Life of a Star

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Goal of the class To understand the life cycle of stars and their fates Question of the day: How do the stages of life differ for average stars and massive stars. Previous answer: Stars are classified by their size, brightness, temperature, color, and composition

Stars Lifetime All stars live between 3 million and 1 trillion years Not infinite The lifespan of the star depends on its mass Greater mass gives a shorter lifespan Smaller stars and massive stars evolve in different ways

Nebulae and Protostars All Stars come from nebulae Dust and gas particles attracted together make a large mass The friction of dust and gas causes enough heat to start nuclear fusion This causes a protostar to form Stage 1- Stars are born in a region of high density Nebula, and condenses into a huge globule of gas and dust and contracts under its own gravity. Stage 2 - A region of condensing matter will begin to heat up and start to glow forming Protostars. If a protostar contains enough matter the central temperature reaches 15 million degrees centigrade.

Hertzsprung-Russel Diagram Most stars are main sequence Main sequence get brighter as they get hotter I showed this last time H-R Diagram. The main sequence is this diagonal line.

Main Sequence During the main sequence stars fuse hydrogen Our sun is a star in its main sequence Stars expand to giants once they run out of hydrogen May expand up to 100 times larger Massive stars become supergiants Stage 3 - At this temperature, nuclear reactions in which hydrogen fuses to form helium can start. Stage 4 - The star begins to release energy, stopping it from contracting even more and causes it to shine. It is now a Main Sequence Star. The nearest main sequence star to Earth, the Sun Stage 5 - A star of one solar mass remains in main sequence for about 10 billion years, until all of the hydrogen has fused to form helium. Stage 6 - The helium core now starts to contract further and reactions begin to occur in a shell around the core. Stage 7 - The core is hot enough for the helium to fuse to form carbon. The outer layers begin to expand, cool and shine less brightly. The expanding star is now called a Red Giant.

White Dwarf The final stage of a medium star The red giant lose all of the gas around it and give off less heat and light The mass will stay the same but the size will shrink to the size of a planet Eventually becomes a black dwarf Class Temperature Sample star O 33,000 K or more Zeta Ophiuchi B 10,500–30,000 K Rigel A 7,500–10,000 K Altair F 6,000–7,200 K Procyon A G 5,500–6,000 K Sun K 4,000–5,250 K Epsilon Indi M 2,600–3,850 K Proxima Centauri

Main Sequence (Massive Stars) During the massive stars life, it fuses hydrogen to helium like an average star It becomes a supergiant once it uses up its hydrogen The temperature of a supergiant goes to 6,000,000,000 as it fuses atoms into iron Average stars fuse atoms into carbon Stage 1 - Massive stars evolve in a simlar way to a small stars until it reaces its main sequence stage (see small stars, stages 1-4). The stars shine steadily until the hydrogen has fused to form helium ( it takes billions of years in a small star, but only millions in a massive star). Stage 2 - The massive star then becomes a Red Supergiant and starts of with a helium core surrounded by a shell of cooling, expanding gas. Stage 3 - In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core.

Supernovae When a massive star runs out of material it goes through a supernova The explosion makes a new nebula around the star Scientist think this is where heavier elements come from Stage 1 - Massive stars evolve in a simlar way to a small stars until it reaces its main sequence stage (see small stars, stages 1-4). The stars shine steadily until the hydrogen has fused to form helium ( it takes billions of years in a small star, but only millions in a massive star). Stage 2 - The massive star then becomes a Red Supergiant and starts of with a helium core surrounded by a shell of cooling, expanding gas. Stage 3 - In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core. Stage 4 - The core collapses in less than a second, causing an explosion called a Supernova, in which a shock wave blows of the outer layers of the star. (The actual supernova shines brighter than the entire galaxy for a short time). The total energy output may be 1044 joules, as much as the total output of the sun during its 10 billion year lifetime. Stage 5 - Sometimes the core survives the explosion. If the surviving core is between 1.5 - 3 solar masses it contracts to become a a tiny, very dense Neutron Star. If the core is much greater than 3 solar masses, the core contracts to become a Black Hole.

Neutron Star After supernova it becomes a neutron star or a black hole at the end of the star’s life A neutron star will have the mass of 1-5 suns in 10 km Made entirely of neutrons

Black Holes Black holes are formed by stars too large to form neutron stars Gravitational power of a black hole is so powerful that light cannot escape The collapse of a black hole causes incredibly dense material at its centre

Vocabulary Nebula – a cloud of dust and gas in space Supernova – the explosion of a star caused by gravitational collapse