Free Astronomy Magazine March-April 2015

SPACE CHRONICLES A t a time when our earliest human ancestors had recent- ly mastered walking upright, the heart of our Milky Way galaxy underwent a titanic eruption, driv- ing gases and other material out- ward at 2 million miles per hour. Now, at least 2 million years later, astronomers are witnessing the aftermath of the explosion: billow- ing clouds of gas towering about 30,000 light-years above and below the plane of our galaxy. The enor- mous structure was discovered five years ago as a gamma-ray glow on the sky in the direction of the galac- tic center. The balloon-like features have since been observed in X-rays and radio waves. But astronomers needed NASA's Hubble Space Tele- scope to measure for the first time the velocity and composition of the mystery lobes. They now seek to calculate the mass of the material being blown out of our galaxy, which could lead them to deter- mine the outburst's cause from sev- eral competing scenarios. Astronomers have proposed two possible origins for the bipolar lobes: a firestorm of star birth at the Milky Way's center or the erup- tion of its supermassive black hole. Although astronomers have seen gas- eous winds, composed of streams of charged particles, emanating from Milky Way core drives wind at 2 million miles per hour by NASA the cores of other galaxies, they are getting a unique, close-up view of our galaxy's own fireworks. "When you look at the centers of other galaxies, the outflows appear much smaller because the galaxies are farther away," said Andrew Fox of the Space Telescope Sci- ence Institute in Balti- more, Maryland, lead researcher of the study. "But the outflowing clouds we're seeing are only 25,000 light-years away in our galaxy. We have a front-row seat. We can study the de- tails of these structures. We can look at how big the bubbles are and can measure how much of the sky they are cov- ering." Fox's results have been published in The Astro- physical Journal Letters and presented at the American Astronomical Society meeting in Se- attle, Washington. The giant lobes, dubbed Fermi Bubbles, initially were spotted using NA- SA's Fermi Gamma-ray Space Telescope. The detection of high-en- ergy gamma rays sug- gested that a violent event in the galaxy's core aggres- sively launched energized gas into space. To provide more information about the outflows, Fox used Hub- ble's Cosmic Origins Spectrograph (COS) to probe the ultraviolet light