The life and death of stars - star formation #
- The life cycle of a star in synopsis [1]
The first stars were born in clouds of the two lightest elements, hydrogen and helium, the residue of the big bang, more than 13 billion years ago. They cannot have had planets, because there was nothing to make planets from—no carbon, oxygen, silicon, iron or any other metals (astronomers call all elements heavier than hydrogen and helium metals). [1.5] In the fullness of time, later stars came to include many other primary elements comprising the heavier metals, the product of supernova explosions successively absorbed by recycled stars with enough mass to sustain nuclear fusion at its core[2]. This fusion of these elements added extra energy to the core, which halted the collapse, and a star would be born. All around, other cloud fragments were also collapsing and forming stars, so our newborn star would be born in a cluster of young stars.
In 2005, NASA’s Spitzer Space Telescope with its infra-red capability was able to peer inside the star nursery known as W5 in the constellation Cassiopeia, 7000 light years[3] away and dominated by a single star. It revealed colossal pillars of coal gas and dust, giving scientists an intimate look at the star-forming process[4]. The largest pillars, fanned by radioactive winds from hot large stars, contained hundreds of new-born stars. At 4.6 billion years old, our sun is estimated to be approximately half way through its life cycle[5]. Its life-sustaining heat and light are generated by the conversion of 4.3 million tons of matter into energy every second[6]. It is a second or third generation star, containing the debris of earlier supernovas.
Just like humans, stars have a life cycle. As we have seen, they are born in gigantic clouds of dust and galactic material in the depths of space, evolve and shine for millions of years and eventually enter a phase of dissolution and extinction. Stars shine by burning their nuclear fuel, which is initially mainly hydrogen; they fuse it into helium, and, later, heavier elements. Each star attains a balance between the forces of gravity, which pulls matter toward the centre, and the outward pressures generated by fusion. This balance keeps the star stable – until all the fuel is converted to iron, which, in nuclear terms, is inert. Fusion then ceases, the all-pervasive force of gravity asserts itself, and the star begins to contract.
Scientists have long been able to observe the formation of stars in so called star nurseries where gravity causes swirling masses of dust and gas to compress together into denser and denser clumps in giant clouds, many light years across. Star formation takes place in the coldest, darkest regions called dark nebulae. The collapsing cloud breaks into hundreds of fragments, each of which continues to collapse. As the density increases, the cloud becomes opaque, trapping the heat within the cloud. This then causes both the temperature and pressure to rise rapidly. The collapsing cloud is now a protostar, surrounded by a disk of gas. These disks will eventually be where planets form.
Hubble telescope image known as the Pillars of Creation where stars are forming in the Eagle Nebula.
In the meantime, the protostar continues to contract and heat up. Eventually, the temperature in the core of the star becomes hot enough for hydrogen to fuse to form helium, just as in the first three minutes after the big bang. This process is known as nuclear fusion. The result is the release of a lot of energy, and the resultant nucleus is smaller in mass than the sum of the ones that made it, in accordance with E=mc². A point is reached where enough outward flowing radiation is generated to stem further gravitational contraction of the gas, and a hot stable, brightly burning star is born.
The star now settles down to life on the main sequence. Fusing hydrogen to helium in its interior, it can produce energy steadily for millions of years. Fusion only takes place at temperatures higher than a few million degrees. Only the core of the star is this hot. The temperature of the surface of the Sun, for instance, is 5800 K, while even at the temperatures of the hottest O-type stars (100,000 K), fusion only takes place in the core. Energy then leaks out to the surface via radiation and/or convection[7]. The energy produced in the star’s core produces enough outward pressure to balance the inward pull of gravity.
A star is only stable as long as it is producing energy in its interior. When the star runs out of hydrogen in its core, its life as a main sequence star is finished. What happens next is a complicated dance as the star tries to hold off gravity, which is trying to make it collapse. Once fusion stops, the core begins to collapse, and as it does so it contracts and heats up. This extra heat forces the outer layers of the star to expand dramatically: it becomes a red giant. The star swells up to as much as 100 times its previous size, and as it expands the outside layers cool. Down in the core, however, the temperature continues to rise.
