Orion

When rotating clouds of gas and dust in the interstellar space collapse, they settle into disks because centrifugal forces prevent the gravitational collapse perpendicular to the rotation axis. This is the reason why spiral galaxies like the Milky Way have disk shape and why the planets orbit the Sun in a common plane. When gas that falls onto stars or black holes has angular momentum, the gas forms a so-called "accretion disk'' around the accreting object.

The Hubble Space Telescope (HST) has a mirror with a diameter of 2.4 m. It orbits Earth at a distance of 600 km from where it sends pictures to the ground. It allows scientists to carry out observations at different wavelengths.

or shock fronts are discontinuities in gas or fluid flows where the density, pressure and velocity jump abruptly. Shock fronts can be created by objects that move supersonically through a medium, for example in the earth atmosphere by a plane flying faster than the speed of sound. The bow shocks of astrophysical jets propagate with up to 10 000 times the speed of sound and with up to 99,99 percent of the speed of light through interstellar and intergalactic space.

Giant gas and dust clouds in the Milky Way are the nurseries for young stars and embryonic planetary systems. The Orion Nebula is one of the closest regions to us where stars are being born. It is about 1500 light-years away and visible to the naked eye as a bright fuzzy patch near the center of the "sword'' in the constellation of Orion the Hunter. The glowing gas of the Orion Nebula is illuminated by the energetic light from a small cluster of hot and massive stars which have formed within the last million years. Let us visit this churning turbulent star factory with the Hubble Space Telescope. Diving into the nebula, we see bright gas filaments, light absorbing dust clouds, and thin curved loops of gas swept up by supersonic shock waves. The Trapezium, the four hottest and most massive stars, passes by at our right. Protoplanetary disks around newborn stars are glowing and stretched out into "teardrops'' due to the intense light from these nearby stars.A small amount of dust can make a disk opaque and appear as dark ring around the infant star at the center. These embryonic solar systems will eventually form planets. The Orion Nebula contains several hundred stars at various stages of formation. A large fraction of the young stars is surrounded by gas and dust disks which may be similar to the disk our own solar system condensed out of 4.5 billion years ago. The abundance of such objects in the Orion Nebula suggests that planet formation is a common occurrence in the Universe. So far nearly 70 planetary systems have been discovered around stars in the solar neighbourhood. In some of them several Jupiter mass planets orbit the central star. Not even the Hubble Space Telescope is able to resolve the formation of such planets directly. Therefore computer simulations have to tell us how such planetary systems form in disks of gas and dust around young stars. A condensing planet begins to accrete the gas from its vicinity. A gap of low density opens up and spiral waves begin to travel through the disk. These spiral arms may fragment and lead to the formation of other planets.