The formation of our solar system: the sun #
# Unless otherwise indicated, this material is drawn from Dr Helen Johnston's Continuing Education course, Origins: From the Big Bang to Life, March 2011, Sydney University Physics Department (CCE, Origins).
Most stars like our sun are born in clusters, containing anywhere from a hundred to a million stars. However, these clusters do not last long, dispersing in only 10 million years and spreading their stars throughout the galactic disk. The sun was born 4.6 billion years ago. It is in an almost circular orbit around the galactic centre in the plane of the galaxy disk, travelling at 235 km/s. This means that since birth it has travelled around the Galaxy some 27 times, travelling more than 3 million light years in that time. Its birthplace would have been at about the same galactic distance in the direction of the Centaurus arm, at right:
There are actually some clues as to what that birth environment must have been like. The presence of the decay products of short-lived radioactive isotopes[1] like 60 Fe[2] (half-life 1.5 million years) means that the sun formed in a region where massive stars were ending their lives. By following one radioisotope, aluminium 26, researchers have been able to trace the sun's family tree back 3 generations. Aluminium 26 has a half life of 730,000 years and is found in meteorites dating back to the earliest days of the solar system and astronomers have assumed that it originated in a supernova that could have exploded near the sun when it was forming [2A]. A nearby supernova must have seeded the pre-solar nebula with these isotopes, which then condensed into rocks within a few million years. This means the sun must have been born in a region which was producing both low-mass and high-mass stars, like the Eagle nebula. Recent research has also indicated that the aluminium 26 could also have formed inside a massive star[2B]. On the other hand, the young Solar System cannot have been too close to hot, massive stars, or the galaxy disk would have been evaporated away at its outer edge by the fierce ultraviolet radiation.
The most likely scenario is that the sun was born in a cluster of 1000 –10,000 stars. A supernova explosion within a few parsecs[3] then enriched the forming disk with the chemicals of life – carbon, nitrogen, oxygen, sodium, iron. Some time later, after the giant planets formed, a close encounter with another star disrupted the orbits of some of the outer Kuiper belt bodies[4], and soon afterwards the sun left its birth cluster. The sun’s siblings are by now spread all over the galaxy, and it should be possible to identify them by looking for stars which have the same age, orbit, and chemical composition as the sun.
Along that path, in April 2018 the GALAH (GALactic Archaeology with HERMES) survey was successful in in obtaining the so-called DNA of some 350,000 stars in our Milky Way and among the extensive data obtained, the hope is to discover the sun’s lost siblings. The HERMES part of the project was launched in late 2013, and is an instrument attached to the Siding Springs Observatory near Coonabarabran, New South Wales, and designed to measure the abundance of 25 chemical elements in a range of stars with a view to tracking down their origins, and whether any have a similar chemical composition, size, temperature, motion and luminosity to our sun, and hence may have come into existence at the same time and in the same region and be its siblings. [5]
In 2014, an astronomer by the name of Iván Ramírez was engaged on a similar quest. Starting with about 30 candidates based on their chemical compositions and the speeds and directions that they travel through the Milky Way, Ramirez narrowed the field to just one star, called HD 162826, now about 110 light years away in the constellation Hercules. [6]
Another investigative tool, the Gaia satellite, is currently engaged in measuring the brightness and precise position of over a billion stars to produce the most detailed 3-D map ever made of the Milky Way, and in the process, it is also hoped to track down more of the sun’s siblings. It is hoped this survey may also tell astronomers more about the sun’s birth environment and its path since then through the Milky Way. [7].
Stars like the sun form from the collapse of a gas cloud. The cloud collapses from the inside out, increasing the temperature of the gas. Eventually the core, which contains most of the mass, reaches the temperature where hydrogen can begin fusing into helium. This produces enough energy to stop the collapse, and marks the birth of the star. Meanwhile, in the disk, planets are starting to form. They need to form quickly, because as the new sun ignites or “turns on”, its wind sweeps away any gas and dust which has not already been incorporated into planets. This would have taken between 3 and 10 million years, which puts a limit on how long it can have taken planets to form. The best evidence that the planets formed in a disk is the arrangement of their orbits: they all have orbits which are in the same direction, in the same plane, and concentric. [8]
[1] An isotope is one of two or more forms of the same element with the same number of protons, but differing numbers of neutrons in their nuclei. The stability of any atom's nucleus depends on the ratio of protons to neutrons. Many isotopes have a ratio of protons to neutrons that renders them unstable, and when some of the protons dislodge, they also lose energy and give off ionizing radiation, known as radioactivity. For radioactivity, see Radioactivity and radioactive decay and E=mc² in practice
[2] Fe is the chemical symbol for iron.
