Introduction
About 4.6 billion years ago, an anonymous interstellar cloud began to collapse, forming many dense cores of dust and gas hidden under a thick, obscuring haze. The cores, perhaps a light year across, continued their slow gravitational implosion. As they spun faster and faster, their shapes changed from roundish globules to flattened pancakes and disks, with most of their mass falling into a central dense ball of contracting gas. Within a million years, the central core temperatures climbed above 10 million degrees and hydrogen atoms began to fuse into deuterium and helium at an accelerated pace. The enormous outward pressure provided by thermonuclear fusion quickly halted the further contraction of the fetal sun, and our sun became a full-fledged star for the first time. But the activity did not cease. As vestigial gas and dust continued to fall into the central core of the orbiting disk, it dragged with it magnetic fields left over from the distant interstellar cloud. The surface of the sun erupted in fantastic plumes and flares of solar energy as magnetic fields tangled and reconnected into newer shapes. The sun had become what astronomers call a T-Tauri star – one of the youngest recognized types of stars discovered over 50 years ago.
These magnetic fields and ejections of matter also caused the rapid spinning of the sun to slow down from its frantic pace of once-per-week to the more sedate monthly cycle we see today. Over the next few million years, a massive wind of ejected gas from the solar surface would blast through the surrounding disk of gas and dust, and clean out a zone nearly as large as our solar system. After 10 to 30 million years, the actions of these innumerable T-Tauri stars that had been born, would begin to dissipate most of the gas and dust in the original interstellar cloud that had been their nursery. As time passed, the powerful wind would slacken to a barely perceptible gale, and then a light blustery wind.
While the sun formed and began its long adolescence, out of the orbiting disk of gas and dust, incipient planet-building processes first created rocky asteroidal bodies, and through innumerable collisions, some of these bodies grew to embryonic size. At this point they grew even faster, not by chance collisions, but by exercising their gravitational pull to sweep-out the space around them like vacuum cleaners. Within 50,000 to 100,000 years the first planets to form were the gas giants, followed millions of years later by the smaller rocky planets in the inner solar system. Once formed, the giant planets felt the friction of the surrounding gaseous disk and they slowly began to fall closer and closer to the sun. Once the T-Tauri winds had completed their work, the frictional decay of orbits ceased and the giant planets took up their present orbital positions. Had the disk been too dense, or the solar winds too enfeebled, the stately progress of giant planets would have ended with their infall to the sun. Along the way, they would have ejected the planets from Mercury to Mars.
The T-Tauri Phase
Stars form out of large collections of gas and dust known as molecular clouds when the densest parts of these clouds collapse gravitationally. One consequence of this collapse is that young stars (T Tauri stars) are usually surrounded by massive, opaque, circumstellar disks. These disks gradually accrete onto the stellar surface, and thereby radiate energy both from the disk (infrared wavelengths), and from the position where material falls onto the star at (optical and ultraviolet wavelengths). Somehow a fraction of the material accreted onto the star is ejected perpendicular to the disk plane in a highly collimated stellar jet. The circumstellar disk eventually dissipates, probably when planets begin to form. Young stars also have dark spots on their surfaces which are analogous to sunspots but cover a much larger fraction of the surface area of the star. The T Tauri phase of stellar evolution lasts 1 - 10 million years. By studying these objects we gain some insight into what conditions must have been like in our solar system soon after the formation of our Sun, but before the formation of the Earth.
Click image for full-size view.
These NASA Hubble Space Telescope views of gaseous jets from three newly forming stars show a new level of detail in the star formation process, and are helping to solve decade-old questions about the secrets of star birth. Jets are a common "exhaust product" of the dynamics of star formation. They are blasted away from a disk of gas and dust falling onto an embryonic star. The scale in the bottom left corner of each picture represents 93 billion miles, or 1,000 times the distance between Earth and the Sun. All images were taken with the Wide Field Planetary Camera 2 in visible light. The HH designation stands for "Herbig-Haro" object — the name for bright patches of nebulosity which appear to be moving away from associated protostars. (Source: Hubblesite.org.)
