OCR Specification focus:
‘Describe star formation from interstellar gas and dust via gravitational collapse and fusion ignition.’
Star formation is a fundamental astrophysical process in which diffuse matter in space becomes concentrated, heated, and ultimately transformed into a luminous star. This topic explains how interstellar clouds collapse, increase in density and temperature, and reach the conditions necessary for nuclear fusion to ignite.
Interstellar Material and Initial Conditions
Star formation begins within regions of the interstellar medium (ISM), consisting mainly of gas (predominantly hydrogen) and dust (micron-scale solid particles). These materials are often concentrated in molecular clouds, large and cold regions where gravitational forces can overcome thermal pressure.
The term interstellar gas refers to the extremely low-density hydrogen and helium found between stars in galaxies.
Interstellar gas: Low-density atomic or molecular gas, primarily hydrogen, occupying the space between stars.
Such clouds remain stable until an external influence—such as a shock wave from a nearby supernova or a collision between clouds—compresses the material, triggering collapse. These influences cause local regions of higher density known as clumps, which can develop into protostars.
Gravitational Collapse
Once a region of a molecular cloud reaches sufficient density, gravitational collapse begins. Gravity pulls matter inward, causing the clump to contract. As the cloud collapses, it fragments into multiple smaller cores, each potentially forming a separate star.
During contraction, the potential energy of the collapsing material converts into thermal energy, and the centre of each fragment begins to heat up. This marks the earliest phase of a protostar, an embryonic stellar object not yet undergoing nuclear fusion.
Protostar: A contracting mass of gas in the earlier stages of star formation before nuclear fusion begins.
The collapse is not uniform. Rotational motion increases due to conservation of angular momentum, leading to the formation of a surrounding accretion disc.
Diagram illustrating a forming protostar embedded in an infalling envelope, with a rotationally supported disc and bipolar jets emerging along the rotational axis. Source.
Material in the disc gradually spirals inward, adding mass to the forming protostar.
Heating and Density Increase
As collapse continues, the protostar’s interior becomes increasingly hot and dense. Radiation trapped by the opaque dust-laden environment further raises the temperature. At this stage, energy is mainly released through gravitational contraction rather than nuclear reactions.
The increasing density allows collisions between particles to become more frequent. These interactions gradually strip electrons from atoms, leading to partial ionisation and altering the internal pressure balance. The growing thermal pressure begins to counteract gravity, slowing the collapse but not halting it until fusion ignition occurs.
Accretion Processes
Accretion is a key mechanism in star formation. Material from the disc continues to fall onto the protostar, increasing its mass and temperature. Accretion flows can be turbulent and may produce strong bipolar jets that eject material along the star’s rotational axis.
JWST image of HH 30 showing an edge-on protoplanetary disc with labelled jets, conical outflows, and a dark midplane; extra scientific context appears on the hosting page. Source.
These jets help remove angular momentum, enabling further contraction.
The strength of accretion and the duration of this phase strongly influence the final mass of the star. Low-mass stars form slowly, while high-mass stars accrete more rapidly and reach fusion conditions sooner.
Fusion Ignition and Hydrostatic Equilibrium
The culmination of star formation occurs when the protostar’s core becomes hot and dense enough for hydrogen fusion. This requires temperatures of around 10 million kelvin, enabling proton–proton chain reactions in low-mass stars. The onset of fusion marks the transition from protostar to main-sequence star.
EQUATION
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Gravitational Potential Energy (U) = −GMm / r
G = Gravitational constant (N m² kg⁻²)
M = Mass of the central object (kg)
m = Mass of the collapsing material (kg)
r = Radius of separation (m)
—-----------------------------------------------------------------
As fusion begins, the stellar core releases energy that generates an outward radiation pressure. When this pressure exactly balances the inward force of gravity, the star achieves hydrostatic equilibrium, establishing long-term stability.
