Ch 12 Vocab: Stellar Evolution
Hydrogen needs to be at what temperature to overcome the electrostatic repulsion of their protons?
10 million K Helium contains more protons, so it requires a much higher temperature to fuse its nuclei together
Helium needs to be at what temperature to overcome the electrostatic repulsion of their protons?
100 million K
White dwarf
A small very dense star that is typically the size of a planet. Core about the size of Earth. A white dwarf is formed when a low-mass star has exhausted all its central nuclear fuel and lost its outer layers as a planetary nebula. Dim, shining only via stored heat. Mostly composed of C and O.
Red Supergiant
A star that has used up all the hydrogen in its core and has a mass much higher than the Sun.
Red giant
A very large star of high luminosity and low surface temperature. Red giants are thought to be in a late stage of evolution when no hydrogen remains in the core to fuel nuclear fusion. The sun will become a red giant.
Supergiant
A very large star that is even brighter than a giant, often despite being relatively cool.
Implosion
As the gravity continues to pull in tighter, the protons and electrons are crushed together creating an entirely neutron composed core As the gravity compresses these neutrons together and it happens so fast, it goes beyond the point at which the neutrons should allow it to The neutrons do not want to be squeezed together any closer, so they produce a huge outward pressure to halt the collapse any further
A star approximately 20 times the mass of our Sun will have the following timeline:Burning of Hydrogen for 10 million years
Burning of Hydrogen for 10 million years Fusion of Helium for 1 million years Fusion of Carbon for 1000 years Fusion of Oxygen for 1 year Fusion of Silicon for 1 week Production of Iron for less than a day
Stellar evolution
Change during the lifetime of an individual star
Helium white dwarf
Cores of very low-mass stars that never become hot enough to fuse Helium, as all of the H has been converted to He via the proton-proton chain and is not hot enough to start He ignition.
Density within a star's shrinking core
DURING STAGE 12 It's density is about 1010 kg/m3 Before the shrinking core can get hot enough for carbon to fuse the rising pressure stops it and stabilizing the pressure not by the same process as the other stages, but because the core now contains a vast sea of free electrons that have been stripped from all the atoms due to the heat
Supernovae
Death of massive stars, gas pressure collapses which results in shockwave that blasts outer shell into space. Supernovae are similar to Novae, except they produce a burst of light billions of times brighter than the Sun and will emit more energy in a couple of months than the Sun does in its lifetime
Photodisintegration
Direct interaction with the nucleus of the atom, causing a state of excitement within the nucleus, followed by the emission of a nuclear fragment. During death, The iron starts to get split into smaller atoms and the smaller atoms get pulled apart into protons, neutrons, and electrons
Stars of higher mass do not undergo ________ and their __________ remain fairly constant as they move back and forth across the top of the H-R Diagram
Helium flashes, luminosities
Novae
Highly luminous, occurs when white dwarf becomes explosively active. Created by a binary star surrounded by common envelope of gas; extra burning caused by mass transfer of hydrogen from companion star onto carbon white dwarf; suddenly brightens periodically.
The universe continually recycles materials through this 4 stage process
Interstellar Medium Clouds of gas and dust dispersed throughout regions of space Star Formation Gas and dust form stars condense under gravity and create stars Stellar Evolution Stars fuse lighter elements into heavier ones and release energy Supernova & Heavy Elements Supernovas and other explosions seed space with more gas and dust to start the cycle over again
When a Carbon-Detonation (Type I) Supernova explodes, what is left?
Nothing!
Carbon-Detonation Supernova
Occurs when a carbon-oxygen white dwarf in a binary system gains mass, collapses and explodes as its carbon ignites. A supernova caused by this carbon fusion.
Star Death
Once the star begins to fuse into iron in its core, it is in trouble Iron is the most stable nucleus of all elements and so the fusion of Iron does not release energy, but rather absorbs massive amounts The absorption of energy in the core stops the outward pressure within the star and the inward gravitational force completely takes over The star completely implodes due to gravity reaching almost 10 billion K At this temperature, the light produced has enough energy to "undo" the fusion the star has been working on for millions of years
When systems have undergone two or more nova-like eruptions during their recorded history, they are referred to as what?
