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STARS https://c2.staticflickr.com/8/7317/10785465034_403dd13691_b.jpg And stellar processes Stars Stars are the things you see most of in the night sky. You already know all about the Sun, which is a pretty good example of an average star But what exactly is a star??? Stars Stars are formed by interstellar dust coming together through mutual gravitational attraction. In a gas cloud (Nebula) all the gas particles are attracted to one another. This means they have potential energy. Over time gas and dust particles will move together. The potential energy of the gas is ultimately converted to kinetic energy leading to high temperatures. The high temperatures created by the loss of potential energy can trigger fusion. The fusion process releases so much energy that the pressure created prevents the star from collapsing due to gravitational pressure. Stars In the middle of the stars the high pressure (due to gravitational squeezing of the huge mass of the star) and high temperatures make it too hot for atoms to hold together, and the interior of a star consists of plasma. Plasma is disassociated ions and electrons. In these conditions, it is possible for two protons (hydrogen nuclei) to overcome their electrostatic repulsion and fuse. During the fusion process large amounts of radiation (in terms of Gamma Radiation) are released. The star also releases radiation in all other areas of the electromagnetic spectrum due to its temperature. Nuclear fusion: Proton Proton Cycle. Very high temperatures are needed in order to begin the fusion process: usually 107 K. Nuclear fusion: Proton Proton Cycle. The proton-proton cycle is the process of fusion of protons (Hydrogen Nucleii) into Helium. Each stage of the cycle involves release of energy, but most is released in the final stage of the cycle. 0 1 1 1 H H H e ve 1 1 H H He 1 1 3 2 2 1 2 1 3 2 0 0 He He He 2 H 3 2 4 2 1 1 Nucleosynthesis All stars begin in similar ways, through the fusion of Hydrogen. Stars of different ages, temperatures and sizes have different processes, and different elements are created through fusion. These different elements gather in the outer layers of the star. The proton proton cycle is one of the processes that is referred to as “neucleosynthesis” and is part of the AHL curriculum. More on Nucleosyntheis later. Equilibrium. The energy created through fusion travels outwards creating increasing the Kinetic Energy of the particles within the star and forcing them outwards creating an outwards force on the surroundings. The outward force created by the radiation is called “radiation pressure”. Radiation Pressure pushes outwards on the outer layers of sun. Gravitation pulls the outer layers inwards. These create a lovely balance, and the star is in equilibrium. A star is in “Hydrostatic Equilibrium” between radiation pressure and gravitational forces. Hydrostatic Equilibrium Radiation Pressure Gravitational Pressure The core supports the weight of the whole star! A star is a big ball of gas, with fusion going on at its center, held together by gravity! Massive Star Sun-like Star Low-mass Star There are variations between stars, but by and large they’re really pretty simple things. What is the most important thing about a star? MASS! Mass • The mass of a normal star almost completely determines everything about the star: • The mass controls the internal pressure of the star. • The pressure controls its TEMPERATURE! • The temperature and pressure control the rate of fusion at the core. • The temperature and rate of fusion control the Luminosity of the star. • The mass controls the lifecycle of the star. • The mass controls the length of time a star burns for. • The mass controls what elements are fused within the star. Note: when we say “normal” star, we mean a star that’s fusing Hydrogen into Helium in its center (we say “hydrogen burning”). Luminosity. The LUMINOSITY of a star is how much ENERGY it gives off every second. You can think of luminosity as the total “power output” of the star. Of course, the energy as star emits is generated by the fusion in its core… This light bulb has a luminosity of 60 Watts The core pressure determines the rate of fusion… MASS PRESSURE & TEMPERATURE RATE OF FUSION …which in turn determines the star’s luminosity! Luminosity Main Sequence (you’ll learn this term in a few slides) stars have a relationship between luminosity and mass: 𝐿 ∝ 𝑀3.5 Luminosity The Luminosity of a star is the total energy that it releases per second. Sun has a luminosity of 3.90x1026 W (often written as L): it emits 3.90x1026 joules per second. The energy that arrives at the Earth is only a very small amount when compared will the total energy released by the Sun. Luminosity. So the Luminosity (energy given off by a star) is controlled by the internal Fusion of the star. The amount of internal fusion is controlled by the mass of a star. The Stefan-Boltzmann law relates the energy given off by a body (really a black body) is related to its temperature and its surface area.. L AT 4 Where: σ =5.67x10-8 Wm-2K-4 T is the temperature (in Kelvin) A is the surface area of the object. Luminosity It is worth noting that the surface area of a star (a sphere) is dependent on its radius… A 4r So… 2 L 4r T 2 4 (This eqn is less important than StefanBoltzmann) Luminosity. But remember, energy from the star spreads out in all directions. By the time it reaches