Date: December 1, 2009

Title: Oh Be A Fine Girl, Kiss Me…


Podcaster: Chris Marr

Organization: The Astronomical Society of Western Australia:

Description: A star is a star, right? Wrong. There are many different types of star, different sizes, different temperatures, different colours. It’s just a jumble, right? Wrong. Amongst all of this variety, there appears to be order and logic. This podcast will attempt to put those 70 sextillion stars in their place and explain the many relationships involved. As you will see, the only part that’s truly jumbled is the human-made part!

Bio: Chris Marr was was raised in the UK, but now he and Viv live in the beautiful city of Perth, Western Australia. Astronomy has always fascinated him but it wasn’t until his 50th birthday that he decided to buy himself a telescope, and only then did he realise he had no idea what to do with it. So he joined his local astronomical society, quickly became the Editor of their newsletter, then Training Officer, and finally President – he loves to write, he loves to talk, and he loves the subject of astronomy, so he’s in heaven!

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the American Association of Variable Star Observers, the world’s leader in variable star data and information, bringing professional and amateur astronomers together to observe and analyze variable stars, and promoting research and education using variable star data. Visit the AAVSO on the web at


Hello, and welcome to this podcast entitled “Oh Be A Fine Girl, Kiss Me”. Firstly, please allow me to apologise for this title. I realise it is a little politically incorrect, but hopefully you will be able to forgive me soon, when the reason for it becomes a bit more apparent. Despite the title, this podcast has nothing to do with kissing, but is all about stars.

A star is a star, right? Wrong. There are a large variety of stars out there. There are different sizes, different temperatures, different brightnesses and different colours, yet despite this great variety there is a rather surprising order and logic to it all. So let’s take a closer look and see how it all fits together.

Firstly, we can see that some stars are much brighter than others. Now brightness, or luminosity, is affected by distance in a known manner called the inverse-square law that was devised in the 17th century. This law states that the intensity of light radiating from a point source is inversely proportional to the square of the distance from the source. In other words if two stars are of the same luminosity but one is twice as far away as the other, the more distant star would appear to be a quarter of the brightness of the closer star. So bearing this in mind, when we see some stars as brighter than others, this could be the result of distance rather than a measure of either star’s intrinsic luminosity.

However, luminosity can be calculated, and there is a mathematical relationship between a starís luminosity, its diameter and its temperature, so knowing two of the values means the third can be calculated. But how do we find any of these variables?

Well, a star’s spectrum tells us a great deal about the star itself. By studying the spectral lines, it is possible estimate the surface temperature of the star and its surface gravity which leads to an estimate of its luminosity. Now we can estimate the star’s diameter as well.

Back in the 1860s, an Italian astronomer named Father Angelo Secchi studied the spectra of around 4,000 stars and classified these stars into 5 distinct classes and subclasses based on their spectral patterns. This was the first stellar classification system. In the 1880s, Williamina Fleming extended this survey at the Harvard College Observatory, and thus was born the Harvard spectral classification system. This system divided Secchi’s classes into a number of more specific classes, using letters of the alphabet. The final result was the 7 classes we use today, which are usually expressed in order of temperature, hottest through to coolest. The classes are O, B, A, F, G, K and M. Now that’s not very easy to remember, so a mnemonic is commonly used to remember it by. What mnemonic you may ask? “Oh be a fine girl, kiss me”, and of course you could also use “Oh be a fine guy, kiss me” depending on your orientation.

In the early 20th century, two astronomers (Ejnar Hertzsprung and Henry Russell) independently plotted a number of starsí luminosity against their temperature.
The resulting chart became known as the Hertzsprung-Russell diagram, and it revealed much about the types of stars that exist in our galaxy. The diagram revealed a rather startling correlation between the two variables such that for the vast majority of stars the hotter the surface, the greater the luminosity. However the truly startling part was that the correlation was so uniform throughout the range of temperatures, from less than 4,000 degrees Kelvin all the way through to 30,000 degrees Kelvin. The plots on the diagram formed a very compact line with surprisingly few aberrations, and this line became known as the “main sequence”. Colour was also found to be closely related to temperature, with the cooler stars being red and as temperature increases the colours change to orange, then yellow, white and finally the hottest stars are blue in colour.

