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Across 4. The measure of the overall power output of a star (10) 7. The general name for a stellar object for which the brightness, or absolute magnitude, is known (8, 6) 8. The series of visible absorption spectral lines due to Hydrogen gas (6) 11. Low luminosity, but hot stars, that lie to the bottom left of an HR diagram (5, 5) 12. The apparent magnitude of a star at a distance of 10 parsec away (8, 9) 13. This can be found from Wien's law and gives us an idea of the appearance of a star, based on its temperature (4, 10) 14. A region of a nebula that is contacting and getting hot but, not yet, in equilibrium (9) 15. The boundary away from a black hole - within this, nothing can escape (5, 7) 16. This is the situation in a stable star (11) 19. Where stars are born (6) 20. These are not big enough to undergo sustained hydrogen fusion. They are failed stars (5, 5) 21. This is a type of star where hydogen fusion is occuring in the core (4, 8) 22. The luminosity of star depends on this to the power 4 (11) 23. This nuclear processes occurs in the core of stars (6) 24. A red supergiant in the constellation of Orion (10) 25. This is the distance to a star where the angle of parallax would be 1 arcsecond (6)

Down 1. The Greek astronomer who ranked stars by brightness on a scale (now named after him) from 1 to 6 (10) 2. Giants will fuse elements up to this (4) 3. This has to be overcome if a star is to become a neutron star (or black hole) rather than a white dwarf (8, 10) 5. This is the name given to processes that require an energy input to take place (eg the formation of heavy nuclei) (9) 6. A small, low mass, main sequence star (3, 5) 9. This star is the next nearest to us, after the Sun. It is a red dwarf (7, 8) 10. Rotating neutron star that gives out periodic beams of radiation in our direction (6) 12. Stellar classification is based on the quality of this from a star (10, 7) 17. The explosion of a large star (9) 18. A singularity surrounded by space from which, not even, light can escape (5, 4)