Stardust and eternity – 3.4.5

The death of a star

Stars positioned on the Main Sequence – the band running from the upper left to the bottom right of the Hertzsprung–Russell (H-R) diagram, that links stars’ temperatures to their luminosities – spend about 90% of their lives fusing hydrogen into helium and producing enough fusion pressure to sustain them against deadly gravitational collapse. For common G-type stars like the Sun, their stable life on the Main Sequence lasts about 10 billion years (we are halfway through right now). However, once the supply of hydrogen in their cores is exhausted, the subtle equilibrium between gravity and outward pressure starts to deteriorate. At this stage, the star’s structure begins to falter, sometimes abruptly: gravity takes over and begins compressing the star, thus rising its temperature and igniting a shell of hydrogen surrounding the inert helium core. Eventually, the star expands up to 100- 1000 times wider, turning into a cooler star shining in the redder part of the electromagnetic spectrum – thus becoming a “red giant”. It is half as hot as the Sun, and moves to the upper right-hand side of the H-R diagram. The red giant phase may last up to a few hundred million years. Meanwhile, as the helium core continues to contract, helium ignites and is rapidly fused into carbon. When the helium is also exhausted, the melting stops, the core shrinks again and a helium shell surrounding the newly formed inert carbon core begins to ignite, leading to further expansion of the star and the ejection of its outer layers into a spherical cloud of matter – misleadingly (for historical reasons) called a “planetary nebula”. What remains, at the center of the nebula, is the hot very compact carbon core, or “white dwarf”, similar to Earth in size and supported against further collapse by a quantum effect called “electron degeneracy pressure”, due to the tightly packed electrons.

This scenario applies to relatively low-mass stars like the Sun. As for more massive stars (between approximately 5 and 10 solar masses), their core continues to progressively fuse into heavier elements besides carbon – thus leading to an onion-like internal structure, which contains an inert iron core. However, unlike all previous stages of nuclear fusion that generated energy, the fusion of iron into heavier elements does not lead to a release of energy, but to the opposite effect, i. e. absorption. The core collapses under its own gravity until the protons of the iron nuclei combine together with the electrons, creating an extremely compact “neutron star”. A neutron star size is about 10 km and contains 1.4 solar masses, supported against further collapse by the so-called “neutron degeneracy pressure” – due to the tightly packed neutrons. While reaching its minimum size, the neutron core begins to rebound, causing a shock wave to move through the outer layers into the infalling stellar material and to explode the outer layers, thus generating a “supernova”. This explosion lefts behind a residue which stays visible for hundreds of years. The Crab nebula (M1) – in the constellation of Taurus – is a remarkable example of a supernova remnant. It exploded in 1054 and was recorded by Chinese astronomers of the time. The supernova was visible for months in the sky as a “guest star” even in daylight.

For stars whose cores are more massive than about 3 solar masses, the neutron degeneracy pressure is NOT able to support their gravitational collapse. The star thus collapses until it becomes a black hole – an object so compact that even light cannot escape it. We cannot actually “see” a black hole, but only what lies beyond the sphere surrounding it, the so-called “event horizon”. The presence of the black hole can be inferred by means of the hot accretion disk that forms around it. The first “direct image” of a black hole was released in 2019, following observations by the Event Horizon Telescope (EHT) of the supermassive black hole in the galaxy M87. In 2022, astronomers were also able to release the first image of the accretion disc around the horizon of Sagittarius A* – a black hole of about 4 million solar masses at the center of our galaxy.

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The Crab Nebula, supernova remnant – *Credit: NASA, ESA and Allison Loll/Jeff Hester (Arizona State University). Acknowledgement: Davide De Martin (ESA/Hubble).*

The Cat’s Eye Nebula – *Credit: ESA, NASA, HEIC and The Hubble Heritage Team (STScI/AURA)*

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Images

Most detailed image of the Crab Nebula (NASA, ESA and A. Loll/J. Hester)