Astronomy/Star and Planet Formation
In order for gas to condense into a star, a set of intermediate steps are usually followed.
Molecular clouds, also known as stellar nurseries, are huge clouds of molecular hydrogen gas within the interstellar medium, ranging from about 1 to 600 light years across. These clouds are dense, at least compared to the rest of empty space, with about 100-1000 molecules per cubic centimeter. Very slowly, gravity will draw particles into a certain area within the cloud; this increase in particles will make an increase in mass of this area, creating a stronger gravitational force that will drag in more particles; this creates a cavity in the cloud, and a protostar will form in the center of this cloud. There are several events that can speed up this process, such as galaxy collisions (which could send matter or energy through the cloud, creating a stronger gravitational force in one area) and supernovae (which can cause the same things as galaxy collisions).
Especially dense parts of a stellar nursery come together to form a ball of gas and plasma, called a protostar. It is so named once it has its own gravity and identity unaffected by the surrounding cloud. Matter and gas continue to flow towards the protostar until it becomes hot enough that molecular hydrogen begins to ionize into hydrogen atoms. These atoms can then fuse together to created helium atoms. Once fusion begins, the star starts producing a stellar wind, which clears the excess material from the star. This matter slowly clears away until the object has become a fully grown new star. Excess mater will accrete into a circumstellar disc, which can result in a planetary system. This disc is also called a "proplyd" or "protoplanetary disk", and can also be found in T Tauri stars.
Exceptionally large stars undergo a slightly different and faster process where the steps are mixed together until a giant star is formed. Sometimes, large protostars will lose their gravitational pull on some parts of the system, and will clump into two different systems, which may result in multiple star systems. If the star does not obtain enough energy to start fusion, it will end up as a brown dwarf (see below).
Pre-Main Sequence Stars
As stars evolve from protostars to fully-fledged main sequence stars, they exist is some pre-main sequence states.
T Tauri objects have not begun fusion yet, and are still in the process of contracting matter, but still give off light because of the contraction. Because they are still contracting, they have larger radii than mature main sequence stars, and are more luminous. They also have more sunspot and emission activity, and tend to be variable stars. In addition, they exhibit lithium burning, caused by the large lithium concentration in the stars.
T Tauri stars exhibit similar physical parts. They all have the central star and an accretion disc of matter surrounding them. This disc is made of matter that still has not fallen into the star, which it may do, or the matter may end up as satellites for the star. They also exhibit polar outflows caused by the continued accretion. These make them more visible and are radiation sources. Sometimes, the bipolar outflow will result in "Herbig-Haro objects", which are cloud-like objects found at the ends of polar gas ejections. They are temporary objects, and will disappear once the star matures.
Herbig Ae/Be stars are pre-main sequence stars which are younger than 10 million years. They are of spectral types A or B, and are usually 2-8 solar masses. They are located to the right of the main sequence on the H-R diagram. They are the intermediary between T Tauri stars (less than 2 solar masses) and massive stars that evolve much more quickly (more than 8 solar masses). There are about 100 confirmed and candidate Herbig Ae/Be stars, but HD 95086b is the only confirmed exoplanet to orbit one of these.
FU Orionis stars are variable stars which undergo extreme fluctuations in magnitude. These eruptions are accompanied by the brightening of an accretion disk around the young star. They are often accompanied by Herbig-Haro objects. They are named for their prototype star, FU Orionis, which was a DSO in 2015.
Brown dwarfs are failed stars. They have an upper limit of around 75 to 80 Jupiter masses, and a lower limit of around 13 Jupiter masses. Low mass brown dwarfs are thought to fuse deuterium, while brown dwarfs of greater than or equal to 65 Jupiter masses may fuse lithium as well. They are thought to be fully convective, with no defined layers. Spectral classes M, L, T, and Y refer to brown dwarfs (M brown dwarfs are usually M6.5 or later, as earlier M classes refer to red dwarfs). Most would appear magenta, orange, or reddish in visible light.
If an object formed in the same manner as most stars, but is less than 13 Jupiter masses, it is called a "sub-brown dwarf".
For more information about exoplanets, see Astronomy/Exoplanets.
Types of Exoplanets
For a more complete list of types of exoplanets, please see Astronomy/Exoplanets#Types of Exoplanets.
There are a couple ways of detecting exoplanets. The main ones, however, are radial velocity, transits, and astrometry. Some of the minor ones are direct detection and gravitational lensing.
When a planet orbits a star, the velocity with respect to the Earth's plane of reference changes depending on where the planet is in its orbit and in what direction the planet is moving. The radial velocity of a planet is positive as it moves towards the Earth and is negative as it moves away from the Earth. Thus, when the planet is just moving straight across our plane of references either to the left or to the right, the radial velocity is zero. We can use the radial velocity values to find the planet's mass: however, it can get tricky as we also need to know the inclination of the planet's orbit around the star, which we cannot know directly from measuring the radial velocity. Also some problems with using the radial velocity method are rotating spots on the star, which can cause the RV to form a wave through the Rossiter-McLaughlin effect, granulation on the star's surface, which can cause blue/red shifts and fluctuations, and oscillations on the star's surface, which can cause similar waves to form. In addition, the oscillation of the star can be complicated through pulsations on different planes on the star's surface.