Recurrent novae
Supernova remnants
Some supernovae can be seen but many can only be detected by the expanding clouds of gas expanding from the supernova explosion ex. Crab, Vela
Why can't stars the size of our Sun fuse elements larger than carbon?
Stars the size of our Sun do not have enough mass to fuse elements larger than carbon.
During ________ explosions, the heat and energy is so great that protons and electrons that are free-moving combine to create elements larger than ______.
Supernova, Fe
Accretion disk
Swirling, flattened disk of matter surrounding a white dwarf. Occurs when the gas swirls around the white dwarf being accelerated by the gravity of the white dwarf as it gets closer and closer.
Main-Sequence Turnoff
The cluster will appear to "peel away" from the main sequence beginning at the top left o The Main-Sequence Turnoff is the area where the stars are leaving the main sequence
Core hydrogen burning
The energy-burning stage for main-sequence stars, in which the helium is produced by hydrogen fusion in the central region of the star. A typical star spends up to 90 percent of its lifetime in hydrostatic equilibrium brought about by the balance between gravity and the energy generated by core hydrogen burning.
Black dwarf
The presumed final stage of a white dwarf, results when a white dwarf stops giving off energy.
Hydrogen shell-burning
The state in which hydrogen is burning at a furious rate in a thin layer surrounding the non-burning inner core of "helium ash" This causes the amount of heat produced to go up, meaning that as the star runs out of hydrogen, it will actually get hotter!
Stage 14
The white dwarf will travel diagonally down its solar mass line as it cools and will change color from white, to yellow, to orange, to red, then it will not have enough heat to radiate visible light and will become dark If the white dwarf is part of a binary system and is close enough to the other star in the system, it will gravitationally pull in hydrogen and helium from the companion star As the gas gets to 107 degrees it will ignite and fuse into helium creating a burst of energy released over a short time seen from Earth as a Nova This can happen several times as the process of pulling in gas can start again after a nova explosion
When a __________ explodes, the rebounding neutrons create a shockwave that blasts the outside of the star into interstellar space, but _________ are left behind
Type II Supernova, the core of neutrons
Clusters of stars
are formed from the same interstellar clouds and have very similar compositions, so only their mass is what separates stars within a cluster These clusters are used to test our models of stellar evolution as the stars will show movement on the H-R Diagram at different times based on their masses As a cluster of stars forms, they all arrive on the main sequence with more low mass than high mass stars As time goes on, the larger massed stars will evolve faster
Type II Supernova
contains lots of hydrogen and have a slightly different light curve than a typical Nova explosion They have a plateau in the light curve a few months after the maximum A Type II Supernova is consistent with the Core-Collapse Supernova
Type I Supernova
contains very little hydrogen and have light curves similar to a typical Nova explosion They have a sharp rise and a slow, steady decrease over time A Type I Supernova has a different mechanism, which gives it the different light curve
Vela
exploded about 9000 BC and is 500 pc from Earth. Type II Supernova that could have been as bright as the moon for months
Stars with____ left over will lie on the right side of the horizontal branch.
high mass
The Chandrasekhar Mass
is the 1.4 solar mass minimum required for a star to have enough mass to go supernova If an accreting white dwarf brings in enough mass to get over the Chandrasekhar Mass, carbon fusion will begin everywhere throughout the white dwarf almost simultaneously, which produces enough energy to overcome its gravity and it will go supernova
Subgiant Branch
is the horizontal track off the main sequence to stage 8. The nonburning outer layers of the sun will expand due to the increase in heat making the star swell and leave the main sequence and is about 3 times bigger during the 8th stage.