[2A] Rebecca Boyle, “The secret life of the Sun”, Scientific American, June 2018, 23-29 at 26.
[2B] Ibid.
[3] A parsec = 3.26 billion light years
[4] The soon-to-be-considered Kuiper belt is a disk-shaped region in the outer solar system lying beyond the orbit of Neptune containing thousands of small icy bodies, some of which are on highly elliptical orbits, periodically visiting the inner solar system as comets.
[5] Ken Croswell, “The sun’s siblings”, New Scientist, 7 April 2012, 40-41.
[6] Rebecca Boyle, op cit at 28.
[7] Ibid. On Gaia, see also Scientific American, June 2018, 18.
[8] https://www.bighistoryproject.com/chapters/2#our-solar-system The formation of elements is dealt with at https://www.bighistoryproject.com/chapters/1#new-elements
The most likely scenario is that the sun was born in a cluster of 1000 –10,000 stars. A supernova explosion within a few parsecs[3] then enriched the forming disk with the chemicals of life – carbon, nitrogen, oxygen, sodium, iron. Some time later, after the giant planets formed, a close encounter with another star disrupted the orbits of some of the outer Kuiper belt bodies[4], and soon afterwards the sun left its birth cluster. The sun’s siblings are by now spread all over the galaxy, and it should be possible to identify them by looking for stars which have the same age, orbit, and chemical composition as the sun.
Along that path, in April 2018 the GALAH (GALactic Archaeology with HERMES) survey was successful in in obtaining the so-called DNA of some 350,000 stars in our Milky Way and among the extensive data obtained, the hope is to discover the sun’s lost siblings. The HERMES part of the project was launched in late 2013, and is an instrument attached to the Siding Springs Observatory near Coonabarabran, New South Wales, and designed to measure the abundance of 25 chemical elements in a range of stars with a view to tracking down their origins, and whether any have a similar chemical composition, size, temperature, motion and luminosity to our sun, and hence may have come into existence at the same time and in the same region and be its siblings. [5]
In 2014, an astronomer by the name of Iván Ramírez was engaged on a similar quest. Starting with about 30 candidates based on their chemical compositions and the speeds and directions that they travel through the Milky Way, Ramirez narrowed the field to just one star, called HD 162826, now about 110 light years away in the constellation Hercules. [6]
Another investigative tool, the Gaia satellite, is currently engaged in measuring the brightness and precise position of over a billion stars to produce the most detailed 3-D map ever made of the Milky Way, and in the process, it is also hoped to track down more of the sun’s siblings. It is hoped this survey may also tell astronomers more about the sun’s birth environment and its path since then through the Milky Way. [7].
Stars like the sun form from the collapse of a gas cloud. The cloud collapses from the inside out, increasing the temperature of the gas. Eventually the core, which contains most of the mass, reaches the temperature where hydrogen can begin fusing into helium. This produces enough energy to stop the collapse, and marks the birth of the star. Meanwhile, in the disk, planets are starting to form. They need to form quickly, because as the new sun ignites or “turns on”, its wind sweeps away any gas and dust which has not already been incorporated into planets. This would have taken between 3 and 10 million years, which puts a limit on how long it can have taken planets to form. The best evidence that the planets formed in a disk is the arrangement of their orbits: they all have orbits which are in the same direction, in the same plane, and concentric. [8]
[1] An isotope is one of two or more forms of the same element with the same number of protons, but differing numbers of neutrons in their nuclei. The stability of any atom's nucleus depends on the ratio of protons to neutrons. Many isotopes have a ratio of protons to neutrons that renders them unstable, and when some of the protons dislodge, they also lose energy and give off ionizing radiation, known as radioactivity. For radioactivity, see Radioactivity and radioactive decay and E=mc² in practice
[2] Fe is the chemical symbol for iron.
[2A] Rebecca Boyle, “The secret life of the Sun”, Scientific American, June 2018, 23-29 at 26.
[2B] Ibid.
[3] A parsec = 3.26 billion light years
[4] The soon-to-be-considered Kuiper belt is a disk-shaped region in the outer solar system lying beyond the orbit of Neptune containing thousands of small icy bodies, some of which are on highly elliptical orbits, periodically visiting the inner solar system as comets.
[5] Ken Croswell, “The sun’s siblings”, New Scientist, 7 April 2012, 40-41.
[6] Rebecca Boyle, op cit at 28.
[7] Ibid. On Gaia, see also Scientific American, June 2018, 18.
[8] https://www.bighistoryproject.com/chapters/2#our-solar-system The formation of elements is dealt with at https://www.bighistoryproject.com/chapters/1#new-elements