previous section), while the accretion events will trigger eruptions that appear as FUors. Such violent events are believed to end when no more gas remains between the two stars, and the two circumstellar disks have become a more stable circumbinary disk.
As seen in the case of FUors, seemingly odd behaviors in stars can be more easily explained when considering multiple stars in groups instead of thinking of stars as isolated objects.
Sisters of the Sun?
Given that stars appear to form close together and within small groups, it seems reasonable to think that our own Sun might have formed in a very similar environment. Traditional models have always depicted the solar system forming in isolation, involving only the Sun itself and the planets that formed around it. However, it is much more likely that the Sun began its life surrounded by other newly born stars, and there is some evidence that points to this more complex picture of the early solar system. If the Sun had formed in isolation, then its undisturbed circumstellar disk would be in the same plane as the Sun's equator. In other words, both the Sun and the disk of gas that formed the planets would both appear to rotate around the same axis. However, when actual measurements are made today, the mean plane of the planetary orbits is tilted from the Sun's rotation axis by 7 degrees. The close passage of a star could explain this tilt, as the passing star would have pulled on the disk with its enormous gravity. Stars rarely come close to one another in the vastness of interstellar space, but close encounters are probable when the stars are still young and clustered together in star-forming regions (Figure 1). Therefore, a tilt of the planetary orbital plane suggests that the Sun was very close to other stars in its early life. If this theory is correct, then it means that there are other stars out there, spread throughout the Galaxy, that formed from the same cloud as our own parent star: the sisters of the Sun.
As astronomers gain more insight into the early life of stars, a more dynamic picture of star formation is emerging. What was once thought to be a centralized and isolated process is now seen as decentralized and interactive. From dramatically shifting orbits, to large outflows and eruptions, young stars are proving to be much more active creatures than the older ones such as the Sun. In reflecting on the large frequency of multiple star systems, it raises an important question for the field of astrobiology: How would life develop on a world with two or more suns? The answer to such a question will come only with the continued study of multiple star systems, and their unique properties.
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Simulation of Bate et al. (2003):
Images of the Herbig-Haro outflows on the Hubble Site:
News Center of the Hubble Site:
accretion : The process of gas falling onto the central (proto)star and adding to the object's total mass.
astronomical unit (AU) : The Earth-Sun distance; approximately 150 million kilometers or 93 million miles.
binary : A system of two stars bound together by their mutual gravity.
bipolar outflow : The flow of material in two opposing directions from a central star. Most young stars go through a violent phase of ejecting material. In some, jets are seen moving away from the star at a speed of many hundreds of kilometers per second.
circumbinary disk : The flattened distribution of gas that orbits a pair of stars. It is composed of the remnants of the circumstellar disks that once orbited each individual star.
circumstellar disk : The flattened distribution of gas that orbits the protostar and is depleted of mass over time as a result of accretion and tidal forces.
class I object: A protostar still partly embedded in a gaseous envelope; less than a million years old.
class II object : A T Tauri star with a large circumstellar disk that is a few million years old.
class III object : A T Tauri star with a depleted circumstellar disk that is less than ten million years old
convection : The circulatory motion that occurs in a fluid at a nonuniform temperature because of the variation of its density and the action of gravity.
eccentricity : A measure of how far as orbit diverges from a circle, that is how elliptical it is.
FUor: An object characterized by a sudden jump in brightness, followed by a long period of slow dimming that is caused by large accretion events stimulated by close encounters in a binary star system.
Galaxy (capitalized): The spiral galaxy containing our solar system and all of the stars visible to the naked eye, often referred to as the Milky Way.
Herbig-Haro (HH) outflow : A large outflow of gas from a binary system caused by tidal interactions between the two stars.
interstellar medium (ISM) : The gas spread across the Galaxy that constitutes the raw material of star formation; composed of mostly hydrogen and helium.
infrared image : An image that is taken at infrared wavelengths between about 7 × 10-7 meters and 1 millimeter, which is just past the red end of the visible spectrum.
light-year : The distance that light travels during one year, approximately 9.5 trillion kilometers or 5.9 million miles.
main sequence : The population of mature stars whose cores are undergoing hydrogen fusion, which stabilizes them against gravitational collapse.
molecular cloud : A cold, dense collection of interstellar gas where stars can form.
molecular cloud core : The densest part of a molecular cloud that fragments under the force of gravity to begin the star formation process.
optical image : An image that is taken at visible wavelengths between about 4 × 10-7 and 7 × 10-7 meters.
periastron : The point in an elliptical orbit around a star that is closest to the star's center.
pre-main sequence (PMS) : The population of protostars that have not yet begun nuclear reactions within their cores.
protostar : A dense object at the center of a cloud core that will grow into a star.
stellar embryo : The gaseous envelope that collapses from the cloud core to form a protostar and its surrounding disk.
tidal stripping : The ability of a massive body to raise tides on another body. Tidal forces can bring about the destruction of a satellite orbiting a planet or a comet approaching too close to the Sun or a planet.
T Tauri star : A young star with a circumstellar disk and a bipolar outflow.
visual binary : A binary star in which the two components are sufficiently well separated to be individually detectable visually or photographically.