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The Hertzsprung-Russell (HR) diagram is a visual representation of stellar evolution. The diagram demonstrates the relationship between stars’ temperature and luminosity, or the relationship between the stars’ color (also known as spectral type) and absolute magnitude. As stars progress through evolutionary stages, these factors change, and these different stages correspond to different regions on the HR diagram. You,too, can learn to use the diagram to learn more about stars’ structure and evolutionary stages - just read on!
By analyzing questions, you can see patterns emerge, patterns that will help you answer questions. Qwiz5 is all about those patterns. In each installment of Qwiz5, we take an answer line and look at its five most common clues. Here we explore five clues that will help you answer a tossup on the Hertzsprung-Russell Diagram.
MAIN SEQUENCE STARS
The HR diagram is divided into three regions corresponding to the evolutionary stages of stars. The most prominent region is known as the main sequence, and it moves from the top left of the HR diagram to the bottom right. For main sequence stars, the hotter they are, the greater their brightness. Main sequence stars fuse hydrogen to helium in their core.
INSTABILITY STRIP
Another region on the HR diagram is a narrow, nearly vertical area known as the instability strip. Stars in the instability strip are usually variable stars, such as Cepheid variables. These stars exhibit instabilities that cause them to pulsate, or change in both luminosity and size. Typically, stars that are more massive than the sun wind up in the instability strip at some point after exiting the main sequence of stellar evolution.
HAYASHI TRACK
The Hayashi track is a path on the HR diagram illustrating the evolution of pre-main sequence stars. Stars in the Hayashi track (typically T Tauri Stars) slowly contract, maintaining their surface temperature but decreasing in luminosity, resulting in a nearly vertical curve on the HR diagram. Eventually stars in the Hayashi track will initiate nuclear fusion and either become main sequence stars or enter the Henyey track.
HENYEY TRACK
The Henyey track is a horizontal path on the HR diagram followed by certain pre-main sequence stars. Stars on the Henyey track have a mass greater than 0.4-0.5 solar masses. These stars become hotter as they contract, but their luminosity remains nearly constant. Stars on the Henyey track eventually reach a sufficient temperature to begin fusion, at which point they join the main sequence stars.
ASYMPTOTIC GIANT BRANCH
Stars with lower mass exit the main sequence eventually and become red giants, which are larger and cooler than stars on the main sequence. The red giant region is in the upper right quadrant of the HR diagram. However, there is an additional region above the red giant region (demonstrating greater luminosity) known as the asymptotic giant branch. This region is reserved for stars that have completed burning helium in their core, unlike red giants, which have typically not initiated helium fusion.
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Quizbowl is about learning, not rote memorization, so we encourage you to use this as a springboard for further reading rather than as an endpoint. Here are a few things to check out:
This website is a good jumping-off point for learning more about variable stars, including the female astronomer who contributed to our understanding of them.
Visit this website to learn more about the different spectral classes.
Before there were stars there were…protostars. What’s a protostar? Find out here!
Ever wonder about the journey of a star from cradle to grave? Watch this video for an overview of the life cycle of a star.
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