A protostar is a young star that has started its collapse but has not yet gotten clear of its circumstellar envelope. This envelope is opaque to optical light and thus protostars are generally invisible to ordinary telescopes. Most of what we know about protostars comes from observations at infrared and millimeter wavelengths.
Protostars have ages of around 100,000 years and are still accreting material onto the central star. At this stage the protostar consists of a central condensation that will become the star, an accretion disk, and an envelope. Another important property of protostars is their outflows. In addition to matter falling on to the star there is also large amounts of material being ejected from the star in the form of powerful bipolar jets. These jets can be seen in Hubble Space Telescope images and are traveling at hundreds of kilometers a second. Bipolar jets are believed to be important for transporting excess angular momentum away from the star.
Observationally, protostars are Class I infrared objects. This means
that the infrared emission from the disk and envelope is dominated by
the cooler material in the envelope. A few protostars have also been
observed at X-ray wavelengths. X-ray emission may be an important source of
ionization, allowing the star, disk and outflow to be coulpled by magnetic
fields. Such magnetic fields may also be responsible for squeezing the
outflow, producing the narrow jets seen above.
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