19 - Stars Flashcards
What is a planet?
An object in orbit around a star with three important characteristics:
- mass large enough for its own gravity to give it a round shape
- no fusion reactions
- cleared its orbit of most other objects
What is the difference between an asteroid and a planet?
Planets are round due to their gravity whereas asteroids are irregularly shaped
What is the difference between a planet and a dwarf planet?
Dwarf planets have not cleared their orbit of other objects.
e.g Pluto has many objects of a similar size to it in its orbit
What is an asteroid?
A small uneven object which orbits the sun
What is a comet?
Small irregular bodies made up of ice, dust and rock which orbit the sun in elliptical orbit.
What happens when a comet approaches the sun?
They develop bright tails
What is the difference between an asteroid and a comet?
Asteroids have a circular orbit around the sun but comets have an elliptical orbit.
Comets contain ice but asteroids do not.
What is a solar system?
A sun and all the objects that orbit it.,
What is a galaxy?
A collection of stars, dust and gas.
What is the average number of stars that a galaxy contains?
100 billlion
What is our galaxy called?
The Milky W`ay
What is a nebula?
Gigantic clouds of dust and gas
What is the main gas that makes up nebulae?
Hydrogen
How big are nebulae?
100s of times the size of our solar system
What is a protostar?
A very hot and dense sphere of dust and gas which is not a star.
What is nuclear fusion?
A process where two smaller nuclei join together to form one larger nucleus.
Where is nuclear fusion most commonly found?
Stars
What does a low mass star become after its main sequence?
Red giants
What does a high pass star become after its main sequence?
Red supergiant
Describe the lifecycle of a low mass star
- Main Sequence
- Fusion stops and star collapses in on itself
- Fusion begins in outer shell
- Red Giant
- Fusion stops in outer shell
- Outer layers float away
- Planetary nebula with white dwarf at centre
What is electron degeneracy pressure?
The pressure exerted by fast moving electrons which prevents core collapse up to the Chandrasekhar limit.
What is the Chandrasekhar liit>
1.44 M⊙ (Solar Mass)
What are the boundaries for the mass of a low mass star?
0.5M⊙ < M < 10M⊙
Why does fusion stop in the core of a star?
Because it runs out of hydrogen.
Why do low mass stars not carry out fusion with elements heavier than hydrogen?
Because they do not have enough pressure or high enough temperature to fuse elements heavier than hydrogen.
Why do white dwarf stars not collapse further?
Because of electron degeneracy pressure.
What are the boundaries for a high mass star?
M > 10M⊙
Describe th4e life cycle of a high mass star.
- Main sequence
- Fusion stops in core and star collapses in on itself
- Pressure and temperature are enough to begin helium fusion in core
- Radiation pressure increases in core so star swells
- Super red giant (fusion in shells)
- Helium runs out in core and star collapses again
- Fusion of carbon begins in core
- Cycle continues until star has a core of iron
- Fusion cannot occur in iron so gravity causes the star to collapse
- Shells bounce off core in a massive explosion - SUPERNOVA
- Either neutron star or black hole
What causes a neutron star to form after a supernova?
If the mass of the star is less than 3M⊙ (but greater than Chandrasekhar limit.
What causes a black hole to form after a supernova?
If the mass of the star is greater than 3M⊙
What stars last for longer on their main sequence?
Low mass stars
What is a neutron star?
A very small star made up of just neutrons with a mass of about 2M⊙ and a very high density, close to the density of an atomic nucleus.
What is a black hole?
If the mass of the star before supernova was greater than 3M⊙ then it continues to compress down into a singularity with a gravitational field so strong not even photons can escape it.
Where are all the elements in the universe made?
Stars
How are elements heavier than iron produced?
In a supernova, all elements are created.
What is stellar (solar) luminosity?
The total radiant power output of the star.
What is the unit of solar luminosity?
L⊙
1L⊙ = 3.85x10^26
Compare the surface temperatures of different types of stars.
- Red (Super) Giant = cool
- Main Sequence = hotter
- White dwarf = hottest
Compare the luminosity of different types of stars.
- Red supergiant = most luminous
- Red giant = more luminous
- Main sequence = luminous
- White dwarf = least luminous
Why are electron energy levels said to be discrete?
Because they can only exist with a specific amount of energy.
No electron can exist between these energy levels
Why are energy levels of electrons always negative?
Because external energy is required to escape an electric field (see electric fields)
How much energy does an electron that is free from an atom have?
0 energy
What is the electron level with the most negative value known as?
Ground state
What is it called when external energy is applied to an electron to raise it to a higher energy level?
Exciting
How could an electron be excited?
Heating
Photons being absorbed
What happens when an electron moves down an energy level?
It releases energy - a photon with the specific energy that the electron has lost
Does each element have the same energy levels?
No - each element has unique energy levels
What equations can we use to calculate energy of a photon?
ΔE = hf
ΔE = hc / λ
What is emission line spectra?
Each element produces a unique emission lie spectrum because of its unique set of energy levels
What is a continuous spectra?
All visible frequencies / wavelengths are present. For example a lamp filament would produce this type of spectrum
What is an absorption line spectra?
Series of dark spectral lines against the background of a continuous spectrum.
What do the spectral lines of an absorbtion line spectra have a wavelength equal to?
The wavelengths of the emission line spectrum for the same gas atoms.
How does an absorbtion spectra form?
Light from a continuous spectra source passes through a cooler gas.
Some photons of specific wavelengths are absorbed, raising the electrons in the gasses to a higher energy level.
A lack of photons for certain frequencies produces dark spectral lines, a negative image of the emmision spectra for the gas the light passed through.
How can we use absorbtion spectra to analyse starlight?
We can look at an absorbtion spectra from a star and match the spectral lines against an emission spectra to see which elements are in the star.
Why do we use a diffraction grating instead of a double slit to split light up?
Because it produces a brighter and clearer pattern - more light passes through.
What is the path difference at the zero order maximum?
0
What is the grating equation?
d sinθ = nλ
What is d in the grating equation?
The distance between the slits in the grating
What is n in the grating equation?
The maxima number
What is λ in the grating equation?
The wavelength of the light passing through the grating.
What is θ in the grating equation?
The angle between the grating and the direction of the light.
What is the largest possible angle that θ can be and why?
90 degrees because sin 90 = 1
How do you calculate the highest order maxima?
n(max) = d/ λ
What is a black body?
An object that emits electromagnetic radiation in a continuous spectrum across all wavelengths.
What is the peak wavelength of a black body?
The most intense wavelength that the object emits.
How does the temperature of an object affect the peak wavelength?
The peak wavelength is smaller for hotter objects.
What is Wein’s displacement law?
λₘₐₓ ∝ 1 / T
Peak wavelength is directly proportional to 1 over the surface temperature of the black body
What can Wein’s displacement law be applied to?
Most objects from mammals to stars
What is Stefan’s law?
L = 4πr²σT⁴
What does Stefan’s law tell us?
Luminosity is directly proportional to:
Radius (L ∝ r²) Surface Area (L ∝ 4πr²) Surface temperature (L ∝ T⁴)
What is σ in Stefan’s law?
Stefan’s constant
5.67 x10⁻⁸ W m⁻² K ⁻⁴
What can be calculated by combining Wien’s Law and Stefan’s law.
The radius of a distant star can be estimated.
What is a white dwarf?
A small very dense star that is typically the size of a planet. They are very hot.
What has a shorter wavelength, blue or red light?
Blue light.
What colour light is closer to the central central maxima when it is diffracted.
Blue light is closer to the central maxima
Red light is further.
