Sunday, March 21, 2021

Star Formation - Part I

10:31 PM | , ,

In the introduction of this section, we saw star formation in a nutshell. Thus, the crucial details of the process as a whole were ignored. In reality, though, star formation is one of the most important subjects in modern Astrophysics, since stars are the blocks that build galaxies, and they are planetary nurseries. Thus, understanding this process is one of the hottest topics at the moment. So to view the subject in detail, let’s see all the steps of star formation.

Jeans mass and star formation

The initial model of star formation was rather “simple”, “quiet”, and “peaceful”. It was based on the initial model of British astrophysicist Sir James Jeans. According to this model, an interstellar’s cloud gravitational inward pull was in balance with its internal heat. Jeans noticed though that this balance was fragile. With just a small external disturbance (e.g. a nearby stellar explosion), gravity would dominate and initiate the collapse of the cloud. Such an event would pile up the matter to densities and temperatures that were high enough to allow hydrogen fusion. Thus a new star (or stars) was born.

Note that this model was used from most of the 20th century, and in the process, details were added to it. Also, this model was treating the infant star as an isolated case, thus influence between stars was considered negligible, and binary systems were not taken into account. The initial main issue back then was understanding the evolution of stars, something that is also complicated. Thus, it would be nice to see the process of star formation as we know it today, through the assistance of modern observations.

Star formation region – Stellar nursery definition

A stellar nursery or a star-forming region is an area within a dense nebula, consisting of gas and dust. Local matter contractions in the region result in the formation of new stars.

Where does star formation take place? – Interstellar cloud definition

An interstellar cloud is an accumulation of gas, dust, and plasma in a galaxy. In other words, an interstellar cloud is denser than the average region in the interstellar medium (ISM), between stars in a galaxy. Depending on its density, size, and temperature, hydrogen can be either neutral, ionized, or molecular. The latter are called molecular clouds. While neutral and ionized hydrogen clouds are called diffuse clouds.

Introduction of star formation – Interstellar clouds and the stellar nurseries

A galaxy like our own consists of stars, stellar remnants (we will review them later on), and a diffuse interstellar medium of gas and dust. The density of the interstellar medium (ISM) varies between 10-4 and 106 particles per cm3 and it is typically composed of roughly 70% hydrogen, with most of the remaining gas consisting of helium. Additionally, the ISM is enriched by trace amounts of various heavier elements that have produced and ejected from stars that ended their lives.

The high-density regions of the ISM form diffuse nebulae (i.e., an extended nebula that has no well-defined boundaries). In dense nebulae, where stars are produced, a significant amount of hydrogen is in molecular (i.e., H2) form, so these nebulae are called molecular clouds.

From observations, we know that the coldest clouds tend to form low-mass stars. Giant molecular clouds which are warmer tend to form stars of any mass range. The latter have typical densities of 100 particles per cm3, diameters around 100 light-years (9.5×1014 km), and masses up to 6 million M☉, and an average interior temperature of 10 K.

Finally, star formation can take place in smaller clouds. These opaque gaseous and dusty clouds are known as Bok globules (named after Dutch Bark Bok). Stars can form either independently or in association with a nearby collapsing molecular cloud. Their mass is in the range of a few solar masses, and they are typically up to one-light year across. More than half of the observed Bok globules have been found to contain newly forming stars.

Note that all these apply to Galaxies with similar morphology to our own (i.e., spiral galaxies). Elliptical galaxies are different, since they lose the cold regions of their ISM, within a time window of one billion years. Thus due to this, the only way for elliptical galaxies to have new diffuse nebulae is through mergers with other galaxies.

Star formation begins – The collapse of the interstellar cloud

As we saw in the introduction, an interstellar cloud is initially at equilibrium. The cloud remains at equilibrium as the gas pressure in its interior balances its gravitational potential (this is known as hydrostatic equilibrium). If a cloud has enough mass so the gas pressure cannot support it, then the cloud will collapse. The mass of such collapse is what defines the Jeans mass as we saw above.

The Jeans mass depends on the density and temperature of the cloud and temperature, but in reality, the mass varies from thousands to tens of thousands of solar masses. When the cloud collapses numerous stars form practically simultaneously. This can be observed in the form of an embedded cluster. The “end-product” of this process is what is known as an open star cluster.

The collapse of an interstellar cloud, hence star-formation, can be triggered by more than one event. A nearby supernova explosion can be a trigger, sending shocked material at very high speeds. Additionally, two molecular clouds may collide between them, which also may initiate transformation. More extreme scenarios evolve galactic collisions, which trigger massive starbursts in each galaxy, prior the merger. Such a scenario could be responsible for the formation of globular clusters.

Assuming that one of the scenarios we saw above takes place (it could be that more than one is evolved), a cloud will break into smaller pieces, until the fragments reach stellar mass. In every fragment, the gas that collapses radiates away the gained energy by the release of gravitational potential energy. The fragment will become less translucent as its density increases thus its efficiency in radiating away its energy becomes less efficient. As a result, the temperature of the cloud rises thus, further fragmentation is stalled. The fragments condense into rotating spheres that are the stellar embryos.

What makes this process even more complicated is the fact that we should take into account the effects of magnetic fields, turbulence, rotation, macroscopic flows, and the geometry of the cloud. Turbulence has an effect on the cloud fragmentation, and on small scales favors collapse. However, rotation and the effects of magnetic fields can prevent the collapse of the cloud.

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