Stellar Physics AH Physics RMA Mr Stewart Birth

Stellar Physics AH Physics RMA Mr Stewart

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 temperature and pressure in the central mass becomes large enough to sustain nuclear fusion. The initial size of a star depends on the amount of matter present when the fusion process begins

H-R diagrams 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

example � In an exam you will only see a simple black and White HR diagram � This is from the 2013 paper: a) What class of star is Sirius B 1 b) Estimate the radius of Betelgeuse 2 c) 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

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. You may have covered this 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

Temperature & peak wavelength The hotter a star is the more “blue” it appears. This is because the emitted peak wavelength shifts more towards the blue end of the spectrum. Also, the hotter it is, the more intense it’s light is. (energy radiated per m 2 per second ) simulation

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 600 nm, what is it’s temperature? T = 2. 9 x 10 -3 / 600 x 10 -9 m = 4830 K

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 in this relationship 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.

Stellar Luminosity � Example: Calculate the luminosity of the sun if it has a surface temperature of 5780 K and a radius of 6. 94 x 108 m Answer: 3. 86 x 1026 W

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 ‘d’ from the star increases Note ‘d’ in this equation is the distance from the star. (This is another example of the inverse square law. ) This assumes there are no energy losses due to interstellar material or atmospheric absorption Example: Calculate the apparent brightness of the Sun on the surface of the Earth. (solar luminosity = 3. 86 x 1026 W)

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

p-p chain

Example –(2017 paper)

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

Death of a star �A star remains on the main sequence until the hydrogen in the core drops below a critical level. This causes hydrogen fusion in the core to stop. �The star contracts due to gravity, temperature and pressure increase until helium starts fusing into carbon in the core. The star then expands as thermal outwards pressure now exceeds the inwards gravitational effects. The star cools as it expands and becomes a red giant. �An average sized star (similar to our Sun) will eventually collapse as fusion stops and gravity takes over and it becomes 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. (see next slide) �Any material left over from a supernova can collapse and become either a neutron star or a Black Hole

Death of a massive star �When a star ten times more massive than the Sun exhausts the helium in the core, the core contracts further and reaches high enough temperature to fuse carbon to oxygen, neon, silicon, sulphur and finally to iron. Iron is the most stable form of nuclear matter and there is no energy to be gained by fusing it to any heavier element. Without any source of heat to balance the gravity, the iron core collapses to a very dense core. 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 shines 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 stars and planets. Without supernova, there would be no carbon, oxygen or other elements that make life possible. https: //map. gsfc. nasa. gov/universe/rel_stars. html

2015 Open-ended question Woodstock - Mattew's Southern Comfort

“Billion year-old stardust” sample answer When a star ten times more massive than Sun exhausts the helium in it’s core, the carbon core contracts further and reaches high enough temperature to fuse carbon to oxygen, neon, silicon, sulphur and finally to iron. The iron core collapses under gravity. 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. 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.