Chapter 19 - Stars and dat

Question 1

What is nuclear fusion?

Answer 1

Where two smaller nuclei join together to form one larger nucleus.

Question 2

What are planets?

Answer 2

An object in orbit around a star

Question 3

What are the three important characteristics of a planet?

Answer 3

• it has a mass large enough for its own gravity to give it a round shape
• it has no fusion reactions
• it has cleared its orbit of most other objects

Question 4

Explain the life cycle of a star up till main sequence?

Answer 4

• nebulae are formed as the gravitation attraction between particles of dust and gas pulls the particles towards each other forming vast clouds.
• they gain mass and get hotter as the gpe is transferred to thermal energy and a protostar forms
• nuclear fusion needs to occur for it to become a star

Question 5

Explain the fusion reaction in a protostar?

Answer 5

Fusion reactions produce energy in the form of kinetic energy. High pressures and temperatures inside the core are needed in order to overcome the electrostatic repulsion between Hydrogen nuclei in order to fuse them together to helium nuclei.
-in some cases more mass is added so that the protostar grows larger and the kinetic energy of the hydrogen nuclei is large enough to overcome the electrostatic repulsion.

Question 6

What is the main sequence ?

Answer 6

• the stable phase of their lives are described as being on their main sequence.
  How long a star remains stable depends on the size and mass of its core.

Question 7

What’s a dwarf planet?

Answer 7

Dwarf planets haven’t clear the orbit of other objects unlike planets.

Question 8

Life of a red giant star?

Answer 8

-stars between 0.5m to 10 M.
Red giants have inert cores. Fusion no longer takes place due to low temperatures so Helium can’t overcome electrostatic repulsion.
Reduction in energy released by fusion in core means gravitational force -> reduced force from radiation and gas pressure.
-Layers of the red giant drift into space as planetary nebula leaving behind a hot core - white dwarf

Question 9

What’s a white dwarf?

Answer 9

Dense hot core that is leftover.

Emits energy due to leaking photons.

Question 10

What’s the life cycle of a red supergiant?

Answer 10

• stars with mass over 10 M.
• hotter core of He fuses into other elements.
• process continues till iron core develops.
• the iron nuclei cannot fuse and this makes the star unstable and leads to the death of the star in a catastrophic explosion called Supernova.
• supernova shockwave ejects all core materials into space.
• if the mass of the core is greater than the Chandrasekhar limit then gravitation collapse continues to form neutron star.
• if the core is above 3M gravitational collapse continues to compress the core. This results in a black hole.

Question 11

Explain why heavier elements are not formed in the core of a red giant star?

Answer 11

Temperature is not high enough therefore the kinetic energy of larger nuclei is not large enough to overcome the electrostatic repulsion and fuse together.

Question 12

What is the Chandrasekhar limit?

Answer 12

The mass of a stars core beneath which the electron degeneracy pressure is sufficient to prevent gravitational collapse, 1.44 solar masses.

Question 13

Describe the process of the creation of heavier elements (nucleosynthesis) and how these elements come to be found distributed throughout the universe?

Answer 13

• elements formed in red supergiants
• core is large enough, therefore hot enough to fuse larger nuclei together
• fusion takes place in shells around the core
• eventually when the core become iron the star will explode in a supernova
• elements above iron created in the supernova
• the supernova distributed heavier elements throughout the universe.

Question 14

What is the hertzprung-Russel diagram?

Answer 14

-a graph showing the relationship between the luminosity of stars (y axis) in our galaxy and their average surface temperature (x -axis, increasing from right to left)

Question 15

Explain why when main sequence star becomes a red giant it moves towards the upper right of an HR diagram?

Answer 15

• it expands so it’s luminosity increases and the surface cools.

Question 16

whats an emission line spectra and how is it produced?

Answer 16

-atoms in a gas are excited then when the electrons drop back into lower energy levels they emit photons with a set of discrete frequencies specific to that element. this produces a characteristic emission line spectra.

Question 17

How is an absorption spectrum formed?

Answer 17

• light emitted from a star as a continuous spectrum
• a gas cloud absorbs specific wavelengths corresponding to the energy levels of the atom it contains.
• the absorbed light appears as black lines in the spectrum.

