they indicate

a particular spectral class.

Decoding the Stars

The table below characterizes the spectral properties of the different classes:

|Class |Spectral characteristics |

|O |ionized and neutral helium, weak hydrogen |

|B |neutral helium, stronger hydrogen |

|A |no helium, strongest hydrogen, some ionized metals |

|F |weaker hydrogen, many ionized metals |

|G |still weaker hydrogen, ionized and neutral metals |

|K |weak hydrogen, many neutral metals |

|M |little or no hydrogen, neutral metals, molecular bands, TiO band |

|L |no hydrogen, metallic hydrides, alkali metals |

|T |methane bands |

Neutral metals are designated by Roman numeral I. Ionized metals have Roman numerals higher than I.

Notes about certain spectral classes:

*Look at the shape of your continuum and compare it to those seen on the last page. Does it peak towards the left, in the middle, or towards the right? This will give you an idea of the general type of star you have.

* The spectrum of a hot star usually looks smoother and has less absorption lines than a cooler star.

*In very hot stars (> 10,000 K) most of the hydrogen gas in the star’s atmosphere will be ionized. Since an ionized hydrogen atom has no electron it cannot produce any spectral lines, so the hydrogen lines are weak in O stars.

*A, B and F stars have the correct range of temperatures to energize their hydrogen gas without ionizing it. Thus the hydrogen Balmer lines are very strong in these stars. At lower temperatures the hydrogen gas isn’t as easily excited, thus the Balmer lines aren’t as strong in G and K stars, and are barely present in M stars.

The Balmer lines are found at these wavelengths in Angstroms: Alpha α =6562, Beta β =4861, Gamma γ =4341, Delta δ =4102, and Epsilon ε = 3970 The graphic at the right shows the location of Balmer lines in a star spectrum.

*Metals are easier to ionize than hydrogen and helium, thus spectral lines from ionized metals (e.g., Fe II, Mg II, etc.) are common in stars of moderate temperatures (roughly 5000 to 9000 K).

*Metals produce many more spectral lines than hydrogen and helium because they have more electrons. As a rule, the cooler the star, the more metal lines it will have.

*Two lines of calcium (CaII 3933A and 3968A) are known as the “Calcium H and K” lines, or the “Calcium Break”, because they often are so pronounced they seem to “break” the smooth continuum curve of the spectrum. They are particularly strong in cooler stars. Below is a spectrum showing a typical calcium break:

*In F stars and cooler, the calcium break lines are stronger than the Balmer lines.

*In the cool G and K stars, lines from ionized metals are less abundant and lines from neutral metals are more common.

*The atmospheres of the very cool M stars contain molecules that produce wide V-shaped absorption bands, which are much wider than regular absorption lines. These absorption bands may radically alter the shape of the continuum, to the point where it is not even clear what the continuum really looks like.

*If you star contains emission lines, then it either is extremely hot or the star light is shining through a hot gas (nebula).

Finally, these are spectra of real stars, not classroom models! It may seem that your star doesn’t obey the rules. Just as with people, some stars reject the status quo and do their own thing.

Sample spectra of some Main Sequence (normal adult) stars.

Note: not all stars in this lab are normal adults!

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