<<On October 4, 2006, a team of astronomers announced the finding of evidence that Sol formed in a fragment (Solar nebula) of a giant molecular cloud (e.g., the Orion Cloud) of gas and dust that gave birth to a large open star cluster with hundreds to thousands of members. According to astronomer Leslie Looney, the evidence for Sol's stellar sisters was found in decayed particles from radioactive isotopes of iron trapped in meteorites, which can be studied as fossil traces of early Solar System conditions. The isotopic evidence indicates that a supernova from a massive star with the mass of at least 20 Solar-masses (probably a very rare, hot, and blue O-type star like Anitak Aa) exploded near the early Sun when it formed 4.6 billion years ago. Measured abundances of the isotopic particle species indicate that the supernova was located only about 0.32 to 5.22 light-years from Sol. Where there are supernovae or any massive star, there should have been hundreds to thousands of low-mass stars like the Sun that were born of the same nebula of gas and dust. Due to insufficient gravitational pull, Sol's surrounding cluster of stars dispersed over the past five billion years as they moved around the developing Milky Way galaxy, and members escaped the cluster due to velocity changes from close encounters with each other, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds crossing their way (press release; and Looney et al, 2006).
On May 24, 2007, a team of astronomers announced that the presence of an isotope of aluminium suggests the Sun was born when an extremely massive star with around 30 Solar-masses released a great amount of energy in winds loaded with aluminium-26. The strong winds of the massive star may have buffeted the Solar nebula sufficiently to initiate the development of the Solar System (Zeeya Merali, New Scientist, May 24, 2007; and Bizzarro et al, 2007; and Shukolyukov and Lugmair, 1993). In addition, the high average abundance of gold in the Solar System
suggests that large amounts of the element was injected into matter that eventually coalesced into the Solar nebula by the collision of two neutron stars in a short-duration gamma ray burst.
In a March 2009, draft pre-print, a computational astrophysicist argued that the chemical abundances found in the Solar System and the observed structure of the Edgeworth-Kuiper Belt constrain the initial mass and radius of Sol's star cluster of birth to between 500 and 3,000 Solar-masses (distributed among an estimated 1,500 to 3,500 stars) within a radius of 5 light-years (1.5 parsecs). Although the cluster dissolved over the past 4.6 billion years with the dispersal of the Sun's sibling stars into the surrounding the Milky Way, the stars should have remained on a similar orbit around the galactic center. While Sol's siblings now lie hidden among many millions of stars, 10 to 60 such stars should still be orbiting the galactic core within a distance of 300 light-years (100 parsecs). With the launch of the European Space Agency's GAIA astrometry mission now scheduled for 2012 (to gather positional and radial velocity measurements for a billion stars within five years and create a 3-dimensional galactic chart of the Milky Way), these sibling stars can be identified with accurate measurements of their level of heavy elements as well as positions and velocities of their motion within the galaxy, and the discovery of even a few such siblings should strongly constrain the original size and location of Sol's birth cluster (Simon P. Portegies Zwart, Scientific American, November 2009, pp. 40-47; and Simon P. Portegies Zwart, 2009).
On April 17, 2009, a team of scientists published a paper discussing their conclusions that the Sun formed from a well-mixed nebula containing dust and gas from two different kinds of supernovae. Titanium isotopes (Ti-46 and Ti-50) in meteorites from the Moon, Mars, and in inclusions found in some meteorites believed to be be the oldest rocks in the Solar System were found in very similar ratios, despite their origins in different types of supernovae. While T-46 (containing 22 protons and 24 neutrons) is believed to be created in core processes within massive collapsing stars (type-II supernovae), Ti-50 (also 22 protons but 28 neutrons) should be created, in theory, from the explosion of white dwarfs as Type Ia Supernovae after attracting too much material from a companion star. That these two isotopes from two sources are found consistently in similar ratios suggest that the Solar Nebula was very well mixed or the developing Solar System absorbed a stray cloud of dust that contained both isotopes (Trinquier et al, 2009; and Rachel Courtland, New Scientist, April 16, 2008).>>