Chapter 16 Evolution of Low Mass Stars

RR Lyrae variables

A low-mass pulsating star. Less luminous and shorter period than Cepheids.

Cassiopeia A

A type 2 supernova visible from Earth, light first reached Earth in late 1600s.

Cepheid variables

A type of pulsating star. High mass stars that are becoming supergiants. Period = amount of time for one pulse. More luminous = longer period, Cepheid variables have periods 1-100 days

How are Type 1a and Type II supernovae different?

Both produce a lot of light very fast, but the light curves are different. Type 1a have a higher peak luminosity, and the light curve of Type II falls at a slower rate. They also have differences in spectra Fig 17.11, textbook p. 479

Post-main sequence evolution

Convection in core moves H molecules around, so more available for fusion. In low mass stars, helium ash collects in core gradually and unevenly. In high mass stars, the convection leads to an even distribution of helium ash throughout the entire core. When the core is 100% helium, it starts collapsing and star cools and expands (will move to the right horizontally on H-R diagram). This is how we get supergiants. Helium in the nondegenerate core will get really hot and basically just burn. High temps = heavier elements can fuse =layers fusing elements that get heavier towards the core. Iron is the heaviest element fused in stars because it just is.

Nucleosynthesis

During type II supernova, intense energy is transferred to interstellar matter and fusion of elements heavier than iron is possible. Neutron capture and helium capture = heavier elements

*When compressed, ordinary gas heats up but degenerate gas does not. Why, then, does a degenerate core heat up as the star continues shell fusion around it?

It is heated by the weight of helium ash falling into it.

Neutron star

Lower mass star will leave behind a neutron degenerate core. Between 1.4-3 solar masses, radius about 10-15 km. Some are found in X-ray binaries, where they take matter from another star into their accretion disk and produce X-rays. Binary neutron stars can merge and produce gamma ray bursts

High mass star life cycle

More mass = generally faster life cycle, because star fuel will be used up at a faster rate (big stars have high luminosity) and star will leave the main sequence ((relatively)) quickly

Type II Supernova

Neutron degeneracy pressure begins working when the core becomes denser than an atomic nucleus. Nuclear forces become repulsive, and about half of the core's material stops falling inward. The other half continues, hits the degenerate core, and bounces back out very fast, sending a shock wave through the uter layers. About 1/5 of the core mass is turned into neutrinos, some of which are trapped by the extreme pressure and make the shock wave even more powerful. Wave reaches the outside of star within a few hours (hella fast because stars are big). Star explodes and it looks cool from a distance. Up close it looks like incineration. Dense core remains.

hydrogen shell fusion

a layer above the core of a star where hydrogen is fused into helium. Created after the star runs out of hydrogen in the core. In low mass stars, will probably be surrounding an electron degenerate core, in high mass stars will be surrounding layers of fusion into heavier elements.

Super helpful websites I used

astronomynotes.com universetoday.com

Star clusters

groups of stars formed from the same material around the same time Globular: dense, up to millions of stars Open: loose, fewer stars H-R diagram of stars in cluster can give relative age of whole cluster. Location on H-R where stars are leaving the main sequence gives cluster age

Stellar populations

groups of stars that share characteristics (particularly age) young clusters are bluer, they contain more massive elements older stars have less massive elements since fewer generations of stars have gone by in these regions

Pulsars

highly magnetized neutron stars that rotate rapidly (not signals from another planet). Beams of radiation hit Earth when their magnetic poles align with it

*When the Sun runs out of hydrogen in its core, it will become larger and more luminous because

it will start fusing hydrogen in a shell around a helium core Explanation ~ (see card 6) helium core will become dense, electron degenerate. Pressure will cause higher fusion rates, so more energy leaving the core. Outward pressure of this leaving energy will create more light and push surrounding material away from core.

*If a main-sequence star suddenly started fusing hydrogen at a faster rate in its core, it would become

larger, cooler, more luminous

Black holes

left over after type II supernova if the star is larger

*If a star follows a horizontal path across the H-R diagram, the star...

maintains the same luminosity. Explanation ~ H-R diagram has luminosity on the vertical axis. If the star doesn't move vertically, then luminosity does not change, but everything else can. HW 9, question 22

*In Latin, nova means "new." That word is used for novae and supernovae because they are

newly visible stars Explanation ~ Before the supernova, the stars in question were too far away, not luminous enough, etc. to be seen from Earth, and the massive explosion in the sky allows us to see the for the first time, which is kind of ironic because we still can't really tell what the star looked like before the supernova. I guess we can make a good estimate though.

degeneracy pressure

pressure exerted by electrons that have been fully squonched (as squonched as quantum physics says they can get) in the collapsing core of a star. The force of this pressure fights against gravity so the star will stop collapsing instead of shrinking forever into a small dot of density.

*A low-mass main-sequence star's climb up the red giant branch is halted by

the beginning of helium fusion in it's core Explanation ~ at the end of the red giant phase, the core of helium is small and dense (helium atoms moving around in a glob of degenerate electrons) and the outer bits of the star are expanded and cool(er, they're still hot as heck). Hydrogen keeps fusing into helium until HE core gets hot enough to start fusing into Carbon (triple alpha process). Helium flash = He fusion creates new energy source, He fusion happens very fast until thermal pressure makes the core expand, outer layers of star contract. We now have a stable smaller, hotter star w/ H fusing layer surrounding He fusing core.

