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Physics > Stars > Flashcards

Stars Flashcards

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1
Q

Define Nebulae

A

Gigantic clouds of dust and hydrogen gas
Will eventually give rise to the formation of stars and planets

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2
Q

Define Planets

A

Spherical bodies that orbit a star

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3
Q

Define Dwarf Planets

A

A planet that has not cleared its orbit of other objects

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4
Q

Define Moons

A

Spherical bodies that orbit planets

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5
Q

Define Asteroids

A

Small, irregularly shaped bodies, composed of dust and metal. Usually in near circular orbits about stars. Remnants of planets and contain no ice.

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6
Q

Define Comets

A

Small, irregularly shaped bodies, composed of dust and ice. Usually in eccentric orbits about stars. Remnants from the formation of a solar system. Contain ice.

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7
Q

Define a Solar System

A

A system of planets orbiting a central star

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8
Q

Define Galaxy

A

A collection of stars and planets (collection of solar systems

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9
Q

Define Universe

A

All the galaxies and all their mass and energy

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10
Q

Steps of the Formation of a Star

A

Nebulae form over millions of years as tiny particles of dust and gas come together under the force of gravitational attraction

Denser regions of a nebula pull in more matter, coming hotter as GPE is transferred to heat energy

A protostar forms in the nebula - a very hot and very dense cloud of dust and gas

If the protostar becomes massive, dense and hot enough, the gravitational force of attraction between particle is able to overcome the electrostatic force of repulsion between hydrogen nuclei

Nuclear Fusion of hydrogen nuclei begins and the protostar becomes a main sequence star

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11
Q

Beginning for all stars

A

Denser regions of a nebula form a protostar

Protostar become hot and dense enough, Nuclear fusion begins and the star becomes a main sequence star

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12
Q

Life Cycle of a low mass star

A

Hydrogen nuclei in core runs out

Rate of fusion decreases - star cools -> radiation pressure drops

Gravitational force collapses star - GPE -> KE -> Causing it to heat up again

Fusion of hydrogen nuclei in shells of core begins -> Shells expand cool, emit lower red EM waves -> Red Giant

Hydrogen nuclei in shells run out

Rate of fusion decreases -> star cools -> mass of shells lost to space and radiation pressure drops

Gravitational force collapses star -> Hot core remains -> White dwarf

White dwarf cools -> black dwarf

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13
Q

Electron Degeneracy Pressure in White Dwarfs

A

When fusion slows down in shells of star, radiation pressure decreases, star core collapses under gravity

As the core collapses, matter/atoms are forced together into a smaller volume

However, electrons are not allowed to occupy the same energy levels within atoms

A pressure is exerted by the electrons as the star collapses - electron degeneracy pressure

This pressure outwards counters the gravitational attraction inwards, and no further collapse of the core is possible

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14
Q

What is the Chandrasekhar Limit

A

Limit to mass of core that will be prevented from total collapse of star due to electron degeneracy pressure

Core/White dwarf can only be as massive as 1.44 times mass of star

A mass of greater than this will mean gravitational collapse is not prevented by electron degeneracy pressure

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15
Q

The Life Cycle of High Mass Stars

A

More mass, more GPE -> KE and hotter core
Leads to faster rate of fusion and shorter life time

Hydrogen nuclei in core run our

Rate of fusion decreases -> star cools -> radiation pressure drops

Gravitational force collapses star - GPE ->KE - causes it to heat up again

Fusion of helium nuclei in the core begins - star heats up and expands - fusion of hydrogen in outer shell begins - outer shells further expand and cool - emit lower red EM waves - Red Super Giant

Star develops an iron core - Iron nuclei in core cannot be fused together - Star becomes unstable and implodes to become a supernova

Outer shells shed

Dense core remains - either a neutron star or black hole

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16
Q

White dwarfs and Neutron Star

A

If mass of remaining core is greater than Chandrasekhar limit - further gravitational collapse forms a neutron star

