Stellar models describing the evolution of a star's composition and internal processes with stellar age, give predictions on the observed luminosity and colour (or equivalently surface temperature) of the star. During a star's lifetime, its surface temperature and luminosity evolve as the star's energy source, the fusion of light elements in its core and shells, progresses through a series of elements.
Although the fundamental principles of stellar evolution are well understood, there are still several aspects of the evolution and interior of stars for which the current theories require further improvement.
As a star evolves it will follow a track in the Hertzsprung-Russel diagram, which is a plot of the star's luminosity versus its colour. The high-precision positions in the Hertzsprung-Russell diagram of stars of known surface abundances, provided by Hipparcos and by high-resolution spectroscopy, have revealed discrepancies between the observations and the predictions of standard stellar models.
Gaia will return accurate luminosities, surface temperatures, chemical abundances, masses, and determinations of the extinction of stellar light by the interstellar medium for all types of stars and hence for the full range covered in the Hertzsprung-Russel diagram.
The large sample of stars over different stellar type gathered by Gaia, will greatly extend our understanding of stellar structure and evolution and will allow further improvement of theoretical models of stellar interiors, for example in the areas of:
- The size of the convective cores
Here the generated heat is dissipated by convection of the gas. The size of the core defines the amount of available nuclear material for sustaining the star's energy output
- The internal diffusion of chemical elements
Diffusion of elements can result in helium being transported to the core, increasing the amount of helium that can be converted into carbon and extending the time spent at this stage by the star
- The outer convective zones
Gaia will greatly enhance our capabilities of dealing with non-local convective models for stellar interiors, as opposed to most of the current stellar models that are still built by treating convection based on a classical theory that works best only for stars in the Main Sequence phase (when they are in the process of burning hydrogen into helium at their cores)
Low mass stars which began their lives with up to a few solar masses, end up as a white dwarf. At this stage, the star no longer produces energy in its interior through nuclear fusion, as the temperature does not rise enough to allow for the next step in the sequence of successive element fusion to kick in. The stellar core collapses and the star's outer layers are expelled which form a planetary nebula.
The gravitational collapse of the star is no longer prevented by radiation pressure but is halted by the electron degeneracy pressure, resulting in a very compact core, a white dwarf.
The electron degenerate pressure is a quantum physical characteristic of degenerate matter where all the electrons in the gas, due to the Pauli exclusion principle, each have to be in a unique state. The filling of all the lower energy states from the centre outward provides a pressure against further compression.
The chemical composition of the core depends on the initial mass of the star as this determines to which level of element fusion the star could progress before the final collapse of the core. Most white dwarfs consist mainly of completely ionised carbon and oxygen.
Models of the balancing of the gravitational contraction versus the electron degeneracy pressure results in a relationship between the mass M and the radius R of a white dwarf, where M×R-3=constant.
As white dwarfs no longer produce radiation, their evolution is mainly by cooling over a period on the order of several 1010 years during which they release their residual energy and steadily decrease in luminosity. During this cooling process, the core material is thought to crystallise from the inside out, forming a compact structure.
Gaia's observations of white dwarfs will significantly advance the study of these compact objects:
- Testing of, and provide constraints for the mass-radius relationship
- By comparing theoretical models with the observed properties of white dwarfs in binary systems, Gaia will be able to constrain the relation between the mass of the star prior to shedding its outer layers, and the mass of the resulting white dwarf
- Observations of white dwarfs in the Solar neighbourhood (accounting for ~5% of the stars within 10 pc) allows for accurate limits to be set on the age of the Solar neighbourhood as derived from the cooling time of white dwarfs
- Distinguish between different populations of white dwarfs in the Galaxy, thereby providing constraints on the models of galactic evolution
During its mission lifetime of nominally 5 years, Gaia will scan the entire sky repeatedly, observing all sources brighter than 20th magnitude. The number of times that a single source is observed depends on its position in the sky, but on average Gaia will have provided 70 photometric measurements for an object at nominal mission end. Gaia's photometric and astrometric instruments will determine the object's brightness in several bands over the wavelength range 320-1000 nm.
From the complete sequence of measurements, sources with variable brightness will be identified. The accuracy of the combined photometric data will allow for detection of diverse variable phenomena from short (seconds) to long (of order 5 years) time scales.
The expected total number of variable objects detected is difficult to predict, but will be in the order of several million. This covers a wide range of different types of variable objects across the different phases of stellar evolution:
- classic periodic variables
- eclipsing binary stars (including those with rotation-induced variability)
- Scuti variables
- RR Lyrae stars (with a significant fraction of these in the bulge)
- Miras and SR variables.
- Derive precise physical and orbital parameters for the observed eclipsing binaries
- The complete sample of variable stars obtained with Gaia, will allow determination of the frequency of peculiar objects, and will accurately calibrate period-luminosity relationships across a wide range of stellar parameters (such as mass, age, and metallicity).