Eventually, the temperature reaches 100 million degrees, and helium begins to fuse to form carbon. But this too cannot last forever. Eventually – and in less time – the core runs out of helium, and the star starts to collapse again. If the star is a similar size to our sun, the core never gets hot enough for the next ignition stage. Instead, the star ejects its outer layers in a series of belches, and this gas expands into space. Lit by the central remnant, which is now a white dwarf, we see this glowing gas as a planetary nebula. However, more massive stars, about three times the mass of our sun, have strong enough gravity that their core can reach higher and higher temperatures. Element after element (hydrogen, helium, carbon, oxygen, neon, silicon, iron) is ignited, then exhausted, until the chain reaches iron, as illustrated at right:
By the time silicon is reached, temperatures are high enough that there are plenty of MeV photons available, which may now cause photonuclear reactions, breaking up some nuclei. "Among other nuclear bits and pieces, neutrons, which do not have to overcome the repulsive Coulomb barrier become available and are readily absorbed, creating a new equilibrium which leads to the copious formation of the most stable nuclei: iron and nickel. [8]
After iron, which is essentially nuclear ash, there is no more energy to extract from fusion. Once the silicon in the core has fused to make iron, the star can no longer support itself against collapse. The star is doomed: it begins to collapse for the last time. Electrons are forced to combine with protons to form neutrons, and the whole core compacts down a million fold in volume, transforming into ‘an ultradense nugget’ called a neutron star: a ball of neutrons only 20 km across [9]. Meanwhile, the outer layers of the star fall inwards until they hit the newborn neutron star. When they meet the core they “bounce” off it so hard that they are ejected outwards in blast wave which explodes the rest of the star outwards. When the blast wave reaches the surface of the star, we see a supernova explosion [10], reaching a peak brightness of about one billion luminosities before fading away.
It has been suggested that stars cannot produce elements heavier than iron and nickel, but clearly they do. Michael Box elaborates [11]:
The star now settles down to life on the main sequence. Fusing hydrogen to helium in its interior, it can produce energy steadily for millions of years. Fusion only takes place at temperatures higher than a few million degrees. Only the core of the star is this hot. The temperature of the surface of the Sun, for instance, is 5800 K, while even at the temperatures of the hottest O-type stars (100,000 K), fusion only takes place in the core. Energy then leaks out to the surface via radiation and/or convection[7]. The energy produced in the star’s core produces enough outward pressure to balance the inward pull of gravity.
A star is only stable as long as it is producing energy in its interior. When the star runs out of hydrogen in its core, its life as a main sequence star is finished. What happens next is a complicated dance as the star tries to hold off gravity, which is trying to make it collapse. Once fusion stops, the core begins to collapse, and as it does so it contracts and heats up. This extra heat forces the outer layers of the star to expand dramatically: it becomes a red giant. The star swells up to as much as 100 times its previous size, and as it expands the outside layers cool. Down in the core, however, the temperature continues to rise.
Eventually, the temperature reaches 100 million degrees, and helium begins to fuse to form carbon. But this too cannot last forever. Eventually – and in less time – the core runs out of helium, and the star starts to collapse again. If the star is a similar size to our sun, the core never gets hot enough for the next ignition stage. Instead, the star ejects its outer layers in a series of belches, and this gas expands into space. Lit by the central remnant, which is now a white dwarf, we see this glowing gas as a planetary nebula. However, more massive stars, about three times the mass of our sun, have strong enough gravity that their core can reach higher and higher temperatures. Element after element (hydrogen, helium, carbon, oxygen, neon, silicon, iron) is ignited, then exhausted, until the chain reaches iron, as illustrated at right:
By the time silicon is reached, temperatures are high enough that there are plenty of MeV photons available, which may now cause photonuclear reactions, breaking up some nuclei. "Among other nuclear bits and pieces, neutrons, which do not have to overcome the repulsive Coulomb barrier become available and are readily absorbed, creating a new equilibrium which leads to the copious formation of the most stable nuclei: iron and nickel. [8]
After iron, which is essentially nuclear ash, there is no more energy to extract from fusion. Once the silicon in the core has fused to make iron, the star can no longer support itself against collapse. The star is doomed: it begins to collapse for the last time. Electrons are forced to combine with protons to form neutrons, and the whole core compacts down a million fold in volume, transforming into ‘an ultradense nugget’ called a neutron star: a ball of neutrons only 20 km across [9]. Meanwhile, the outer layers of the star fall inwards until they hit the newborn neutron star. When they meet the core they “bounce” off it so hard that they are ejected outwards in blast wave which explodes the rest of the star outwards. When the blast wave reaches the surface of the star, we see a supernova explosion [10], reaching a peak brightness of about one billion luminosities before fading away.
