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Stars
Stars are the most observable form of normal matter in the cosmos. They have produced about
90% of all the radiant energy emitted since the big bang (with black holes accounting for most of the
balance). Through the nuclear reactions that power them, they have taken the primordial hydrogen and
helium produced during the big bang, converted this into heavier elements like carbon, oxygen and iron,
and then dispersed this material so that it can be incorporated in subsequent generations of stars and of the
planets that accompany them (Figure 2-4-3). Such recycling is proceeding continuously within galaxies
like our own.
We now have a mature theory for the structure and evolution of stars. This is based on a synthesis
of known physical processes (nuclear reactions, the outward flow of matter, radiation, and energy). We
now know that the overall life story of a star depends primarily on its mass and, secondarily, on its
chemical composition. The mass of a star has a pronounced effect on the rate at which it consumes its
nuclear fuel: the more mass the star contains, the shorter its life will be (live fast, die young), and the
more violent and spectacular its death with explosive heating of the surrounding gas and production of a
legacy corpse in the form of a neutron star or black hole.
Yet challenges remain. We know that as stars like the Sun age they lose mass in the form of a
relatively steady wind, or more episodically during violent pulsations and explosive eruptions in the late
stages of the star’s life. Indeed, the final stage of a star’s life depends quite sensitively on the amount of
mass it retains following its evolution beyond the hydrogen-burning stage. It will also depend strongly on
how rapidly the star is rotating and on the strength and nature of the magnetic fields that it has built.
This has far-reaching implications because the end states of massive stars (supernovae) determine
the chemical composition of a galaxy and hence the properties of the subsequent generations of stars and
planets. In order to understand the lives of stars and the role they play in cosmic evolution we must
understand the roles of mass loss, rotation, and magnetic fields in stellar evolution. Prospects are bright
for the coming decade. All three phenomena can be assessed through high dispersion spectroscopy.
Rotational studies are possible with detailed long-term photometric monitoring. It is now becoming
possible to study the structure and strength of magnetic fields on the surfaces of nearby stars, and changes
in the magnetic fields can be diagnosed with X rays. At the same time, the major advance provided by the
Advanced Technology Solar Telescope (ATST) will provide an improved ability to observe and
understand the rich array of magnetic activity exhibited by our nearest star: the Sun. Solar radio emission
will be observed at high time and wavelength resolution on a continuous basis, providing unique data to
combine with that of ATST.
Indeed, following the successful launch and commissioning of the Solar Dynamics Observatory
(SDO; Figure 2-9) we are poised to understand the origin of the eleven year solar cycle, which underlies
“space climate,” by relating the surface behavior of the Sun to its interior properties, in particular at the
tachocline located at seventy percent of the solar radius where the hot gas begins to undergo convective
motion. In addition the high resolution, all disk imaging combined with the ability to map the surface
magnetic field in three dimensions, as it erupts into the solar chromosphere and corona, are providing
unprecedented understanding of how magnetic fields behave above the solar surface both in the “quiet”
Sun and during massive flares associated with active regions. This understanding is of major importance
for astrophysics beyond the solar system as the Sun is the best large scale magnetic field laboratory we
have. Meanwhile, ATST will come on line in 2017 and will provide complementary diagnostics for
similar science goals to space observatories, specifically high resolution imaging⎯it will have the
capability of seeing down to 30 km scales⎯and detailed spectroscopy. It will be able to see the strange
ways that magnetic field lines twist and braid themselves as well as how they mediate the flow of energy.
Understanding these physical processes is a key step towards explaining how the solar wind⎯the outflow
of gas that blows past the Earth and has such a large effect upon our atmosphere⎯is powered.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
2-22