19, Stars

Question 1

What is a planet?

Answer 1

An object orbiting the sun,
which is big enough to be roughly spherical
and has cleared its orbit.

Question 2

What is a planetary satellite?

Answer 2

An object in orbit of a planet including moons and man-made satellites like the Hubble space telescope.

Question 3

What is a comet?

Answer 3

A small object orbiting the sun made of ice and rock, often with highly elliptical orbits

Question 4

What is a solar system?

Answer 4

The set of objects including the sun which orbit the sun (systems for other stars are just planetary systems).

Question 5

What is a galaxy?

Answer 5

A system of stars and dust held together by gravitational attraction.

Question 6

What is the universe?

Answer 6

All existing matter and space.

Question 7

What stops gravity causing the star to continue shrinking?

Answer 7

The radiation pressure from fusion and the gas pressure from the compact gas.

Question 8

What stops gravity causing the star to continue shrinking?

Answer 8

The radiation pressure from fusion and the gas pressure from the compact gas.

Question 9

How is 1 solar mass denoted?

Answer 9

M with a subscript circle with a dot in the centre.

Question 10

What is a red giant and what is the required solar mass for it to form?

Answer 10

A red giant is a star which is running out of hydrogen fuel and expands in size, stars of 0.5 - 10 solar masses become red giants.

Question 11

What causes white dwarfs and what stops them from collapsing?

Answer 11

White dwarfs are the left-overs of a red giant which couldn’t be fused, they are kept apart by electron degeneracy pressure (which comes from the fact that electrons cannot exist in the same energy state).

Question 12

What is the Chandrasekhar limit and its value?

Answer 12

With a value of 1.44 solar masses, the Chandrasekhar limit is the mass of a star core which if higher than it a supernovae will occur.

Question 13

What is the difference between a red giant and red supergiant?

Answer 13

Red supergiants are much larger and can fuse heavier elements up to iron.

Question 14

What is a neutron star?

Answer 14

The remains of a small supernovae, made entirely of neutrons and very dense.

Question 15

What is a black hole?

Answer 15

Caused by a supernovae where the core is greater than 3 solar masses, black holes have an event horizon (the radius beyond which light cannot escape the gravitational field) which are larger than the object themselves.

Question 16

What is the Hertzsprung-Russell diagram, what are the axis and what does it show?

Answer 16

A plot of stars of luminosity (y) against temperature (or colour) (x) with hotter temperatures closer to the origin (both scales are logarithmic), it shows a central s shaped band which contains main sequence stars, a line of supergiants across the top and white dwarfs towards the bottom left.

Question 17

What path do normal stars make in the HR diagram?

Answer 17

Starting in the main sequence band (where they stay until the red giant phase) they move up to the right when they are red giants and move back down to the left when they become dwarfs.

Question 18

What path do supergiants take in the HR diagram?

Answer 18

Move from the top left to top right.

Question 19

How do neon lights work?

Answer 19

A P.D. is placed across neon gas, the energy provided will raise electrons from lower to higher energy states, as the electron returns to the lower energy level it releases a photon of a specific wavelength.

Question 20

Why are energy levels of electrons in atoms negative?

Answer 20

Because it shows the electron is trapped in a bound system, i.e. energy is required to remove it.

Question 21

What to the equations ΔE = hf = hc/λ show in spectra?

Answer 21

The change in energy of an electron is equal to the energy of the photon produced.

Question 22

How can helium be identified in stars and why does this work?

Answer 22

Helium can be identified from the emission lines produced by electrons in helium changing energy levels, elements can be found this way because electrons can only be found at specific (different) energy levels in (different) atoms.

Question 23

What causes emission line spectra?

Answer 23

Photons produced from electrons in atoms changing energy level.

Question 24

What is a continuous spectra?

Answer 24

All wavelengths of light are produced, e.g. from a very hot metal.

Question 25

What is absorption line spectra?

Answer 25

When electrons in atoms absorb the photon required for them to increase energy level (photon will be re-emitted but it will do so in a random direction).

Question 26

What is diffraction grating?

Answer 26

Lots of very close together slits which will diffract light splitting it into the component colours allowing you to measure emission/absorption spectra.

Question 27

What does the equation d sinθ = nλ show?

Answer 27

The distance between slits * sin of the angle made by the perpendicular to the diffraction grating and the line between the maxima and the grating = the nth order maxima * wavelength of the light.

Question 28

Why is the highest order maxima that is visible is about d/λ?

Answer 28

Because when θ is close to 90, sinθ is close to 1 so d/λ(sinθ) = n.

Question 29

What does λmax is proportional to T show?

Answer 29

The temperature of a black body is inversely proportional to the peak wavelength (the wavelength at intensity is maximum).

Question 30

What is the definition of luminosity?

Answer 30

The total power radiated by an object.

Question 31

What does the equation L = 4πr^2σT^4 show?

Answer 31

Luminosity of a star = 4π * radius of star^2 * the Stefan constant * surface temperature^4.

Question 32

How can the main wavelength and luminosity of a star be used to give the radius?

Answer 32

Use λmax is proportional to T to find the temperature and solve L = 4πr^2σT^4 for r.