Perhaps the most important pre-main-sequence stars are the T Tauri stars, named after the 20th cataloged variable star in the constellation Taurus. They can be found scattered in star forming regions all across the sky. These transitional objects allow us to study the transition between a young star being completely encased in a cocoon of dust and gas, and a newly minted hydrogen burning star joining its cohorts on the main sequence.
T Tauri stars represent a transitional stage between infrared stars surrounded by opaque nebulae and stable stars that have lost their "cocoons" and settled on the main sequence. These stars are so prevalent in star forming regions that the number of T Tauri stars in a star-forming cubic parsec of space may exceed by a factor of 10 the number of all stars per cubic parsec found in the more boring area near the Sun. T Tauri stars vary irregularly in brightness and are typically only 20,000 to a million years old — younger than the human species!
After T Tauri stars form, they emerge slowly from their cocoon of dust and debris. Many T Tauri stars this jet set their fed by in falling matter from the still collapsing and accreting stellar nebula. For years, astronomers speculated that this pre-main-sequence stage of stellar evolution might come prior to the disk-shaped solar nebula forming planets. Starting in 1984, astronomers began discovering disk-shaped nebulae of cool dust near young stars. This infrared-emitting dust extends hundreds of astronomical units from the star. These discoveries seem to provide a "missing link" between initial collapsing gas cloud and the later dusty disk that is required to form planets. The edge-on disk around Beta Pictoris is an excellent example. Beta Pictoris is an older star somewhat more massive than the Sun, giving evidence that the debris left over from star formation can form disks in stars with a wide range of ages.
In addition to their dusty disks, many T Tauri stars have, as mentioned above, jets in the form of bipolar outflows. During bipolar outflows, the gas shoots out in opposite directions, perpendicular to the disk. Stellar winds may also blow some material away from some T Tauri stars in all directions. In many T Tauri stars, however, some gas and dust apparently spirals inward toward the star, getting caught in its magnetic field and squirted "upward" and "downward" in two diffuse jets away from the disk.
A nearby example of a young star cluster is NGC 2264. Most of the low-mass stars (spectral classes A to K) in this cluster lie distinctly to the right of the main sequence in an H-R diagram, and many of these are identified as T Tauri stars. The calculated lines of constant age show that the T Tauri stars match the positions predicted for ages in the range of 3 to 30 million years. This demonstrates how the H-R diagram can be used, together with physical models of stars, to determine the ages of open clusters.