The same diagram shews that two stars of the same weight do not usually have the same luminosity. In general, giant stars on the spur branch leading out to the red giants have substantially higher luminosities than the main-sequence stars of equal weight. We have already noticed how a red giant may emit as much as 10 or 20 times the radiation of an equally massive main-sequence star. The same story is repeated when we pass from the main-sequence stars to the white dwarfs. Main-sequence stars emit enormously more radiation—anything up to 500 times more—than white dwarfs of equal weight. This is illustrated by the three following white dwarfs, which may be compared with the last three stars of the preceding table: +-----------------+-----------------+--------------------+ | Star | Weight |Generation of energy| | |(in terms of sun)| (ergs per gramme) | +-----------------+-----------------+--------------------+ |Sirius _B_ | 0·85 | 0·0027 | |ο₂ Eridani _B_ | 0·44 | 0·002 | |van Maanen’s star| (0·20) | (0·00055) | +-----------------+-----------------+--------------------+ We have hitherto supposed generation of energy to be spontaneous and so unaffected by changes of physical conditions. Yet the facts just mentioned seem to suggest that this can hardly be the whole truth of the matter. To state the objection in terms of a concrete instance, Sirius _A_ and its white dwarf companion Sirius _B_ must in all probability have been born at the same time out of the same nebula (p. 284), yet the former generates 4000 times as much energy per ton as the latter. It seems improbable that so great a difference can be attributed to different types of atoms; the common origin of the two stars almost precludes this. We know that the atoms are in different physical conditions in the two stars; in Sirius _A_ they have retained their _K_-rings intact, while in Sirius _B_, the white dwarf, they are completely broken up into bare nuclei and free electrons. If the two components of Sirius consist of essentially the same types of atoms, as their common origin would lead us to expect, then the enormous difference in the rates at which these atoms generate energy would seem to depend on the different physical conditions of their atoms.