H-R Diagrams? More like Hella-Rad Diagrams!

The Hertzsprung-Russell diagram is a tool used by scientists to classify stars by their luminosity, spectral types, and temperatures. The diagram design was developed in 1910 by Ejnar Hertzsprung and Henry Norris Russell. Along the x-axis are surface temperatures in Kelvin, and along the y-axis are solar units of luminosity. These diagrams can be used to trace out the life sequences of stars and can even help estimate a star’s distance from our planet.

hr diagram

When looking at an H-R diagram, you’ll notice a line of stars clustered together from the top left to the bottom right. This is known as the main sequence, and this is where most stars fall. Most of these stars are known as dwarf stars. Hotter dwarf stars have larger radius’s than cooler dwarf stars. “Red dwarfs” are cool, dim, small stars found on the main sequence.

Below the main sequence are stars classified as “white dwarfs,” which are low-mass stars that have died. They are hotter, dimmer, and smaller than stars on the main sequence.

Above the main sequence and to the left are “super giants,” which are very large and very cool stars. One example of a supergiant is Betelgeuse, which is a red supergiant nearing the end of its life. In between the super giants and the main sequence are “giant stars,” which are larger and cooler than main sequence stars but not as large or cool as super giants.

REVISION: An article posted on arstechnica.com titled, “Massive survey makes sense of the diversity of quasars” describes how scientists have recently begun to classify quasars on a quasar-specific version of the H-R diagram. The quasar H-R diagram even has a main sequence like the diagrams we learned about in class, with most quasars falling onto this main line. This connects to what we learned in class because it shows how H-R diagrams can be used to organize and classify celestial bodies beyond just stars.

REFLECTION: I thought it was really interesting how scientists thought to modify the diagram to fit a different type of celestial body (quasars) in order to learn more about something that is still surrounded by so much mystery. It shows how organization and classification of the data that you do have can help in obtaining data that you don’t have.


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