Birth + 1 million years
Based on a study of young solar analogues [Feigelson etal, 2001] our sun started life as a weak T-Tauri star with the following properties. Its mass is 1 solar mass, but its radius is 2.5 Rsun and its luminosity is 1.7 Lsun .Its surface temperature is 4500 K and it has an effective spectral type of K5 IV. Its rotation rate is 25 km/sec compared with its surrent 2.0 km/sec and its rotation period is 5 days compared to its current 25. It is covered by sunspot regions with a total area of 25% of its surface, sompared to a maximum coverage today near sunspot maximum of 0.2%. Its X-ray luminosity is 10^30 ergs/sec compared to its modern level of 10^27 - 10^28 ergs/sec. As a Weak T-Tauri (WTT) star, the sun has just emerged from an enshrouding solar nebula in which it was in a 'Classic T-Tauri' (CTT) phase. It was during the CTT phase that planets were formed, and during the WTT phase, this process of planet building came to an end as the protoplanetary gas cloud was ejected. The fast rotation of the youn sun is based on considerable evidence that stars are born as fast rotators, and that initially some kind of magnetic braking slows their rotations by the WTT phase. A likely source of the magnetic braking is the solar magnetic field coupled to the protoplanetary gas cloud. Once the cloud dissipates, stars continue to slow down as a result of the coupling of rotation to convective zone evolution.
Stage III begins when infall onto the solar nebula has stopped but the viscous dissipation of the accretion disk is continuing. Meteoritic chondrites recovered from this time show millimeter-sized inclusions 'chondrules' embedded in a matrix. The matrix rarely appears to be directly derived from interstellar dust grains and have been processed in some way. Chondrules account for half the mass of the chrondrite so a large fraction of the solar nebula was processed by a chondrule 'factory'. The chondrules have been heated to temperatures well above the ambient solar nebula, and crystal sizes are small so they cooled in only a few minutes. Magnetic fields of 0.1-10 Gauss are also found embedded in them. This factory is consistent with powerful magnetic flares discharging energy and producing local heating and grain melting.
Birth+10 million years
According to studies of stars in young clusters such as the Alpha Persei cluster (50 million years), stars in the Pleiades (80 million years) such as HD129333 an early G-type star [Feigelson et al., 2001], the young sun probably continued to rotate much faster that at present, and that this fast rotation was correlated with enhanced solar activity and optical variability. Pleiades dwarfs in the K3 V - M0 V range experience rapid and extreme variability at levels of 5-15% and over hours to days. Some extreme jumps to 20-30% were also seen on occasion during multi-year studies. Rotation rates from 1/4 to1 day were also seen. Year to year changes in the activity level were also seen suggesting that the activity was modulated, but far faster than a solar activity cycle.
Birth+30 million years
Radioactive dating of meteoritic material and earth mantle radioisotopes tungsten-182 shows that the core of earth formed in a hurry. Previous estimates suggested 60 million years, but Alan Boss and Thorsten Klein show a faster formation time. Any tungsten-182 in the mantle must have formed by Hafnium-182 after the formation of the core and mantle. The level of tungsten-182 in meteorites from undifferentiated bodies was used as the comparison. This study also suggests a 13 milion year formation time for mars and a 3 million year formation time for Vesta. The moon however is as old as earth and this is consistent with its not being formed by accretion, but from the impact with an already formed earth. [Science News, 2002]
Birth+100 million years
The properties of solar analogues HD39587 (270 million years, G1 V), Hyades Cluster (660 million years) , Coma Bernices Cluster (600 million years) give us some insight into this period in solar evolution [Feigelson et al]. Brightness variations of 3%, rotation periods of 5 days, and activity cycles of about 6 year period. Hyades rotation curves show 6-8 day rotation periods for 19 stars. Rotational slow down is now caused primarily by magnetic braking against a solar wind.
