5.5 Astrophysics and Cosmology Flashcards
What are Nebulae?
Nebulae are gigantic clouds of dust and gas (mainly consisting of hydrogen)
How are Nebulae formed?
Nebulae are formed over millions of years, as the tiny gravitational attraction between particles of dust and gas pull the particles towards each other, eventually forming the large clouds of dust.
Describe how a star is born
- As particles of dust and gas exert forces of gravitational attraction on each other, they form gigantic clouds together called a nebula.
- As the dust and gas get closer together, this gravitational collapse accelerates.
- Due to tiny variations in the nebula, denser regions begin to form
- These regions pull in more dust and gas, gaining mass and getting denser, and also getting hotter as gravitational energy is eventually transferred to thermal energy
- In one part of the cloud, a protostar forms - this is not yet a star but a very hot, very dense sphere of dust and gas
- For a protostar to become a star, nuclear fusion of hydrogen nuclei into helium nuclei needs to start in its core
- Many protostars never reach this stage, as high pressures and temperatures in the core are needed in order to overcome the electrostatic repulsion between hydrogen nuclei - in order to fuse them together to form helium nuclei
- In some cases, more and more mass is added to the protostar; it grows so large and the core becomes so hot that the kinetic energy of the hydrogen nuclei is large enough to overcome the electrostatic repulsion.
- The hydrogen nuclei are forced together to form helium nuclei as nuclear fusion begins.
A star is born
What happens once a star is formed?
- Once a star is formed it maintains equilibrium with almost a constant size
- Gravitational forces act to compress a star
- But the radiation pressure from the photons emitted during fusion and the gas pressure from the nuclei in the core push outwards
- The force from the radiation and gas pressure balances the force from the gravitational attraction and maintains equilibrium
Stars in this stable phase are describes as being on their main sequence
What are the factors that affect how long a star is and in its stable main sequence phase?
The factors that affect how long a star is in its main sequence phase are its the size of its core and mass of its core
- The cores of massive supergiant stars are much hotter than those of smaller stars, releasing more power and converting the available hydrogen into helium in a much shorter time.
Answer: The size and mass of its core
How long are really massive stars stable (in their main sequence) compared to smaller stars like the Sun?
Really massive stars are only stable for a few million years, whereas smaller stars like our Sun are stable for tens of billions of years
Explain what a Planet is
A planet is an object in orbit around a star with three important characteristics:
1. It has a mass large enough for its own gravity to give it a round shape (unlike the irregular shape of asteroids)
2. It has no fusion reactions
3. It has cleared its orbit of most other objects (asteroids, etc.)
What are planetary satellites?
A planetary satellite is a body in orbit around a planet. This includes moons and man-made satellites.
Explain what Comets are
Comets are small irrelgular bodies made up of ice, dust and small pieces of rock. All comets orbit the Sun, many in highly eccentric eliptical orbits. They range from a few hunred metres to tens of kilometres across. As they approach the Sun, some comets develop spectacular tails.
What does our Solar System contain
Our solar system contains the Sun and all objects that orbit it (planets, comets, etc.)
Explain what galaxies are
Brain dump
A galaxy is a collection of stars, and interstellar dust and gas.
On average a galaxy will contain 100 billion stars, a significant proportional of which have their own solar systems.
Our galaxy is known as the Milky Way
Explain what the Universe is
The universe is absolutely everything. It is all the electromagnetic radiation, energy, matter, all of space-time and everything that exists within it - including all the galaxies
What happens to low-mass stars like our Sun after the main sequence phase?
Low mass stars like our Sun, specifically between 0.5 Solar Mass and 10 Solar Mass will evolve into red giants.
Explain the evolution of a low-mass star (like our Sun) from main sequence into a red giant
- Main sequence stars eventually run low on hydrogen fuel in their core
- The low hydrogen fuel in their core results in a reduction of energy released by fusion
- Once the core runs out of hydrogen, fusion stops in the core, so there’s no more outward pressure.
- This means that the gravitational force is now greater than the force from radiation pressure and gas pressure
- This causes the core of the star to collapse
- As the core shrinks and becomes denser, it also heats up (due to XYZ).
- This hydrogen shell surrounding the core also heats up, which increases the pressure enough to start fusion around the core
- This causes the periphery/border of the star to expand as the layers slowly move away from the core
- As these layers expand, they cool, giving the star its characteristic red colour.
Describe the core of a red giant
- Red giant stars have inert cores (inert meaning chemically inactive, static, motionless)
- Fusion no longer takes place since very little to no hydrogen remains and the temperature is not high enough for the helium nuclei (which are both positively charged) to overcome the electrostatic repulsion between them
Explain the evolution of a star from a red giant into a white dwarf and planetary nebula
- In the red giant phase, the layers of the star are expanding and slowly moving away from the core
- As these layers expand, they cool, giving the star its characteristic red colour.
- Eventually most of the layers of the red giant around the core drift off into space as a planetary nebula, leaving behind the hot core as a white dwarf
Describe the characteristics of a white dwarf
- A white dwarf is very dense, often with a mass like our Sun but with the volume of the Earth
- No fusion reactions take place inside a white dwarf. It only emits energy because it leaks photons created in its earlier evolution
- The surface temperature of a white dwarf can be as much as 30 000K
Describe the stages a massive star goes through
- Massive stars, reffering to those above 10 Solar Mass, consume the hydrogen in their core very quickly and in much less time than Low Mass stars, some in only a few million years.
- This is because their mass is much greater, and so their cores are much hotter. So they convert the available hydrogen into helium in a much shorter time.
- Eventually the core of the star runs low on hydrogen fuel. This results in a reduction of energy and pressure released by fusion.
- This means that the gravitational force is now greater than the force from the radiation pressure and gas pressure released by fusion
——————————-STUCK HERE——————————–
- This causes the core of the star to collapse
- As the core shrinks and becomes denser, it also heats up (due to XYZ).
- This hydrogen shell surrounding the core also heats up, which increases the pressure enough to start fusion around the core
- This causes the periphery/border of the star to expand as the layers slowly move away from the core
- As these layers expand, they cool, giving the star its characteristic red colour.
