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Units to cover: 59, 60, 61, 62 The H-R Diagram • • A star’s location on the HR diagram is given by its temperature (x-axis) and luminosity (y-axis) We see that many stars are located on a diagonal line running from cool, dim stars to hot bright stars – • Other stars are cooler and more luminous than main sequence stars – – • The Main Sequence Must have large diameters (Red and Blue) Giant stars Some stars are hotter, yet less luminous than main sequence stars – – Must have small diameters White Dwarf stars The Family of Stars Stars come in all sizes… The Mass-Luminosity Relation • If we look for trends in stellar masses, we notice something interesting – Low mass main sequence stars tend to be cooler and dimmer – High mass main sequence stars tend to be hotter and brighter • The Mass-Luminosity Relation: L » M 3.5 Massive stars burn brighter! Massive stars burn brighter L~M3.5 Luminosity Classes Stellar Evolution – Models and Observation • • • • • Stars change very little over a human lifespan, so it is impossible to follow a single star from birth to death. We observe stars at various stages of evolution, and can piece together a description of the evolution of stars in general Computer models provide a “fast-forward” look at the evolution of stars. Stars begin as clouds of gas and dust, which collapse to form a stellar disk. This disk eventually becomes a star. The star eventually runs out of nuclear fuel and dies. The manner of its death depends on its mass. Evolution of low-mass stars Evolution of high-mass stars Tracking changes with the HR Diagram • As a star evolves, its temperature and luminosity change. • We can follow a stars evolution on the HR diagram. • Lower mass stars move on to the main sequence, stay for a while, and eventually move through giant stages before becoming white dwarfs • Higher mass stars move rapidly off the main sequence and into the giant stages, eventually exploding in a supernova Interstellar Gas Clouds • Stars begin as a cloud of cold gas and interstellar dust, a molecular cloud • The cloud begins to collapse in on itself – Collapse is triggered by a variety of phenomena – Stellar winds, explosions, etc. – Collapse heats the center of the cloud – gravitational energy is being converted to heat. • Rotation of the cloud forces it into a disk-shape • After a million years or so, the center of the disk develops a hot, dense core called a protostar Protostars • Once a dense core forms in the disk, the system has entered the protostar stage • Protostars are difficult to find – they are shrouded by gas and dust • Infrared telescopes can detect them. The Eagle Nebula Protostars Bipolar Flows • Once the protostar heats to around 1 million K, some nuclear fusion begins • Narrow jets of gas can form, flinging stellar material more than a lightyear away! • These jets can heat other clouds of gas and dust Jets are launched from young stars A. Due to nuclear blasts in the star B. Due to magnetic forces acting on accreting material C. Due to radiation forces from the hot nuclear burning star core D. Due to gravitational pull of the star on the jet material Why is it that the majority of stars in the sky are in the main sequence phase of their lives? • a. Because this is the only phase that is common to all stars • b. because most stars die at the end of main sequence phase • c. because most stars in the sky are created at about the same time • d. because this is the longest lasting phase in each star life Tracking the birth of stars The birth tracks of lowand high-mass stars From Protostar to Star • Low-mass protostars become stars very slowly – Weaker gravity causes them to contract slowly, so they heat up gradually – Weaker gravity requires low-mass stars to compress their cores more to get hot enough for fusion – Low-mass stars have higher density! • High-mass protostars become stars relatively quickly – They contract quickly due to stronger gravity – Core becomes hot enough for fusion at a lower density – High-mass stars are less dense! The CNO cycle • Low-mass stars rely on the protonproton cycle for their internal energy • Higher mass stars have much higher internal temperatures (20 million K!), so another fusion process dominates – An interaction involving Carbon, Nitrogen and Oxygen absorbs protons and releases helium nuclei – Roughly the same energy released per interaction as in the proton-proton cycle. – The C-N-O cycle! Internal Structure of Stars - Convection • Convection occurs in the interiors of stars whenever energy transport away from the core becomes too slow – Radiation carries away energy in regions where the photons are not readily absorbed by stellar gas – Close to the cores of massive stars, there is enough material to impede the flow of energy through radiation – In less massive stars like the Sun, cooler upper layers of the Sun’s interior absorb radiation, so convection kicks in – The lowest-mass stars are fully convective, and are well mixed in the interior. The Main-Sequence Lifetime of a Star • The length of time a star spends fusing hydrogen into helium is called its main sequence lifetime – Stars spend most of their lives on the main sequence – Lifetime depends on the star’s mass and luminosity • More luminous stars burn their energy more rapidly than less luminous stars. • High-mass stars are more luminous than low-mass stars • High mass stars are therefore shorter-lived! • Cooler, smaller red stars have been around for a long time • Hot, blue stars are relatively young. Two Young Star Clusters How do we know these clusters are young?