Approximate model for sun. Hydrogen ignition occurred 4.5 Gyr ago. The young Sun started out with slightly different properties than we see today: 0.90 Rsun , 0.70 Lsun , 5586 K Young stars are known to be dimmer and cooler. The young sun would have been no more than 70 percent as bright as today.
Age of Solar System: 4.9 billion years
![A recent study in Earth and Planetary Science Letters from NAI’s Teams at NASA Goddard Space Flight Center, Carnegie Institution of Washington, and University of Wisconsin, shows that nucleic acids of extraterrestrial origin are present in the Murchison meteorite.]()
A recent study in Earth and Planetary Science Letters from NAI’s Teams at NASA Goddard Space Flight Center, Carnegie Institution of Washington, and University of Wisconsin, shows that nucleic acids of extraterrestrial origin are present in the Murchison meteorite.
Murchison Meteorites, like the one pictured to the left, have been dated by Cyril Ponnaperuma and his colleagues at Ames Research Center, and found to be 4.5 billion years old, making it the oldest known remnant of the pre-Earth solar system environment. More recent dating sets its age at nearly 4.95 billion years; nearly 500 million years older than the age of the Earth! A study of the contents of the available pieces [Hoppe et al, 1994] uncovered 720 silicon carbide dust grains embedded in the meteorite, and these seem to fall within several well-defined isotopic and chemical families. propose that these different families indicate separate stellar origins for the dust grains in very old stars on the so-called Asymptotic Giant Branch, and in Wolf-Rayet stars. We know that the interstellar medium is rich in dust grains from thousands of different astronomical sources spread over billions of years, so it is perhaps no wonder that our own solar nebula was contaminated by dust grains from many different sources over time. To find so many separate signatures in a single 200 pound meteorite is exciting!
4.58 billion
The maximum age for solar nebula/meteorites is between 4.53 and 4.58 billion years ago. The best age for the Earth comes not from dating individual rocks but by considering the Earth and meteorites as part of the same evolving system in which the isotopic composition of lead, specifically the ratio of lead-207 to lead-206 changes over time owing to the decay of radioactive uranium-235 and uranium-238, respectively. Scientists have used this approach to determine the time required for the isotopes in the Earth's oldest lead ores, of which there are only a few, to evolve from its primordial composition, as measured in uranium-free phases of iron meteorites, to its compositions at the time these lead ores separated from their mantle reservoirs. These calculations result in an age for the Earth and meteorites, and hence the Solar System, of 4.54 billion years with an uncertainty of less than 1 percent. To be precise, this age represents the last time that lead isotopes were homogeneous throughout the inner Solar System and the time that lead and uranium was incorporated into the solid bodies of the Solar System.
References
- "X-Ray Observations of Young Stars Address a Puzzle of the Solar System's Origin," Feigelson et al, 2001, Physics Today, Vol. 54, No. 11, p. 19.
- "Planetary Beginnings: Data reveal Earth's quick gestation," Two new studies confirm that Earth's core formed in a hurry—during the first 30 million years after the solar system's birth. Science News, August 31, 2002 p. 131.
- Hoppe, P. et al., 1994, Astrophysical Journal, vol. 430, page 870.
Related EoC Articles
External Links
- "The Evolution of Stars" - Educational Web Sites on Astronomy, Physics, Spaceflight and the Earth's Magnetism, David P. Stern, Ds.C.
Preview Image
As the early stages of star formation proceeds, the cloud tends to gather around the star in a more isolated manner, removed from neighboring gas and dust nebula. It may then enter the T Tauri phase at which the growing star starts to generate strong stellar winds. The cloud disk still can exceed 150 A.U. in dimension. This telescope image shows the glowing cloud (rendered here in blue, but actually of a different color) around the incipient, still poorly organized central star (a binary pair). (Source: NASA, GSFC, Remote Sensing Tutorial-Hubble Space Telescope.)
Citation
Odenwald, Sten, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Sun: Formation." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 26, 2007].