Question 18

What’s electron degeneracy pressure?

Answer 18

A quantum mechanical pressure created by the electrons in the core of a collapsing star due to the Pauli exclusion principle.

Question 19

How does a star maintain equilibrium ?

Answer 19

-gravitation forces act to compress the star but the radiation pressure from the photons emitted during fusion and the gas pressure from the nuclei in the core push forward. This balances the gravitational attraction and maintains equilibrium.

Question 20

What is Pauli exclusions principle?

Answer 20

Two electrons cannot exist in the same energy state.

Question 21

What is the Chandrasekhar limit and what’s it used for?

Answer 21

1.44 solar masses. The electron degeneracy pressure is only sufficient to prevent gravitational collapse is the core has a mass less than this.

Question 22

What do supernovas create?

Answer 22

Everything above iron in the periodic table was created in a supernova and such events help to distribute these heavier elements throughout the universe.

Question 23

4 points on the energy levels?

Answer 23

• an electron cannot have a quantity of energy between two levels.
• the energy levels are negative because external energy is required to remove an electron from the atom, the negative values also indicate that the electrons are trapped within the atom or bound the positive nuclei.
• an electron with 0 energy is free from the atom
• the energy level with the most negative value is the ground state

Question 24

What is a black body?

Answer 24

An idealised object that absorbs all the em radiation incident on it and when in thermal equilibrium emits a characteristic distribution of wavelengths at a specific temperature.

Question 25

Wien displacement law?

Answer 25

The peak wavelength at which the intensity of radiation from a black body is a maximum is inversely proportional to the absolute Temperature of the black body.

Question 26

What is Stefan law?

Answer 26

The total power radiate per unit surface area of a black body is directly proportional to the fourth power of the absolute temperature of the black body.

Question 27

Explain why heavier elements are not formed in the core of a red giant star?

Answer 27

• temperature is not high enough, therefore the kinetic energy of larger nuclei is not large enough to overcome the electrostatic repulsion and fuse together.

Question 28

Describe the processes of the creation of heavier elements and how these elements come to be found distributed throughout the universe?

Answer 28

• elements are formed in red supergiants
• core is large enough, therefore hot enough to use larger nuclei together
• fusion takes place in shells around the core
• eventually when the core becomes iron the star will explode in a supernova
• elements above iron created in the supernova
• the supernova distributes heavier elements through the universe.

Question 29

Explain why larger stars tend to spend less time in their main-sequence Phase?

Answer 29

-core is hotter therefore greater rate of fusion in the core. Star depletes it’s hydrogen in the core in a shorter time.

Question 30

Explain why nuclear fusion in the core of a star prevents further gravitational collapse?

Answer 30

• fusion produces gas and radiation pressure.

- this pushes outwards, against the gravitational collapse.

Question 31

Explain why when a main sequence star becomes a red giant it moves towards the upper right of an HR diagram?

Answer 31

• expands so it’s luminosity increases

- surface cools (moving it to the right)

Question 32

Describe the differences between a continuous spectrum and an emission line spectrum?

Answer 32

Continuous spectrum - all frequencies and wavelengths present
Emission spectrum - only certain/specific frequencies and wavelengths present.

Question 33

Suggest why the maxima produced from a diffraction gratings are brighter than those produced via the double-slit experiment.

Answer 33

-more light passing through grating therefore maxima are brighter.

Question 34

Describe the way in which the sun was formed and it’s most probable future evolution?

Answer 34

• matter/gas attracted by gravitational forces
• GPE converted to gain in KE (rise in temperature)
• temperature high enough for hydrogen to begin fusion process
• hydrogen runs out so core of the sun begins to collapse.
• hydrogen fusion continues in the outer layers of sun
• sun expands to form red giant
• outer layers drift away form planetary nebula
• core forms a white dwarf and slowly cools.

Question 35

Describe three characteristics of a red supergiant star?

Answer 35

• appears red
• have very large luminosity
• relatively low surface temperature
• large mass

Question 36

How to detect elements within stars?

Answer 36

If we know the line spectrum of a particular element, we can check whether the element is present in the star, even for extremely distant stars. If a particular element is present then it’s characteristic pattern of spectral lines will appear as dark lines in the absorption line spectrum