Astronomy 103: The Evolving Universe Lecture Notes PDF

Astronomy 103: The Evolving Universe Stars, Galaxies, & Cosmology Lecture 16: The Evolution of Massive Stars Monday, Nov 4  Top Hat Question Immediately after the Sun converts all of its core hydrogen into helium... A) helium begins fusing into carbon B) its...

Astronomy 103: The Evolving Universe Stars, Galaxies, & Cosmology Lecture 16: The Evolution of Massive Stars Monday, Nov 4  Top Hat Question Immediately after the Sun converts all of its core hydrogen into helium... A) helium begins fusing into carbon B) its core will contract and heat up C) its core will expand and cool D) it becomes a white dwarf  Today Stellar evolution of massive stars  Top Hat Question What power do you have to shape the communities you live in and society overall? Answer what you honestly believe! A) None – what I do won’t matter B) I have individual power through my actions C) I have political power through my vote D) Both B and C November 5 2025 Election vote.wisc.edu 5 Election Day: November 5 Polls open 7:00am–8:00pm Why Vote? Go to your assigned polling place Ensuring a pluralistic, You can register at the polls democratic society is a Have approved ID ready fundamental right and responsibility of citizens of Am I eligible to vote in Wisconsin? our country. Yes, if you: Are a U.S. citizen; Will be 18 years of age on or before Election Day; Have resided in Wisconsin for at least 28 consecutive days before Election Day; and Are not currently serving a felony sentence (includes probation/parole) Voting on Election Day in Wisconsin Polls open at 7 a.m. and close at 8 p.m. Find your polling place on myvote.wi.gov. What should I bring to vote? Be prepared to show your voter identification (required) Be ready to register or re-register (if applicable) Bring a sample ballot (optional) Questions? The BadgersVote Coalition The BadgersVote Coalition is a non-partisan initiative that strives to equip the UW–Madison community with the knowledge and resources necessary to vote and be active participants in democracy. BadgersVote serves as the central hub for voter resources at UW–Madison. Vote.wisc.edu UW–Madison’s resource for voter information. Visit vote.wisc.edu for information on voter registration, voter ID requirements and upcoming elections. Exam 40 Exam 2 scores Exam 2 median: 21/26 (81%) scores 30 # Students 20 Individual scores will be posted 10 this evening. 0 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 # Questions Correct 0/28 28/28 Exam 1 median: 19/28 (68%)  Top Hat Question Which of the following main sequence stars has the longest hydrogen-burning lifetime? A) O star (40 Msun) B) G star (1 Msun) C) M star (0.1 Msun) D) All have approximately the same lifetime Intentionally blank  More massive stars have shorter lifetimes More mass means there’s more H to burn But more massive stars are much more luminous and burn H at a higher rate.  A star’s post-main sequence evolution depends on its mass We can divide stars into three basic groups: low mass star: < 2 solar masses intermediate mass star: 2 - 8 solar masses high mass star: > 8 solar masses Recap from last Monday The Evolution of a Low Mass Star The images above are showing the core not the whole star!  Top Hat Question When a star becomes a red giant, how does its luminosity (L), surface temperature (T) and radius (R) change? ↑ = increase ↓ = decrease A) L:↑ T: ↑ R: ↑ B) L: ↓ T: ↑ R: ↑ C) L:↑ T: ↓ R: ↑ D) L: ↑ T: ↓ R: ↓ In a red giant phase, stars are so large and luminous that they start to drive off their outer layers The force due to gravity on the surface is tens of thousands of times less than it was on the main sequence This process is intensified in the second red giant phase (double shell burning) when shell He fusion doesn’t reach equilibrium and proceeds in series of thermal pulses. Stars can lose up to 80% of their mass!  The outer layers of the star can form a planetary nebula When they were first observed through telescopes they appeared round, so people thought they were distant planets They are actually expanding shells of gas from a dying star  The core is laid bare After ejection of the outer layers, all that’s left is a tiny, hot, dense electron degenerate carbon core The core is very hot (about 100,000 K) so it’s spectrum peaks in the ultraviolet. We call such a star a white dwarf. The ultraviolet radiation from core keeps the nebula hot glowing Planetary nebulae have emission line spectra Different emission lines are stronger in different parts of the nebula, giving it a range of colors. The Evolution of a Low Mass Star Note the images above are showing the core not the whole star! A low mass star ends its life as a slowly cooling white dwarf — a ball of ultra-dense carbon. Why doesn’t carbon fuse to something else? A low mass star ends its life as a slowly cooling white dwarf — a ball of ultra-dense carbon. Why doesn’t carbon fuse to something else? The stars white dwarfs form from aren’t massive enough to produce the high core temperatures needed for carbon fusion If there’s no fusion to keep the pressure high, what prevents gravity from collapsing the white dwarf? Degeneracy It’s held up by Electron Degeneracy Pressure: a quantum physics effect