It has been suggested that stars cannot produce elements heavier than iron and nickel, but clearly they do. Michael Box elaborates [11]:
The key is the neutron capture process: as the neutron number increases by the nucleus capturing one or more neutrons, the nucleus becomes β unstable, and decays to the next element in the periodic table. In this way we can, at least in principle, build up almost all the heavy stable elements – or at least their neutron rich isotopes... (T)hese isotopes have much larger natural abundances than the proton rich isotopes of the same elements.
During the main helium and silicon burning phases, these neutron captures take place sufficiently slowly that any necessary β-decays can take place before the next capture. Thus", Box interpolates, "we are effectively moving along the bottom of the β-stability valley [12]. This is known as the s-process (for slow). On the other hand, if the neutron captures take place too rapidly for the necessary β-decays to take place (for example, in 'a supernova), then some highly unstable neutron-rich isotopes are produced.... This is known as the r-process (for rapid). The observed nuclear abundances on Earth suggest that both the s-process and the r-process were involved in producing the material which made up the original solar system. |
A supernova explosion expels into interstellar space not only the elements formed inside the star, but elements forged in the supernova blast wave itself. In the explosion, nuclei are bombarded with neutrons, until elements all the way up to uranium are formed within seconds. All these heavier elements are then spread throughout the galaxy by the immense force of the supernova. Supernovas are responsible for changing the composition of gas from which each generation of stars form. Without supernova explosions, there would be no heavy elements in the interstellar gas. In particular, there would be no silicon to form rocky planets, no oxygen to form water, none of the elements we depend on here on Earth. It is the stuff of which we are made. The creation of new elements is a Big History Project threshold, so described [13].
When our sun runs out of fuel, its core will contract under its own gravity until it is no bigger than Earth, at which point, it will be supported by the force exerted by fast-moving electrons, called electron degeneracy pressure. The resulting object is called a white dwarf.
Stars 3 to 5 times the mass of the sun settle to a different final state: a neutron star. Formed when a star explodes and the core, mainly iron at this stage, collapses in on itself, neutron stars are only about 10-20 kms in diameter, but contain the mass of about 500,000 Earths. Neutron star gravity is about one billion times stronger than Earth gravity, crushing the protons and electrons in its own atoms together so they form neutrons, hence "neutron star", supported by the pressure not of electrons but of neutrons. That same gravity would destroy the Earth if a neutron star were to enter our planetary neighbourhood [14].
Neutron stars were originally postulated by astronomers Walter Baade and Fritz Zwicky in 1934 (only two years after British physicist James Chadwick had discovered the neutron), as an answer to the question of what might be left over after a supernova. For some time, there was much speculation about the concept. Then in 1967, Jocelyn Bell Burnell and her colleagues observed the first radio pulsars (PSR B1919+21 was the first) in 1967, which researchers postulated over the next year must be rapidly rotating neutron stars. Only then was the idea was widely accepted[15].
When our sun runs out of fuel, its core will contract under its own gravity until it is no bigger than Earth, at which point, it will be supported by the force exerted by fast-moving electrons, called electron degeneracy pressure. The resulting object is called a white dwarf.
Stars 3 to 5 times the mass of the sun settle to a different final state: a neutron star. Formed when a star explodes and the core, mainly iron at this stage, collapses in on itself, neutron stars are only about 10-20 kms in diameter, but contain the mass of about 500,000 Earths. Neutron star gravity is about one billion times stronger than Earth gravity, crushing the protons and electrons in its own atoms together so they form neutrons, hence "neutron star", supported by the pressure not of electrons but of neutrons. That same gravity would destroy the Earth if a neutron star were to enter our planetary neighbourhood [14].
