This opening activity will introduce you to the visual variety of stars, and show you their different spectra. Relax: You don’t have to have any prior knowledge about these stars or spectrography. We simply want to introduce you to the variety that exists among stars.

Look at the chart on the Colors of Stars Worksheet. This chart shows the spectra of stars; that is, how the light looks when viewed through a spectroscope. A spectroscope is an instrument astronomers use to look at a star’s spectrum. Similar to how we can use a prism to look at the different colors of light in white light, a spectroscope spreads a star’s light into different wavelengths.

When you look at the chart, what do you notice? What questions come up? Write down your observations and any questions you have. Be sure to save this worksheet—you’re going to use it again at the end of this lesson.

Stars can be categorized by their spectra. The gases in a star’s atmosphere absorb some of the light in a star’s spectrum, showing up as black bands. Size and temperature determine a star’s luminosity, and the HR diagram plots stars’ luminosity versus temperature. Most stars fall along the main sequence of the HR chart. Giants and supergiants are large and redder (cooler), and white dwarfs are very small and bluer (hotter).

This video will introduce you to the concept of a star’s spectrum, and explains how astronomers categorize stars by their spectra. You’ll also learn about a number of astronomers whose work collectively shed light on how a star’s spectrum is like a fingerprint. Our understanding of different types of stars and their life cycles, including that of our own Sun, emerges from our study of the spectra of stars.

Preview

Stars give off light in a continuous spectrum, which includes light of all wavelengths. But when viewed through a spectroscope, we see black bands, or parts of the spectrum that appear to be missing. These gaps are caused by the composition of a star’s atmosphere. When we combine what we know about a star’s distance and its spectrum, we can better understand its luminosity, size, temperature, and ultimately what kind of star it is and where it is in its life cycle.

Key Ideas—Factual

Think about the following questions as you watch the video:

1. What are two different reasons that stars appear to differ in brightness?
2. What is a spectrum?
3. What does it mean that a star gives off a continuous spectrum?
4. What does a star’s atmosphere do to its continuous wavelength?
5. What did Max Planck determine about bluer versus redder stars?
6. How does today’s star classification system arrange stars?
7. What letter classification is our Sun?
8. Why are there no green stars?
9. What is luminosity?
10. What two things does a star’s luminosity depend upon?
11. Describe the three groups of stars Hertzsprung and Russell saw when they plotted a chart of stars’ luminosity versus its temperature.

Thinking Conceptually

Why do you think the most common stars in the universe are red dwarfs? Red dwarfs are the smallest and dimmest stars on the Hertzsprung-Russell (HR) diagram, compared to the rarest and massive hyper-giant stars, which emit hundreds of thousands of times more energy than our Sun, but have a (relatively) short lifespan of only a few million years?

The MK luminosity class is a system for categorizing stars by their luminosity, or actual brightness, and arranges stars from brightest to dimmest. This system generally uses Roman numerals and follows the letters of the Harvard spectral classification system, which uses the mnemonic “Oh Be A Fine Girl/Guy, Kiss Me” (O-B-A -F-G-K-M). Our Sun’s full classification is G2V.

This short article will give you an overview of the Morgan-Keenan (MK) luminosity class, which allows astronomers to divide stars by their luminosity.

Skimming for Gist

The spectral classification doesn’t tell the whole story about stars; in order to distinguish between dwarf and supergiant stars, we need to classify them by their luminosity, or how bright they are. The MK luminosity class ranks stars in order of their actual luminosity, rather than how bright they appear to us from Earth.

Understanding Content

By the end of the second close read, you should be able to answer the following questions:

1. What is the primary way we classify stars?
2. How is a star’s spectral type determined?
3. Why was the Morgan-Keenan luminosity class established?
4. What luminosity class is our G2V Sun?

Thinking Conceptually

In Lesson 2.3, you learned that astronomers study dark matter by looking at the gravitational pull it has on objects surrounding it. Considering that massive gravitational forces can distort light, which stars would you expect to show some distortion in its spectral information?

Annie Jump Cannon’s early interest in the stars led her to a career in astronomy, which was highly unusual for women in her time. She classified hundreds of thousands of stars in her lifetime, and earned several prestigious awards due to her contributions to science. Today, we still use the spectral classification scheme that she devised.

This comic strip will introduce you to more details about the life and work of astronomer Annie Jump Canon. It was originally published in 1949, in issue 33 of the Wonder Woman comic.

Skimming for Gist

Annie Jump Cannon was born in 1863. In her time, there were very few opportunities for women to pursue education beyond high school. She is famous for developing the spectral star classification system and for identifying and classifying hundreds of thousands of stars—more than anyone else in the world.

Understanding Content

By the end of the second close read, you should be able to answer the following questions:

1. What were some childhood experiences that shaped Annie Jump Cannon’s future as an astronomer, as depicted in this comic strip?
2. What were some of the honors she earned?

Thinking Conceptually

This comic was featured in the 1940s, along with the stories of other women in science. The comic doesn’t include three interesting events in Annie Jump Cannon’s life:

1. Throughout her career, she was completely deaf.
2. She was made an honorary member of the Royal Astronomical Society, because at the time, women could not become regular members.
3. Harvard never hired her as a faculty member but named her the “William Cranch Bond Astronomer,” a non-faculty appointment, in 1938 when she was 75 years old.

Does this information make you think any differently about Annie Jump Cannon?

This activity will give you the opportunity to reenact some of the work done by the astronomers they have learned about in this lesson. You will be given a group of stars with a variety of properties, and together with your group you will arrange the stars by different criteria. They your group will make observations and notice any trends that form as a result of their arrangements.

Get into small groups and get a set of Sample Stars from your teacher. Your task is to imagine that you are astronomers working together to sort the stars by color, temperature, and luminosity. Together, write your observations about the stars in each grouping, looking for trends and patterns.

When you are done grouping the stars and writing down your observations, look at the HR chart. Work together to determine where you think each of the stars belongs on this chart, according to their luminosity and temperature. Be prepared to report to the class where you placed each of the stars.

Now, as a class, use the HR diagram your teacher has displayed to determine which stars among the Sample Stars you’ve been working with are supergiants, giants, white dwarfs, and on the main sequence.

Now it’s time to review some of what you learned about star classification systems by returning to the images you looked at in the opening activity of this lesson. You’ll apply what you have learned about how we classify stars, and determine where these stars are along the main sequence.

Look at the opening activity stars again, including their spectra. Notice that these stars share something similar in their classification: the Roman numeral V. This indicates that they are all on the main sequence.

Most of the stars that we see are on the main sequence. Stars spend most of their lifetime on this sequence. Moving up the scale toward the upper left quadrant means moving up in mass; moving down the scale toward the lower right means moving down in mass. Stars don’t move much along this diagonal line, but they do move as they burn their fuel.

Think about these questions and be prepared to share your answers with the class:

1. Which way do you think main sequence stars move along this line during their lifetime?
2. Where does each of the stars on this worksheet belong on the HR chart?
3. Which one is the hottest? The coolest?