Animation of the processes around young stars

This animation sketches an approach to a very young, newborn star of solar mass, which is surrounded by a flat, cold circumstellar disk of dust and gas. Our planetary system once originated in a disk like this. Stars like these are called T Tauri stars, and they are found inside cold, molecular interstellar clouds in our galaxy. Since our solar system presently moves through a rather empty region of space, the most nearby molecular clouds are relatively distant, typically 500 light years away. Therefore these disks appear as very small objects in the sky when seen from the Earth.

In the animation the angles corresponding to 1 arcsecond, 0.1 arcsecond and further down to 0.001 arcsecond as resolved from the earth are marked and compared to where the orbits of Pluto, Jupiter and Venus would fall at the distance of 500 light years. 1 arcsecond of spatial resolution is what can be achieved with a single telescope from the ground, but subarcsecond resolutions are nowadays retrieved through special techniques including vibrating optics or with two or several interacting telescopes forming an interferometer. With the Hubble Space Telescope resolutions slightly better than 1 arcsecond is reached, but to resolve structures smaller than 0.001 arcseconds, as marked in the final close up, we will need future special interferometers placed in space. It is within this smaller limit that most of the observed activity of T Tauri stars originates. Strong energetic eruptions, possibly due to surface flares, to blobs of gas falling into the star along dipole magnetic field lines or even to magnetic energy released in the disk which is rotating differentially and build up stresses between layers moving at different velocities. The dipole magnetic field, it is thought, truncates the disk and creates an inner gap.

The stellar surface is covered with dark spots, and some are much larger than the sunspots. The spots move around with stellar rotation, and the T Tauri stars rotate much faster than our present Sun. The spots on the stellar disk can indirectly be observed from the ground, since the dark areas that come and go modulate the brightness of the star regularly following the rotational period.

At the beginning of the animation we also see the disk turning edge-on. Fluffy blobs above and below the disk can occasionally pass the star and obscure it. From our ground based telescope the star then fades dramatically in brightness, and becomes redder. When the star fades the degree of linear polarization of light increases. This is due to the fact that we see less of the star but more of the light scattered from disk. This scattered light is polarized because of the flat geometry. Notice also the bright jets that shoot out in two directions perpendicular to the disk. The jets are fast moving warm gas, which has been accelerated through some mechanism related to the release of gravitational energy when matter in the disk after slow migration inwards finally is dumped under almost free-fall velocity towards the central star.

Most stars in the sky are double or multiple - so also the T Tauri stars. In the case of a close binary, the disk will be truncated further out, and form a circumbinary disk. If the separations are very large between the stars, each star can host a disk of its own. The case shown at the very end of the animation is the binary DQ Tauri, which has an orbital period of 15.7 days. The two stars move around each other in very elongated elliptical orbits.