| 21. | However, the characteristic broad H? emission line has rarely, if ever, disappeared.
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| 22. | On occasion, however, BL Lacertae has been observed to display weak emission lines.
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| 23. | It was the 433rd entry in their 1977 catalog of stars with strong emission lines.
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| 24. | The emission lines are being generated by a decreted circumstellar disk of hot hydrogen gas.
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| 25. | The emission lines are generated in the stellar wind and the photosphere is completely hidden.
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| 26. | These emission lines were first discovered in the spectra of planetary nebulae in the 1860s.
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| 27. | During an eclipse, emission lines can be seen in the spectrum of this system.
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| 28. | The power radiated by the emission lines is 10 27 erg s " 1.
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| 29. | The hydrogen-alpha emission line becomes far more prominent as the star becomes brighter.
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| 30. | The spectrum then develops hydrogen emission lines.
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