Stars and Steller Evolution Lecture 12

Hertzsprung-Russell Diagram

Empty graph Add our sun and other stars main sequence stars Red Giant stars Red Supergiants stars Blue Giant stars White Dwarf stars

• The obvious main feature is the Main Sequence of stars. This includes the sun and many others. It covers a band from dull, yellow stars right up to bright blue/white stars.
• The grouping known as Supergiants containing Rigel and Deneb. very bright stars - and yellow-red
• Red giants - a group of red bright stars, notably Betalgues, Antares, Aldebaron.
• White Dwarves - stars on the ``blue side'' of main sequence. Basically wimpish but white stars. Includes Sirius B.

Main Sequence Stars

In the sun there are two opposing forces at work. Firstly there is gravity - this force will always try to contract the star. Opposing this force there is pressure from within. In the sun these two forces is in stable equilibrium. The sun is almost entirely hydrogen and helium. Since these, in the core, are very hot they produce a pressure which balances the gravitational attraction. Changing the temperature would change the balance. Since the Sun radiates heat something must produce energy in the sun - otherwise it would cool. The ``powerhouse'' at the centre of the sun is thought to be nuclear fusion reactions. To understand how the sun works we must understand nuclear fission and fusion. The radius of the central core where fusion occurs is about 0.2 of the suns radius.

Nuclear Fusion and Fission

• Breaking up a large nucleus (twice as big as iron say) into two peices closer to iron plus energy. This is Nuclear Fission
• Taking two small nuclei and joining them together to make a larger nucleus (closer to iron). This is Nuclear Fusion
• All current nuclear power station use fission - breaking up a very large nucleus such as uranium.
• Nuclear weapons come in both varieties. Hiroshima/Nagasaki were both fission bombs. The more powerful hydrogen bombs use fusion
• Main sequence stars are powered by the reaction 4H -> He That is joining together four hydrogen nuclei (via various intermediate steps) to form a helium nucleus. (positrons and neutrinos are also produced)
• Hydrogen nuclei (a single proton!) have positive electric charge. Thus they naturaly repel. It is difficult to get the Hydrogens close enough to ``stick''. This is why fusion power generation has so far eluded us. It needs high ebnergies/pressures/temperatueres to get the hydrogen close.
• The intense pressures/temperatures at the suns core allow fusion to occur.

  ---

  The sun is a complicated object. It does not have a "hard" edge like the earth but has layers of increasing density/pressure.
• The core is where the fusion takes place temperature reach 14 million degrees,
• The remainder of the interior cobsists of Hydrogen/Helium but pressure/temperature are not enough for fusion.
• there is a 200-300 km layer called the photosphere. Here the temperature is around 6000C. This is where all the light that we see it emmited. It is not a solid boundary but is an optical boundary -we cannot see through it.
• Above the photosphere is the Chromosphere, (transparent)
• Finally we have the Corona which is very hot but very dilute gas. Visable during solar eclipses.

• sun in ``hydrogen-alpha light -with huge primensance
• ditto
• sun in ultraviolet light -spot the spots!
• sun in x-rays -again with prominent spots
• sun in x-rays -with false colours
• close up of prominence (in x-rays)
• picture of corona with centre of picture blanked off
• another of the same
• picture showing how the sun spots change over time

Solar Model

Mass	Surface T	Luminosity	Lifetime ( million yrs)
0.5	4000	0.03	200,000
0.75	5000	0.5	15,000
1	6000	1	10,000
1.5	7000	5	3,000
3	11,000	60	500
15	30,000	10,000	15
25	35,000	80,000	3

I have introduced a new concept - Luminosity . This is the energy ( in light of its various forms) emitted by the star. I use units in terms of the sun's luminosity.

© Dave Dunbar 2020