Our story begins with a type of event called a nova (plural: novae). Novae are typically thought to be nuclear explosions on the surface of white dwarf stars. White dwarfs are the remains of stars that have used all of their nuclear fuel. If a white dwarf can collect enough hydrogen, such as a very near neighbor star, that hydrogen can begin an uncontrolled nuclear fusion reaction. The result is a dramatic brightening of the white dwarf (sometimes making the otherwise faint star visible to the naked eye). Careful measurements of novae tend to detect debris from the eruption expanding out into space at speeds of thousands of miles per second. The white dwarf escapes more or less unscathed, and usually begins collecting more hydrogen for another explosion decades to centuries later. [Note that novae are different from supernovae, which are tremendous explosions that mark the end of the life of a massive star or the explosion of an entire white dwarf; supernovae are far, far brighter and far more energetic than novae.]
Over the past few decades, a handful of novae have been seen, both in our Milky Way galaxy and in neighboring galaxies, that don't quite match these characteristics. Sometimes called "red novae", these are brighter than normal novae and spew out material at much slower speeds. They are also redder in color (hence the name). I've seen some vigorous debate as to whether red novae are similar to normal novae, or perhaps something different altogether. One of the most famous red novae was an even known as V838 Monocerotis, and the Hubble Space Telescope has taken a series of dramatic images of the flash of light from this event lighting up the dust in space surrounding the nova:
Image Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA) / H. Bond
In September of 2008, a nova appeared in the constellation Scorpius. Called V1309 Scorpii, it was soon recognized that this nove belonged to the exotic class of red novae. A group of astronomers led by Romuald Tylenda of the Nicolaus Copernicus Astronomical Center in Poland also recognized that V1309 Scorpii was located in the field of an experiment called OGLE. OGLE is an experiment that images certain interesting portions of the sky night after night, looking for brightening of stars due to the bending of light by gravity, called gravitational microlensing. Since it is impossible to predict which of the millions of stars in OGLE's sights will brighten until it actually happens, data is collected and stored on every visible star in the experiment's view.
So, Prof. Tylenda's group checked OGLE's database at the position of V1309 Scorpii, and they found that there was a star visible before the nova. This star varied in brightness every 1.4 days in a way that suggests the star was a contact binary star, two stars that orbit each other so closely that their outer layers are actually touching. The stars may look something like a peanut, like this. And over the six years that OGLE had been looking at the field containing this star, the period of the stars' orbits had decreased -- the stars were slowly moving closer together!
From 2002 to 2006, the variations in light from the star looked fairly normal. But in 2007, the variations changed. Instead of something that looked like two stars constantly passing in front of each other as seen from the Earth, the variations looked more like a single star that had one side hotter than the other. About a year after this, the nova occurred. And now that the nova has faded away, astronomers see what looks like a single star, with no eclipses, no hot and cool side of the star, just a single red star.
So, through hard work and more than a little luck, Professor Tylenda's team seems to have observed the merger of two stars into a single one. Before 2007, the two stars were circling each other, getting closer and closer together. Sometime in 2007, the stars got close enough that the two stars shared a single atmosphere. Finally, a year later, the centers of the two stars merged together, leading to a tremendous outburst of energy that looked similar to these curious red novae. And it seems clear that there was no white dwarf involved here.
There's still more work that needs to be done to confirm this description of V1309 Scorpii, so this hypothesis explaining this nova could be wrong. And there are some differences between V1309 Scorpii and the other known red novae, so perhaps other red novae are not merging stars.
The crucial evidence for cracking the case of V1309 Scorpii was the several years of astronomical images collected by the OGLE project. Now that we know what to look for, perhaps we can search existing databases for similar objects. Better yet, in a few years, a telescope called the LSST will begin taking series of images of the entire southern hemisphere sky, not just selected areas like OGLE has done. With some legwork and a little luck, LSST may discover dozens of future red novae so they can be studied in even more detail before the two stars merge together. The key is knowing what to look for, and Tylenda's team may have unearthed that important clue.
R. Tylenda, M. Hajduk, T. Kamiński, A. Udalski, I. Soszyński, M. K. Szymański, M. Kubiak, G. Pietrzyński, R. Poleski, Ł. Wyrzykowski, & K. Ulaczyk (2010). V1309 Scorpii: merger of a contact binary Astronomy & Astrophysics arXiv: 1012.0163v1