| Date | May 2015 | Marks available | 2 | Reference code | 15M.3.HL.TZ2.4 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Explain | Question number | 4 | Adapted from | N/A |
Question
This question is about stellar evolution.
The mass of a main sequence star is two solar masses. Estimate, in terms of the solar luminosity, the range of possible values for the luminosity of this star.
The star in (a) will eventually leave the main sequence.
State
(i) the condition that must be satisfied for this star to eventually become a white dwarf.
(ii) the source of the energy that the white dwarf star radiates into space.
(iii) one likely element, other than hydrogen and helium, that may be found in a white dwarf.
Explain why a white dwarf maintains a constant radius.
Markscheme
\(\frac{L}{{{L_ \odot }}} = {2^n}\) with n between 3 and 4;
so \(8{L_ \odot } < L < 16{L_ \odot }\);
Award [2] for a bald correct answer.
(i) the core/remnant mass must be less than the Chandrasekhar limit/1.4 solar masses; } (must see core or remnant or similar term)
(ii) residual/thermal/internal energy of the star / OWTTE;
Do not allow fusion.
(iii) C/O/Ne/Mg; (accept no others)
gravitational attraction/pressure is balanced by;
electron (degeneracy) pressure/repulsion / pressure/force due to Pauli exclusion principle;
Award the first marking point independently of the second.
Examiners report
In part (a) most candidates correctly referred to the mass–luminosity equation and used it to determine the luminosity range for the star.
Part (b)(i) was answered well by many, but there were also many who did not refer to the remnant or core mass being below the Chandrasekhar limit. In (b)(ii) there were far too many candidates who referred to fusion continuing in a white dwarf. In part (b)(iii) carbon or oxygen were almost always correctly stated.
In (c) it was expected that electron degeneracy pressure would be mentioned, many did so but fusion radiation pressure was also incorrectly mentioned.
