“We are a way for the universe to know itself… We’re made of star stuff.” From hydrogen and helium, early stars fused the rest of the elements, including the carbon and oxygen that form most of the human body. To get to know our universe — and by extension, ourselves — a little better, let’s take a look at how stars themselves form.
Stellar Nurseries: The Interstellar Medium and Cloud Collapse
From here on Earth, the stars probably seem lonely. After all, apart from binary star systems, each star is light years away from the next. In reality, though, most stars don’t form alone — they begin in large interstellar clouds and eventually become star clusters up to 300 light years in diameter.
Star formation is usually more common in spiral galaxies than elliptical galaxies. Spiral galaxies contain large amounts of gas and dust, known as the interstellar medium (ISM), which can stay in a spiral galaxy for billions of years due to the gravitational force from the center. In contrast, the structure of elliptical galaxies causes them to lose most of their diffuse (scattered) ISM after 1 billion years, seriously slowing star formation.
In galaxies that favor star formation, the diffuse interstellar medium can form higher-density regions that look like clouds. These are called diffuse nebulae, and they become nurseries for stars to form. The ISM, by mass, is 70% hydrogen, and most of the other 30% consists of helium. Trace amount of heavier elements, such as oxygen, nitrogen and carbon, can also be found in the ISM.
Since the type of diffuse nebulae that foster star formation contain molecular hydrogen (H2, unlike ionized hydrogen, H+) they’re primarily known as molecular clouds. Smaller, colder molecular clouds usually form smaller stars, while warm giant molecular clouds are more inclusive and form stars of all sizes.
The matter in a molecular cloud is perfect for the formation of stars, but nothing will happen without adding a little bit of pressure. As the giant molecular cloud accumulates more gas and dust, the force of gravity within the cloud also increases. When the cloud passes a mass limit called the Jeans mass, it will collapse on itself because the gas pressure pushing outward can no longer overpower the gravitational force pulling inward. A lot of the time, cloud collapse is triggered by one of two events: a shockwave from a nearby supernova, or a collision with another cloud.
When a cloud undergoes gravitational collapse, as many as tens of thousands of stars form at once, resulting in a young open cluster of stars. The formation of individual stars occurs because the molecular cloud continually fragments until each piece reaches stellar mass. These pieces are much denser and hotter than the original interstellar medium, so they’ll gradually condense into spheres that will turn into protostars, or the beginnings of actual stars.
The Formation of Protostars
As a molecular cloud collapses, it eventually becomes opaque. The dust within a fragment of the original cloud can reach initial temperatures of 100 K, and each cloud fragment will continue condensing until its core temperature reaches about 2000 K.
At this point, hydrogen and helium atoms can become ionized, meaning that their nuclei and electrons are separated. Ionization allows for convection in the core of the protostar, while energy is radiated from the outside of the core. These two processes allow the stellar core to keep collapsing until its internal pressure is at equilibrium with its gravitational force. This state is known as hydrostatic equilibrium. At this point, the object becomes known as a protostar.
Just because a protostar’s core is no longer collapsing doesn’t mean that it isn’t accumulating any more matter. At this stage, there’s still a lot of matter, known as a circumstellar disc, around the protostar. The matter from this disc falls quickly toward the protostar in a process known as accretion — think of rain falling onto Earth — causing the temperature to continually increase.
PMSing: the In-Between Phase
Eventually, the matter in the circumstellar disc becomes dispersed, halting the process of accretion. At this stage, the star is known as a pre-main-sequence (PMS) star. It isn’t a main-sequence star yet, mainly because of its energy source. The PMS star creates energy from gravitational contraction (note that gravitational contraction is different from gravitational collapse) and convection, while main sequence stars fuse hydrogen.
The gravitational contraction of the protostar eventually slows down. While this is happening, the PMS star becomes less luminous, or bright, because contraction decreases the surface area available for light emission. The protostar also keeps the same temperature while its luminosity is decreasing.
Eventually, the PMS star will become dense enough that a radiative zone forms around it, where energy produced by the star is electromagnetically radiated outward, in contrast to the convection that has been taking place in its core.
Once the radiative zone forms, the PMS star’s temperature increases while the luminosity stays the same. When the temperature of the protostar’s core reaches about 15,000,000 K (roughly the temperature of the Sun’s core *wink*), nuclear fusion, specifically hydrogen fusion, begins.
The start of nuclear fusion creates a huge blast of energy, clearing away any excess dust surrounding the PMS star. With hydrogen fusion underway and the space dust cleared away, the star is now officially full-grown and enters its main sequence phase.
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