Composed of neutrons - small volume - very dense

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17
Q

White dwarfs and Black holes

A

If mass of the remaining core is greater than 3 solar masses - further gravitational collapse of core creates a very dense body

Gravitational pull is very strong - photons/light unable to escape the gravitational pull

18
Q

Low Mass Star life cycle

A

Protostar -> Main sequence star -> Red giant -> White dwarf -> Black dwarf

19
Q

High mass star life cycle

A

Protostar -> Main sequence star -> Red supergiant -> Supernova -> Neutron Star/Black hole

20
Q

What does the Hertzsprung - Russell Diagram show

A

The position of stars at various stages or their life cycle as a plot of their luminosity against temperature

Both axis are log scales

Temperature axis runs in reverse

Luminosity is often plotted in units relative to the luminosity of the sun

21
Q

For what is the Hertzsprung Russel diagram used for

A

To determine what stage of its life cycle a star is

22
Q

What is Luminosity

A

An absolute measure of the radiated EM power (Watts)

How much EM radiation is emitted per second

23
Q

What does the luminosity of a star depend in

A

Both temperature and mass

High temperature and low mass - low luminosity - low output of energy per unit time

Low temperature but large mass - high luminosity - output lots of energy per unit time

24
Q

Where are the hottest most luminous stars located on the Hertzsprung Russel diagram

A

Top left

25
Q

Where are the coolest least luminous stars located on the Hertzsprung Russell diagram

A

Bottom right

26
Q

Where are electrons only allowed to exist

A

At discrete energy levels about a nucleus in an atom

27
Q

What is the lowest energy level of an atom

A

Ground state

n=1

28
Q

How much energy must be transferred to an electron to escape the atom

A

The energy at a given level to attain zero electrostatic potential energy

29
Q

How can an electron excite to a higher level

A

By absorption of a photon E=hf only if the E matches is exactly the difference between levels

30
Q

How can orbital electrons be excited to higher energy levels

A

Interaction with accelerated electrons

Accelerated electrons energy does not have to match the difference between energy levels to cause excitation - simply has to have at least energy matching the difference

31
Q

What happens to any electrons that excite to a higher energy level

A

Must de-excite to the original level either directly or in steps

Every de excitation / step down between energy levels emits a photon of energy equivalent to the difference in energy between the two levels

32
Q

What happens during every de-excitation between energy levels

A

A photon is emitted with the energy equivalent to the difference in energy between the two levels

33
Q

What is a Black Body

A

A theoretical body which absorbs all EM radiation of all frequencies incident upon it

A perfect absorber of EM radiation and a perfect emitter of EM radiation

34
Q

Describe Black Body Radiation Curves

A

Illustrate how the intensity of radiation emitted by a black body varies with its temperature

Every black body emits a continuous range of EM radiation

As temperature increases - the peak intensity of EM radiation shifts to lower wavelengths and increases in relative magnitude

35
Q

What is Wien’s Displacement Law

A

The wavelength at peak intensity is inversely proportional to the absolute temperature of the body

36
Q

What is Stefans Law

A

The total power radiated by a black body at absolute temperature is proportional to its surface area and to its temperature^4

37
Q

Wiens Displacement Law Equation

A

λmax T = constant​
Wien’s Constant = (0.0029)m K = (2.9x10-3) metre-Kelvin​

38
Q

Stefan’s Law Equation

A

P = σAT^4​

σ = Stefan’s Constant​

5.67 x 10^-8 Wm^-2K^-4​

39
Q

For a star of radius, r its total power radiated is a measure of its luminosity, L

A

L = 4πr^2σT^4

40
Q

What can Wien’s Law and Stefan’s Law be usedfor

A

Wien’s Law & Stefan’s Law can be used together​
to estimate the radius of a distant star

Once radius is known, mass and density of the star can be determined using Newton’s Law of Gravitation.

Physics (19 decks)

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