Astronomy and stars chap 19 Flashcards
Define Nebulae
Gigantic clouds of dust and gas, usually hundreds of times larger than our solar system
How are nebulae formed?
Over millions of years as tiny gravitational attraction between dust and gas particles pull towards each other forming clouds
How is a protostar formed?
As dust and gas get closer together gravitational collapse accelerates
Dense regions form
These regions pull in more dust/gas gaining mass and density
gravitational energy is transformed to thermal energy
These regions form protostars
What is a protostar?
A very hot dense sphere of dust and gas
What process is needed for a protostar to form a star?
Nuclear fusion
How does a protostar become a star?
Fusion reactions between hydrogen nuclei produce helium nuclei and kinetic energy
More and more mass is gained which increases the size and temperature of the protostar so electrostatic repulsion is over come
Why are very high temperatures and densities needed for nuclear fusion?
To overcome strong electrostatic repulsion between nuclei
What forces keep a main sequence star in equillibrium?
Gravity, radiation pressure and gas pressure
How are main sequence stars kept in equillibrium?
Gravitational forces compress the star
Radiation pressure from emitted photons and gas pressure from the nuclei counteract gravity
What decides how long a star stays stable for?
The size and mass of it’s core
Are bigger or smaller stars likely to stay stable for longer?
Smaller as they don’t burn as hot
Define Planet
An object in orbit around a star
Must have a mass large enough for it’s own gravity to give it a round shape
No fusion reactions occur in the body
It’s cleared its orbit of most other objects
What’s a dwarf planet?
Like a planet except they haven’t cleared their orbit of other objects
Define asteroid
Objects too small and uneven to be planets but have near circular orbits
No ice present
What is a planetary satellite?
A body in orbit round a planet
What’s a comet?
Small irregular bodies of ice, dust and pieces of rock
Between a few hundred metres to tens of kilometres across
All comets orbit the sun
Highly elliptical orbits
Some develop tails when approaching the sun
Define solar system
A system containing a large central star and all the objects that orbit it
What is a galaxy?
A collection of stars and interstellar dust and gas
Usually contain about 100 billion stars
What mass does a star that’s going to be a red giant have?
Between 0.5 solar masses and 10 solar masses
How does a star become a red giant?
Energy released via fusion is low
So gravitational force is greater than outward pressure
This causes the core to collapse
As the core shrinks pressure increases- fusion starts in a shell around the core
How does a red star become a white dwarf?
Red giants have inert cores
Fusion no longer takes place as the temperature is too low
A shell of helium nuclei has formed around the core
Layers start to move away from the core giving the star it’s red colour
Characteristics of a white dwarf
Very dense- volume of earth with the mass of the sun
Very hot but inert
Only emits photons that were created in earlier evolution
Surface temperature of up to 30 000K
What is the Pauli exclusion principle?
Two electrons cannot exists in the same energy state
How does Pauli apply to stars?
When the core of a star collapses electrons are squeezed together creating a pressure that prevents the core from further gravitational collapse
What is electron degeneracy pressure?
Pressure created by squeezing electrons together
What is the Chandrasekhar limit?
A core must be less than 1.44 solar masses for electron degeneracy pressure to prevent gravitational collapse
How do red super giants occur?
They have a high enough temperature and pressure to fuse helium nuclei
This causes the core to expand to fuse heavier elements
This happens until the star develops an iron core- iron can’t fuse
This makes the star very unstable eventually leading to implosion called a type 2 supernova
When is a neutron star formed?
If the mass of the core is greater than the Chandrasekhar limit
List the characteristics of a neutron star
Almost entirely neutrons
Often very small
Mass of around 2 solar masses
Density similar to a nucleus
When do black holes occur?
If the mass of an imploding core is above 3 solar masses
What is a black hole?
A core still undergoing gravitational collapse resulting in a gravitational field so strong nothing can escape
They vary in mass
Define luminosity
The total radiant power output of a star
Which way does temperature increase on a HS diagram?
Right to left
Define energy levels
A discrete amount of energy an electron within an atom is allowed to possess
Name the 4 rules of energy levels
- An electron cannot have a quantity of energy between 2 levels
- Energy levels are negative as external energy is required to remove an electron and it indicates that electrons are bound
- An electron with zero energy is free from the atom
- The energy level with the most negative value is know as the ground state/ level
When is an atom excited?
When an electron moves from a lower to a higher energy level in a gas
What is de-excitation?
When an electron moves to a lower energy level releasing a photon to conserve energy
Change in energy =
h*frequency or hc/lambda
What is spectroscopy?
A technique in which spectral lines are identified and measure to identify elements
What is emission line spectra?
Each element produces a unique emission line spectrum because of its unique set of energy levels
What are continuous spectra?
All visible frequencies or wavelengths are present
Atoms of a heated solid metal produce this spectrum
What are absorption line spectra?
It has a series of dark spectral lines against the background of a continuous spectrum, the dark lines have exactly the same wavelengths as the bright emission spectral lines for the same gas atoms
When are emission line spectra produced?
When atoms in a gas are excited, then electrons drop back down to their original energy levels they emit photons with specific frequencies unique to each element
When are absorption line spectra made?
When light from a source that produces a continuous spectrum passes through a cooler gas
Photons pass through the gas, some are absorbed increasing their energy levels and exciting the atom
Only specific wavelengths are absorbed creating dark lines
How do we use spectra to identify elements in stars?
By looking for the absence or presence of a specific spectral line
What is a diffraction grating?
An optical component with regularly spaced slits or lines that diffract and split light
What is the diffraction grating equation?
dsin(theta) = n*lambda
What is a black body?
An idealised object that absorbs all the electromagnetic radiation that shines onto it
When in thermal equilibrium emits wavelengths at a specific temperature
What is Wien’s displacement law?
Max Wavelength is inversely proportional to T
What is Stefan’s law?
Total power radiated per unit surface area is directly proportional to the fourth power of the absolute temperature of the black body