Neutron stars were originally postulated by astronomers Walter Baade and Fritz Zwicky in 1934 (only two years after British physicist James Chadwick had discovered the neutron), as an answer to the question of what might be left over after a supernova. For some time, there was much speculation about the concept. Then in 1967, Jocelyn Bell Burnell and her colleagues observed the first radio pulsars (PSR B1919+21 was the first) in 1967, which researchers postulated over the next year must be rapidly rotating neutron stars. Only then was the idea was widely accepted[15].
"On 28 November 1967, (Bell) detected a "bit of scruff" on her chart-recorder papers that tracked across the sky with the stars. The signal had been visible in data taken in August, but as the papers had to be checked by hand, it took her three months to find it.[28] She established that the signal was pulsing with great regularity, at a rate of about one pulse every one and a third seconds. Temporarily dubbed "Little Green Man 1" (LGM-1) the source (now known as PSR B1919+21) was identified after several years as a rapidly rotating neutron star. This was later documented by the BBC Horizon series.[29] In a 2020 lecture at Harvard, she related how the media was covering the discovery pulsars, with interviews taking a standard 'disgusting' format: Hewish (Bell's Ph D supervisor, who received a Nobel prize for the discovery) would be asked on the astrophysics, and she would be the 'human interest' part, asked about vital statistics, how many boyfriends she had, what colour is her hair, and asked to undo some buttons for the photographs.[30] The Daily Telegraph science reporter shortened 'pulsating radio source' to pulsar." Wikipedia, https://en.wikipedia.org/wiki/Jocelyn_Bell_Burnell
Neutron stars have a slightly gaseous atmosphere above a thin crust layer made of heavy atomic nuclei and some floating electrons. What lays beneath is a matter of speculation. One theory is that particles in the inner core are squeezed in so tight that hey form a so-called superfluid that flows without resistance. Another that some form of exotic quarks come into existence.[16]
Still more massive stars cannot settle either to a white dwarf or to a neutron star because these forms of pressure are just not sufficient. Unless some other unknown form of pressure comes into play, gravitational collapse becomes unstoppable. Gravity is now the sole operative force, and the final fate of the star is determined by Einstein’s theory of gravitation. The theory indicated that the outcome is a singularity – either a clothed singularity, otherwise known as black hole or a naked singularity, which still remains visible to an outside observer.
Recent research tends to indicate that supermassive black holes may inhibit star formation.
# This page has changed in appearance and content over the years. Its original formulation was the product of Dr Helen Johnston's Continuing Education course Origins: From the Big Bang to Life, 16.3.11, Sydney University Physics Department, and published with her kind permission. A full account may also be found in Chapter 3 of Caleb Scharf's The Zoomable Universe, Scientific American (2017). Other sources are as noted in the narrative.
[1] Pankaj S. Joshi, “Naked Singularities”, Scientific American – Special Collector’s edition, op cit, August 2013, 74 at 81.
[1.5] John Gribbin, "Alone in the Milky Way - Why we are probably the only intelligent life in the galaxy", Scientific American, September 2015, 86-91, at 88.
[2] Edward J Weiler, Hubble – A Journey through Space and Time, Abrams, New York, 29.
[3] A light year is the distance covered by light traveling in empty space for year. It is about 9.5 trillion kilometers, or 5.8 trillion miles or 63 times the average distance from Sun to earth.
[4] ‘It’s cool, it’s a gas, it’s the birthplace of the stars’, SMH, 11 Nov 2005. See also ‘Astronomers spot ‘super-earth’ 80 light years away’, SMH, 9 Jan 2010.
[5] From Earth to the Universe, Power House Museum exhibition, Sydney, op cit
[6] Greene (2005), 354.
[7] An O-type star is a main-sequence (hydrogen-burning) star of spectral type O and luminosity class V. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and 52,000 K. They are between 30,000 and 1,000,000 times as luminous as the Sun. These stars are rare; it is estimated that there are no more than 20,000 in the entire Milky Way: http://en.wikipedia.org/wiki/O-type_main_sequence_star
[8] Associate Professor Michael Box, WEA course: "What are atoms made of? Session 5, segment 5.1.6: "Beyond silicon".
