Module 5: C19 - Stars Flashcards
How are Nebulae formed?
Nebulae are formed over millions of years as the tiny gravitational attraction between particles of dust and gas pulls the particles towards each other, eventually forming the vast clouds.
How is a Protostar formed?
As dust and gas particles get closed 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.
How does a Protostar become a Star?
For a protostar to become a star, nuclear fusion needs to start in its core. Many protostars never reach this stage. Fusion reactions produce energy in the form of kinetic energy. Extremely 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 form helium nuclei. In some cases, it grows large and hot enough that the kinetic energy of the hydrogen nuclei is large enough to overcome this electrostatic repulsion, forcing them together to make helium nuclei as nuclear fusion begins, forming a star.
What happens to a Star in its Main Sequence
Once a star is formed, it remains in a stable equilibrium with almost a constant size. Gravitational 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 outwards. The force from this radiation and gas pressure balances the force from the gravitational attraction and maintains equilibrium.
What factors affect how long a star remains stable (in its main sequence) for?
How long a star remains stable depends on the size and mass of its core. The cores of large, massive supergiant stars are much hotter than those of small stars, releasing more power and converting the available hydrogen into helium in a much shorter time.
(Really massive stars are only stable from a few millions years, whereas smaller stars like the sun are stable for tens of billions of years).
Description of a Planet
A planet is an object in orbit around a star with 3 important characteristics:
- It has a mass large enough for its own gravity to give it a round shape (unlike irregular shape of asteroids)
- It has no fusion reactions (unlike a star)
- It has cleared its orbit of most other objects (asteroids, e.c.t)
Description of a Dwarf Planet
Dwarf planets (like Pluto) have not cleared their orbit of other objects. In Pluto’s case there are many other bodies of comparable size close to its orbit.
Description of an Asteroid
Asteroids are objects too small and uneven to be planets, usually in near-circular orbits around the Sun without the ice present in comets.
Description of a Planetary Satellite
A planetary satellite is a body in orbit around a planet. This includes moon and man-made satellites.
Description of a Comet
Comets range from a few hundred meters to tens of kilometres across. They are small irregular bodies made up of ice, dust, and small pieces of rock. All comets orbit the Sun, many in highly eccentric elliptical orbits. As they approach the Sun, some comets develop spectacular tails.
Description of Solar Systems
Our Solar System contains the Sun and all objects that orbit it (planets, comets, e.c.t). It is one of many. In 2014 over 1100 other solar systems (sometimes called planetary systems) have been discovered.
Description of a Galaxy
A galaxy is a collection of stars, and interstellar dust and gas. On average a galaxy will contain 100 billion stars, a significant proportion of which have their own solar systems.
What is the Cosmological Principle
The idea that the universe has the same large scale structure when observed from any point within it is known as the cosmological principle.
What are the 3 Assumptions of the Universe
⚫ The universe is homogeneous (it’s density is the same everywhere)
⚫ The universe is isotropic (it’s the same in all directions)
⚫ The laws of physics are universal (all laws of physics on Earth can be applied to other places in the universe)
Evolution Stages of Lower Mass Stars
- Main Sequence
- Red Giant
- White Dwarf (rest of mass projected out in a planetary nebula)
The evolution of stars of lower mass, from main sequence to red giant and ending with a white dwarf. The planetary nebula may collapse again to form another star, or even a solar system with its own planets.
Evolution Stages of More Massive Stars
- Main Sequence
- Red Supergiant
- Supernova
- Neutron Star or Black Hole
What is a Red Supergiant made up of?
Inside a red supergiant, the core is made up of onion-like layers in which different elements are created by fusion, with heavier elements deeper in, up to the central core, made of stable iron nuclei that cannot fuse any further.
Solar Mass required to form a White Dwarf
< 1.4 Mo
Solar Mass required to form a Neutron Star
1.4Mo - 2.0Mo
Solar Mass required to form a Black Hole
2Mo <
What makes up a Neutron Star?
After a 1.4 to 2 solar mass star has exploded in a supernova, only the inner core of the star remains.
This core will have a radius of only 10 km, and a density more than 14 orders of magnitude higher than that of the Sun, and close to the density of an atomic nucleus. This is a neutron star.
The outer shell is thought to be composed of a solid crust of atomic nuclei. Inside this crust is a liquid interior composed almost entirely of neutrons, increasing in density towards the centre, to reach nearly 10^18 kgm^–3.
How is a Black Hole Formed?
When a large star ends its life in a supernova, the central core that is left behind is so massive that the neutrons inside it are destroyed by gravitational forces.
It becomes smaller and more dense than a neutron star, and eventually its centre collapses into a point of infinite density called a singularity.
Its gravitational field is now so strong that nothing can escape it, including light, so it appears black. This is a black hole.
What is an Event Horizon?
Every black hole is surrounded by an event horizon. Nothing that occurs within this boundary can ever affect the Universe outside it, and anything that crosses this horizon will fall into the black hole.
What is the Schwarzchild Radius
Light can escape from the vicinity of a black hole if it is outside a radius called the Schwarzschild radius.
Inside this boundary, the escape velocity from the black hole is greater than the speed of light. Nothing can move faster than light, so nothing can escape.
Equation for the Schwarzchild Radius
The Schwarzschild radius, Rs, is the minimum value of r in this inequality:
1/2 mv^2 ≥ GMm/r
r ≥ 2GM/v^2
Therefore,
Rs = 2GM/c^2
What is the Schwarzschild radius of a black hole four times the mass of the Sun?
(1 solar mass = 2.0x10^30 kg)
Rs = 2GM/c^2
Rs = 2 x 6.67x10^-11 x 4 x 2.0x10^30 / (3x10^8)^2
Rs = 11857m
Rs = 12km
How does an object falling into a black hole look for an outsider, as well as the object itself
To an outside observer, an object falling into the black hole slows down as it approaches the event horizon, never quite crossing it.
From the object’s point of view, it crosses the event horizon and falls towards the singularity.
What happens to Red Giants
At the start of the red giant phase, the reduction in energy released by fusion in the core means that the gravitational force is now greater than the reduced force from radiation and gas pressure. The core of the star therefore begins to collapse. As the core shrinks, the pressure increases enough to start fusion in a shell around the core.
Red giant stars have inert cores. Fusion no longer takes place, since very little hydrogen remains and the temperature is not high enough for the helium nuclei to overcome the electrostatic repulsion between them. However, fusion of hydrogen unto helium continues in the shell around the core. This causes the periphery of the star to expand as layers slowly move away from the core. As these layers expand, they cool giving the star its red colour
What happens to White Dwarfs (and how are they formed)
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. The white dwarf is very dense, often with a mass around that of our Sun, but with the volume of Earth. No fusion reactions take place inside a white dwarf. It emits energy only because it leaks photons created in its earlier evolution.
