D.2 Stellar Characteristics And Stellar Evolution

Question 1

What is Wien’s law?

Answer 1

The wavelength at which the emitted energy of a black body is a maximum is related to its surface temperature.

Question 2

What is a cepheid star/variable?

Answer 2

A star whose brightness varies with a well defined period. This period is related to the stars luminosity and so can be used to estimate the distance to the star.

Question 3

What is the hertzsprung russell diagram?

Answer 3

A plot of the luminosity of stars versus their surface temperature.

Question 4

What is the main sequence star?

Answer 4

A star undergoing fusion of hydrogen into helium. Such stars occupy a diagonal strip on a HR diagram from top left to bottom right.

Question 5

What is a red giant?

Answer 5

A large, relatively cool, reddish star which occupies the upper right region of the HR diagram.

Question 6

What is a red supergiant?

Answer 6

A very large, relatively cool, reddish star which occupies the extreme upper right region of the HR diagram.

Question 7

What is a white dwarf?

Answer 7

A very small, hot, dense star occupying the lower left region of the HR diagram this is the end stage in the evolution of low mass stars.

Question 8

What is a neutron star?

Answer 8

A very hot, very dense star which is the end stage in the evolution of massive stars.

Question 9

What is the chandrasekhar limit?

Answer 9

The largest mass a white dwarf star can have it is about 1.4 solar masses.

Question 10

What is the oppenheimer volkoff limit?

Answer 10

The largest mass a neutron star can have it is about 3 solar masses.

Question 11

When stars radiate as black bodies how does their intensity vary with wavelength?

Answer 11

Shows a typical black body curve the peak wavelength is related to surface temperature according to Wien’s law.

Question 12

What happens when light has been passed trough a gas?

Answer 12

Dark lines appear on a background of the colours of the rainbow. The dark lines correspond to photon wavelengths that have been absorbed by the atoms of the gas. A similar thing happens when the light that is produced within a star moves outwards through the outer layer of the star. Specific elements absorb specific wavelengths and so the dark lines give us information about what elements exist in the star.

Question 13

What dark lines often dominate in the star spectra?

Answer 13

Hydrogen and helium lines - although many other elements are present as well.

Question 14

How does the hertz sprung Russell diagram plot stars?

Answer 14

According to their luminosity on the vertical axis and surface temperature on the horizontal axis. Temperature increases as e move to the left. The dashed lines represent stars of equal radius, so Ro is the radius of the sun.

Question 15

What are the three main groups of stars on the diagram?

Answer 15

Main sequence stars, the broad curve running from upper left to lower right, white dwarfs at lower left and red giants and red supergiants at the upper right.

Question 16

What is a cepheid star/variable?

Answer 16

Stars whose brightness varies periodically with time because the stars expand and contract.

Question 17

What is the period of variation of a Cepheid’s brightness related to?

Answer 17

Its average luminosity, the longer the period the higher the luminosity. So by observing a Cepheid over time we can measure its period and thus deduce its average luminosity, this makes

Question 18

What makes Cepheid’s standard candles?

Answer 18

By observing a Cepheid over time we can measure its period and thus deduce its average luminosity, this makes Cepheids standard candles since they are stars of known luminosity. By measuring its average brightness we can deduce the distance to it, distances up to 40Mpc can be measured.

Question 19

How do stars stay on the main sequence for most of their life?

Answer 19

They fuse hydrogen into helium.

Question 20

What happens when the star has used up about 12% of its hydrogen?

Answer 20

The star developed instabilities and will start to expand, as it expands the surface temperature of the star decrease but the increase in surface area more than compensates and so its luminosity and actually increases. The star will become a red giant if its mass is less than about 8 solar masses or a red supergiant if its mass is greater than about 8 solar masses.

Question 21

what happens to the reactions in the higher mass stars?

Answer 21

In higher mass stars nuclear reactions continue in the core but now involve heavier and heavier elements. The more massive the star the heavier the elements produced in the nuclear reactions. The process stops at the production of iron. No heavier elements can be produced by nuclear fusion as this would require that additional energy be supplied from outside.

Question 22

What happens after the star finished producing iron?

Answer 22

The star undergoes a catastrophic explosion prodding s plantar nebula if the mass of the star is low and a supernova if it is high.

Question 23

What happens to the core of the star once most of its mass has been ejected into space via the explosions if it less than 1.4Msun?

Answer 23

It will become a stable white dwarf star. It does not collapse further because of electron pressure. The limiting mass of 1.4Msun is known as the Chandrasekhar limit. Even a very massive star can end up as a white dwarf if the mass of its core is less than the Chandrasekhar limit.

Question 24

What happens to the core of the star once most of its mass has been ejected into space via the explosions if it is between 1.4Msun and 3Msun?

Answer 24

It will become a neutron star. It does not collapse further because of neutron pressure. The limiting mass of 3Msun is known as the Oppenhemier Volkoff limit.

Question 25

What happens if the mass of the core is larger than the Oppenheimer Volkoff limit?

Answer 25

No known mechanism exists to stop its further collapse and it becomes a black hole a star from which nothing can escape.

Question 26

What is the relation that shows the relationship of a main sequence stars mass and luminosity?

Answer 26

L = kM^3.5 where k is a constant. This only work for main sequence stars.