detail (measure their mass, chemical composition, and ages). There is also growing evidence that many gamma-ray bursts are the explosive deaths of very massive stars and sometimes resulting in the formation of the first generation of black holes with the unusual chemical compositions expected for the first stars (nearly devoid of elements heavier than hydrogen and helium). The study of the coolant deaths of these stars offers another way to learn about the first stars.

The small protogalactic fragments containing the first stars were embedded in halos of dark matter, which formed first and provided most of the total mass. Through their mutual gravitational attraction, these small fragments of gas and dark matter would have fallen slowly toward other such objects, collided, and then merged into larger objects. This process continued over the entire history of the universe: in the densest regions, small objects merged to form medium-size objects that later merged to form large objects (Figure 2.7). Over time even larger

FIGURE 2.7 This enlargement of part of the Hubble Ultra Deep Field shows distant young galaxies in the process of forming; several galaxy mergers and unusual structures are evident. SOURCE: NASA, ESA, S. Beckwith, and the Hubble Ultra Deep Field team.