Next Lecture Last Lecture Physics Dept

Stars and Stellar Evolution: Lecture 14

Steller Maturity

A main sequence star is one burning hydrogen in its core. A main sequence is very stable. Of course eventually the hydrogen in the core will be used up and the star will have a core almost entirely composed of helium. At this point the core will contract and the hydrogen rich areas outside the core will also contract- this shell will next ignite and the star will become shell hydrogen burning. Such a star works differently from a core hydrogen burning star and is called a red giant

Red Giants

As shell hydrogen takes over the star becomes rather schizophrenic. Although the core is collapseing the outer reaches above the burning shells expand and the whole star spreads out. For our sun the surface of the star will expand until it reaches the orbit of the earth. (at present the suns diameter is about 1/100 A.U.) The outer reaches of the red giant are quite cool - it is red in colour. However because of its huge surface area ( about 10,000 that of the sun ) it is very luminous. Red giants are hence found in the top right side of the HR diagram. Examples of red giants visable late at night are Pollux (in Gemini) and Aldebaron (in taurus)

Evolutionary tracks

A star such as ours will head upwards and rightwards in the HR diagram as it becomes a red giant. At this moment it will be shell hydrogen burning. However temperatures/densities in the core will rise and eventually core helium burning will take place. For small stars this can be explosive and the onset is a helium flash . At this point the evolutionary track takes a sharp turn and moves horizontally towards the left (becoming much hotter). Eventually the core helium burning will run out and the star will start shell helium burning and the star will ascend upwards and rightwards again to become a red supergiant.
HR diagram
Evolutionary track of our Sun

Red Supergiants

Red supergiants are shell helium burning. They are truly huge. An example of a red supergiant is betelgeuse which has a size about the orbit of jupiter. In general a star such as betalguese is rather unstable - its apparant magnitude varies between 0.4 and 1.2. In general after leaving the main sequence stars become much less reliable and stable objects. In general large stars have a fairly complicated evolutionary track in the supergiant area.

Death of Stars

We must determine what happens to stars after helium burning ceases. This depends critically upon the size of the original star.

Small Stars M < 4.0 Ms

onsider such a star when it reaches Red supergiant status. It is burning He to carbon and Oxygen and has a large exterior far from the core. Whilst burning it is probably suffering serious mass-loss The far away gases can be lost in jets and can drift away from the star. In a small star these blown away gasescan form a planetary nebulae (a BAD name - nothing to do with planets!) As helium runs out, because of the small size, pressures never get high enough to "burn" carbon and oxygen. Such stars then become White Dwarves (see pics. and another ). A white dwarf is the left over Carbon/Oxygen core at the centre of a planerary nebula. The outside material has blown off exposing the very-hot carbon/oxygen core. The core then basically just cools down over time. Such an object lies tothe bottom left of a HR diagram since it is very hot but not very bright. Left over will be a "lump" of carbon/Oxygen. As it cools it will move downwards to the right in the HR diagram- eventually becoming a black dwarf . Sirius B is a good example of a white dwarf - it is 0.98 the mass of the sun but very dim. The white dwarf is not as heavy as the original star because of the amount of material ejected. For example a star of mass 3.0 Ms will leave a white dwarf of 1.2 Ms behind ejecting 1.8 Ms in the planetary nebula. A star of mass 1.5 Ms will leave a white dwarf of 0.8 M_s and a star of 0.8 Ms will leave a white dwarf of 0.6 Ms. A white dwarf is extremely dense. The sun will leave behind a white dwarf about the size of the earth. This is a density of around 109 kg/m3 (the earth is 5000).

We have images of a variety of planetary nebula (look for white dwarf in most )

© Dave Dunbar 2019