Astrophysics Flashcards
An astronomer claims to have discovered a white dwarf with a mass twice the size of our sun. Suggest why this claim is wrong.
The white dwarf can only have a maximum mass of 1.44 solar mass. (Chandrasekhar limit) therefore it cannot be twice the mass
What is the mass of our sun in solar mass?
1 solar mass
What does one solar mass equal to?
2.0 x 10^40 kg
State two ways a black hole is different to a star
No fusion occurs
Black holes have smaller masses compared to stars
State how emission lines is produced?
Electrons make transitions to lower energy levels which makes them lose energy and emitting a photon
What is a nebula?
Giant clouds of dust and gas.
How are nebulae formed?
Tiny gravitational attraction between particles of dust and gas pulls toward each other and eventually forms a cloud.
How are protostars formed?
Dust and gas get closer together and denser regions begin to form. These regions pull lore dust and gas and gain more mass becoming more denser. It becomes hotter as the gravitational energy converts to thermal energy.
For a protostar to become a star what has to happen?
Nuclear fusion
Why are many protostars not able to become a star?
In order for nuclear fusion to occur high pressure and temperature is required to overcome electrostatic repulsion between hydrogen nuclei. Hence they remain as protostars
When hydrogen nuclei are forced together what do they form?
Helium nuclei
How does a star remain in a stable equilibrium?
Gravitational forces compress the star, whilst the radiation pressure from the photons emitted during fusion and the gas pressure from the nuclei push outwards. The forces balance out and maintain equilibrium.
What is the stable phase of a star?
Main sequence star
What affects how long a star is stable?
The mass and size of its core
Are bigger stars or smaller stars likely to be stable for longer?
Smaller stars.
Why are bigger stars stable less then smaller stars?
Larger stars are much hotter so release more power and convert the available hydrogen into helium in a much shorter time.
What do main sequence stars with 0.5-10 solar masses turn into?
Red giants
What happens at the start of a red giant?
The reduction in energy released by fusion results in the core of the star to start collapsing due to stronger gravitational force.
As a red giant starts to collapse what happens next?
As the core shrinks the pressure increase for fusion to occur around the core.
What happens after the red giant starts to collapse
As the pressure increases, the pressure increases to start fusion in a shell around the core.
Why does fusion no longer occur in the core of a red giant?
The core is inert as there is very little hydrogen and the temperature isn’t high enough for helium to overcome its electrostatic repulsion.
As fusion still occurs around the core of a red giant what happens?
The star starts to expand as the core slowly moves away from the core. The layers then cool and become red in colour.
Once the layers around the core drift off into space what is left behind?
A planetary nebula and white dward
Characteristics of a white dwarf?
Very dense
About the mass of our sun
No fusion occurs inside a white dwarf
Why does a white dwarf emit energy if no fusion occurs?
As it leaks photons created in its earlier stages.
What stops a white dward from further collapsing?
Two electrons cannot exist in the same energy state so when the core starts to collapse the electrons are squeezed. This created pressure that prevents it from collapsing further. (Electron degeneracy pressure)
When does the electron degeneracy pressure not work?
When the mass is above a certain value
What is Chandrasekhar limit?
This limit is the maximum mass of a stable white dwarf -1.44solar masses
If a main sequence star has a mass greater then 10 solar masses what happens?
Red SUPER GIANT
As the core of larger stars are hotter shattering happens?
The helium nuclei is moving fast enough to overcome electrostatic repulsion and form heavier elements.
What happens to red super giants?
They have high temp and presure so can fuse to make heavier elements and shells. Eventually it will make an iron core which in inert. After this point it cannot produce any more energy. Lead to the DEATH of red super giant
What can the remnant core of a supernova turn into?
Neutron star
Or a black hole
When does a supernova become a neutron star?
If the mass of the core is greater then Chandrasekhars limit it continue to collapse and eventually form a neutron star.
Properties of a neutron star?
Entirely made up of neutrons
Very small
Mass of about 2 solar masses
When are black holes formed?
When a supernova dies and the core has a mass greater the 3 solar masses it continues to collapse.
Properties of black holes:
Has a such a strong gravitational file that in order to escape an object will need the escape velocity greater then the speed of light.
What is a hertz-sprung Russell diagram?
A graph of stars in our galaxy showing the relationship between their luminosity and surface temperature.
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Why are electron levels negative?
As external energy is required to remove an electron from the atom. It also indicates that the electrons are trapped within the atom or bound to the positive nuclei.
When are electrons free from the atom
When they have zero energy
What is the energy level with the most negative state known as?
The ground state
When electrons move from a lower energy level to a higher one ?
It gets excited.
When an electron moves from a higher energy level to a lower energy level what happens?
It loses energy
When an electrons transitions between levels what is emitted?
A photon
How do you find the change in energy?
Lower energy level minus higher energy level
What does each element has a different set of?
Energy levels
How can the starlight from a star help identify the elements within the star?
Using spectra’s
What are the three types of spectras?
Emission line
Continuous spectra
Absorption line spectra
How does an emission line spextra get produced?
When an elextron transition from a high energy ti a lower energy level it emits photons wirh a set frequency.
Each line represents a photon with a specific wavelength. These can be observed from heater gases.
How are absorption line spectra produced?
When a light from a source passes that would produce a continuous spectrum passes through a cooler gas. As the photon passes through the gas some are absorbed by the gas atoms raising the energy level of the electrons. Only photons that are exactly equal to the difference between energy levels are absorbed so specific wavelength are absorbed creating dark lines in the spectrum.
What did the dark lines in an absorption spectrum show?
Which photons have ben absorbed by the gas atoms. Although the photons can be re emitted when the electrons drop back down they are emitted in all directions so the internist is reduced.
What is a diffraction grating?
An optical component is regularly spaced slits or lines that diffract and split light into beams of different colour travelling in different direction.
What is a black body?
An idealised object that absorbs all the electromagnetic radiation that shines onto it and when in thermal equilibrium emits a characteristic distribution of wavelengths at specific temperature
What is wien’s law?
Wavelength max is inversely proportional to T
What is the value for the constant?
2.90 x 10-3 mK
What happens to the peak wavelength as the temperature increases?
Becomes sharper and taller.
The wavelength decreases.
Stefans law
Total power radiated per unit surface area of a black body us directly proportional to the fourth power of the surface temperature
An objectaway from the wave source will experince what?
Longer wavelength with shorter frequency
