Cartoon of a T Tauri Star
Stars form out of large collections of gas and dust known as molecular clouds when
the densest parts of these clouds collapse gravitationally. One consequence of
this collapse is that young stars (T Tauri stars) are usually surrounded by massive,
opaque, circumstellar disks. These disks gradually accrete onto the stellar surface, and
thereby radiate energy both from the disk (infrared wavelengths), and from the position
where material falls onto the star at (optical and ultraviolet wavelengths). Somehow
a fraction of the material accreted onto the star is ejected perpendicular to the
disk plane in a highly collimated stellar jet. The circumstellar
disk eventually dissipates, probably when planets begin to form. Young stars
also have dark spots on their surfaces which are analogous to sunspots but cover a much
larger fraction of the surface area of the star.
The figure above illustrates the current conceptual picture of a young star.
The star is the orange ball at the center, and has dark spots on its surface.
Columns of material accrete from the disk onto the star, funneled along stellar
magnetic fields. The accreting columns form hot spots where they impact the
stellar surface (white/purple), giving rise to excess UV and optical light.
Instabilities in the disk may have a spiral pattern as suggested in the diagram.