Most stars, like our Sun, spend the majority of their lives as main sequence stars, only appearing anomalous in very early life, and again at the end of their lives when they balloon outwards in size. At this point they can still have a high luminosity because of their immense size, even though their surface temperature has dropped considerably. These stars form their own groupings outside the main sequence, dependant on their size. One group is known as the subgiants, then there are the giants, the bright giants and finally the supergiants. The larger the star, the greater its luminosity, but just like those smaller stars in the main sequence, the colour of each star is based on the surface temperature.

Of course stars also reach a point at which they are unable to maintain that great size and gravity takes over, compressing them down to a relatively small size. A few might form objects like neutron stars or even black holes, but the majority will be reduced to white dwarf stars. At this point the surface temperature can still be very high, but the high reduction in size means that there is also a huge reduction in luminosity, creating another grouping outside the main sequence. As time passes, the temperatures of these dwarfs slowly drops and as it does so, the luminosity also diminishes and the colour changes from blue, to white, then down to yellow, orange, red and darker as it slowly fades away over billions of years.

Anyway, back to our stellar classifications, let’s have a look at them in a bit more detail. As I said, the O class stars are the hottest, and they’re also the largest, the most massive and the brightest, but as far as main sequence stars go, O is the rarest type. The colour of an O class star is blue. With a surface temperature of more than 30,000 degrees Kelvin, an O class star’s luminosity is more than 30,000 times that of the Sun. Of main sequence stars, only three in every 10 million is O class, and because they burn their fuel so furiously to maintain their incredibly high temperatures, they burn out in the shortest time and head off into the giant stage after just a few million years. Zeta Puppis, Zeta Orionis and several other bright stars in Orion are O class stars.

B class stars are slightly more common with 13 of them in every ten thousand stars. They are blue white in colour, being between 10,000 and 30,000 degrees Kelvin at the surface. Bs have quite an extensive range of sizes, so understandably their range of luminosities is vast at anywhere between 25 times that of the Sun and 30,000 times the luminosity of our Sun. Rigel is a B class star, as is Spica and several of the brighter stars in the Pleiades cluster.

When it comes to luminosity, A class stars have a much smaller range of between 5 and 25 times the Sun. Their surface temperature of between 7,500 and 10,000 degrees Kelvin means they appear white to blue white in colour, but the size and mass of an A class star is only just over 1.4 times that of our Sun. Nevertheless, these are still pretty rare with only about 6 in every thousand main sequence stars fitting into this class. Some examples of A class stars are Vega, Sirius, Deneb and Altair.

Next come the F class stars which are only slightly larger and more massive than the Sun but have anywhere between 1.5 and 5 times the luminosity. They appear white thanks to their temperature of between 6,000 and 7,500 degrees Kelvin. F class stars make up around 3% of main sequence stars. Both Canopus and Procyon are F class stars.

What comes next … Oh Be A Fine Girl … that’s right, G class. Now this class contains around 7.6% of the stars in the main sequence, including the Sun itself, so we tend to know quite a bit about this class. Surface temperature is between 5,200 and 6,000 degrees kelvin, and the mass, size and luminosity are all around the same as the Sun. And of course as any kid who’s ever drawn the Sun will tell you, it’s pale yellow in colour. This class of star can expect to remain on the main sequence for around 10 billion years. Apart from our Sun, Alpha Centauri A and Capella have G class attributes.

K class stars are more of an orange-yellow colour thanks to their relatively cool temperature of 3,700 to 5,200 Kelvins. They are slightly smaller, dimmer and less massive than the Sun but they make up about one eighth of all main sequence stars, so we see a lot of them including Arcturus, Aldebaran and Alpha Centauri B.

The last grouping in the Harvard spectral classification system is that of the M class stars. These stars are the coolest at less than 3,700 degrees Kelvin at the surface. They have a distinctly red-orange look about them. They can be up to 70% of the size of the Sun, but contain less than half the solar mass, and can be difficult to see with a luminosity of only around one twelfth that of the Sun. A lot of the stars in the class are considered red dwarf stars, like Proxima Centauri.

Off the main sequence in the area of giant stars, the spectral classes still exist, but things are slightly different. These stars still keep the same groupings as the main sequence stars based on temperatures and therefore colours, but their other attributes are quite different. An M class giant star will still have a surface temperature of up to 3,700 degrees Kelvin, and will still appear reddy-orange, but it will have a luminosity 1 million times that of an M class star on the main sequence, primarily because of its size. Huge bright stars like Antares and Betelgeuse are giant M class stars, and in fact most giant stars are classified as M class.

My name’s Chris Marr. I’m with the Astronomical Society of Western Australia, and I hope that was helpful.

End of podcast:

365 Days of Astronomy
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