Chapter 26.3 LIFE CYCLES OF STARS Pearson Prentice Hall Physical Science: Concepts in Action Chapter 26 Exploring the Universe Chapter 26.3 LIFE CYCLES OF STARS Welcome back again. This presentation will concentrate on how stars form and how they die. We will talk about some of the main differences and the two paths stars can take, depending on their mass. We will also talk about nucleosynthesis and how it relates to stars and their lifecycles.

How Stars Form Def: a nebula is a large cloud of gas and dust spread out over a large volume of space Stars form in the densest regions of nebulae and are created by gravity Stars form from huge clouds of gas and dust. These clouds are called nebula. As the nebula gets more dense gravity begins to pull more and more gas and dust into itself.

Nebulae As the nebula contracts, it heats up, producing a shrinking cloud and gas and dust with enough mass to form a star called a protostar A star is formed when a shrinking cloud of gas and dust becomes so dense and hot that nuclear fusion begins As the gravity and the dense cloud continue it begins to heat up. This heat makes the cloud shrink and condense into a protostar. Once the protostar gets dense enough and hot enough the process of nuclear fusion begins.

Formation of Stars This diagram shows the formation of the stars. You see over in the top left corner hydrogen gas and dust in a nebula. Then you see that the gases and dust are being pulled into the center by strong gravitational forces until it becomes extremely compact and dense. This forms the core, which is extremely hot. As the core becomes hotter and hotter nuclear reactions start to take place and as a result a star is formed. https://image.slidesharecdn.com/chapter9starsandgalaxies-140408230052-phpapp01/95/form-3-chapter-9-stars-and-galaxies-12-638.jpg?cb=1396998100

The Main Sequence of a Star (Mass) A star’s mass: determines the star’s place on the main sequence and how long it will stay there Mass is determined by the amount of gas and dust available The most massive stars have the most energy High mass: produce bright blue stars use up their fuel quickly last only a few million years The mass of a star is a huge determining factor in determining the type of star. There are main sequence stars, giant stars, super giants, and dwarf stars. Most stars, like our Sun can be found along the Main Sequence continuum. Mass of stars are determined by the amount of gas and dust that are available when they begin to form. The stars with the higher mass are brighter stars and burn more intensely, but that also means they die out quicker. http://s4.thingpic.com/images/6Y/GFShu79m3SFb3Mw24Y7pNoiG.jpeg

Main Sequence Star Diagram Absolute magnitude: this is a scale that astronomers use ensure at standard scale is used when determining the characteristics of stars. This is a Hertzsprung Russel Diagram. It is used to show the classification of stars. In the middle you can see the Main Sequence stars above the main sequence you can see the giants and the super giants. Below you can see the white dwarfs. You can also see they are classified by absolute magnitude, temperature, luminosity, and spectral class. Absolute magnitude is a scale used by all astronomers in classifying characteristics to ensure consistency. http://phillips.seti.org/kids/images/definitions/hertzsprung-russell-diagram.png

Fuel For Stars The fuel is hydrogen and helium which is converted to energy by nuclear fusion producing heavier elements Middle-sized yellow stars like the sun remain stable for about 10 billion years Our sun is estimated to be in its prime life at about 4.6 billion years Small, cool red stars are long-lived for more than 100 billion years since they use their fuel slowly In the main sequence of stars the fuel stars is fusing hydrogen to helium. As stars move out of the main sequence they begin to fuse other elements such as carbon, oxygen, silicon, neon, iron, and other heavier elements.

So, what happens when the core runs out of hydrogen? Star begins to collapse, heats up Core contains He, continues to collapse But H fuses to He in shell– greatly inflating star  RED GIANT (low mass) or SUPERGIANT (high mass) When a star begins to die it starts to heat up and the core starts to collapse. The core contains helium and the collapse continues in low mass stars the star will then enter the Red Giant stage, higher mass stars the star will progress to a Red Supergiant.

