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The sun formed in a dark molecular cloud after a super nova shock wave triggered the collapse of the cloud. It heated up due to the energy of contraction until the initiation of nuclear fusion in the core and the main sequence phase. The sun was 20% cooler when the Earth formed and is slowly growing in luminosity. The main sequence phase will last for 9 billion years.
The sun formed in a stellar nursery along with tens of thousands of other stars. As with the Eagle nebula (stellar nursery), large stars formed first. When the large stars exploded as supernovae, they triggered the formation of the sun and other stars in an uncollapsed cloud section such as the Pillars of Creation (Figure 3‑11). The evidence that a supernova triggered the sun’s birth is the presence of nickel-60 in meteorites. Iron-60 is a short-lived isotope (half-life = 1.5 million years) emitted by supernovae that decay to cobalt-60, which then decays to nickel-60, which is not radioactive and is stable. Because scientists found nickel-60 in meteorites, they know that iron-60 was once in the meteorite. Scientists calculated that the fraction of iron-60 in solar system meteorites was 10 times the normal concentration of iron-60 in interstellar space, which indicates that one or more supernova exploded within a million years and less than 0.3 parsecs (0.9 light-years) away from of the protosun.
Once triggered by supernovae or other shocks, giant molecular clouds break into cloud cores that form protostars. Protostar formation from contracting cloud cores is a long process. As cloud cores contract, the energy of contraction increases the temperature of the gas. This increased energy would normally prevent further contraction; however, dust in the cloud radiates energy into interstellar space. Clouds in the early universe did not contain dust, which is why dark matter was necessary in the early universe to cause cloud contraction and form the first stars. As cloud cores contract, gravity increases and overcomes the increased pressure and temperature, allowing contraction to continue. The cloud core temperature eventually reaches 10,000 K and ionization begins, which is the formation of plasma by the dissociation of electrons from their nuclei (protons and neutrons). The conversion of hydrogen and helium gas to plasma consume energy, which means that the internal temperature and pressure do not rise as the core contracts. Thus, the core has no resistance to collapse and collapses catastrophically. For the sun, this catastrophic collapse phase took about a decade, and the molecular cloud core shrank from the diameter of Jupiter’s orbit to the diameter of Mercury’s orbit and formed the protosun. Once ionization ended and stopped absorbing energy, the core temperature of the protosun quickly rose to more 200,000 K, and the surface temperature rose to 3,500 K. At this point, the sun's core was not hot enough for nuclear fusion. Energy came from contraction.
After the rapid ionization and collapse stage, the core of the protosun was still not hot enough to burn hydrogen in nuclear fusion. The solar evolutionary track begins at point a in Figure 3‑17, the rapid ionization collapse phase. At that point, the protosun’s surface temperature was 3,500 K, and the luminosity (brightness) was between 100 and 1,000 times brighter than the sun. At this point, the sun’s core was not hot enough for nuclear fusion. Energy came from contraction.
During its first 10,000 years, the protosun shrank, and the luminosity dropped to approximately 100 times as bright as the current sun (point b in Figure 3‑17). The temperature of the outer part of the protosun remained approximately 3,500 K, but the outer part of the molecular cloud envelope that surrounded it was much cooler, which is why astronomers view this stage in the infrared. During the next 100,000 years (point c), the sun continued to shrink and the luminosity dropped to 10 times as bright as the sun, with the surface temperature slightly increasing. At the same time, the protoplanetary disk formed (third image in Figure 3‑18). After 100,000 years (point c in Figure 3‑17), the protosun entered the T-Tauri stage when the protostar emits vertical jets from the poles. During this period, planetesimals formed in the disk and solar wind gradually blew the envelope into interstellar space (4th image in Figure 3‑18). The temperature and radiation during this phase were unstable, with the high solar wind, flash heating, and x-ray emission from the protosun. The protosun emerged from its molecular cloud cocoon (envelope) and became visible at this stage, which classifies it as a young stellar object.
Figure 3‑17. Stellar evolutionary tracks on an HR diagram. The sun’s path to the main sequence line during the first 10 million years is shown in yellow. Credit: Lithopsian. Used here per CC BY-SA 3.0
Figure 3‑18. Standard Model of solar system formation. Credit NASA: Original image (right side) by Frank Shu. NASA.
The sun continued to contract for another nine million years before the core reached 10 million K and became hot enough to begin nuclear fusion (point d in Figure 3‑17). Hydrogen fusion eventually heated the internal core to 15 million K with a surface temperature of 6,000 K, approximately 30 million years after the ionization collapse phase. After the surface heated up to 6,000 K, there was another decrease in luminosity due to contraction (endpoint e in Figure 3‑17). Although planets had formed by this time, the solar system was still unstable with infrequent but catastrophic collisions between planetismals and planets during the first 200 million years. The collision that formed the moon took place after 30 million years.
Most stars are in binary systems, two stars orbiting each other, because cloud cores spin and break apart, forming two or more stars that orbit each other. Based on the circular rotation of the planets in a single plane in our solar system, scientists do not think that the core that formed the sun split into a binary star system. The Standard Model of solar system formation (Figure 3‑18) is that a single star (the sun) formed within a single molecular cloud core and the protoplanetary disk formed around it.
Figure 3‑19. Gravity and pressure equilibrium in a star.
The main sequence phase of the sun began 4.5 billion years ago and will continue for another 4.5 billion years. During the main sequence phase, stars like the sun have a relatively stable temperature because the outward radiation pressure from nuclear fusion is balanced by the inward gravitational pull. Stars tend to be stable because the rate of nuclear fusion (energy output) in a main-sequence star is a function of the pressure and temperature in the core. When nuclear fusion increases in the core, there is an increase in radiation energy. This causes the star to slightly expand and decrease in the nuclear fusion rate and temperature. When the outward pressure drops, the star compresses. This compression, in turn, increases the rate of nuclear fusion and drives up the core temperature and pressure and (Figure 3‑19).
The sun radiates an amazing amount of power into space: 4 x 1023 kW. A car engine might produce 100 kW. The sun produces more energy in 1 second than all the people on Earth would use in 100,000 years. The solar power that reaches Earth is 10,000 times greater than the world’s power requirements [1].
Figure 3‑20. Sun’s luminosity vs. time. Credit: RJ Hall. Used here per CC BY-SA 3.0
The sun’s energy output has increased by 20% over the billions of years since the sun formed because the sun’s radius increased by 10% while the surface temperature remained constant (Figure 3‑20). Scientists have calculated that if the sun delivered 20% less energy to the Earth during the first billion years, then the water on Earth’s surface should have been completely frozen. However, geologic evidence shows that there was liquid water on Earth during the early Archaean Eon. This discrepancy between observation and theory is called the Faint Young Sun paradox, which is discussed in more detail in Chapter 7.
As a long-lasting G star, the sun is ideal for intelligent life. Apparently, intelligent life required 4 to 5 billion years to appear on earth. The sun will go on steadily fusing hydrogen into helium for another 4.5 billion years during its 9-billion-year main sequence phase. The sun also does not have gamma ray bursts like small stars.
[1] Van den Heuvel, Chapter 2. The Sun’s Backyard: Our Solar System. In The Amazing Unity of the Universe, 2016. Springer International. Pg. 14
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Life cycle of the sun. Credit: ESO/S. Steinhöfel
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