14-Feb-18 INSIDE STARS. Matter and Energy. Matter and Energy. Matter and Energy. What is matter?

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1 What is matter? What is matter? It's what everything that occupies space and has mass is made of And everything like that is made out of chemical elements There are 92 naturally occurring chemical elements, and a number of synthetic ones Where were all the naturally occurring chemical elements made? What is matter? It's what everything that occupies space and has mass is made of And everything like that is made out of chemical elements There are 92 naturally occurring chemical elements, and a number of synthetic ones: Where were all the naturally occurring chemical elements made? INSIDE STARS 1

2 What is energy? It s what makes things happen It s what makes matter move We buy energy every day What are some forms of energy you ve bought this week? So you can see that energy comes in a variety of forms Forms of Energy Kinetic energy -- energy of motion Potential energy -- energy stored for later release as kinetic or radiative there are several types: gravitational chemical electrical nuclear Radiative energy -- energy carried by electromagnetic waves The different forms of energy can be converted into one another Understanding the conversions is essential to understanding astronomy And this is tied to another fundamental conservation law: Conservation of Energy 2

3 Conservation of Energy In an isolated system, energy can change form, but the total amount never changes Anything that happens involves an exchange of energy between material objects and/or the conversion of energy from one form to another. Here's an example Chemical PE (food) KE (lifting weight) gravitational PE (holding weight) KE (weight falling) various KEs (thermal energy, work, sound) Kinetic energy and thermal energy Kinetic energy = energy of motion, already encountered in orbital energy Thermal energy is the total kinetic energy in a system of randomly moving particles Thermal energy is related to the temperature of the system The average kinetic energy of the particles in the system is given by But which temperature scale? It has to be the Kelvin scale Thermal motion and thermal energy Thermal motion can be surprising How fast do you think air molecules move? 3

4 Thermal motion and thermal energy Thermal motion can be surprising How fast do you think air molecules move? Thermal motion and thermal energy Thermal motion can be surprising How fast do you think air molecules move? Thermal motion and thermal energy Thermal motion can be surprising How fast do you think air molecules move? 4

5 all the 5

6 all the all the Which box has the higher temperature? all the Which box has more thermal energy? 6

7 all the Which box has the higher temperature now? all the Which box has more thermal energy? all the Systems can have the same temperature but different thermal energies And vice versa 7

8 all the It is thermal energy, not temperature, that causes burns Think about this kitchen example The 212ºF oven has less thermal energy than the boiling water More Potential energy Gravitational potential energy, already encountered in orbital energy Mass-energy Important for Astronomy Kinetic energy energy of motion Thermal energy randomized kinetic energy of collection of particles This is the average KE of the particles Gravitational potential energy energy due to position in a gravity field Mass-energy energy equivalent of mass 8

9 How much mass do you think is equivalent to the amount of energy released by metabolizing a ~2 ounce candy bar 1 x 10 6 J? merely half a billionth of an ounce How much is contained in 1 kg of mass? 9 x J, the equivalent of an 18 megaton nuclear bomb, 1200 times the bomb that wiped out Hiroshima in X more than is released by fissioning 1 kg of uranium! Energy moves matter What is matter made of? It is composed of the chemical elements There are 92 naturally occurring ones But there are many more substances than that (???)... Why? Because atoms combine to form molecules and compounds These have very different properties than the elements they are made of H 2 gas O 2 gas S solid H 2 O liquid H 2 SO 4 liquid Atoms are very small If a typical atom were the size of this room, its nucleus would be a fraction of a millimeter in diameter The tiny nucleus contains nearly all of the atom's mass and determines the identity of the element 9

10 What distinguishes one element from another is the number of protons in the nucleus The atoms themselves come in different forms called isotopes What distinguishes one element from another is the number of protons in the nucleus The atoms themselves come in different forms called isotopes The carbon isotopes shown above are examples Isotopes differ in the number of neutrons in the nucleus Isotopes are often radioactive, an astronomically important property Different combinations of atoms form different substances Once a substance forms, it can exist in different states or phases Most of us are familiar with the solid, liquid, and gas phases These differ in how much the particles are moving Which depends on the temperature Their motion affects how strongly they stick together Adding or extracting thermal energy, which changes the temperature, can cause "phase transitions, generally in the sequence shown for water at left 10

11 but phase transitions can also happen out of sequence Sublimation: solid gas And sometimes they occur with no change in overall thermal energy Evaporation: liquid gas But all phase changes are caused by particles gaining or losing energy and how this affects the particles motions which can affect the possible types of bonds between particles If the temperature gets really high, there will be transitions to phases that are not so familiar... These are called plasmas In plasmas, because of the high temperatures, the particles themselves dissociate Plasmas are important in astronomy because they are found in all stars Do you understand phase changes? Suppose that a chunk of an unknown solid substance was found on an alien world in which the bonds between atoms were unbreakable. Would phase changes be possible for this substance? A. Yes B. No 11

12 Do you understand phase changes? Suppose that a chunk of an unknown solid substance was found on an alien world in which the bonds between atoms were unbreakable. Would phase changes be possible for this substance? A. Yes B. No, because if the bonds can t be broken, the phase can t change An individual atom has several forms of energy: Mass-energy (E = mc 2 ), by virtue of the mass the atom (mostly the nucleus) has Kinetic energy (KE = ½mv 2 ), if the atom is moving Electron orbital energy (internal to the atom), very important in astronomy An electron in an atom can have different energies But it can t have just any energy Instead, the energy states are quantized (the basis of quantum theory) This diagram shows the energy states for a hydrogen atom An electron in a hydrogen atom can only have these energies 12

13 The electron can move between these energy states But transitions only happen if the energy gained or lost exactly equals the difference between energy states And this leads to one of the most important techniques used in astronomy Spectroscopy Energy lost in a downward transition is emitted as a photon of light Absorption of photons can supply the energy for an upward transition This is where emission and absorption spectra come from And these spectra provide astronomers with a surprising wealth of information 13