Properties of stars Hydrogen and helium fusion reactions Stellar evolution

1. Stellar Physics (pages 22-25) AH Physics • Properties of stars • Hydrogen and helium fusion reactions • Stellar evolution • The Hertzsprung-Russell (H-R) diagram

2. Stellar Physics is the study of stars through their lifecycle. There are estimated to be about 1011 galaxies in the Universe, each estimated to contain 1011 stars, so our best guess is that there are about 1022 stars. We can see just a few thousand from Earth with the naked eye.

3. Astronomical Distances One Astronomical Unit (AU) is the average orbital distance of the Earth from the centre of the Sun. It is about 1.5 x 1011 m. The AU is usually used for measuring distances inside the Solar System. One light-year (ly) is the distance travelled by light in one year. It is equivalent to 9.47 x 1015 m. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. It is about 3 x 1016 m, and is used for measuring distances outside the Solar System.

4. Apparent Brightness ‘b’ • Luminosity is the total power emitted from the surface of the star. • The power we receive here on earth per square metre from a star is called the apparent brightness ‘b’(Wm-2) • The light energy emitted from a star spreads out over the surface area of an ever increasing sphere as the distance ‘r’ from the star increases Example: Calculate the apparent brightness of the Sun on the surface of the Earth. (solar luminosity = 3.86 x1026 W) Note ‘r’ in this equation is the distance from the star. (This is another example of the inverse square law.) b = 1.37 MWm-2 This assumes there are no energy losses due to interstellar material or atmospheric absorption

5. Problems 128 to 130.

6. Stellar Luminosity • Luminosity of a star is a measure of the total energy given off per second. • Luminosity is measured in watts (It’s the total power output of the star!) • Luminosity only depends on the temperature and surface area of the star. The temperature of a star can be determined from the spectrum of light it emits. This was mentioned in Higher Physics. The hotter a star is, the further the peak wavelength in its spectrum is shifted towards the blue end of the visible spectrum. The relationship between peak wavelength and temperature is known as Wien’s Law

7. Temperature & peak wavelength The hotter a star is the more “blue” it appears. This is because the peak wavelength it emits shifts more towards the blue end of the spectrum. Also, the hotter it is, the brighter or more intense it is. simulation

8. Wien’s Law (revision from Higher Physics – not in the AH course) • The temperature of a star is related to the peak wavelength it emits by this relationship: Temperature is measured in kelvin (K) wavelength is measured in meters. (m) Example: if a star has a peak wavelength of 600nm, what is it’s temperature? T = 2.9 x10-3 / 600 x 10-9m = 4830 K

9. Temperature and Power per unit area The greater the temperature of a hot object, the more energy it radiates per second (Power) per unit area. The relationship between temperature and power emitted from hot objects was investigated by 2 scientists: Josef Stefan (1879) and Ludwig Boltzmann. They both found that the power per unit area (in Wm-2) was directly proportional to the fourth power of the Kelvin temperature. (The Stefan-Boltzmann Law) (The constant of proportionality is called the Stefan-Boltzmann constant. (σ) “sigma”) Kelvin temperature Wm-2 The value of the Stefan-Boltzmann constant (σ) pronounced “sigma” is approximately 5.67 x 10 -8 W m-2 K-4 This equation is on the relationships sheet and the constant is on the data sheet.

10. Luminosity • The luminosity of a star is a measure of it’s total power output. • It is the power per unit area x surface area of the star. • Considering the star to be a sphere: surface area = • Therefore luminosity Example: Calculate the luminosity of the sun if it has a surface temperature of 5780K and a radius of 6.94 x 108m Answer: 3.86 x 1026 W

11. Problems 132 to 135.

12. The Life cycle of stars Stars go through a range of changes over a period of time. The various stages of the life of a star, and it’s final outcome, depends on the initial mass of the star https://map.gsfc.nasa.gov/universe/rel_stars.html

13. Birth of Stars • Stars are born in interstellar clouds that are particularly cold and dense (relative to the rest of space). Stars form when gravity causes a molecular cloud called a nebula to contract until the central object becomes hot enough to sustain nuclear fusion. The initial size of a star depends on the amount of matter present when the fusion process begins

14. Proton-proton chain • Fusion occurs when the temperature and pressure is high enough • Protons fuse together in the sun. This produces a chain of nuclear reactions that finally results in the production of helium. • Energy is released at various stages during the fusion reaction

15. proton-proton chain in stellar fusion reactions converts hydrogen to helium.

16. H-R diagrams Tap for simulation • A Hertzsprung Russell (H-R) diagram is a scatter graph showing the relationship between a stars brightness (luminosity) and its temperature • There are 4 main regions • Main sequence • White dwarfs • Giants • Supergiants • Our sun is a main sequence star • Eventually it will become a red giant

17. Tap for video (1:39) Tap for video (10:40)

18. example • In an exam you will only see a simple black and White HR diagram • This is from the 2013 paper: What class of star is Sirius B 1 Estimate the radius of Betelgeuse 2 Ross 128 and Barnard’s star have similar temperatures but Barnard’s star has a slightly greater lumiosity. What other information does this tell you about the two stars? 1 (white) dwarf 6.955 x 1011 m Barnard’s star is larger / more massive

19. Example –(2017 paper)

20. Problems 136 to 142.

21. Death of a star • Fusion continues until the source of hydrogen in the star drops below a critical level. This causes the star to expand as the thermal outwards pressure exceeds the inwards gravitational effects. • The star will cool as it expands and become a red giant. • An average sized star (similar to our Sun) will then collapse as gravity takes over and become a super dense, super hot white dwarf • Very large stars (10 times bigger than our Sun) become red supergiants and then end their lives in a massive explosion called a supernova. • Any material left over from a supernova can collapse and become either a neutron star or a Black Hole

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23. Stellar Nucleosynthesis The process of stellar nucleosynthesis was first postulated by the British astronomer, Fred Hoyle, in the late 1940s when he was trying to understand how elements heavier then hydrogen and helium could have arisen in the Universe. The production of these heavier elements by nuclear fusion in a star of mass 25MSun is summarised opposite.

24. Supernova Nucleosynthesis Nuclear fusion in stars produces elements with nucleon numbers up to iron and nickel. Further fusion does not release energy, in fact it requires energy. It is thought that this (endothermic) energy absorption at the instant of a supernova event is responsible for the creation of the rarer elements heavier than nickel. This is known as supernova nucleosynthesis.

26. 2015 Open-ended question Woodstock - Crosby Stills Nash & Young

27. “Billion year-old stardust” sample answer When a star ten times more massive than Sun exhaust the helium in the core, the nuclear fusion cycle continues. The carbon core contracts further and reaches high enough temperature to fuse carbon to oxygen, neon, silicon, sulphur and finally to iron. Without any source of heat to balance the gravity, the iron core collapses until it reaches nuclear densities. This high density core resists further collapse causing the in falling matter to "bounce" off the core. This sudden core bounce produces a supernova explosion. For one brilliant month, a single star burns brighter than a whole galaxy of a billion stars. Supernova explosions inject carbon, oxygen, silicon and other heavy elements up to iron into interstellar space. They are also the site where most of the elements heavier than iron are produced. This heavy element enriched gas will be incorporated into future generations of stars and planets. Without supernova, the fiery death of massive stars, there would be no carbon, oxygen or other elements that make life possible.