Quantum Mechanics: the study of matter and its interactions with energy on the scale of (sub)atomic particles. On very small scales many properties of objects (position, speed, angular momentum, energy) that appear continuous turn out to be “quantized” — they must take on one of a set of small, discrete allowable values Example: Electron Orbitals The behavior of particles at the quantum scale is governed by new rules The Heisenberg uncertainty principle tells us that as you closely pin down one measurement (such as the position of a particle), there is a fundamental limit to how accurately you can measure a complementary property of the same particle (such as its speed). The Pauli exclusion principle says that two particles cannot have the same quantum state (position, momentum, spin) * Lol? But in reality the uncertainty principle only matters for sub-atomic objects!  Electron Degeneracy Pressure in Stars In a very dense star like a white dwarf, the positions of electrons are tightly constrained (they’re really packed in). To obey the uncertainty and exclusion principles, the electrons must move fast — this creates pressure which can counteract gravity For stars with initial mass < 2 MSun, the core runs out of helium and stops fusing, and survives as a white dwarf supported by electron degeneracy pressure.  A star’s post-main sequence evolution depends on its mass We can divide stars into three basic groups: low mass star: < 2 solar masses intermediate mass star: 2 - 8 solar masses high mass star: > 8 solar masses Now let’s consider how intermediate (2 - 8 Msun) and high (> 8 Msun) mass stars evolve The evolution of intermediate and high mass stars is similar to a low mass stars, but their cores are so hot that helium fusion can begin very quickly. Whereas low-mass stars have an intermediate step of hydrogen fusion in a shell while the core is still too cold to fuse helium.  Intermediate and massive stars evolve off the main sequence quickly, and briefly become supergiants. Their temperature and radius change but their overall luminosity is relatively stable. The evolution of intermediate and high mass stars is similar to a low mass stars, but their cores are so hot that helium fusion can begin very quickly. Intermediate mass stars (2-8 MSun) stop fusion after creating a carbon core (and some will have just enough fusion to create oxygen too). Just like a low-mass star, the outer layers get blown off, and the star becomes a white dwarf supported by electron degeneracy pressure. Massive stars (> 8 Msun) keep fusing one element into the next in their cores and building up an onion-like set of shells. Each stage of core fusion occurs for a progressively shorter amount of time  Core fusions stops when the star’s core is composed of iron. The fusion of iron into heavier elements does not release any energy The iron core contracts until it reaches the quantum mechanical limit - the degenerate electrons cannot be packed any tighter  For a massive star’s core, gravity is stronger than the degeneracy pressure. As the core gets crushed further, electrons combine with protons to form neutrons and neutrinos Electron degeneracy no longer supports the star and in an instant it shrinks until it is one big ball of neutrons supported by neutron degeneracy pressure. In really massive stars, gravity is even stronger than neutron degeneracy pressure and the star collapses to become a black hole! (More on this on Wednesday)  This catastrophic stellar collapse is what causes a supernova explosion In an instant, the star releases more energy than the sun will over its 10 billion year lifetime!  This catastrophic stellar collapse is what causes a supernova explosion In an instant, the star releases more energy than the sun will over its 10 billion year lifetime! Energy is released in several forms: Light Lots of neutrinos A shock wave that disrupts the entire star and pushes stellar material out into space  Top Hat Question Which element below is not created in the core of a massive star? A) Iron B) Carbon C) Oxygen D) Platinum  Supernova Remnants Depending on the mass of the star, the supernova will leave behind either a neutron star or a black hole. (More on these in the next lecture!) The outer layers of the star are ejected in the explosion and can form a large diffuse nebula. This material is very rich in newly synthesized chemical elements. Massive stars and supernovae are where a large fraction of the most common elements are made  Stellar Recycling Both low and high mass stars return chemically enriched material to interstellar gas, where it can form new stars and planets We are star stuff!  There hasn’t been a supernova (that we know of) in the Milky Way since 1604 but we observe lots of them going off in other galaxies Supernova 1987 A in the large Magellanic Cloud  Low-mass Evolution vs. High-mass Evolution (% is approximate percent of lifetime) 1. Main Sequence (80%) 2. Red Giant (18%) 1. Main Sequence (80%) H-burning shell; degenerate He core 2. Blue/Red Supergiant (20%) 3. Helium Burning Star (1%) H-burning shell and He-burning core H-burning shell; He-burning core 4. Double Shell Burning Red Giant Star expands and number of burning layers gradually grow until iron in the core Phase (