Recently, the Harvard-Smithsonian Center for Astrophysics discovered an extreme mass-ratio binary system that is especially interesting because their finding introduced a new class of binary stars: while one of the stars was fully formed, the other star was still in the process of formation.
Before we can appreciate what this discovery means, however, we need to have a better understanding of binary stars. Binary stars are a gravitationally bound pair of stars, meaning that the two stars are orbiting around each other without escaping or falling into their gravitational pull on each other. These stars are useful because we can use their orbits to find the mass of each star, and then knowing their masses, we can indirectly find their radii and densities. Most stars in our galaxy are in binary systems, and particularly, more massive stars tend to have companion stars. But stars in binary systems usually have similar masses, but not always. This is why astronomers have been looking for binary systems of stars with very different masses, called extreme mass-ratio binaries, so that they can better understand how binary stars are formed and perhaps how they evolve. To give you an idea of the difference in masses in extreme mass-ratio binary systems, the more massive stars tend to be from 6-16 times the mass of the sun, and the and the less massive stars are around 1-2 times the mass of the sun.
The discovery of extreme mass-ratio binaries is somewhat nuanced since the more massive star would be much brighter, causing it to outshine and therefore hide the lighter star in its overpowering light. So in order to see the lighter star, astronomers look for eclipsing systems since when the lighter star is directly in front of the more massive star, the total brightness seen drops significantly. But for this to happen, there must be a perfect alignment of the two stars with respect to Earth.
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| Image Credit: Robert Gendler and Josch Hambsch 2005 http://www.sciencedaily.com/releases/2015/02/150212102850.htm |
This recent discovery of the extreme mass-ratio binaries in formation, depicted in the image above, is especially important because, not only have we found another class of binary stars, we also get to observe the beginning and evolution of a binary system in order to better understand massive stars and binaries in general.
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