61 research outputs found
Nuclear and gravitational energies in stars
The force that governs the evolution of stars is gravity. Indeed this force
drives star formation, imposes thermal and density gradients into stars at
hydrostatic equilibrium and finally plays the key role in the last phases of
their evolution. Nuclear power in stars governs their lifetimes and of course
the stellar nucleosynthesis. The nuclear reactions are at the heart of the
changes of composition of the baryonic matter in the Universe. This change of
composition, in its turn, has profound consequences on the evolution of stars
and galaxies.
The energy extracted from the gravitational, respectively nuclear reservoirs
during the lifetimes of stars of different masses are estimated. It is shown
that low and intermediate mass stars (M < 8 Msol) extract roughly 90 times more
energy from their nuclear reservoir than from their gravitational one, while
massive stars (M > 8 Msol), which explode in a supernova explosion, extract
more than 5 times more energy from the gravitational reservoir than from the
nuclear one. We conclude by discussing a few important nuclear reactions and
their link to topical astrophysical questions.Comment: 8 pages, 2 figures, Seventh European Summer School on Experimental
Nuclear Astrophysics, Santa Tecla, AIP proceeding
Astronomy, space science and geopolitics
Astronomy has played a major part in the development of civilisations, not only through conceptual developments, but most importantly through the very practical gains obtained through the observation of Sun, Moon planets and stars. Space sciences, including astronomy, have also played a major rôle in the development of modern societies, as an engine for most subsequent space technology developments. Present trends tend to decrease the rôle of science in space development. This trend should be reversed to give modern "societies” their independence in space-related matters that permeate the lives of all inhabitants of the Eart
The Bright Quasar 3C 273
We review the observed properties of the bright quasar 3C~273 and discuss the
implications of these observations for the emission processes and in view of
gaining a more global understanding of the object. Continuum and line emission
are discussed. The emission from the radio domain to gamma rays are reviewed.
Emphasis is given to variability studies across the spectrum as a means to gain
some understanding on the relationships between the emission components. 3C~273
has a small scale jet and a large scale jet. The properties of these jets are
described. It is also attempted to relate the activity in the small scale jet
to that observed in the radio and infrared continuum.Comment: 47 pages, 12 figures. to appear in Astronomy and Astrophysics revie
What May We Learn from Multi-wavelength Observations of Active Galactic Nuclei
We discuss how several of the questions that remain unclear on the physics of
Active Galactic Nuclei may find elements of answers when using in the coming
years the extraordinary set of instruments that will be flying simultaneously
to observe in all bands of the electromagnetic spectrum.
The choice of questions mentioned here is personal and not exhaustive.Comment: 10 pages, no figures, to appear in "Quasars, AGNs and related
research across 2000", Rome May 200
Surprises in the Hard X-ray Sky
Cosmic objects emit throughout the electro-magnetic spectrum, from radio
waves to very high-energy gamma rays. Some spectral regions can be observed
from the ground, but space instrumentation is needed for most of them. Some
spectral regions are particularly difficult, for example hard X-rays and MeV
gamma rays. For one thing, in this spectral region photons interact least with
matter. Moreover, no focussing optics was available until very recently.
INTEGRAL, the gamma ray observatory mission led by ESA, was designed to observe
the sky in this very domain
- …