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 schematic 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 distances are to scale, but their directions are arbitrary and the star sizes are exaggerated.) Credit: Penn State University and P. Eisenhardt/J.D. Kirkpatrick. 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. It actually dipped 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 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 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. Scientists expect red and brown dwarfs to be the most common kinds of stars. But they are so small and dim that they cannot be detected at large distances. WISE’s all-sky infrared survey allows the nearest of these to the sun to be discovered. If the sun’s neighborhood is typical, this allows scientists to better characterize this stellar population. And if these stars are among the most common kinds, they are the most likely to pass near the sun. Finding all of the nearby stars gives us a benchmark on how common they really are, and a sense of the longer term implications of close encounters with the solar system and Earth.

5. What are the Implications? 4 Space is so vast that the odds of one star colliding with another are very small. But close encounters can produce gravitational effects. A passing star’s gravity could send Oort Cloud objects toward the sun in a “rain” of comets, risking collisions with the planets including 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.org/abs/1311.2716http://arXiv.org/abs/1311.2716 (Sholz discovery paper) http://adsabs.harvard.edu/abs/2015AJ....149..104Bhttp://adsabs.harvard.edu/abs/2015AJ....149..104B and http://arxiv.org/abs/1410.4288http://arxiv.org/abs/1410.4288 (Burgasser et al measurements paper) News Stories: http://www.rochester.edu/newscenter/scholz-star/

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 close. 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. 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.