Unit 4 (Chapters 12-14) Flashcards
What 4 laws can be used to describe the internal structure of a star?
- The law of hydrostatic equilibrium the balance between weight and pressure
- The law of energy transport: energy flows from hot to cold regions by radiation, convection, and conduction
- The law of mass conservation: the total mass of the star must equal of the sum of the masses in its shells with the added requirement that the mass must be smoothly distributed through the star.
- The law of energy conservation: the amount of energy flowing out from the top of a shell in the star must be equal to the amount of energy coming in at the bottom of the shell plus any energy generated with in the shell.
What is the mass luminosity relation?
More massive stars have more luminosity
- increased mass increases central pressure and temp because of the weight pushing down on the inner layers
- they also have hotter cores with more rapid reactions
What are the characteristics of stars in the upper end of the main sequence?
- These are high mass stars with up to 150 solar masses
- They are less common because massive clouds are more likely to fragment into multiple stars than form one massive star.
- More massive stars are also more unstable (high energy = high pressure which = radiation that blows the gas away with strong stellar winds)
What are characteristics of stars in the lower end of the main sequence?
These stars are low mass and very common but difficult to study because they are so dim.
- objects less than 0.8 solar masses are brown dwarfs and can’t ignite hydrogen because they are not hot enough
- instead, brown dwarfs contract slowly and convert gravitational energy into thermal energy which radiates away.
- Brown dwarfs blur the lines between stars and planets as they are capable of having their own weather system and sometimes are cool enough to get solid grains in their atmosphere as well as methane bands.
What happens to a star as it converts hydrogen to helium?
Because it takes 4 hydrogen to make 1 helium, the amount of pressure in the star drops which creats an imbalance so gravity is able to compress the star’s core.
As the core compresses, it increases in density and temperature which then increases the nuclear reactions/energy output.
the increased energy flow outward causes the star to expand and cool, making it larger and brighter but cooler.
How long do stars spend on the main sequence?
Most stars spend 90% of their life on the main sequence
Massive stars live shorter than smaller stars. for example 25 solar mass star might live 4 million years but a red dwarf could live hundreds of billions of years.
What happens when a star leaves the main sequence?
These stars have converted the last of their hydrogen into helium which lowers energy production and causes the outer layers to contract the core
Contracting helium can’t generate nuclear fusion but it converts gravitational energy into thermal energy which ignites a thin shell of hydrogen outside the core. This shell burns leaving more helium and increasing the mass of the core.
During this stage, the star produces more energy than it needs to balance its gravity which causes the star to expand into a giant. The giant has a contracting core and expanding outer region.
If the star was a massive star in the upper region of the H-R diagram, it will become a supergiant, if it was in the mid-range of the hr diagram it will become a giant.
What are the key characteristics of giants and supergiants.
They are very large with a contracting core and expanding outer region. They have low densities and there is no mass-luminosity relationship.
What happens to a star when it is a giant/supergiant and the temperature rises?
As the temperature rises, the core continues to contract. these combined cause helium fusion where 3 helium get converted into 1 carbon + gamma ray photons.
Depending on the mass of the star, some helium fusion is gradual but stars with solar masses between 0.5 and 3 do helium flashes
What is nucleosynthesis
This is when stars build elements and when they die, these elements get blasted into space.
For example, carbon and oxygen produce silicon which can also fuse to other elements at the right temperature.
What are 2 types of star clusters?
- open clusters: has about 10-1000 stars in a region of 25 parsecs; the stars are not crowded together
- Globular clusters: have 10^5 to 10^6 stars in only 10-30 parsecs. These clusters are spherical and the stars are very close together.
What are variable stars?
These are stars that change brightness significantly and consistently.
They can be variable due to eclipsing variables or they can be intrinsic variables
What are Cepheid Variable stars? What are the 2 types? What is the period luminosity relation?
These are supergians or bright giants of spectral F or G. They cycle through faint to bright every 2-60 days.
Cepheid type 1: have a composition similar to the sun
Cepheid type 2: have very little in elements heavier than helium
The period luminosity relation is the idea that the longer a period the cepheid has, the more luminous the star is.
What happens to low mass stars as they die?
When a star exhausts one nuclear fuel, its core contracts and heats until the next nuclear fuel ignites. Low mass stars have limited gravitational energy so they are limited in how hot their cores gets which limits the fuel they can ignite. There are two categories of low mass stars for death:
- Low mass Brown dwarfs with a mass of 0.08 solar masses or less. Unable to ignite hydrogen fusion. They cool and fade after formation
- 0.08-0.5 solar mass stars (Red Dwarfs spectral type M). Can ignite hydrogen fusion but burn very slowly so they last a long time. They are entirely convective and can last trillions of years.
What happens to medium mass stars as they die (e.g. our sun)?
Medium mass stars (0.5-8 solar masses) can fuse hydrogen and helium but not carbon. They use convection and radiation so they have a helium shell left.
The helium shell burns leaving carbon and oxygen behind. This increases the mass and cause the core to contract. The contracting core and expanding shell cause the star to expand and cool. The star is now a red giant where it is large and cool and prone to losing its surface due to strong winds.