<http://www.cosmosportal.org/articles/view/138657/>
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Introduction
About 4.6 billion years ago, an anonymous interstellar cloud began to collapse, forming many dense cores of dust and gas hidden under a thick, obscuring haze. The cores, perhaps a light year across, continued their slow gravitational implosion. As they spun faster and faster, their shapes changed from roundish globules to flattened pancakes and disks, with most of their mass falling into a central dense ball of contracting gas. Within a million years, the central core temperatures climbed above 10 million degrees and hydrogen atoms began to fuse into deuterium and helium at an accelerated pace. The enormous outward pressure provided by thermonuclear fusion quickly halted the further contraction of the fetal sun, and our sun became a full-fledged star for the first time. But the activity did not cease. As vestigial gas and dust continued to fall into the central core of the orbiting disk, it dragged with it magnetic fields left over from the distant interstellar cloud. The surface of the sun erupted in fantastic plumes and flares of solar energy as magnetic fields tangled and reconnected into newer shapes. The sun had become what astronomers call a T-Tauri star – one of the youngest recognized types of stars discovered over 50 years ago.
These magnetic fields and ejections of matter also caused the rapid spinning of the sun to slow down from its frantic pace of once-per-week to the more sedate monthly cycle we see today. Over the next few million years, a massive wind of ejected gas from the solar surface would blast through the surrounding disk of gas and dust, and clean out a zone nearly as large as our solar system. After 10 to 30 million years, the actions of these innumerable T-Tauri stars that had been born, would begin to dissipate most of the gas and dust in the original interstellar cloud that had been their nursery. As time passed, the powerful wind would slacken to a barely perceptible gale, and then a light blustery wind.
While the sun formed and began its long adolescence, out of the orbiting disk of gas and dust, incipient planet-building processes first created rocky asteroidal bodies, and through innumerable collisions, some of these bodies grew to embryonic size. At this point they grew even faster, not by chance collisions, but by exercising their gravitational pull to sweep-out the space around them like vacuum cleaners. Within 50,000 to 100,000 years the first planets to form were the gas giants, followed millions of years later by the smaller rocky planets in the inner solar system. Once formed, the giant planets felt the friction of the surrounding gaseous disk and they slowly began to fall closer and closer to the sun. Once the T-Tauri winds had completed their work, the frictional decay of orbits ceased and the giant planets took up their present orbital positions. Had the disk been too dense, or the solar winds too enfeebled, the stately progress of giant planets would have ended with their infall to the sun. Along the way, they would have ejected the planets from Mercury to Mars.
The T-Tauri Phase
Stars form out of large collections of gas and dust known as molecular clouds when the densest parts of these clouds collapse gravitationally. One consequence of this collapse is that young stars (T Tauri stars) are usually surrounded by massive, opaque, circumstellar disks. These disks gradually accrete onto the stellar surface, and thereby radiate energy both from the disk (infrared wavelengths), and from the position where material falls onto the star at (optical and ultraviolet wavelengths). Somehow a fraction of the material accreted onto the star is ejected perpendicular to the disk plane in a highly collimated stellar jet. The circumstellar disk eventually dissipates, probably when planets begin to form. Young stars also have dark spots on their surfaces which are analogous to sunspots but cover a much larger fraction of the surface area of the star. The T Tauri phase of stellar evolution lasts 1 - 10 million years. By studying these objects we gain some insight into what conditions must have been like in our solar system soon after the formation of our Sun, but before the formation of the Earth.
Click image for full-size view.
These NASA Hubble Space Telescope views of gaseous jets from three newly forming stars show a new level of detail in the star formation process, and are helping to solve decade-old questions about the secrets of star birth. Jets are a common "exhaust product" of the dynamics of star formation. They are blasted away from a disk of gas and dust falling onto an embryonic star. The scale in the bottom left corner of each picture represents 93 billion miles, or 1,000 times the distance between Earth and the Sun. All images were taken with the Wide Field Planetary Camera 2 in visible light. The HH designation stands for "Herbig-Haro" object — the name for bright patches of nebulosity which appear to be moving away from associated protostars. (Source: Hubblesite.org.)
