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--- courses:ast100:3 [2026/02/24 07:59] – [2. Event details] asad
+++ courses:ast100:3 [2026/02/24 08:21] (current) – [3. Stars and humans: birth and death] asad
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 For our local cosmic neighborhood, the Stellar Age culminated 4.6 billion years ago when a chemically enriched interstellar cloud collapsed under its own gravity. Likely triggered by a nearby supernova shock wave, this solar nebula shrank, spun, and flattened into a rotating protoplanetary disk. As material plunged inward, the center grew hot and dense enough to ignite nuclear fusion, birthing our Sun. Meanwhile, the surrounding disk debris accreted to form the planets of our Solar System.
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+This 2014 Madau & Dickinson diagram illustrates the cosmic history of star formation and black hole growth. The plot utilizes a dual x-axis: the bottom tracks cosmological redshift from 0 to 6, while the top shows corresponding lookback time from 0 to 12 billion years (Gyr). The logarithmic y-axis measures activity density in Solar masses per year per cubic Gigalight-year. Two primary trends are plotted: a thick black line for the Star Formation Rate (SFR) and a red line for the Black Hole Accretion Rate (BHAR). Surrounding shaded regions indicate observational data uncertainties.
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+The graph's defining feature is the strongly correlated trajectory of both curves, highlighting a synchronized cosmic evolution. Moving from the early universe toward the present, both the SFR and BHAR rise steeply to a dramatic, shared peak around a redshift of 2. This maximum activity phase, occurring roughly 10 billion years ago, represents the era known as "Cosmic Noon." Following this incredibly fertile epoch of cosmic birth, both rates undergo a steady, parallel decline toward the present day. This synchronization demonstrates that the fundamental gas reservoirs fueling stellar nurseries simultaneously drove the massive growth of central black holes.
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+We can draw a striking allegory between this cosmic timeline and human demographic evolution (shown above), specifically the inverse relationship between human fertility and life expectancy. Just as global demographic charts reveal that societies with the highest birth rates paradoxically experience the lowest life expectancies, the universe displays a similarly intertwined fate of creation and consumption. During "Cosmic Noon," the cosmos was in its own demographic extreme—fervently birthing stars at an unprecedented rate, yet simultaneously feeding the dark, consumptive engines of supermassive black holes. For both mortals and milky ways, explosive, prolific birth is inextricably bound to aggressive consumption and accelerated mortality. As these systems mature—whether a human civilization transitioning to smaller families and longer lives, or our universe settling into a less fertile epoch—the frantic pace of creation wanes, trading the volatile fires of youth for a cooler, more enduring stillness.