Super wind refers to the massive mass loss that red giants go through.
What happens to larger stars as they die?
Larger stars are able to fuse beyond helium
Helium converts to carbon to neon, to oxygen to silicone to nickle and iron.
Only 8 solar mass stars can fuse carbon and only 10 solar mass stars can fuse neon, and only 11 solar mass stars can fuse oxygen and silicone.
What is a planetary nebula? What are they made of and what is their size? How long do they last? Where do they come from?
This is an expanding shell of gas ejected from a star during the later stages of its evolution (when its surface has cooled and become more opaque)
It is not a planet, it is a dying star. They are made up of excited low density gas. They are aout 0.2-3ly in size and expand at 10-20km/s. They can last 100 000 years and come from medium mass stars (8-10 solar masses)
What happens at the end stages of a planetary nebula?
At the end stages, the planetary nebula loses its outer layer and contracts, becoming a white dwarf.
What are white dwarfs? What is the Chandrasekhar limit?
These are very dense objects about the size of earth. They convert gravitational energy to thermal energy which heats them but they never get hot enough to fuse its carbon or oxygen. They continue to compress until they become degenerate.
Theory predicts they will eventually crystalize but the universe is not old enough for that to have happened yet.
The white dwarf will gain mass but it will just cause them to shrink. The Chandrasekhar limit is the idea that a white dwarf with a mass of 1.4 solar masses will have a radius of ZERO. Essentially this means that stars with a solar mass higher than 1.4 have to shed mass before they become a white dwarf.
What happens when a star in a binary system evolves?
Stars in binary systems affect their partner. According to the Roche Lobe, larger stars will have more distance between themselves which means they will more easily hold on to their own matter. Smaller stars will have less distance and so there lost matter can be absorbed by their partner.
Sometimes binary stars expand together and merge to form one giant, rapidly rotating star which is unique as most giants rotate slowly.
What is an Accretion Disk?
This is a whirling disk of gas that forms around a compact object such as a white dwarf, a neutron star, or black whole as they draw matter in. It gets very hot and shifts the angular momentum outward.
What is a Nova?
A Nova is the eruption of a white dwarf in a binary system. It is caused by the mass from a regular star transferring to the accretion disk of the white dwarf causing degeneration and rapid heating. This in turn causes rapid and uncontrolled hydrogen fusion. The Nova will repeat until all the hydrogen is fused
What happens to massive stars as they die?
Massive stars will fuse hydrogen, expand to either a red giant or super giant then contract their core and fuse helium and then continue to fuse elements until it reaches iron.
Fusion increases in speed through succession of elements where it could take a 25solar mass star 3 million years to fuse it’s hydrogen but only a day to fuse its silicone
What happens to stars with iron cores?
Iron cores contract and grow hotter and hotter. This ignites all of the other elemental shells causing them to fuse to iron too, making the star heavier. When it reaches the Chandrasekhar limit (1.4solar masses) the star will collapse.
Because it has an iron core, it collapses in a few thousandths of a second in a violent explosion called a super nova.
What are the two types of supernovas?
Type 1 Supernovas: have no hydrogen lines in its spectrum and are 4 billion times brighter than the sun. They decline fast at first and then slow down. There are two subtypes of type 1 super novas
- Type 1a: these occur when a white dwarf exceeds its Chandrasekhar limit due to gaining mass from a binary star.
- Type 1b: these occur when a massive star loses its hydrogen rich outer layer due to super winds.
Type 2 Supernovas: these spectra contain hydrogen lines and are much dimmer than type 1 super novas. They decline to a temporary standstill and then fade rapidly.
What is the difference in the death of a a medium star versus a massive star?
When medium stars die, they leave behind white dwarfs.
When massive stars die they leave behind neutron stars and black holes.
What is a neutron star?
Neutron stars are a small highly dense star composed almost entirely out of packed neutrons and it has the radius of only 10km but is 1 solar mass. These stars would spin rapidly, have a surface temp equal to the sun’s interior, and have a magnetic feild 1 trillion x stronger than earths.
These stars are formed during supernovas when the collapsing core is greater than the Chandrasekhar limit of 1.4 solar masses. This density forces the electrons and protons to merge to form neutrons and neutrinos as a byproduct
Stars between 10-20 solar masses form neutron stars.
Neutron stars are always less than 3 solar masses because 3+ become black holes.
What is a Pulsar?
A pulsar is pulsing star that actually a neutron star.
What is a black hole?
A lack hole is when a star with a core greater than 3 solar masses leads to it exceeding the Chandrasekhar limit and therefore collapsing in on itself. It then has an infinite density and gravity (called a singularity) and is unable to emit light.
Essentially, if matter is packed into a small enough volume, space time will curve back on itself.
What is time dilation and how does it relate to black holes?
Time dilation is the idea that time slows down in curved spacetime. In terms of black holes this means that if someone fell into a black hole, to an observer they would fall slower and slower until they barely seemed to move. Meanwhile the person in the black hole would experience “normal” time and would feel like they were speeding up as they fell towards the event horizon
What is a hyper nova?
This is when a very massive star collapses into a black hole and creates an explosion which causes some gamma-ray bursts.
