[Widdekind]

Stars form by the gravitational collapse of Giant Molecular Clouds (GMC). As that cloud's gas and dust is consumed by the forming stars, or driven away by their Stellar Winds, the GMCis converted, into an Open Cluster of around one thousand young stars. Open Clusters are loosely gravitationally bound, like the GMC from which they formed, and the stars remain associated for around 300 million years (cf. the Sun's Galactic Orbital Period is about 200 million years)*.

* History Channel The Universe -- Milky Way Galaxy (TV) ; http://en.wikipedia.org/wiki/Open_cluster

Now, Long Period Comets are believed to appear, in the Inner Solar System, after a nearby star perturbs their distant orbit, far out in the Oort Cloud, some 50,000 AU away (~1/3rd Light Year)*.

* http://en.wikipedia.org/wiki/Oort_cloud

CONCLUSION (?): The early Solar System was associated with hundreds of nearby stellar siblings, which all formed from the same GMC "crib", about 4.6 billion years ago. For hundreds of millions of years, the sun, and its stellar siblings, stayed comparatively close together. Their mutual gravitational interactions could have perturbed the orbits of copious quantities of comets, sending them careening towards the core of the Solar System, where they eventually collided with the Earth, raining down for hundreds of millions of years. Perhaps this explains (part of) the Late Heavy Bombardment (~4.6 to 3.8 Ga).

[Geosciences-Forum.com]

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[Widdekind]

Identifying all of the Sun's "stellar siblings", which formed from the same GMC, about 4.6 billion years ago, could indicate which stars are most likely to have Habitable Planetary Systems, or, perhaps, even Inhabited Planetary Systems*.

* http://www.astronomyforum.net/showthread.php?t=84613

[Geosciences-Forum.com]

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[Widdekind]

About 160 million years ago, two giant Asteroids collided, in the Asteroid Belt. The fragments of these 50 km, and 150 km, Asteroids slowly sprayed out through space, forming the Baptistina Asteroid Family. About 100 million years later (~65 Ma), one of these pieces of debris slammed into the Earth, forming the Chicxulub Crater on the Yucatan Peninsula, and killing off the Dinosaurs, paving the way for the rise of Mammals, and Mankind*. Indeed, the initial collision (~160 Ma) likely caused a series of similar such collisions, for roughly 50 million years**.

* History Channel Jurassic Fight Club -- Armageddon (TV) ; http://www.npr.org/templates/story/story.php?storyId=14181539. The Chemical Fingerprint of the Chicxulub Impactor closely matches those of the Baptistina Family of Carbonaceous Chondrite Asteroids.
** http://en.wikipedia.org/wiki/Baptistina_family

CONCLUSION: It took about 100 million years, for the effects of a collision, comparatively close by, in the Asteroid Belt, only a few AU away, to play out upon the Earth. This strongly suggests, that it could conceivably take many hundreds of millions of years, for gravitationally disturbed Comets, out in the Oort Cloud, roughly 50,000 AU away, to play out upon the Earth. This could explain why the Late Heavy Bombardment (~4.6 to 3.8 Ga) continued for hundreds of millions of years after the Sun's Open Cluster was dispersed (~4.3 Ga).