*Post-main-sequence stars lose up to half their mass because

the star swells until the surface gravity is too weak to hold material. Explanation ~ If hydrogen is burning around a helium core, the star will become a red giant. Core becomes hella dense (electron degenerate) and the weight of outer layers put even more pressure on the core. Increased pressure = increased fusion = energy leaving core at a higher rate. This energy makes the star more luminous and fights against gravity to push the outer layers outwards, so far that some of the mass can escape the core's gravity.

*The most massive stars have the shortest lifetimes because

the temperature is higher in the core, so they fuse faster Explanation ~ High mass stars have a lot of stuff in them, so more fuel available, and also more heat because of high gravity in the core. They also start out with a higher luminosity, which is caused by the material of the star being burned faster, so they will go through their hydrogen faster.

*As an AGB star evolves into a white dwarf, it runs out of nuclear fuel, and one might guess that the star should cool off and move to the right on the H-R diagram. Why does the star move instead to the left?

As outer layers are lost, deeper layers are exposed

What happens when a star forms an iron core?

Fe requires energy input to fuse, so the star can no longer generate energy in the core. This means that the star has no way to make up for the energy lost in the form of neutrinos (neutrino cooling). Star now has less energy to create internal pressure, so it collapses. Density and temperature will increase. The core becomes electron degenerate, but the weight of a high mass star outweighs electron degeneracy pressure. Core becomes ridiculously hot and dense, to the point where the thermal radiation is in the gamma-ray part of spectrum. Once the iron core begins collapse, the process only takes a fraction of a second.

Crab nebula

First recorded type II supernova, has a pulsar at it's center that lights the nebula

Photodisintegration

Gamma ray photons (high energy) in the core of collapsing star have enough energy to break down nuclei in the core. Process uses thermal energy and reverses the fusion process

fusion shells

He core in a high mass star will not be degenerate, so it can continue fusion at high enough temperatures. He burns to Carbon C burns to sodium, neon, and magnesium (Na, Ne, Mg) Ne burns to oxygen, magnesium oxygen burns to sulfur and silicon (S, Si) S and Si burn to iron (Fe) Fe core will be degenerate each stage of burning gets progressively shorter

What happens to high mass protostars?

They collapse quickly, so particles are heated and ionized. Ionized particles create a radiative zone in outer bits of star (around convection zone), which causes increase in temperature. Luminosity doesn't change much (star is getting hotter while getting smaller)

Instability strip

a region on the H-R diagram where stars will grow and shrink, so their light will appear to pulse (pulsating variable stars). Time for one pulse is called a period. Instead of reaching a balance between outwards pressure and gravity, the star will keep overshooting it's equilibrium radius. Period-luminosity relationship

*All Type Ia supernovae

are extremely luminous Mass-transfer binary stars ~ If you've got 2 low mass stars in a binary system, the bigger one will evolve first, and eventually expand past it's Roche lobe (the thing that looks like an uneven infinity) and small star will steal some of it's mass. Small star will then become bigger star while original big star becomes a white dwarf. Second big star will evolve past Roche lobe, and white dwarf will steal it's mass. WD creates a hydrogen outer layer that gets very hot. If mass of WD reaches Chandrasekhar limit, carbon will ignite and explode. Big explosion, consumes entire WD, other star might not get exploded though. I'd put a picture but I'm not paying for quizlet plus.

*A planetary nebula glows because

light from the central star causes emission lines Explanation ~ cloud of gas surrounds a very dense, hot, small star (phase between red giant and white dwarf). High temperature means the star emits mostly ultraviolet and is dim in the visible spectrum, but the gas particles intercept UV and converts it to visible. Discrete emission lines from atoms/ions mean lots of separate colors. These were first noted by Messier in 1780s, and were named "planetary nebulae" because they kind of looked like the outer planets, but they don't actually have anything to do with planets. http://www.scholarpedia.org/article/Planetary_nebulae

*Degenerate matter is different from normal matter because as the mass of degenerate material goes up,

the radius goes down Explanation ~ degenerate matter is hella dense, there's basically no space between particles, so they resist compression. Pressure in degenerate matter depends on particle movement (NOT temperature). An increase in mass of degenerate core = increase in material in core = increase in gravity = the whole thing gets squonched up into a smaller circle which is now considerably more dense.

*A white dwarf is located in the lower left of the H-R diagram. From that information alone, you can determine that the star is

very hot left side (horizontal) = high temp bottom (vertical) = low luminosity White dwarfs are dense, but you can't tell this from only temperature and luminosity

Fusion in high-mass stars

while star is on the main sequence, the CNO cycle (carbon catalyst assisted fusion) is used t fuse hydrogen into helium. Low probability that two hydrogen nuclei will just fuse, so the CNO cycle speeds up the process. begin with 4 H and 2 e⁻, using Carbon-12 as a catalyst, end with 4 He. Release gamma rays and neutrinos through 6 stages. Also releases positrons that will collide with nearby electron. Homework 9, question 12

Charge destruction

while the core of a star is undergoing photodisintegration, it is so dense that electrons are shoved into protons, resulting in neutrons and neutrinos. Uses up a lot of the energy that was supporting the star. As neutrinos yeet themselves out of the star, some of the stars energy is yeeted with them. Photodisintegration, charge destruction, and physical collapse of core all occur withing a fraction of a second.