STELLAR FORMATION

Although stars are inanimate objects, we tend to describe their stages of evolution as if they were alive. Just like us, they are born, live, and then die. Of course, their lifetimes are much longer than ours and they can ‘live’ for billions of years. And during their lives, stars produce monumental amounts of energy through nuclear processes in their interior, giving them their characteristic shine. So let’s start at the beginning. Where do stars come from?

A Giant Gas Cloud

A star begins life as a giant cloud of gas which is generally an accumulation of dust, gas, and plasma.

Stars form inside relatively dense concentrations of interstellar gas and dust known as molecular clouds. At these temperatures, gases become molecular meaning that atoms bind together. CO and H2 are the most common molecules in interstellar gas clouds.

Pillars of Creation. An interstellar cloud of gas and dust in the Eagle Nebula,

 known for its complexity and beauty.

A Protostar Is a Baby Star

A protostar looks like a star but its core is not yet hot enough for fusion to take place. The luminosity comes exclusively from the heating of the protostar as it contracts. Protostars are usually surrounded by dust, which blocks the light that they emit, so they are difficult to observe in the visible spectrum.

As the cloud collapses, it begins to spin and by the time a protostar is formed, the cloud flattens and there is a protostellar disk spinning around the protostar. These disks probably slow the rotation of the protostar, and sometimes coalesce into planetary systems.

A protostar becomes a main sequence star when its core temperature exceeds 10 million K. This is the temperature needed for efficient hydrogen fusion.

Protostar L1527

The T-Tauri Phase

When the star is still in the earliest stages of formation, it doesn’t have enough temperature in its core to ignite the fusion of hydrogen and helium. Instead, the star shines with the gravitational energy of its continued collapse. Astronomers call this pre-star a T Tauri star. 

In this stage, a young star begins to produce strong winds, which push away the surrounding gas and molecules. This allows the forming star to become visible for the first time. A star in its T-Tauri phase can be spotted without the help of infrared or radio waves.

 T-Tauri Star

Main Sequence Stars

Eventually, the young star reaches hydrostatic equilibrium, in which its gravity compression is balanced by its outward pressure, giving it a solid shape. The star then becomes a main sequence star. 

Main sequence stars fuse hydrogen atoms to form helium atoms in their cores. About 90 percent of the stars in the universe are main-sequence stars. These stars can range from about a tenth of the mass of the sun to up to 200 times as massive.

Our Sun is currently in its main sequence phase.

Our Sun is a main sequence star (Image source: NASA)

Expansion into Red Giant

When all the hydrogen in a star is fused to helium, the core contracts, and its temperature increases. This increased core temperature and pressure cause helium to fuse into carbon via the triple-alpha process.

This fusion releases more energy than hydrogen-helium fusion, causing an increase in radiation pressure. This increased radiation pressure pushes matter outwards, thus expanding the star. As the star expands its surface cools and becomes redder — a red giant is formed.

     Star's actual expansion into a red giant.

Fusion of Heavier Elements

As it expands, the star begins fusing helium molecules in its core, and the energy of this reaction prevents the core from collapsing. Once helium fusion ends, the core shrinks, and the star begins fusing carbon. This process repeats until iron begins appearing in the core. Iron fusion absorbs energy, so the presence of iron causes the core to collapse. If the star is massive enough, the implosion creates a supernova. Smaller stars like the sun contract peacefully into white dwarfs while their outer shells radiate away as planetary nebulae.

The fusion of heavier elements in a star.

Supernovae and Planetary Nebulae

A supernova explosion is one of the brightest events in the universe. Most of the star's material is blown into space, but the core implodes rapidly into a neutron star or a singularity known as a black hole. Less massive stars don't explode like this. Their cores contract into tiny, hot stars called white dwarfs while the outer material drifts away. Stars smaller than the sun don't have enough mass to burn with anything but a red glow during their main sequence. These red dwarves, which are difficult to spot but may be the most common stars out there, can burn for trillions of years. Astronomers suspect that some red dwarves have been in their main sequence since shortly after the Big Bang.

Supernova