Birth + 1 million years
Based on a study of young solar analogues [Feigelson etal, 2001] our sun started life as a weak T-Tauri star with the following properties. Its mass is 1 solar mass, but its radius is 2.5 Rsun and its luminosity is 1.7 Lsun .Its surface temperature is 4500 K and it has an effective spectral type of K5 IV. Its rotation rate is 25 km/sec compared with its surrent 2.0 km/sec and its rotation period is 5 days compared to its current 25. It is covered by sunspot regions with a total area of 25% of its surface, sompared to a maximum coverage today near sunspot maximum of 0.2%. Its X-ray luminosity is 10^30 ergs/sec compared to its modern level of 10^27 - 10^28 ergs/sec. As a Weak T-Tauri (WTT) star, the sun has just emerged from an enshrouding solar nebula in which it was in a 'Classic T-Tauri' (CTT) phase. It was during the CTT phase that planets were formed, and during the WTT phase, this process of planet building came to an end as the protoplanetary gas cloud was ejected. The fast rotation of the youn sun is based on considerable evidence that stars are born as fast rotators, and that initially some kind of magnetic braking slows their rotations by the WTT phase. A likely source of the magnetic braking is the solar magnetic field coupled to the protoplanetary gas cloud. Once the cloud dissipates, stars continue to slow down as a result of the coupling of rotation to convective zone evolution.
Stage III begins when infall onto the solar nebula has stopped but the viscous dissipation of the accretion disk is continuing. Meteoritic chondrites recovered from this time show millimeter-sized inclusions 'chondrules' embedded in a matrix. The matrix rarely appears to be directly derived from interstellar dust grains and have been processed in some way. Chondrules account for half the mass of the chrondrite so a large fraction of the solar nebula was processed by a chondrule 'factory'. The chondrules have been heated to temperatures well above the ambient solar nebula, and crystal sizes are small so they cooled in only a few minutes. Magnetic fields of 0.1-10 Gauss are also found embedded in them. This factory is consistent with powerful magnetic flares discharging energy and producing local heating and grain melting.
Birth+10 million years
According to studies of stars in young clusters such as the Alpha Persei cluster (50 million years), stars in the Pleiades (80 million years) such as HD129333 an early G-type star [Feigelson et al., 2001], the young sun probably continued to rotate much faster that at present, and that this fast rotation was correlated with enhanced solar activity and optical variability. Pleiades dwarfs in the K3 V - M0 V range experience rapid and extreme variability at levels of 5-15% and over hours to days. Some extreme jumps to 20-30% were also seen on occasion during multi-year studies. Rotation rates from 1/4 to1 day were also seen. Year to year changes in the activity level were also seen suggesting that the activity was modulated, but far faster than a solar activity cycle.
Birth+30 million years
Radioactive dating of meteoritic material and earth mantle radioisotopes tungsten-182 shows that the core of earth formed in a hurry. Previous estimates suggested 60 million years, but Alan Boss and Thorsten Klein show a faster formation time. Any tungsten-182 in the mantle must have formed by Hafnium-182 after the formation of the core and mantle. The level of tungsten-182 in meteorites from undifferentiated bodies was used as the comparison. This study also suggests a 13 milion year formation time for mars and a 3 million year formation time for Vesta. The moon however is as old as earth and this is consistent with its not being formed by accretion, but from the impact with an already formed earth. [Science News, 2002]
Birth+100 million years
The properties of solar analogues HD39587 (270 million years, G1 V), Hyades Cluster (660 million years) , Coma Bernices Cluster (600 million years) give us some insight into this period in solar evolution [Feigelson et al]. Brightness variations of 3%, rotation periods of 5 days, and activity cycles of about 6 year period. Hyades rotation curves show 6-8 day rotation periods for 19 stars. Rotational slow down is now caused primarily by magnetic braking against a solar wind.
