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The Life Cycles of Stars A star’s life cycle is determined by its mass. The larger the mass, the shorter the life cycle. A star’s mass is determined by the amount of matter that is available in its nebula, the giant cloud of gas and dust in which it is born. Over time, gravity pulls the hydrogen gas in the nebula together and it begins to spin. As the gas spins faster and faster, it heats up and is known as a protostar. Eventually the temperature reaches 15,000,000 oC and nuclear fusion occurs in the cloud’s core. The cloud begins to glow brightly. At this stage, it contracts (gets smaller) a little and becomes stable. It is now called a main sequence star and will remain in this stage, shining for millions or billions of years to come. As the main sequence star glows, hydrogen in the core is converted into helium by nuclear fusion. When the hydrogen supply in the core begins to run out, the core becomes unstable and contracts. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands (gets bigger), it cools and glows red. The star has now reached the red giant phase. It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths it will take after the red giant phase. Lower mass stars like our sun range from 0.5 times the mass of our sun up to 4 times the mass of our sun. A medium mass star has a mass ranging from about 4 solar masses up to about 8 solar masses. A high mass star is typically 8 times or larger with the largest found to date (R136a1) coming in at 265x the mass of our sun. SMALLER STARS Throughout the red giant phase, the hydrogen gas in the outer shell continues to burn and the temperature in the core continues to increase. At 200,000,000 oC the helium atoms in the core fuse to form carbon atoms. The last of the hydrogen gas in the outer shell is blown away to form a ring around the core. This ring is called a planetary nebula. What’s funny about this name is that planetary nebula cannot form planets. The name was an accident because past scientists thought planets could form from this type of nebula. Unfortunately, the name stuck. When the last of the helium atoms in the core are fused into carbon atoms, the star begins to die. Gravity causes the last of the star’s matter to collapse inward and compact. This is the white dwarf stage. At this stage, the star’s matter is extremely dense. White dwarfs shine with a white hot light. Once all of their energy is gone, they no longer emit light. The star has now reached the black dwarf phase in which it will forever remain. MORE MASSIVE STARS Once medium and high mass stars reach the red giant phase, the core temperature increases as carbon atoms are formed from the fusion of helium atoms. Gravity continues to pull carbon atoms together as the temperature increases forming oxygen, nitrogen, and eventually iron. At this point, fusion stops and the iron atoms start to absorb energy. This energy is eventually released in a powerful explosion called a supernova. The temperature in a supernova can reach 1,000,000,000 oC (that’s one billion degrees!). A supernova is so bright that it can light up the night sky for several years. A supernova that occurred in 1040 AD was so bright that it was visible during the day for ~3 months. The core of a medium star that is 4 to 8 times as massive as our Sun ends up as a neutron star after the supernova. Neutron stars spin rapidly giving off radio waves. If the radio waves are emitted in pulses (due to the star’s spin), these neutron stars are called pulsars. The core of a massive star that has 8 or more times the mass of our Sun remains massive after the supernova. No nuclear fusion is taking place to support the core, so it is swallowed by its own gravity. It has now become a black hole which readily attracts any matter and energy that comes near it. Black holes are not visible. They are detected by the X-rays which are given off as matter falls into the hole. Just like living things and humans, stars have a life cycle, which consists of birth, growth, development, middle age, old age, and death. The life cycle of a star spans over billions of years. Section One - Sequencing The stages below are not in the right order. Number the stages in the correct order (1-6). _____ The star begins to run out of fuel and expands into a red giant or red super giant. _____ Stars start out as diffused clouds of gas and dust drifting through space. A single one of these clouds is called a nebula _____ What happens next depends on the mass of the star. _____ Heat and pressure build in the core of the protostar until nuclear fusion takes place. _____ The force of gravity pulls a nebula together forming clumps called protostars. _____ Hydrogen atoms are fused together generating an enormous amount of energy igniting the star causing it to shine. Section Two - Vocabulary Match the word on the left with the definition on the right. ____ black dwarf a. star left at the core of a planetary nebula ____ white dwarf b. a red super giant star explodes ____ nebula c. what a low-mass star becomes at the very end of its life ____ protostar d. a large cloud of gas or dust in space ____ supernova e. exerts such a strong gravitational pull that no light escapes ____ neutron star f. the earliest stage of a star ’s life ____ black hole g. the remains of a medium mass star Section Three – Graphic Organizer – Putting it all Together Section Three – Understanding Main Ideas - Low Mass Star ____ 1. Red giant ____ 2. Where fusion begins ____ 3. Nebula ____ 4. Black dwarf ____ 5. The stage the sun is in ____ 6. White dwarf ____ 7. Planetary Nebula Section Four – Understanding Main Ideas - High Mass Star ____ 1. Black Hole ____ 2. Supernova ____ 3. Protostar ____ 4. Gravity causes this to condense into a protostar ____ 5. Main sequence star ____ 6. When a star begins to run out of fuel and grows larger ____ 7. Neutron star THOSE A-MAZE-ING STARS Use what you have learned about stars to find your way through the maze below. Begin at the start box, carefully read the statement in each box and decide if it is true or false. Circle the correct answer. You will move from box to box by following the directional arrows (T = TRUE, F = FALSE). Continue to follow the arrows until you reach the true end of the maze (END E). All other exits are incorrect. If you exit at A, B, C, or D, retrace your steps to find where you got off track.