First Light by Emma Chapman

Author:Emma Chapman
Language: eng
Format: epub
ISBN: 9781472962904
Publisher: Bloomsbury Publishing

Figure 19  The current nomenclature of stellar metallicity,4 adapted from Frebel 2018.11

The discovery of metal-poor stars

The first metal-poor stars were discovered in 1951, by American astronomer Joseph Chamberlain and his PhD supervisor Lawrence Aller,5 who measured the iron abundance in two stars and found they were about 1/10th the solar value.13 This may seem a small difference from our Sun, but at the time it was revolutionary, and Chamberlain struck gold in the academic sense, too, as this was his first ever paper and it turned out to be one the most important astronomical papers of the twentieth century. Until that point it had been assumed that all stars had roughly the same elemental make-up as the Sun. Stars with different strengths of metal lines had been observed, but this was explained away by the stars being of different spectral type only, and not indicative of a different generation entirely. Spectra were, after all, the tool that was used to separate out the stellar zoo into the OBAFGKM classification system in the first place, with the strength of the hydrogen lines becoming a proxy for temperature. In Chamberlain and Aller’s paper you can feel their disbelief as they reveal their results, book-ended with phrases such as ‘the observed [iron] abundance appears to be smaller than in the sun, although this conclusion must be taken with caution’. They refer to the measured iron abundances as ‘the one undesirable factor in our interpretation’. Another case, then, of the undesirable noise becoming the paradigm-shattering signal, though with less pigeon poo this time.

It’s easy to look back in hindsight and cherry-pick these kinds of statements from the unassuming papers that changed our world view, but what we are looking at are good scientists. They had a sample size of two metal-poor stars against the accepted knowledge that stars were all the same, all like our Sun, and while a scientist should be confident in their analysis to publish, rarely does a eureka moment come so clearly. More often, instead of ‘Eureka! I’ve got it!’, it is ‘That looks weird! Hang on, what is that? I think that might be … huh. Hey, can you all check this to make sure I haven’t done something stupid?’ It often requires a few eureka moments for people to be comfortable leaving their comfy, but wrong, theories. When you read backwards from Chamberlain and Aller’s work you see hints that the ground was already shifting under the homogenous star theory, for example in their reference to American astronomer Nancy Roman’s work the year before. Nancy Roman had a major role in planning the Hubble Space Telescope and was the first woman in an executive position at the National Aeronautics and Space Administration (NASA), but before all that her work provided a hint that stars were not all the same. What she did was take a sample of stars and categorise them firstly by how weak their metal lines were, and secondly by their velocities.14 As far back as

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