What is a Collapsar?

Illustration of a collapsar — Artist's impression of a collapsar: At the heart of a supernova remnant, the core of a massive star collapses into a gravitational black hole. This process produces a swirling accretion disk and propels two relativistic jets of ultra-energetic matter through the stellar envelope, triggering a high-intensity gamma-ray burst.
Image source: astronoo.com (new window) - AI-generated image, public domain.

Scientific summary

The Astronoo article presents the collapsar, an astrophysical phenomenon born from the gravitational collapse of a massive star. The term, coined in the 1990s, designates a star whose core collapses into a black hole, releasing relativistic jets and long-duration gamma-ray bursts. The text describes the mechanism: exhaustion of nuclear fuel, formation of an iron core, exceeding the Chandrasekhar limit, then collapse. It then details the role of the accretion disk, the emission of jets at relativistic speeds and the origin of gamma-ray bursts, among the most energetic events in the Universe.

What is a collapsar and how does it form?

This article is devoted to the collapsar, an astrophysical object resulting from the gravitational collapse of a massive star. But what is a collapsar and how does it form? The term combines "collapse" and "quasar": it designates a star whose core, deprived of nuclear fuel, collapses in on itself to give birth to a black hole. When the mass of the core exceeds the Chandrasekhar limit, about 1.4 times the mass of the Sun, matter collapses abruptly. If the remnant exceeds three solar masses, a black hole forms. Around it, an accretion disk settles, where matter, heated to extreme temperatures, emits jets of particles at speeds close to that of light. These jets, piercing the outer layers of the star, produce gamma-ray bursts of long duration, among the most luminous events in the Universe. The study of collapsars thus allows us to probe general relativity, plasma physics and the evolution of the most massive stars.

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Origin and scientific definition of the collapsar

Etymology and birth of the concept

The term "collapsar" combines the words "collapse" and "quasar". It was proposed in the 1990s to describe massive stars whose gravitational collapse generates a black hole, causing relativistic jets and gamma-ray emissions.

A class of objects linked to long gamma-ray bursts

Collapsars are particularly associated with gamma-ray bursts of long duration (more than two seconds), resulting from the collapse of massive stars and the rapid formation of black holes.

Gravitational collapse mechanism

A massive star at the end of its life

A collapsar is an astrophysical phenomenon resulting from the gravitational collapse of a massive star, generally with a mass greater than 20 times that of the Sun. This process leads to the formation of a black hole and can be at the origin of gamma-ray bursts, among the most energetic events observed in the universe.

From the iron core to the Chandrasekhar limit

At the end of its life, a massive star exhausts its nuclear fuel, resulting in the production of an iron core. Unlike lighter elements, the fusion of iron does not release energy. The core therefore ceases to produce the pressure necessary to counter gravitational attraction. When the mass of the core exceeds the Chandrasekhar limit (~1.4 times the solar mass), it collapses under its own weight. If the mass of the residual core exceeds about 3 solar masses, the star becomes a collapsar, forming a black hole.

Formation of black holes and accretion disk dynamics

Birth of the accretion disk

When a collapsar forms, the collapsing stellar core continues to accumulate matter, forming an accretion disk around the black hole in formation. The disk, heated by extreme gravitational forces and internal frictions, releases enormous quantities of energy.

Ejection of relativistic jets

The matter in the disk is accelerated to relativistic speeds, and part of it is ejected in the form of powerful jets, often aligned with the magnetic poles of the star.

Associated gamma-ray bursts

An extreme energetic phenomenon

The gamma-ray bursts (GRB) associated with collapsars are luminous explosions of gamma radiation. They are generated when the relativistic matter jets penetrate the outer layers of the star and escape into space. The energy released by a gamma-ray burst is colossal, on the order of $10^{44}$ joules, in just a few seconds.

A laboratory for extreme physics

These events allow the study of extreme phenomena such as general relativity, plasma physics and the evolution of massive stars.

In summary

A key phenomenon of modern astrophysics

Collapses in a galaxy are numerous, so the scenario of the collapse is known. Collapsars, through their link with the formation of black holes and gamma-ray bursts, are key astrophysical phenomena for understanding the evolution of massive stars and the extreme mechanisms of the universe. The observation of gamma-ray bursts and collapsars allows us to explore energies and gravities so extreme that they push the limits of current physics.

References

FAQ: Everything you need to know about collapsars

What exactly is a collapsar?

A collapsar is a massive star whose core collapses into a black hole at the end of its life. The term, a contraction of "collapse" and "quasar", was proposed in the 1990s to designate these objects whose collapse produces relativistic jets and long-duration gamma-ray bursts. It is therefore not an ordinary star, but an ultimate stage of stellar evolution.

How does a collapsar form?

Formation begins when the star, after exhausting its nuclear fuel, develops an iron core that can no longer produce energy by fusion. Deprived of radiation pressure, this core collapses under its own gravity. If its mass exceeds the Chandrasekhar limit (~1.4 solar masses), the collapse becomes irreversible. Beyond about 3 solar masses for the remnant, a black hole forms, and the accretion disk surrounding it ejects powerful jets.

What is the link between a collapsar and a gamma-ray burst?

Long-duration gamma-ray bursts (more than two seconds) are most likely produced by collapsars. When the relativistic matter jets pierce the outer layers of the star and escape into space, they emit an intense flash of gamma rays. These bursts are among the most luminous events in the Universe and can be detected billions of light-years away.

Are a collapsar and a supernova the same thing?

No. A classical supernova marks the explosive destruction of a massive star, often leaving behind a neutron star. A collapsar corresponds to a more extreme scenario: direct collapse into a black hole, accompanied by relativistic jets and a gamma-ray burst. Some collapsars may produce a faint or "failed" supernova, but the two phenomena are not equivalent.

What is the minimum mass of a star to become a collapsar?

It is generally estimated that the progenitor star must be very massive, often greater than 20 or 30 solar masses. Below this threshold, the collapse more frequently leads to a neutron star. The mass of the core at the time of collapse also plays a determining role: beyond about 3 residual solar masses, the formation of a black hole is inevitable.

Can a collapsar be observed from Earth?

A collapsar itself is not directly visible: the black hole does not emit light. However, the gamma-ray bursts it produces can be detected by satellites such as Swift (NASA) or Integral (ESA). The relativistic jets also emit radiation at other wavelengths (X-ray, optical, radio) that astronomers observe after the fact, allowing them to reconstruct the scenario of the collapse.

What is a Collapsar?

Scientific summary