September 29, 2011

Massive stars and super star clusters – The John Griffiths Memorial Lecture

Prof Allan Willis
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.

Several more slides followed: a beautiful picture of Kepler Supernova 1604; Orion nebula M42; plus several Chandra X-ray images, finishing with an impressive image of The Whirlpool Galaxy M51. Massive ‘O’ stars have Main Sequence lifetimes of a few million years.  (Our sun will have lasted for 10 billion years). Massive O stars are formed in the spiral arms of galaxies and in some galaxies (like M51) are found in Massive Super Star Clusters, where concentrations of hundreds of O stars have been discovered in regions only a few parsecs across. Galaxies like M82, called Starburst galaxies, contains thousands of O stars in their core, which are producing galactic winds and spewing processed material into the Inter-Galactic medium.

Allan then went on to talk about Gamma Ray Bursters which were discovered in 1963 by Vela satellites.  A review in 1973 of Vela data indicates “…interesting signals that do not originate on the Earth, Sun, or Moon.” Recent work has shown that the Long Duration Gamma ray Bursts result from jets of gamma rays produced in the core-collapse supernova of Wolf-Rayet stars.

This led on to his penultimate slide which is worth reproducing here:-

Hot Massive Stars ( O and WR ) are first stars formed after Big Bang

Their strong UV radiation re-ionizes the Universe

Their nuclear burning produces the first generation of elements which enriches the Universe through winds and Super Nova

Enriched material produces new stars and Galaxy formation

The final slide of the evening was of Dr John Griffiths relaxing at his villa in Spain where he had just completed building a home observatory – something that had been an ambition of his for many years, and it is fitting that many of John’s friends, who are Flamsteed Members, are planning to visit the Observatory for a few days next year.

This first John Griffiths Memorial Lecture was a worthy tribute to a great man, a great astronomer, a great friend, and an inspirational teacher.

Posted under: Flamsteed, Flamsteed Lecture, Meeting Report