A long held misunderstanding of stellar brightness is being corrected, thanks to a new study published in the Monthly Notices of the Royal Astronomical Society based on International Astronomical Union (IAU) General Assembly Resolution B2.
“Stars have been observed for millennia and were crucial in navigating across deserts, seas, and oceans,” noted Zeki Eker, the study’s lead author. Stars are even more crucial today for guiding us to the secrets of universe. This is, however, assured only if their radiating power is measured correctly.
Brightness differences as small as hundredths—even thousandths—of magnitude is measurable today since the advent of Charge Coupled Devices, or CCDs, in space-born observations. There were only visual magnitudes until photography met telescopes at the end of the nineteenth century. Soon sensitivity difference in photographs was discovered; as soon as photo-plates like human eyes were invented, the two kinds of magnitudes, blue (photographic) and visual, started to be used. Today, there are many filters to see how a star shines at various colours, including those the eye cannot see. Filtered magnitudes are calibrated by a star called “Vega,” the zero point for all magnitudes. Measuring the magnitude of a star at various filters is needed for obtaining its surface temperature. Its absolute size or distance and angular size are also needed to obtain its absolute power: luminosity, energy radiated per second (Watt) from a star. But, these are only accessible in certain types of binary stars. Astronomers had to find a way to assign luminosity to each star, in the same way as light bulbs sold in markets would be useless if the power (Watt) is not written on them.
By the end of 1930s, a new kind of magnitude with an arbitrary zero point flourished. Unlike blue and visual, this new magnitude, named bolometric, represents the luminosity of a star, including all its colours—visible and not visible. This new magnitude is for calculating the luminosity of a star in one step, but it is not observable; no telescope could observe total radiation of a star including gamma-rays, X-rays, UV, visible, infrared, and radio at once. Bolometric magnitudes, therefore, were computed first from known luminosities of a very limited number of binary stars. Because luminosity is more than one of its parts, called visual luminosity, the idea that “bolometric magnitudes should be brighter than visual magnitudes” entered astrophysical textbooks and literature as the first paradigm and went unchallenged for 80 years.
The difference between bolometric and visual magnitudes is called bolometric correction (BC). BC is a useful concept: if it is added to a visual, the bolometric magnitudes can be obtained, and so there is no need to observe a star of various colours at once. Using the limited number of existing BC as a calibrating sample, many tables are published to give BC as a function of stellar effective temperature, a parameter available from multicolour photometry, simply from blue and visual magnitudes.
BC is like a missing part; when added to visual magnitude, the bolometric magnitude is obtained. The magnitude scale is in reverse order, like first class is more valuable (brighter) than the second, and second is brighter than the third, so on. That is, the smaller the number the brighter it is. Therefore, the BC values were recognized as negative numbers. Consequently, “the BC of a star must be negative” became the second paradigm.
Inconsistencies between paradigms become obvious if one pays attention to the third paradigm; the arbitrariness attributed to the zero point of BC scale. Studying BC tables published, one realizes that there are two groups: a group of BC tables with all negative numbers, and a group of BC tables with mostly negative but also containing a limited number of positive. Obviously, the first group of producers took the arbitrariness granted and felt free to change computed BC in such a way that no single positive BC was left in the table in order to avoid dilemmas; while the other group trusted their calculations by keeping them as calculated and did not care about the paradigms. It is obvious that both cannot be right: either one of them is wrong. Another inconstancy is that there are occurrences that the same star is found to have different bolometric magnitude and luminosity, which is not true, among the scientists who use different BC tables with different zero point.
A solution was suggested by the IAU General Assembly in 2015, where the zero point of bolometric magnitude scale is fixed by a convention of worldwide astronomers and astrophysicists, called 2015 IAU General Assembly Resolution B2. Apparently, the established paradigms must be very strong, however, as even after the resolution published, an article appeared in one of the major journals still defending arbitrariness of the zero point of BC scale. IAU’s solution, therefore, stayed hidden for six years. Finally, proving that fixing the zero point of bolometric magnitudes has a firm consequence, the zero point of BC scales must also be fixed by a zero-point constant equal to the difference of the zero-point constants of bolometric and visual magnitudes. This firm positive zero-point constant of BC scale, on the other hand, requires a limited number of positive BC. So, “bolometric magnitudes should be brighter than, visual magnitudes” is not necessarily true.
Now, it is time to remove those paradigms from the textbooks, teaching minds, and researchers. This is needed urgently for consistent astrophysics as well as achieving accurate standard BC and stellar luminosities. Accurate standard stellar luminosities are needed not only by stellar structure and evolution theories, but also by galactic and extragalactic astrophysics to search amount of luminous matter in galaxies, to determine galactic and extragalactic distances, and thus even it could be useful for recalibrating Hubble law and restudying cosmological models of universe with better galactic and extragalactic luminosities, because galaxies are made of stars. This is how stars would lead us to the secrets of the universe.
Featured image by Faruk Soydugan
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