STARS Option D 2 Stellar Characteristics Stars A

STARS Option D. 2 Stellar Characteristics

Stars A star is a big ball of gas with fusion going on in its center that is held together by gravity Stars are formed by interstellar dust coming together through mutual gravitational attraction The loss of potential energy is responsible for the initial high temperature necessary for fusion Fusion process releases so much energy that the pressure created prevents the star from collapsing due to gravitational pressure Called Stellar Equilibrium Thermal Pressure out = Gravity in

Nuclear Fusion in Stars emit a great deal of energy Source for all energy is the fusion of hydrogen into helium (sometimes referred to hydrogen burning) Rxn is nuclear not chemical

Nuclear Fusion in Stars Mass of the products is less than the mass of the reactants Using ∆E=∆mc 2 we can work out that the sun is losing mass at the rate of 4 × 109 kg s-1 Takes place in the core of the star Eventually all this energy is radiated from the surface Calculate the rate at which energy is being emitted

Stellar Spectra Radiation from stars is not a perfect continuous spectrum There are particular wavelengths that are missing The missing wavelengths correspond to the absorption spectrum of a number of elements Although is seems sensible to assume that the elements concerned are in the Earth’s atmosphere, this assumption is incorrect Wavelengths would still be absent if light from the star was analyzed in space. Absorption is taking place in the outer layers of the star This means that we have a way of telling what elements exist in the star- at least in its outer layers

Stellar Spectra Absorption Lines and Classifications

Stellar Spectra Wavelengths absorbed are characteristic of atoms present Absorption spectrum can be used to identify the elements present in the outer layers of the star Deduce both chemical and physical data about stars Chemical composition and surface temperature Speed and direction of motion using the Doppler Effect If source of waves is moving towards us, their frequency is shifted upwards Stars moving towards us are called blue-shifted If source of waves is moving away from us, their frequency is shifted down Stars moving away from us are called red-shifted

Luminosity is total power radiated by a star (Watts) Different than power received by an observer on Earth (brightness) Depends on surface temperature of the star and radius L α T 4 L α r

Apparent Brightness of a star is the power per unit area received by an observer on Earth (Wm-2) If two stars were at the same distance away from the Earth then the one with the greatest luminosity would be brighter as the distance increases, the brightness decreases since the light is spread over a bigger area But, stars are at different distances from the Earth (d) It is thus possible for two very different stars to have the same apparent brightness all depends on how far away the stars are

Temperature of Stars If the energy emitted from a star is analyzed over a range of wavelengths, the peak wavelength can be determined Using Wien’s Law, the surface temperature of the star can be determined As temperature increased, total energy increases (area under curve) Luminosity is proportional to T 4 Stefan-Boltzmann law

Binary Stars A binary star is a stellar system consisting of two stars orbiting around their common center of mass Called companion stars A large percentage of stars are part of systems with at least two stars Binary star systems are very important in astrophysics, because observing their mutual orbits allows their mass to be determined The masses of many single stars can then be determined by extrapolations made from the observation of binaries Hubble image of the Sirius binary system, in which Sirius B can be distinguished at the lower left

Single Stars The source of energy for our sun is the fusion of hydrogen into helium This is also true for many other stars There are however, other types of stars that are known to exist in the universe RED GIANT STARS WHITE DWARF STARS CEPHEID VARIABLES

Red Giants As the name suggests, these stars are large in size and red in color High Luminosity Since they are red, they are comparatively cool Source of energy is the fusion of some elements other than hydrogen (He to heavier elements) Red Supergiants are like red giants only cooler, more luminous and bigger These stars are the largest structures in the universe, although they are not the most massive

White Dwarfs & Brown Dwarfs These stars are small and white in color Low Luminosity Since they are white they are comparatively hot Fusion is no longer taking place, and a white dwarf is just a hot remnant that is cooling down They are usually composed of oxygen and carbon in an extremely dense form Eventually it will cease to give out light when it becomes sufficiently cold It is then known as a brown or black dwarf

Cepheid Variables Unstable stars that have completed its hydrogen burning phase Have regular variations in brightness and hence luminosity due to the oscillation in the size of the star A stellar layer loses hydrostatic equilibrium Cycle of Cepheid Star: 1. Outer layer becomes compressed due to gravity 2. Temperature inside increases, thus increasing outward pressure 3. Layer pushed out ward by increasing pressure 4. Expansion caused layer to cool and less dense 5. More radiation escapes and pressure decreases 6. Outer layer becomes more compressed due to gravity

Hertzsprung-Russell Diagram Hertzsprung and Russell discovered the relationship between surface temperature and luminosity around 1910 H-R diagram is an attempt to look for patterns in the stars like the periodic table was constructed to look for patterns in the elements Each dot on the diagram represents a star The scales are NOT LINEAR Temperature scale runs backwards (high temp on left) 90% of stars fall on the diagonal band known as main sequence Ordinary stars still producing energy through fusion (like sun)

Absolute magnitude Betelgeuse Rigel Sirius B Color index, or spectral class

Hertzsprung-Russell Diagram Starting at lower right are the coolest stars, reddish in color Further up towards the left are hotter and more luminous stars that are yellow and white Still further up are the more luminous blue stars Mass of a star increases moving up the main sequence Gravitational pressure increases with mass, so to maintain equilibrium, fusion reactions in the core must generate a greater radiation pressure Star has to ‘burn’ at a higher temperature, giving it a greater luminosity L α M 3. 5

Stars in the vicinity of the Sun

Specific segments of the main sequence are occupied by stars of a specific mass Majority of stars are here

Hertzsprung-Russell Diagram About 1% of stars are red giants and supergiants Their high luminosity and low temperature means they have a very large area About 9% of stars are white dwarfs White dwarfs are very hot and not luminous Low luminosity and high temperature means they are much smaller than main sequence Final group of stars (cepheids) congregate in a great band of instability that appears between main sequence and red giants