Studying the Lives of Stars  Stars don’t last forever  Each star is born, goes through its life cycle, and eventually die

Life Cycle of a Star

A Star is Born  A star is made up of a large amount of gas in a relatively small volume  A nebula, is a large amount of gas and dust spread out in in an immense volume  All stars begin their lives as parts of nebulas  Gravity can pull some of the gas and dust in a nebula together and it begins to spin – this is called a stellar nebulla  The contracting cloud is then called a protostar

Protostar – The Earliest Stage  A protostar is the earliest stage of a star’s life  A protostar is born when the contracting gas and dust become so hot that nuclear fusion starts

Main Sequence  Once nuclear fusion occurs in the cloud, it will begin to glow brightly  It contracts a little and becomes stable  It is now a main sequence star and shines as nuclear fusion produces heat and light The star may remain in this stage for millions or billions of years

Red Giant  As the main sequence star glows, hydrogen is converted to helium  When the hydrogen supply begins to run out, the core becomes unstable and contracts  The outer shell starts to expands and glows red as it cools  It is now a red giant Red Giant – expands and glows red as it cools

 The outer shell begins to expand, cools, and glows red Red Super Giant

Medium vs. Massive Stars  The amount of mass a star has determines the path it will take after the red giant phase

Medium Stars – Planetary Nebula  At 200,000,000 °C 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  The ring is a planetary nebula

White Dwarf  When all the helium atoms are fused, the star begins to die  In white dwarfs, gravity causes the last of the star’s matter to collapse inward and compact  It is now a white dwarf The star’s matter is extremely dense White dwarfs are the size of Earth, but have the mass of the Sun  Once all the energy is gone, it becomes a black dwarf, which is a star that is dead and no longer emits light

Massive Stars - Supernova Stephen Hawking - Supernova  Once massive stars reach the red giant phase, the core temperature continues to increase  The energy is eventually released in a powerful explosion called a supernova  Supernova – super giant explodes  The temperature of a supernova can reach 1,000,000,000 °C

Supernova

Famous Supernova Remnant - Crab Nebula

Neutron Stars  A star that is 1.5 – 4 times as massive as our sun becomes a neutron star after a supernova  Neutron stars spin rapidly giving off radio waves  Neutron star – core collapses and becomes very dense  If the radio waves are emitted in pulses, they are called pulsars

Black Holes  The most massive stars become black holes when they die  In a black hole, no nuclear fusion is taking place, so it is swallowed by its own gravity making it a black hole  Black holes are not visible They must be detected by the x-rays which are given off The Largest Black Holes in the Universe Naked Science Spaghettification

Lifetime of Stars  A star’s life cycle is determined by its mass!  The larger the mass, the shorter the life cycle  Small stars use up their fuel more slowly than large stars, so they have much longer lives Small stars can live up to 200 billion years Medium stars (like the sun) can live for about 10 billion years Giant stars usually live for about 10 million years

Checkpoint  If a star is twice as massive as the sun, will it have a longer or shorter life than the sun? Shorter!

Deaths of Stars  When a star begins to run out of fuel, the center of the star shrinks and the outer part of the star expands The star becomes a red giant or supergiant  When a star runs out of fuel, it becomes a white dwarf, a neutron star, or a black hole

Life Cycle AnimationLife Cycle Animation - classzone BBC - Describe StarsStars: Life and Death