Numerical answer
The four sentences below, when properly sequenced, form a coherent paragraph. Decide the correct order and key in the sequence (e.g., 3142) as your answer.
1. Early spectral classifications relied only on the visible strength of absorption lines, grouping stars in an order that seemed logical at the time but did not track any single underlying physical property.
2. The modern classification of stars by spectral type traces back to a scheme developed for a nineteenth-century photographic survey.
3. It was only later, once astronomers connected these spectral patterns to stellar temperature, that the categories were reordered into the sequence still used today.
4. That survey's original alphabetical ordering of spectral classes has since been substantially revised, though its letter labels persist as a historical residue.
1. Early spectral classifications relied only on the visible strength of absorption lines, grouping stars in an order that seemed logical at the time but did not track any single underlying physical property.
2. The modern classification of stars by spectral type traces back to a scheme developed for a nineteenth-century photographic survey.
3. It was only later, once astronomers connected these spectral patterns to stellar temperature, that the categories were reordered into the sequence still used today.
4. That survey's original alphabetical ordering of spectral classes has since been substantially revised, though its letter labels persist as a historical residue.
Show answer and solution
Answer: 2143
Sentence 2 opens by identifying the historical origin (a nineteenth-century photographic survey). Sentence 1 explains how that original scheme was built (visible line strength, not a single physical property). Sentence 4 notes the ordering was later revised, while the labels persisted. Sentence 3 explains why and how the reordering happened (connection to stellar temperature), functioning as the paragraph's concluding explanation. Order: 2-1-4-3.
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