Reading and Announcements Read Chapter 14.1, 14.2 Homework #6 due Tuesday, March 26 Exam #2, Thursday, March 28
The life of the Sun The Sun started as a cloud of gas. Gravity caused the cloud to collapse. The central temperature and density became high enough to start nuclear fusion. The Sun joined the main sequence.
Main Sequence Stars Sequence stars are all fusing H to He in their cores. The lifetime of a star is determined by its mass. The sun will spend about 10 billion years on the main sequence. Main
The Structure of Stars Inner convective, outer radiative zone Inner radiative, outer convective zone CNO cycle dominant PP chain dominant
What happens when the H in the core runs out? The sun is not mixed in its core. A big ball of Helium forms in the core. No more heat is produced in the core from fusion. Thermal pressure from fusion Gravitational Contraction Balance is lost and gravity causes the Helium core to collapse.
How does the Helium core push back? As matter compresses it becomes denser (and heats up). Eventually the electrons are forced to be too close together. A quantum mechanical law called the Pauli Exclusion Principle restricts electrons from being in the same state, i.e. it keeps them from being too close together. The resulting outward pressure that keeps the electrons apart is called electron degeneracy pressure; this is what supports the core against collapse. Indistinguishable particles are not allowed to stay in the same quantum state.
When core hydrogen fusion ceases, the sun leaves the main sequence and becomes a giant Electron degeneracy pressure supports the Helium core. The enormous weight from the outer layers compresses hydrogen in the layers just outside the core enough to initiate shell hydrogen fusion. The sun overproduces energy and the new thermal pressure causes the outer layers to expand. As it expands, the surface cools, and it becomes a luminous red giant.
Up the red giant branch Hydrogen fuses only in a shell around the helium core. The sun will swell to an enormous size, as large as the earth's orbit. Sun in ~5 Gyr Sun today
Anatomy of a Star that is leaving the Main Sequence Hydrogen fuel Hydrogen burning core shell ABSOLUTELY NOT TO SCALE: In a 5 Msun star, Helium ash if core has size of a quarter, envelope has size of a baseball diamond. Yet, core contains 12% of mass
Helium fusion begins at the center of a giant While the exterior layers expand, the helium core continues to contract and eventually becomes hot enough (100,000,000 K) for helium to begin to fuse into carbon. He fuses through a number of reactions, generally referred to as the Triple process. He + He + He = C + energy and produces an element crucial to our existence: CARBON
After helium fusion gets going Carbon builds up in the core and eventually a helium-burning shell develops. This shell is itself surrounded by a shell of hydrogen undergoing nuclear fusion.
After helium fusion gets going The Sun will expand and cool again, becoming a red (super) giant. Earth, cooked to a cinder during the red giant phase, will be engulfed and vaporized within the Sun. At the end of this stage, the Sun s core will consist mostly of carbon, with a little oxygen.
The life of the Sun For a star like the sun the carbon core never gets hot enough to ignite nuclear fusion (needs 600,000,000 K ). The outer layers are shed forming a Planetary nebula.
Planetary Nebula At the center of the nebula there is a remnant stellar core. Destiny of stars with roughly M < 7 Msun
White Dwarfs The remnant core is called a White Dwarf. No longer making energy just slowly cooling off like a giant ember. Very dense! Carbon and Oxygen held up by electron degeneracy pressure. One teaspoon weighs 15 tons. Maximum size: 1.4 MSUN. Called the Chandresakhar Mass. Largest mass that can be supported by electron degeneracy pressure.
The Life of the Sun
The life of a massive star More massive stars, M>7MSUN start out like the sun. However, the central temperature and density becomes high enough to fuse carbon and eventually heavier elements.
Nuclear burning in massive stars Many successive fusion stages. Many successive red giant phases. Many shells of fusion. Eventually builds up an iron core.
Cannot fuse iron and release energy The lead-up to disaster in massive stars Iron cores do not immediately collapse owing to electron degeneracy pressure. If the core mass becomes bigger than 1.4 MSUN eventually the electrons are forced to combine with the protons resulting in neutrons. What comes next is core collapse: a Supernova explosion.
The time spent in each phase: Each successive stage takes less and less time.
Massive Star Explosions: Supernovae The gravitational collapse of the core releases an enormous amount of energy. All the shells ignite, and the star literally explodes 100 times the total amount of energy produced by the Sun in its lifetime is released in a matter of seconds. A neutron star or black hole is left behind. For a few days, the star is as luminous as a whole galaxy!!! Then the luminosity decays in the following months.
Supernova 1987a before/after
Remnant from a supernova: The Crab Nebula The supernova explosion that created the Crab was seen on about July 4, 1054 AD.
Supernova Remnant Cassiopeia A
The life of a massive star
Stellar Evolution in a Nutshell M < 7 MSun M > 7 MSun Mcore < 3MSun Mass controls the evolution of a star! Mcore > 3MSun
Brown Dwarfs Stars with masses < 0.08MSUN Gravity not strong. Central temperature and density low. Too low to fuse hydrogen into helium. Called brown dwarfs.
Low Mass stars: M<0.4MSun The lifetime is 1010/(M/MSUN)2.5 years. Older than the age of the Universe Nature also makes more low mass stars than high mass stars. That is why these are the most common stars. Not massive enough to fuse Helium into Carbon Will eventually end as Helium white dwarfs.
Production of Elements in the Universe Hydrogen and Helium are initially created in the Big Bang. Stars process Hydrogen and Helium into heavier elements (elements up to iron). Elements heavier than iron are generated only in the deaths of high mass stars (supernovae). We were all once star stuff. Parts of us were formed in a supernova.
Big Bang Stars Supernovae