1. Astro 101 Slide Set: Binary System Makes Near Miss Developed by the WISE team 0 Topic: Close pass of binary system Concepts: Solar neighborhood, red dwarfs, brown dwarfs, cosmic scale, gravitational interactions. Missions: WISE Coordinated by the NASA Astrophysics Forum An Instructor’s Guide for using the slide sets is available at the ASP website https://www.astrosociety.org/e ducation/resources-for-the- higher-education-audience/ https://www.astrosociety.org/e ducation/resources-for-the- higher-education-audience/

2. The Discovery 1 The diagram shows the trajectory of WISE 0720-0846 past the sun 70,000 years ago, as well as the distances of the four nearest stellar neighbors to the sun (and the years their distances were determined): the triple system Alpha Centauri/Proxima Centauri, Barnard’s Star, and newly discovered brown dwarfs. (The Oort Cloud boundary is the outer edge of the gigantic reservoir of icy leftovers surrounding the sun from which long-period comets originate. The distances are to scale, but the sizes are exaggerated.) Credit: Penn State University and P. Eisenhardt. A small binary system called WISE 0720-0846, discovered in 2013 in the data of the Wide Field Infrared Survey Explorer (WISE), has turned out to be a daredevil. The system, consisting of a red dwarf star and its brown dwarf companion, buzzed the sun 70,000 years ago, passing just 0.8 light years away and dipping into the outer Oort cloud, where icy leftovers from the solar system’s formation lurk. It is the sun’s closest known stellar encounter.

3. How was the Discovery Made? 2 Artist’s conception of WISE 0720-0846—also called Scholz’s Star after its discoverer, Ralf Dieter-Scholz, using WISE data—during its flyby of the solar system 70,000 years ago. The brown dwarf—a body too small to fuse hydrogen like a regular star—is in the foreground, the red dwarf star beyond. The sun appears as a bright star in the left background. Credit: Michael Osadciw/University of Rochester. After the binary was discovered in the WISE data, scientists studying it noted that although it was relatively close—20 light years away—it showed very little “tangential” motion—motion across our line of sight. Most of its motion was “radial”—along our line of sight, and it was moving away. By extrapolating backward the binary’s trajectory and velocity, they were able to determine that it had made a close pass of the sun—the closest yet known.

4. The Big Picture 3 Artist’s conception of a brown dwarf (foreground, with hypothetical moon.) Credit: NASA/JPL. Red and brown dwarfs are so small and dim that they cannot be detected at large distances. But scientists expect these “small fry” to be the most common kinds of stars. WISE’s all-sky infrared survey allows the nearest of these to the sun to be discovered. Assuming the sun’s neighborhood is typical, this allows scientists to better characterize these smallest members of our galaxy’s stellar population. And if these stars are among the most common kinds of stars, they are the most likely to pass near the sun. Finding all of the nearby stars gives us a benchmark on how common brown dwarfs really are, and the longer term implications for the dynamics of close encounters with the solar system and the Earth.

5. What are the Implications? 4 Space is so vast that the odds of one star actually colliding with another are very small. But close encounters can produce gravitational effects. If a passing star is massive enough and the encounter lasts long enough, its gravitational pull could dislodge Oort Cloud objects from their positions, sending some of them toward the sun in a “rain” of comets, risking collisions with the planets including the Earth. It could take millions of years for these comets to reach the inner solar system, long after the star has passed. Scientists calculate that the WISE 0720-0846 binary wasn’t massive enough and the encounter was too brief to cause such a comet rain—but we may not know for certain for two million years! Artist’s conception of a comet storm. Credit: NASA/JPL-Caltech.

6. Resources 5 Papers: http://arxiv.org/abs/1502.04655http://arxiv.org/abs/1502.04655 (Mamajek et al analysis paper) http://adsabs.harvard.edu/abs/2015ApJ...800L..17M http://adsabs.harvard.edu/abs/2014A%26A...561A.113Shttp://adsabs.harvard.edu/abs/2014A%26A...561A.113S and http://arXiv:1311.2716http://arXiv:1311.2716 (Sholz discovery paper) http://adsabs.harvard.edu/abs/2015AJ....149..104Bhttp://adsabs.harvard.edu/abs/2015AJ....149..104B and http://arXiv:1410.4288http://arXiv:1410.4288 (Burgasser et al measurements paper) News Stories: http://www.rochester.edu/newscenter/scholz-star/ http://www.upi.com/Science_News/2015/02/18/Neighboring-star-once- came-within-a-single-light-year-of-the-sun/3581424273371/ http://www.zmescience.com/space/observations/star-flyby-red-dwarf- 18022015/

7. Binary System Makes Near Miss BONUS CONTENT 6

8. When Stars Do Collide... The most likely scenario for stellar collisions occurs in binary systems—systems of two stars gravitationally bound to each other and which are already relatively close to each other. 7 For example, astronomers at Keele University are studying a red giant star that grew so large it collided with its companion, lost up to 90% of its mass, and became a new kind of pulsating star. NASA Goddard astronomers have simulated the collision of two neutron stars, which end up ripping each other apart and forming a black hole. The Swift spacecraft has recorded the high-energy blasts created when neutron stars collide, and follow-up studies with Hubble and other telescopes suggest that in such blasts matter reaches conditions extreme enough for the heaviest elements to synthesize--such as mercury, lead, and gold! Simulated view of a neutron star collision. Credit: NASA Goddard.