How do you deal with making sense 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, and then divide the data into different categories. That’s what gets done in the census. That is also how we made sense of a population of thousands, or millions, or billions of stars.
There are two things that are obvious: firstly stars come in a range of brightnesses, and secondly, they come in a variety of colours. So we could tabulate them in terms of brightness and colour. However, since stars are at a wide range of distances from us, the brightness alone means little. So we start measuring how far away they are. If we know the distance and how bright a star looks, we can estimate its energy output, which we call its luminosity. Now we are getting somewhere. 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 U.S.A. The result was what is now known as the Hertzsprung-Russell Diagram, has become one of the most important tools in the study of stars and how they work.
What surprised them both is 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 at 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 made possible by Hertzsprung and Russell, people had categorized stars into classes: A, B, C, D and so on. However, the H-R diagram as it is now known, 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 starhood 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 order. However, if there is no other mnemonic, here is my suggestion. “Oh, be a fine girl, kiss me! Like this? Yes!”
Jupiter, Mars, Venus and Mercury are all clustered low in the sunrise glare, and are hard to see. The only bright planet available is Saturn, which rises about 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 the 24th, and be new on the 1st of June.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at DRAO, Penticton.