The Asymptotic Giant Branch
is the the 2nd vertical branch a star takes as it leaves the horizontal branch as the carbon ash core continues to get dense
Stage 12
( A Planetary Nebula) The hydrogen and helium burning shells consume fuel at increasing rates as the star ages As it expands, cools, and then reascends the giant branch the star begins to fall apart The radiation from within causes the gas to drift away into interstellar space Starts off slowly at first, but then more rapidly as the core luminosity increases It loses its entire envelope in about a million years As the core exhaust its last remaining fuel, it contracts and heats up moving to the left in the H-R Diagram Eventually it becomes so hot that its UV radiation ionizes the inner parts of the surrounding cloud creating a spectacular display known as a Planetary Nebula
Stage 11
(A Red Giant Again) Just as it did on its way to stage 9, the stars inner carbon core continues to shrink and become more dense, pushing the hydrogen and helium fusion further out in the core making the star expand once again to a red giant The star is much larger and more luminous than it was in the 1st red giant stage
Stage 13
(A White Dwarf) The carbon core is now shining by its stored heat as all NUCLEAR FUSION HAS STOPPED. It appears as a bright white-hot surface The path is a large sweeping arc around the main sequence on the H-R Diagram going from red giant around to the white dwarf region of the chart The white dwarf will radiate its heat into space until it runs out
Stage 10
(Helium Fusion) The contraction of the inner core and the expansion of the envelope (nonburning layers) cannot continue forever and only goes on for a few hundred million years. When the helium fusion begins, the core changes drastically. The Helium ignition expands the core rapidly, making the density go down and the balance of outward pressure comes back to balance with the inward gravitational pressure. Temperatures go well above the 108 K and helium begins to fuse into carbon. The flash stops the climb up the vertical red giant branch and drops the star down and left on the H-R Diagram which results in a drop in luminosity as the star shrinks back down to a smaller size
Stages 8 and 9
(Subgiant to Red Giant) The hydrogen shell-burning phase: As hydrogen starts to run out, the core builds up with Helium and fusion moves away from the core's center. Without as much fusion occurring, the outward pressure starts to weaken and the core will start to shrink due to its steady gravitational pull. As the core shrinks, it releases gravitational energy, making the overlying layers of the core hotter, which makes the hydrogen fuse faster than before. The nonburning outer layers of the sun will expand due to the increase in heat making the star swell and leave the main sequence and is about 3 times bigger. As the hydrogen fusion moves further out in the core, the nonburning outer layers continue to expand more quickly, taking the star to about 21 times its original size. It now emits 160 times the energy it did while in the main sequence
Pauli Exclusion Principle
A law of quantum physics that comes into play in the environment of a star's shrinking, dense carbon core. The Pauli Exclusion Principle does not allow the electrons in the core to be crushed any closer together, which makes it reach equilibrium. Quantum mechanical principle that states that two identical fermions cannot occupy the same quantum state simultaneously, most commonly known for forcing atomic electrons to fill orbitals in pairs.
Planetary Nebula
A nebula that was once thought to be a star with its planets but is now thought to be a very hot star surrounded by an expanding envelope of ionized gases that emit a fluorescent glow because of intense radiation from the star
Red Giant Branch
is the vertical track up the H-R Diagram to stage 9. As the hydrogen fusion moves further out in the core, the nonburning outer layers continue to expand more quickly, taking the star to about 21 times its original size during the 9th stage. The star now emits 160 times the energy it did while in the main sequence.
Helium Flash
is the violent result of the core's conditions changing within it after Helium fusion begins at 100 million K.
Crab Nebula
is what is left over from a star that went supernova in 1054 AD and is still expanding at several thousand kilometers per second Type II Supernova that was documented by Chinese and Middle Eastern astronomers as it was visible even during the day for almost a month
Core-collapsing supernova
is when the core "bounces" back outward and produces an enormous energy shock wave that blasts all the overlying layers out into space Because of the amount of mass most stars emit during their red supergiant phase, only stars >12 times the mass of the Sun will have enough mass to go supernova One of the most energetic events known. The death of a high mass star.
Stars with _______ left over will lie on the left side of the horizontal branch.
low mass
A star much more massive than the sun that has swollen out after the main sequence stage to become a ________ before it collapses.
red supergiant
The Horizontal Branch
the region on the H-R Diagram that stage 10 stars lie on Depending on the mass and size of the red giant, it can lose a lot of its mass before coming down to the horizontal branch
