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Most stars with masses smaller than that of the Sun will live even longer than the current age of
the universe. This means that low-mass stars that formed at any time over the history of the universe are
still present in galaxies today. Thus, detailed studies of the populations of stars within a galaxy provide a
fossil record that traces the history of star formation over the whole course of the galaxy’s evolution. Such
studies also trace the build up of the heavy elements in the galaxy as successive generations of stars
formed, converted their light elements into heavier ones, and then exploded, contributing their newly
formed heavier elements to their surroundings. This observational approach is currently practical only in
the Milky Way and its nearest neighbors. Future generations of optical telescopes in space and large
ground-based telescopes will enable us to extend this technique farther afield and study the histories of
the full range of galaxies by imaging their stellar populations.
The Origin of Black Holes
In the past decade we have discovered two remarkable things about black holes. The first is that
supermassive black holes—objects with masses of a million to billions of times the mass of the Sun—are
found in the centers of all galaxies at least as massive as our Milky Way. This means that the formation of
black holes is strongly related to the formation of galaxies. The second is that supermassive black holes
were already present, and growing rapidly, at a time less than a billion years after the big bang, when the
first galaxies were being assembled. This strains our understanding of the early universe: how could such
dense and massive objects have formed so rapidly Which formed first: the black hole or the galaxy
around it Radio observations of star-forming molecular gas in some of the most distant galaxies suggest
a black hole is present before the formation of a massive galactic halo. ALMA and the EVLA may
provide more such examples.
But we cannot answer these questions definitively yet, because we do not have a robust theory for
how supermassive black holes form. In the coming decade we expect a major breakthrough in our
understanding. A space-based observatory to detect gravitational radiation will allow us to measure the
rate at which mergers between less-massive black holes contributed to the formation process. Are the
supermassive black holes we can now detect only the ‘tip-of-the-iceberg’ (the biggest members of a vast
unseen population) Deep imaging surveys in the near-infrared and X-ray, with follow-up spectroscopy
with JWST and ground-based extremely-large telescopes, will detect and study the growth of the less
massive objects through the capture of gas and accompanying emission of electromagnetic radiation.
These surveys will also allow us to search for such black holes at even earlier eras: back to the end of the
dark ages.
The Origin of Stars and Planets
Looking up on a clear night from a dark location, we see that the sky is full of stars. Telescopic
observations by Galileo revealed that the Milky Way’s white band traversing high across the summer and
fall sky can be resolved into countless stars. Gazing upon the winter constellation of Orion, the sharp eye
will note the fuzzy Orion Nebula (Figure 2-4-3) with its nursery of stars born “yesterday” in cosmic
time—not long after the first humans walked. Nearby is the famed Pleiades star cluster—formed when
dinosaurs still roamed the Earth. In contrast, some stars of our galaxy are nearly as old as the universe
itself. The story of how successive generations of stars form out of the gas and dust in the interstellar
medium in both benign and exotic environments is fundamental to our understanding of, on the larger
scale, the galaxies in which stars reside and, on the smaller scale, the planetary systems they might host.
What was it about the Sun’s birth environment or its star formation process that determined the
final properties of our solar system versus that of other planetary systems (See Box 2-2.) How and on
what time scale did the Solar mass build up, and how much gas and dust were left over for planet
formation How rapidly did the high-energy radiation of young stars disperse their gas disks, ending the
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