[9] Daniel Kasen, “Stellar Fireworks”, Scientific American, June 2016, 28 at 31; George Dodd, op cit, 5 at 7.
[10] Ibid. Type 1a supernovae are considered later.
[11] Associate Professor Michael Box, op.cit.
[12] Explained on the page Radioactivity and radioactive decay
[13] https://www.bighistoryproject.com/chapters/1#new-elements
[14] George Dodd, "Incredible Star Collision"', SAM, Sydney Alumni Magazine, Issue 07, Semester 1 2018, 5 at 7.
[15] Clara Moskowitz, "The inner lives of neutron stars", Scientific American, March 2019, 18 at 20. Jocelyn Bell's role in the discovery of pulsar PSR B 1919+21 is also explored in Einstein and Hawking, Masters of the Universe, (2 Parts), BBC 2019; precis at https://in.mashable.com/science/10936/einstein-and-hawking-masters-of-our-universe-tells-a-brief-history-of-relativity
[16] Ibid.
Still more massive stars cannot settle either to a white dwarf or to a neutron star because these forms of pressure are just not sufficient. Unless some other unknown form of pressure comes into play, gravitational collapse becomes unstoppable. Gravity is now the sole operative force, and the final fate of the star is determined by Einstein’s theory of gravitation. The theory indicated that the outcome is a singularity – either a clothed singularity, otherwise known as black hole or a naked singularity, which still remains visible to an outside observer.
Recent research tends to indicate that supermassive black holes may inhibit star formation.
# This page has changed in appearance and content over the years. Its original formulation was the product of Dr Helen Johnston's Continuing Education course Origins: From the Big Bang to Life, 16.3.11, Sydney University Physics Department, and published with her kind permission. A full account may also be found in Chapter 3 of Caleb Scharf's The Zoomable Universe, Scientific American (2017). Other sources are as noted in the narrative.
[1] Pankaj S. Joshi, “Naked Singularities”, Scientific American – Special Collector’s edition, op cit, August 2013, 74 at 81.
[1.5] John Gribbin, "Alone in the Milky Way - Why we are probably the only intelligent life in the galaxy", Scientific American, September 2015, 86-91, at 88.
[2] Edward J Weiler, Hubble – A Journey through Space and Time, Abrams, New York, 29.
[3] A light year is the distance covered by light traveling in empty space for year. It is about 9.5 trillion kilometers, or 5.8 trillion miles or 63 times the average distance from Sun to earth.
[4] ‘It’s cool, it’s a gas, it’s the birthplace of the stars’, SMH, 11 Nov 2005. See also ‘Astronomers spot ‘super-earth’ 80 light years away’, SMH, 9 Jan 2010.
[5] From Earth to the Universe, Power House Museum exhibition, Sydney, op cit
[6] Greene (2005), 354.
[7] An O-type star is a main-sequence (hydrogen-burning) star of spectral type O and luminosity class V. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and 52,000 K. They are between 30,000 and 1,000,000 times as luminous as the Sun. These stars are rare; it is estimated that there are no more than 20,000 in the entire Milky Way: http://en.wikipedia.org/wiki/O-type_main_sequence_star
[8] Associate Professor Michael Box, WEA course: "What are atoms made of? Session 5, segment 5.1.6: "Beyond silicon".
[9] Daniel Kasen, “Stellar Fireworks”, Scientific American, June 2016, 28 at 31; George Dodd, op cit, 5 at 7.
[10] Ibid. Type 1a supernovae are considered later.
[11] Associate Professor Michael Box, op.cit.
[12] Explained on the page Radioactivity and radioactive decay
[13] https://www.bighistoryproject.com/chapters/1#new-elements
[14] George Dodd, "Incredible Star Collision"', SAM, Sydney Alumni Magazine, Issue 07, Semester 1 2018, 5 at 7.
[15] Clara Moskowitz, "The inner lives of neutron stars", Scientific American, March 2019, 18 at 20. Jocelyn Bell's role in the discovery of pulsar PSR B 1919+21 is also explored in Einstein and Hawking, Masters of the Universe, (2 Parts), BBC 2019; precis at https://in.mashable.com/science/10936/einstein-and-hawking-masters-of-our-universe-tells-a-brief-history-of-relativity
[16] Ibid.