Flamsteed Astronomy Society

The John Griffiths Memorial Lecture— 29 September 2011

‘Massive stars and super star clusters’ by Prof Allan Willis

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Report by Chris Gadsden

This was the first John Griffiths Memorial Lecture, presented in honour of Dr John Griffiths who sadly passed away in 2010.   John was a passionate astronomy teacher and a good friend and mentor to many Flamsteed Members.  The lecturer on this important occasion was Professor Allan Willis who was a colleague and friend of John’s since they were both undergraduates at UCL 40 years ago.

Prof Willis projected some slides of their student life together in the early 1970’s, including shots of them playing football, John’s wedding to Kath (at which Allan was Best Man) and the title page of John’s Doctoral Thesis.

After graduation both John and Allan stayed at UCL to do their PhDs - John went on to study stars in the infra-red group while Allan joined the Ultraviolet group, concentrating on the investigation of the most massive and luminous stars, with particular emphasis on the physical and chemical nature of Wolf-Rayet stars.

Allan’s talk encompassed the following topics:-

What are massive stars?

The HR Diagram

Stellar Evolution Time Scales

Properties of Hot Massive Stars

Evolutionary links (O stars -> Wolf Rayet stars)

Massive stars formation

Massive stars in Galaxies

Super star Clusters

The first topic – What are massive stars? – can be answered quite simply: a star is defined as “massive” if its mass is between 20 and 100 solar masses.  It will also be very hot, typically between 30,000 and 60,000 ºK and with a luminosity of between 100,000 and 1,000,000 solar units.

Allan projected the HR diagram on which the principal characteristics of the various star groups are shown.  The massive stars of the ‘O’ group are shown at the top left hand side of the diagram.

A feature of massive stars is their mass loss and stellar winds.  The mass loss affects the evolution of the star by “peeling down” the outer atmospheres to reveal interior nuclear processed material.

There followed a spectrograph of an ‘O’ star showing flux plotted against wavelength. Spectrographs are among the most useful tools for the study of the stars.  As Prof Willis so aptly put it: “If a picture is worth 1000 words, a spectrograph to an astrophysicist is worth 1000 pictures”.  Several more spectrographs were shown including UV spectra of O stars obtained with the  FUSE satellite (Far Ultraviolet Spectroscopic Explorer), WN and WC stars (also obtained by FUSE). It is now known that the WR stars are the chemically evolved descendents of massive O stars.

Another very interesting slide came up next – the schematic chemical structure of a massive star just prior to a supernova explosion, depicting the composition of the star from the centre outwards:

Fe > Si > O > Ne > C > He > H. 

This nuclear-processed material is returned to the Interstellar medium via stellar winds, outbursts from Luminous Blue Variables – like Eta Carina - and from supernova explosions.

All the material that makes up the planet Earth comes from the stars.


Prof Allan Willis

[Pic: Grey Lipley]

John & Allan


HR Diagram


Structure of massive star prior to supernova explosion


John, Kath, & Allan

Kath & John’s wedding


‘Astronomy 3 UCL’ 1972/3