Like this? Yes!

In the sky this week…

  • Jupiter, Mars, Venus and Mercury are all clustered low in the sunrise glare, and are hard to see.
  • The only bright planet visible is Saturn, which rises around 5 p.m. and is well up in the eastern sky by dark. Look for a moderately bright, yellowish “star.”
  • The Moon will reach Last Quarter on May 24 and be New on June 1.

Ken Tapping, May 25, 2011

How do you make sense out of data describing a large number of individuals, all of which differ slightly or dramatically from one another? One way is to pick some properties that are easily measurable and differ from individual to individual, then divide that data into different categories. This same approach is used to conduct the census. This is also how we managed to sort out a population of thousands, millions, or billions of stars.

There are two things that are obvious about stars; they vary in brightness and come in a variety of colours. We could tabulate stars according to these two criteria, however since stars are at a wide range of distances from us, the brightness alone means little. We then start measuring how far away they are. If we know the distance and how bright a star looks, we can estimate its energy output or “luminosity.”

Imagine plotting each star on a graph, with luminosity along one axis and colour on the other. Around 1910 two scientists did this, completely independently of one another — Ejnar Hertzprung in Denmark, and Henry Norris Russell in the USA. The result was the “Hertzsprung-Russell Diagram,” and has become one of the most important tools in the study of stars and how they work.

Skygazing by Ken Tapping

Hertzsprung-Russell diagram. A plot of luminosity (absolute magnitude) against the colour of the stars ranging from the high-temperature blue-white stars on the left side of the diagram to the low temperature red stars on the right side. Image: Richard Powell

What surprised both Hertzsprung and Russell, was that most stars lie on a track going from the dim-red corner to the bright-blue corner. This track is now called the “Main Sequence.” There are also stars not on the Main Sequence, but they are a minority. This famous diagram is an important tool in understanding the lives of stars. We now know that most stars lie on the Main Sequence because they spend most of their lives there. They drop onto it soon after they are born and then spend most of their lives moving up it a little, in the hot-blue direction, and then leave it in their old age.

Before the breakthrough of Hertzsprung and Russell, people had categorized stars into classes: A, B, C, D and so on. However, the H-R diagram, together with other research, showed the order of types was wrong. If we arrange the classes running from bright-blue to dim-red, we get O, B, A, F, G, K and M. Astronomy students remember this using the mnemonic “Oh, be a fine girl, kiss me”.

Over the last couple of decades, improving telescopes have shown us dimmer and cooler stars, and those that never quite graduated to stars because they didn’t have enough mass to start nuclear fusion in their cores. Now we are adding new classes to the Main Sequence; the latest are progressively cooler and fainter stars in classes L, T and Y. Unless those who named these classes have a particularly witty mnemonic up their sleeves, it is hard to see why they chose those letters in that specific order. However, if there is no other mnemonic, here is my suggestion — “Oh, be a fine girl, kiss me! Like this? Yes!”

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mail: ken.tapping@nrc-cnrc.gc.ca

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