Approximate model for sun. Hydrogen ignition occurred 4.5 Gyr ago. The young Sun started out with slightly different properties than we see today: 0.90 Rsun , 0.70 Lsun , 5586 K Young stars are known to be dimmer and cooler. The young sun would have been no more than 70 percent as bright as today.
Age of Solar System: 4.9 billion years
![A recent study in Earth and Planetary Science Letters from NAI’s Teams at NASA Goddard Space Flight Center, Carnegie Institution of Washington, and University of Wisconsin, shows that nucleic acids of extraterrestrial origin are present in the Murchison meteorite.]()
A recent study in Earth and Planetary Science Letters from NAI’s Teams at NASA Goddard Space Flight Center, Carnegie Institution of Washington, and University of Wisconsin, shows that nucleic acids of extraterrestrial origin are present in the Murchison meteorite.
Murchison Meteorites, like the one pictured to the left, have been dated by Cyril Ponnaperuma and his colleagues at Ames Research Center, and found to be 4.5 billion years old, making it the oldest known remnant of the pre-Earth solar system environment. More recent dating sets its age at nearly 4.95 billion years; nearly 500 million years older than the age of the Earth! A study of the contents of the available pieces [Hoppe et al, 1994] uncovered 720 silicon carbide dust grains embedded in the meteorite, and these seem to fall within several well-defined isotopic and chemical families. propose that these different families indicate separate stellar origins for the dust grains in very old stars on the so-called Asymptotic Giant Branch, and in Wolf-Rayet stars. We know that the interstellar medium is rich in dust grains from thousands of different astronomical sources spread over billions of years, so it is perhaps no wonder that our own solar nebula was contaminated by dust grains from many different sources over time. To find so many separate signatures in a single 200 pound meteorite is exciting!
4.58 billion
The maximum age for solar nebula/meteorites is between 4.53 and 4.58 billion years ago. The best age for the Earth comes not from dating individual rocks but by considering the Earth and meteorites as part of the same evolving system in which the isotopic composition of lead, specifically the ratio of lead-207 to lead-206 changes over time owing to the decay of radioactive uranium-235 and uranium-238, respectively. Scientists have used this approach to determine the time required for the isotopes in the Earth's oldest lead ores, of which there are only a few, to evolve from its primordial composition, as measured in uranium-free phases of iron meteorites, to its compositions at the time these lead ores separated from their mantle reservoirs. These calculations result in an age for the Earth and meteorites, and hence the Solar System, of 4.54 billion years with an uncertainty of less than 1 percent. To be precise, this age represents the last time that lead isotopes were homogeneous throughout the inner Solar System and the time that lead and uranium was incorporated into the solid bodies of the Solar System.
References
- "X-Ray Observations of Young Stars Address a Puzzle of the Solar System's Origin," Feigelson et al, 2001, Physics Today, Vol. 54, No. 11, p. 19.
- "Planetary Beginnings: Data reveal Earth's quick gestation," Two new studies confirm that Earth's core formed in a hurry—during the first 30 million years after the solar system's birth. Science News, August 31, 2002 p. 131.
- Hoppe, P. et al., 1994, Astrophysical Journal, vol. 430, page 870.
Related EoC Articles
External Links
- "The Evolution of Stars" - Educational Web Sites on Astronomy, Physics, Spaceflight and the Earth's Magnetism, David P. Stern, Ds.C.
Preview Image
As the early stages of star formation proceeds, the cloud tends to gather around the star in a more isolated manner, removed from neighboring gas and dust nebula. It may then enter the T Tauri phase at which the growing star starts to generate strong stellar winds. The cloud disk still can exceed 150 A.U. in dimension. This telescope image shows the glowing cloud (rendered here in blue, but actually of a different color) around the incipient, still poorly organized central star (a binary pair). (Source: NASA, GSFC, Remote Sensing Tutorial-Hubble Space Telescope.)
Citation
Odenwald, Sten, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Sun: Formation." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 26, 2007].
<http://www.cosmosportal.org/articles/view/138657/>
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