After fusion starts, the surrounding disc gradually disperses, either accreted by the star, forming planets, or ejected by stellar winds. What remains is a fully formed main-sequence star shining steadily due to continuous nuclear reactions.
Key Stages of Star Formation (Summary List)
Although each forming star follows a unique path shaped by its environment, most follow these broad stages:
Molecular cloud formation
Cold, dense regions rich in hydrogen and dust serve as stellar nurseries.
Cloud fragmentation
Disturbances create clumps that reach the conditions for gravitational collapse.
Protostar formation
Collapsing clumps heat up, forming a central object surrounded by an accretion disc.
Accretion and heating
Material falls inward, raising temperature and density while jets regulate angular momentum.
Fusion ignition
Core temperature becomes high enough for hydrogen nuclei to fuse.
Main-sequence entry
Hydrostatic equilibrium is established, and the star becomes stable and luminous.
Influence of Mass on Formation
The mass of a collapsing clump largely determines the star’s future behaviour. Higher-mass protostars contract more quickly and ignite fusion earlier, whereas low-mass protostars evolve more slowly and radiate away less energy during collapse. Mass also influences disc lifetime, likelihood of planet formation, and the strength of stellar winds.
The mass accumulation phase ends when stellar radiation and winds become strong enough to halt further accretion. Once accretion ceases, the star enters a long period of main-sequence stability, shining as a result of nuclear fusion sustained in its core.
FAQ
The degree of fragmentation during gravitational collapse determines whether one or several stars form.
Fragmentation depends on:
• Local variations in density within the cloud
• The cloud’s temperature, with cooler regions fragmenting more readily
• Turbulence that seeds small-scale density differences
If fragments remain gravitationally bound to each other, they may form binary or higher-order multiple-star systems.
Protostars form inside dense regions rich in dust, which strongly absorbs and scatters visible light.
They are instead detected in infrared wavelengths, which pass more easily through dust.
This obscuration is why infrared telescopes such as JWST and previous missions like Spitzer are especially important for observing early star formation stages.
Magnetic fields slow down the collapse of molecular clouds by providing magnetic pressure that counteracts gravity.
As collapse proceeds, the field lines become compressed, guiding the flow of material and helping launch bipolar jets.
Magnetic braking also removes angular momentum from the infalling gas, allowing material to accrete more efficiently onto the protostar.
Infalling material carries angular momentum, which cannot simply vanish.
As a result, gas settles into a rotating disc where frictional processes and magnetic interactions gradually transport angular momentum outward.
This allows matter to slowly spiral inward while preventing it from collapsing directly into the centre in a single step.
The duration depends mainly on the mass of the forming star.
• Low-mass protostars contract slowly and can spend tens of millions of years before fusion begins.
• Higher-mass protostars reach fusion temperatures far sooner due to stronger gravitational forces and faster accretion.
Environmental factors such as turbulence, radiation from nearby stars, and the density of the surrounding cloud also modify the timeline.
Practice Questions
Question 1 (2 marks)
Describe the role of gravitational collapse in the formation of a protostar.
Mark Scheme:
• Gravity causes regions of higher density within a molecular cloud to contract, pulling material inward. (1)
• The collapse increases the temperature and pressure at the centre, leading to the formation of a protostar. (1)
Question 2 (5 marks)
Explain how a molecular cloud evolves into a main-sequence star. In your answer, include the processes of collapse, heating, accretion, and the conditions required for fusion ignition.
Question 2 (5 marks)
• Molecular cloud becomes unstable due to a disturbance, allowing gravitational collapse to begin. (1)
• Collapse leads to increasing density and temperature in the centre of the forming star. (1)
• Rotation forms an accretion disc; material spirals inward and adds mass to the protostar. (1)
• Continued heating brings the core to the conditions needed for hydrogen fusion (very high temperature and pressure, around 10 million kelvin). (1)
• Fusion ignition establishes hydrostatic equilibrium, marking the start of the main-sequence phase. (1)