What happens next depends on stellar mass Here you see the progression again as at the left are all the main sequence stars and their masses. The lower mass stars will turn into giants, Planetary Nebulae, and ending in a White Dwarf. Higher mass stars will move from the Main Sequence to Super Giants, Supernova, and then can either turn into either a Black Hole or a Neutron Star. http://web.pdx.edu/~ruzickaa/meteorites/stellarevol&nucleosynthesis.ppt

Running Out of Fuel…the Death Star? When stars run out of fuel in the core (hydrogen and helium for nuclear fusion reactions) the star dies Depending on the star’s mass dead stars will be: a white dwarf, neutron star, or black hole Low and medium mass stars (red and yellow) can be up to eight times as massive as our sun So again just repeating what we’ve said as a star starts dying it’s because it has burned through all of the hydrogen and turned it into helium. The size of the star will determine it’s fate, it can either end in a White Dwarf state (low mass stars) or if it was a high mass star can turn into a black hole or a neutron star.

Life Cycle of Star The life cycle of stars depends on the mass of the star Here is a diagram of the life cycles of low and high mass stars. The upper levels are low mass stars and the lower level shows the fate of the higher mass stars.

Life Cycle of Low-Medium Mass Star nebula: protostar: main sequence star: red giant: planetary nebula: white dwarf http://slideplayer.com/slide/10416117/35/images/3/Life+Cycle+of+Small+&+Medium+Mass+Stars.jpg Here is another sequence of the lower mass stars. They start as a nebula, then create a protostar, main sequence star, red giant, planetary nebula, and end with a white dwarf.

Red Giant As quantity of hydrogen dwindles, gravity becomes stronger than pressure & core shrinks Core temperature rises causing the hydrogen outside the shell to begin fusion Energy flows outward making the star expand & atmosphere cooling in the outer regions causes the star to glow red Def: a red giant is a large reddish star late in its life cycle that fuses helium into carbon or oxygen http://www.antonine-education.com/Image_library/Physics_5_Options/Astrophysics/giant.gif Red Giant are the first stage of death of a low mass star. Here the hydrogen has all burned out to helium and the gravity brings the core in shrinking it further. The temperature in the core raises causing all the hydrogen outside the shell to start fusing. Energy then starts to flow outward from the star expanding and the star’s atmosphere begins to cool causing the star to glow red in color. These new elements are being formed by fusion this is nucleosynthesis.

Planetary Nebula Red Giants can lose its outer layers—ultimately a planetary nebula forms, leaving a white dwarf in the center With decreasing energy from the core and less outward pressure to support the star against gravity’s inward pull, the star collapses producing a glowing cloud of gas (nebula) Def: a planetary nebula is the glowing cloud of gas produced by a dying star Planetary nebula When the star reaches the planetary nebula all the gas is ejected from the star. All of the gas in these outer layers leaves nothing left but a core, which is our White Dwarf. White dwarf http://web.pdx.edu/~ruzickaa/meteorites/stellarevol&nucleosynthesis.ppt

White Dwarf The star blows off most of its mass leaving only its hot core Def: A white dwarf is a very dense star that remains after the fusion in a red giant stops and the star will die The white dwarf cannot fuse and cools slowly for an estimated 20 billion years (longer than the current age of the universe) Our sun will end its life as a white dwarf Once all the mass is blown off only a hot dense core remains. This is called the White Dwarf. Here there is no more fusion taking place and it’s just they dying embers left over continuing to cool. Our sun will eventually end in a White Dwarf. http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit3/Images/nstar.gif

High Mass Stars Thus ends the Life Cycle of Low Mass Stars. Now we turn to the High Mass Stars

Life Cycle of High Mass Star High mass stars have mass greater than eight times that of our sun Their early life and main sequence are similar to low and medium mass stars For high mass stars: nebula: protostar: main sequence: red supergiant: supernova: neutron star or black hole Higher mass stars have higher gas and dust available when they form. This means they can create larger stars. They will be much larger than our smaller mass Sun. In the early stages of their life they are very similar to the low mass stars. They start from nebula, they both create protostar, and they both enter the main sequence. Then they turn into Red Supergiants, Supernova explosions, and then either will go into a neutron star or a black hole. http://linus.highpoint.edu/~mdewitt/phy1050/images/week5/high-mass-cycle.jpg

RED SUPER GIANT As high mass stars start to die, they become red supergiants Def: red supergiants are extremely large stars that create elements as heavy as iron As gravity overcomes pressure in massive stars, the collapse is dramatic creating a supernova Def: A supernova is a powerful explosion that occurs when a massive star dies The explosion can create elements heavier then iron Again high mass stars are also on the main sequence but once they start to die these stars are different from the low mass stars because they will now enter the Red Supergiant instead of the Red Giant stage. These Supergiants are extremely large and start to create (nucleosynthesis) heavier elements such as iron. The gravity will continue to grow until the final collapse in a huge explosion called a supernova. These supernova are powerful explosions which can create even more of the heavier elements (nucleosynthesis) https://dangthatscool.files.wordpress.com/2009/07/stardiagram3.jpg

