Physics chapter 19 - Stars

How are nebulae formed?

They are formed over millions of years as the tiny gravitational attraction between particles of dust and gas pull the particles towards each other.

How are protostars formed?

They are formed due to denser regions forming inside a nebula, pulling in more dust and gas, gaining mass and density, causing it to get hotter.

What needs to happen for protostars to become stars?

Nuclear fusion needs to start in its core.

What conditions are needed for nuclear fusion to occur?

Extremely high temperatures and pressures inside the core are needed to overcome the electrostatic repulsion forces between hydrogen nuclei to fuse them together to form helium nuclei.

What sized stars become red giants after their main sequence?

Stars between 0.5 and 10 solar masses (0.5M - 10M). Stars larger than 10M will become red supergiants.

What is the mass of the sun?

1.989 × 10³⁰ Kg

Why does fusion in red giants not occur?

This is because there is very little hydrogen left in the core and the pressure and temperature is not high enough for heavier elements to be fused together.

Why are red giants red?

This is because the outer layers of the star expand as nuclear fusion takes place in those layers. This causes the layers to cool down, giving the star a red colour.

What is electron degeneracy pressure?

This is a pressure caused by the fact that two electrons cannot exist in the same energy level at the same time. When a stars core collapses, the gravitational force is matched by the pressure exerted by the electrons, stabilizing the core.

What is the Chandrasekhar limit?

This is the maximum mass of a stable white dwarf. If the mass of a stars core is greater than this limit (1.44M (1.44 × 1.989 × 10³⁰ )) then the core will collapse and become a neutron star or a black hole.

Why can you not fuse iron nuclei?

This is because the reaction does not produce any energy. Elements heavier than iron cannot be fused as well.

What is the typical mass of a neutron star?

2M (2 solar masses)

How heavy does the core of a star have to be to become a black hole?

Greater than around 3M (3 solar masses).

What is the Schwarzschild radius of an object?

This is the radius of an imaginary sphere sized so that, if all the masses of the object is compressed into the sphere, the escape velocity for that object would be greater than the speed of light.

What is the equation for the Schwarzschild radius?

r_s = 2GM / C²

(where r_s is the Schwarzschild radius, G is the gravitational constant, M is the mass of the object and C is the speed of light).

What is the value of the gravitational constant?

6.67 × 10^-11 Nm²kg^-2

What is a Hertzsprung Russell diagram?

This is a graph of stars in our galaxy showing the relationship between their luminosity and their temperature. Y-axis = luminosity, X-axis = temperature

Temperature increases from right to left.

What is luminosity?

The luminosity of any star is the total radiant power output of the star.

What is the luminosity of the sun?

3.85 × 10²⁶w

Why are energy levels negative values?

This is because eternal energy is required to remove an electron from the atom.

What is the name of the energy level with the most negative value?

The ground state (or ground level).

What are the three types of spectra?

Emission line spectra

Continuous spectra

Absorption line spectra