us we can only measure the energy arriving at us… we only measure “Apparent Luminosity.” (The amount of energy we receive at Earth is spread out over a vast area… remember from our Climate Change Unit???) Apparent Luminosity (the power we receive at Earth per meter squared) is called “Brightness” (b). (This is the energy per metre squared arriving a the Earth every second from a star). L AT b 2 2 4d 4d 4 Where d is the distance to the star. Information from Light. We can figure out distance (d) to a star, (but we will discuss how in a later slide show). So if we can figure out temperature, we can determine the size of the star. Black Body Radiation • The intensity of light emitted by a black body is distributed over an infinite range of wavelengths. • The peak wavelength emitted is dependent on temperature. • Notice also that different temperatures have different shape curves. Black body radiation Remember: A black body is a perfect emitter. A good model for a black body is a filament light bulb: the light bulb emits in a very large region of the electromagnetic spectrum. The interior of a star is generally considered to be a “black body” There is a clear relationship between the temperature of an object and the wavelength for which the emission is maximum. That relationship is known as Wien’s law: max T constant max T 2.9x10 m K -3 Wien Displacement law By analysing a star’s spectrum, we can know in what wavelength the star emits more energy. The Sun emits more energy at λ=500 nm. According to Wien’s law, the temperature at the Sun’s surface is inversely proportional to the maximum wavelength. So: T 2.9x10 -3 max 2.9x10 -3 5800 K -9 500x10 Black body radiation and Wien Law Star’s Colour and Temperature Why is this important? The spectrum of stars is similar to the spectrum emitted by a black body. We can therefore use Wien Law to find the temperature of a star from its spectrum. If we know its temperature and its luminosity then its radius can be found from StephanBoltzmann law. Real spectra Blackbody Spectrum Emission and Absorption Lines Real spectra are more complicated than this (remember emission and absorption lines?). The elements in the outer layer of stars absorb some of the energy emitted and you get some wavelengths missing. Stellar Classification It has been noticed that stars of certain temperatures also have the same absorption spectra. Suggesting that the same mechanisms are going on inside stars of similar temperatures. Based on the “colour” (ie the temperature) of stars, and their emission/absorbtion spectra. We have classified stars into 7 (ok there are more, but you need to know 7…) classifications. These are : O.B.A.F.G.K.M Stellar classification Each classification has a unique temperature (Class O being the hottest, and Class M being the coolest). You will need to know the order of these. I suggest another Mnemonic. Oloomis Banjo Annoys Friends and Gives Kids Migraines. Or use the ones most commonly used by others… (I did not make the next ones up…) OBAFGKM - Mnemonics O Be A Fine Girl/Guy Kiss Me Only Boring Astronomers Find Gratification in Knowing Mnemonics! The Spectral Sequence Class Spectrum Color Temperature O ionized and neutral helium, weakened hydrogen bluish 31,000-49,000 K B A F G neutral helium, stronger hydrogen blue-white 10,000-31,000 K strong hydrogen, ionized metals white 7400-10,000 K weaker hydrogen, ionized metals yellowish white 6000-7400 K still weaker hydrogen, ionized and neutral metals yellowish 5300-6000 K K M weak hydrogen, neutral metals orange 3900-5300 K little or no hydrogen, neutral metals, molecules reddish 2200-3900 K L no hydrogen, metallic hydrides, alkalai metals red-infrared 1200-2200 K T methane bands infrared under 1200 K Spectra. Spectra tell us about the physics of the star and how those physics affect the atoms in it The Hertzsprung-Russell diagram When all the observed stars were plotted in a graph of Luminosity vs Spectral Class (surface temperature), we start to see some “grouping” of different types of stars. The Hertzsprung-Russell diagram • This diagram shows a correlation between the luminosity of a star and its temperature. • The scale on the axes is not linear as the temperature varies from 3000 to 25000 K whereas the luminosity varies from 10-4 to 106, 10 orders of magnitude. • It’s also important to realize that temperature (along the X axis) is hottest on the LEFT and Coldest on the right. H-R diagram The stars are not randomly distributed on the diagram. There are 3 features that emerge from the H-R diagram: Most stars fall on a strip extending diagonally across the diagram from top left to bottom right. This is called the MAIN SEQUENCE. Some large stars, reddish in colour occupy the top right – these are red giants (large, cool stars). The bottom left is a region of small stars known as white dwarfs (small and hot) Hint… Know your way around the H/R diagram. Look. Types of Stars Main Sequence Stars These are “ordinary” stars that produce energy from fusion of hydrogen and other light nuclei such as carbon and helium. Most (90%) of all stars are main sequence stars. The sun is a main sequence star (G class). Types of Stars Red Giants Red Giant stars are stars (of similar mass to our Sun) approaching the end of their life cycles. In these stars, there is less hydrogen fusion occurring in the core. The reduced fusion leads to reduced radiation pressure from the core and leads to gravitational