Star Formation

Star Formation 2
Star Formation 2

Stars form inside relatively dense concentrations of interstellar gas and dust known as molecular clouds. These regions are extremely cold (temperature around 10 to 20K, just above absolute zero). At these temperatures, gases become molecular, meaning the atoms bond together. CO and H2 are the most common molecules in interstellar gas clouds. Deep cold also causes gas to agglomerate at high densities. When the density reaches a certain point, stars are formed.

Because the regions are dense, they are opaque to visible light and are known as dark nebulae. We must use IR and radio telescopes to probe them, as they do not glow with optical light.

Star formation begins when the denser parts of the cloud core collapse under their own weight/gravity. These cores typically have about 104 solar masses in the form of gas and dust. Cores are denser than the outer cloud, so they collapse first. As the cores collapse, they break up into clumps of around 0.1 parsec in size and 10 to 50 solar masses in mass. These clusters then turn into protostars, and the whole process takes about 10 million years.

How do we know it's happening if it's taking this long and lurking in dark clouds? Many of these cloud cores have IR sources (potential energy converted into kinetic energy), which is evidence of energy from collapsing protostars. Also, where we find the young stars (see below), we find them surrounded by clouds of gas, a remnant dark molecular cloud. And clusters of the same cloud core occur as groups of stars.

Protostars (Protostar):

When a cluster breaks away from other parts of the cloud core, it has its own gravity and identity, and we call it a protostar. As the protostar forms, the loose gas falls into its centre. The incoming gas emits kinetic energy in the form of heat, and the temperature and pressure in the center of the protostar rise. As its temperature approaches thousands of degrees, it becomes an IR (infrared) source.
Several candidate protostars have been found by the Hubble Space Telescope in the Orion Nebula.
During the initial collapse, the stack is transparent to radiation and the collapse progresses quite quickly. As the clump becomes denser, it becomes opaque. The escaping IR radiation is trapped and the temperature and pressure in the core begin to rise. At some point, the pressure stops more gas from entering the core and the object stabilizes as a protostar.

Protostar initially only has 1% of its final mass. However, the envelope of the star continues to grow as the falling material accumulates. After a few million years, thermonuclear fusion begins in its core, after which a strong stellar wind is produced that stops new mass from penetrating in. The first star is considered a young star, as its mass is fixed and its future evolution is now set.

T-Tauri Stars:

When a protostar becomes a hydrogen-burning star, a strong stellar wind usually occurs along its spin axis. For this reason, many young stars have a bipolar outlet, which is a flow of gas outward from the poles of the star. This is a feature easily seen with radio telescopes. This early phase in a star's life is called the T-Taurus phase.
One consequence of this collapse is that young T Tauri stars are often surrounded by large, opaque, circumferential disks. These disks gradually gather onto the star's surface, thereby radiating energy from both the disk (infrared wavelengths) and the location where the material falls onto the star (optical and ultraviolet wavelengths). Somehow, some of the material added to the star is ejected perpendicular to the disk plane in a highly parallelized stellar jet. The circumstellar disk eventually disintegrates, possibly as planets begin to form.

Young stars also have dark spots on their surfaces that look like sunspots but take up a much larger portion of the star's surface area.

The T-Tauri phase is when a star has:

  • Strong surface activity (flashes, explosions)
  • strong stellar winds
  • Variable and irregular light curves

A star in the T-Taurus phase can lose up to 50% of its mass before settling down as a main sequence star, so we call them pre-main sequence stars. Their positions in the HR diagram are shown below:
Arrows show how T-Tauri stars would evolve into the main sequence. They begin their lives as slightly cooler stars, then warm up and become bluer and slightly dimmer depending on their initial mass. Very massive young stars are born so quickly that they appear with such a short T-Taurus phase in the main sequence that they are never observed.

T-Tauri stars are always found buried in the gas clouds from which they were born. An example is the Trapezoid star cluster in the Orion Nebula.
The evolution of young stars is from a protostar cluster deep within a molecular cloud core to a T-Tauri star cluster (HII, pronounced H-two) whose hot surface and stellar winds heat the surrounding gas to form an HII region. ionized hydrogen). Then the cluster explodes, gas is blown up and stars develop as shown below.

This composite image of the Kleinmann-Low Nebula, part of the Orion Nebula complex, consists of several points from the NASA/ESA Hubble Space Telescope in optical and near infrared light. Infrared light allows you to peer through the nebula's dust and see the stars within. The resulting stars are shown in a bright red color in the image. With this image showing the central region of the Orion Nebula, scientists were looking for rogue planets and brown dwarfs. They found a fast-moving runaway star as a side effect.

We often find young star clusters in galaxies alongside other young stars. This phenomenon is called supernova-induced star formation. First, very massive stars form and explode into supernovae. This converts the shock waves into a molecular cloud, causing the nearby gas to become compressed and form more stars.

This creates a kind of stellar coherence (young stars are found alongside other young stars) and is responsible for the pinwheel patterns we see in galaxies.


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