Stellar Evolution

Stars form from interstellar clouds rich in hydrogen....they shine for billions of years, gradually converting hydrogen into helium....when hydrogen is exhausted, they need to find other energy sources....they expand to become giants and supergiants....low-mass stars expel planetary nebulae and become white dwarfs....high-mass stars explode as supernovae, leaving a neutron star or black hole behind. That is the simple picture, reality is more bizarre. The sky is full of exotic stars which don't seem to fit. Why not? What can they tell us about normal stellar evolution?

(from Stephen Tonkin's tutorial on stellar evolution)

In Armagh, we study highly evolved stars, representing the last stages of stellar evolution. Many of these are peculiar because they have virtually no hydrogen on their surfaces. Some represent groups of extremely rare stars. Many show pulsations. Many present challenges to stellar evolution, stellar pulsation and stellar atmospheres theory. In many cases, the basic question is "What is their origin?" Our WWW pages give a flavour of current research.

Extreme helium stars are B- and A-type supergiants, rich in carbon and nitrogen but practically devoid of hydrogen.

Hot subdwarfs are O and B-type stars lying just beneath the main-sequence. They are less massive than the Sun. Some are extremely helium rich.

Hydrogen-deficient binaries are A-type supergiants in binary systems with orbital periods 1 - 12 months. They are comparatively young stars, with masses 1 - 3 times the Sun. Are they the precursors of type Ib supernovae?

R Coronae Borealis stars fade unpredictably by factors of up to 1000. What is the obscuring dust and how does it condense?

The evolution of normal stars compared with
stellar remnants studied in Armagh (schematic)

Stellar atmospheres are the outermost layers of a star from which light is radiated. The passage of radiation through the atmosphere modifies the spectrum. This can tells us about the conditions and composition within the atmosphere. Modelling stellar spectra requires sophisticated techniques, and lots of atomic data.

Stellar pulsations are observed in many stars across the Hertzsprung-Russell diagram, and in all of the classes of star described above. We can learn about the fundamental properties of stars from their pulsations.

A simulation of pulsating helium star V652 Her (300kB mpeg)

To find out more, try Simon Jeffery's Homepage