collapse (within the core). The core then starts to fuse Helium, and fusion that is still occurring happens in a shell surrounding the core. This leads to an increase in the luminosity of the star, and causes the outer layers to expand. These outer layers cool down, and the star appears red (due to lower temperature). Larger than the sun. Cooler than the sun. Similar or Slightly greater Luminosity than the sun. Types of Stars Supergiants MUCH Larger than the sun (radii ~30-500 times greater radii than the sun, or more.) Cooler than the sun (red!). Much greater Luminosity ( up to >1,000,000 times) than the sun. These stars are later stages of the life cycle of mainsequence stars that are considerably more massive than the sun. Supergiants Types of Stars White dwarf stars Small and very hot stars. Since they are so hot, they appear white. These stars are last stage of the life cycle of smaller mass main sequence stars. Fusion no longer occurs in white dwarf stars, they are stellar remnants that are cooling. As they cool and give out less light, they become “brown dwarfs” Types of Stars Neutron Star Neutron stars are the remnants of large mass stars (supergiants) after they go “supernova”. Neutron stars are very dense due to the gravitational collapse of these large mass stars. Neutron stars are composed mainly of neutrons. Pulsars Pulsars are highly magnetized rotating neutron stars which emit a beam of detectable electromagnetic radiation in the form of radio waves. Periods of rotation vary from a few milliseconds to seconds. They emits a beam of radiation from their poles. This acts a little like a lighthouse, it appears to “pulse” when the pole points at us. Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission beams Types of Stars Black hole Ok not really a “star”. (some) Black holes are the remnants of the very largest mass stars, after they have gone supernova. The gravitational collapse cannot be stopped by any known process. The gravitational pull is so great that nothing (including Electromagnetic Radiation) can get away from them, and so we cannot directly observe them. Physicists call anything we can’t see “black”, since the colour black is the result of no EM radiation. We know they exist due to modelling, and due to the fact that they bend light that travels past them. Types of Stars Cepheid variables Cepheid variables are stars of variable luminosity. The luminosity increases sharply and falls of gently with a welldefined period. The period is related to the absolute luminosity of the star and so can be used to estimate the distance to the star. A Cepheid is usually a giant yellow star, pulsing regularly by expanding and contracting, resulting in a regular oscillation of its luminosity. The luminosity of Cepheid stars range from 103 to 104 times that of the Sun. By studying the period of brightness of a Cepheid variable, you can determine it’s luminosity. Once you know its luminosity you can know how far away it is (more on this later). Types of stars Cepheid Variables, are stars which brighten and dim periodically. The time period of variation is proportional to the Luminosity of the star. Types of Stars Binary stars A binary star is a stellar system consisting of two stars orbiting around their centre of mass. For each star, the other is its companion star. 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 clearly distinguished (lower left). Binary Stars Binary Stars are stars are systems of two stars that orbit a common centre (the centre of mass of the two stars combined!). We classify three types of binary stars: Visual Binaries. Eclipsing Binaries. Spectroscopic Binaries. Binary stars Binary stars are useful because we can figure out their masses (or their combined mass) by measuring how long they take to complete one orbit around their centre of mass (the orbital period) Their orbital period (T) is related to their masses by: 2 3 4 d 2 T G(M1 M 2 ) Where d is the distance between the two stars. We can measure T directly (through observation), so this allows us to calculate the mass of the stars. 3 Ways of knowing: Binary stars Some binary stars, you can just see. Visual Binaries. These appear as two separate stars when viewed through a telescope. Binary stars Eclipsing binaries – some binaries are too far away from us to be resolved visually as two separate stars. At big distances two stars may seem to be one. But if the plane of the orbit of the two stars is suitably oriented relative to that of the Earth, the light of one of the stars in the binary may be blocked by the other, resulting in an eclipse of the star, dimming the light from the system for a while. This allows us to determine the period. Binary stars Spectroscopic binaries – this system is detected by analysing the light from one or both of its members and observing that there is a periodic Doppler shifting of the lines in the spectrum. As a moves away it it’s light is slightly more redshifted. As it moves towards us it is slightly more blueshifted. This gives us period. Binary stars A blue shift is expected as the star approaches the Earth and a red shift as it moves away from the Earth in its orbit around its companion. If λ0 is the wavelength of a spectral line and λ the wavelength received on earth, the shift, z, is defined as: 0 z 0 If the speed of the source is small compared with the speed of light, it can be shown that: v z c The speed is proportional to the shift