The Crab Nebula. (original) (raw)
NASA/ADS
Abstract
The central star of the Crab nebula is probably the south preceding of the two stars near the center of the nebula. The spectrum of this star is continuous. The nebula consists of two parts with different spectra. The filaments on the outside of the nebula produce emission lines of H, He i, He n, [N u] [0 i], [0 ii], [0 `I'], and [S II]. The hydrogen lines are faint. The spectrum of the main diffuse nebu- losity is continuous except for a faint discontinuity at the Balmer limit. The continuous spectrum con- tains practically the entire energy emitted by the nebula. Its intensity distribution deviates from that of a black body; the color temperature is about 84000 at X 4500 and about 6700° at X 6000. In the absence of absorption in the nebula the continuous spectrum cannot be due to scattering and must be a true emission spectrum. The only physically justified assumption is that the continuous spec- trum is produced by free-free and free-bound transitions of electrons in the highly ionized gas. On this basis the observed intensity distribution in the continuous spectrum of the nebula finds a satisfactory explanation. The analysis leads to an electron density of the order 1O~ cm-3 and an electron temperature of the order of 50,0000. The mass contained in the nebula is about 15 solar masses. The hydrogen abun- dance seems to be low. For the central star a temperature of the order of 500,000° is obtained, a radius of 0.020 solar radius, and a total luminosity of 30,000 solar units. A comparison with spectra of planetary nebulae indicates that the unusually high intensity of the [0 ii] lines in the filaments is compatible with a high temperature for the central star. The faintness of the hydrogen lines may be interpreted as due to low abundance of hydrogen. The results as a whole support Chandrasekhar's suggestion that supernovae of type I are due to the transition of stars heavier than the limiting mass into the degenerate state. Supernovae of type II might possibly represent this transition for stars of smaller mass
Publication:
The Astrophysical Journal
Pub Date:
September 1942
DOI:
Bibcode: