The Sun, as well as other "late-type" stars (with surface temperatures less than about 8000 K), possesses an expanding outer atmosphere known as a stellar wind. Continual mass loss has a significant impact on stellar evolution, on the chemical evolution of galaxies (including the mass and energy budgets of the interstellar medium), and even on the long-term evolution of planetary atmospheres. By studying the physical mechanisms that drive these outflows, as well as their interaction with stellar convection, rotation, pulsation, and magnetic fields, we are able to better delineate the importance of stellar winds to astrophysics as a whole.
The H-R Diagram below shows various regions of stellar effective temperature and luminosity that share similar kinds of winds. Cool-star winds (on the right-hand side of the diagram) have been classified into two broad categories, linked by a possibly distinct intermediate state.
"Hot" solar-type winds are accelerated in extended coronae (temperatures exceeding 1 million K) and typically have low mass loss rates (less than 10^(-12) solar masses per year). Our understanding of solar wind acceleration has increased dramatically over the past decade thanks in part to new remote-sensing and in situ measurements. Cool, evolved stars exhibit chromospheres (temperatures around 10,000 K and lower), winds with high mass loss rates (at least 10^(-7) solar masses per year), and terminal wind speeds seemingly smaller than their surface escape speeds. In between these two groups, the so-called hybrid-chromosphere stars exhibit moderately hot outer atmospheres (temperatures greater than about 100,000 K) with wind speeds and mass loss rates between those of the "cool" and "hot" classes. Despite more than a half-century of study, though, the basic physical processes responsible for driving the various types of stellar winds are still largely unknown.
A great deal of research is being done in modeling the complex multi-dimensional and time-varying properties of cool-star atmospheres and winds. Click on the images below for links to other modeling groups:
For further information about cool-star winds and coronae, see:
This section is under construction. Topics will include:
GO BACK to Steven Cranmer's Home Page, or to the Harvard-Smithsonian CfA Home Page.