2013
What kind of star is it?
- Round
- Theory
- Points
- 10
- Level
- Bronze
- Taught in
- Section 4.7
Adapted from IOAA 2013 (Volos), theory problem S7. We have restated it in our own words and kept the original data. The solution was last checked on 20 October 2026.
A star has visual apparent magnitude 12.2, parallax 0.001″ and effective temperature 4000 K. Its bolometric correction is −0.6. (a) Find its luminosity in units of the Sun's. (b) Is it a red giant, a blue giant or a red dwarf?
Find
- The star is
- By (4.15),
- The star is 18 times as luminous as the Sun but much cooler, at 4000 K against 5772 K. By (4.12) it must be much larger,
AnswerThe star is about 18 times as luminous as the Sun and about nine times its size, so it is a red giant.
This problem also appears among the olympiad problems of Section 4.7, Colour, temperature and bolometric correction, which teaches the ideas it uses. Equation and figure numbers refer to the course.