Supernovae The elements get ejected into space eventually becoming part of other solar systems (like ours) The iron that exists on Earth came from supernovae that occurred billions of years ago When the supernova event occurs all of the elements will get ejected into space and will wander into some other solar system. It’s believed that the iron here on Earth came from supernova billions of years ago. The larger picture is a video, be patient and it will load so you can see it. http://www.dailygalaxy.com/.a/6a00d8341bf7f753ef01b8d2762be6970c-pi https://www.youtube.com/watch?v=aysiMbgml5g

End for high mass star comes as it tries to fuse core Fe into heavier elements– and finds this absorbs energy STAR COLLAPSES & EXPLODES AS SUPERNOVA and the elements created spread out from the explosion Here is another picture of an actual supernova caught on film. http://web.pdx.edu/~ruzickaa/meteorites/stellarevol&nucleosynthesis.ppt

Neutron Star If the mass after the supernova is less than three times the sun’s mass, the star will die as a neutron star Def: A neutron star is a dead star with the density of atomic nuclei Neutron stars are only a few kilometers in diameter but so dense that they are sometimes detected as pulsars Def: Pulsars are rapidly rotating sources of radio waves given off by rapidly rotating neutron stars If the supernova is smaller than 3 suns the star will end as a neutron star with a high density. https://d2gne97vdumgn3.cloudfront.net/api/file/c2EBRQFcQBi501EfaQqU

Black Holes Def: A black hole is an object so massive that not even light can escape its gravity Black holes are observed indirectly (since no light escapes) by gravitational influence of objects around them If the supernova is larger than 3 suns it will create a black hole. A black hole is so massive that not even light can escape the pull of gravity. We can’t actually see the black holes we can only see how they influence objects around them as shown in this clip. https://upload.wikimedia.org/wikipedia/commons/thumb/d/d6/BlackHole_Lensing.gif/290px-BlackHole_Lensing.gif

Nucleosynthesis: Nucleosynthesis: formation of elements Except for H, He (created in Big Bang), all other elements created by fusion processes in stars  Stars are said to evolve with changes in the abundances of the elements within Here you can see the evolution of elements in a supergiant star. We start with hydrogen to helium, helium to carbon, carbon to nitrogen, nitrogen to oxygen, and so on. All of these elements created through the process of fusion in stars. https://helios.gsfc.nasa.gov/onion.gif

Summary of nucleosynthesis processes Process (what’s burning in the core) What it produces Star Responsible Hydrogen Helium Main sequence stars Carbon and Oxygen Red Giant Carbon/Oxygen/Neon/ Silicon Neon through Iron Super Giant Higher processes Many elements Red Super Giants though Supernovae (supernovae produce heavy elements) Here is a quick table showing the progression of elements and which stars are responsible for the production of those elements. http://web.pdx.edu/~ruzickaa/meteorites/stellarevol&nucleosynthesis.ppt

Hydrogen Carbon oxygen Neon through Iron Many heavy elements Hydrogen One more overview of the life cycle of stars and where elements are formed. Both high and low mass stars will start off with a stellar nebula. Here, depending on the mass of the dust and particles around it will either form an average (low-medium) mass star or a high mass star. Both low and high mass stars will enter the Main Sequence and burn hydrogen into helium. Once the hydrogen burns it they both will start the death sequence. Low-Medium mass stars will start fusing carbon and oxygen and the star will become larger and cooler (making it a giant and red in color) from there the outward gases will be ejected in what is called a planetary nebula. Once the material is ejected all that is left is the core, which we call a White Dwarf. The White Dwarf will continue to cool but after the Red Giant phase no more fusion is taking place. The large mass stars will again start in the stellar nebula, move into a protostar (where fusion begins) and enter the Main sequence. On the main sequence it will also burn hydrogen to helium. As it starts to die it will enter a Red Supergiant (super because it’s mass is so large and it is also expanding making it cooler and turning it red). Here it will begin fusing neon and iron until a great explosion occurs, supernova, and depending on the size of the supernova it can create a neutron star or a black hole. The creation of new elements is called nucleosynthesis.