I started out Monday with a press conference on exoplanets, and then followed it with a science session on the Milky Way. After that I attended a press conference about brown dwarfs, one about new observations of the Milky Way, and another about the International Year of Astronomy. I took more notes than any of you probably want to read, so I’ll just pick and choose some of the news stories that interested me.
Brown dwarfs — those odd spheres that don’t really fit in anywhere. They’re too cold and not massive enough to be full-fledged stars (they can’t convert hydrogen into helium in their cores), but they’re too massive and too hot to be planets. The upper mass limit for brown dwarfs is around 70 to 80 Jupiter masses (about 7 to 8 percent of the Sun’s mass). Because our stellar neighborhood (some 32.6 light-years) has a plethora of low-mass M-type red dwarf stars, astronomers had thought the same would be true of brown dwarfs. Well, it seems that’s just not the case. Various groups have used different techniques to locate nearby brown dwarfs, and they’re not finding many.
In the first study announced during the press conference, a group of astronomers led by Sergio Dieterich of Georgia State University observed 233 nearby multiple-star systems looking for brown-dwarf companions. They found only two brown dwarfs as companions. Those 233 stars are part of the Research Consortium on Nearby Stars (RECONS) survey. This survey is meant to create a census of the Sun’s neighbors within 32.6 light-years. The numbers are a bit lopsided: Astronomers know of some 239 red dwarfs but only 12 brown dwarfs.
In the press conference that focused on the Milky Way, three groups discussed their findings. One was a pretty picture release of an infrared panorama of the Milky Way. Another group, led by Elizabeth Humphreys of the Harvard Smithsonian Center for Astrophysics (CfA), looked for stars forming near our galaxy’s center. There’s been a long-standing mystery in astronomy about why young stars exist near our supermassive black hole. Tidal forces from the black hole would, theoretically, rip anything apart. How could collapsing clouds in this environment stay together to form a star? Some theories suggest the young stars form near the black hole; other theories suggest the stars form far away and somehow migrate in to the galaxy’s center.
Humphreys and colleagues used the Very Large Array of radio telescopes to look for a special type of microwave laser light (called “water masers”) within a 12 light-year radius around the central black hole. Because masers need an energetic source and particular conditions of gas pressure and density, they serve as signposts for protostars.
The astronomers detected two sites that are associated with gas clouds surrounding the black hole. One site is 7 light-years from the galactic center and the other 10 light-years. This finding suggests that young stars do in fact form very close to the central black hole. In addition, the density of some gas orbiting the black hole is 10 to 1,000 times denser than previously thought. This is further evidence that gas clouds collapse into protostars near the black hole.
Also at the Milky Way press conference, astronomers announced that our galaxy is actually rotating about 15 percent faster than previously thought. This revised speed means the galaxy must contain additional mass — 50 percent more, to be exact. That pushes the Milky Way’s size to the same size as Andromeda. We’re not the little sibling anymore!
The astronomers, led by Mark Reid of the CfA, used the Very Long Baseline Array radio telescopes to study massive star-forming regions in the Milky Way. By measuring distances to massive star-forming regions, the astronomers can trace the locations and number of spiral arms. They could also determine the motions of those star-forming regions, which tells them how fast the galaxy rotates. From the Milky Way’s rotation, they could then determine a mass.
The astronomers used a simple distance measurement, one that’s been around in astronomy for centuries: trigonometric parallax. This is the same effect you observe if you hold your finger out in front of your face at arms-length. Now close one eye, keeping the other open. Now switch which eye is open and which is closed. Do you notice how your finger appears to move compared to the background? That’s the same idea as Reid and colleagues used to measure the distances to star-forming regions. With this direct measurement, they learned the following about the Milky Way:
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They mapped how tightly wound the spiral arms are and found the arms can wrap around the center only once, which requires that there are four arms, not two.
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We’re about 27,000 light-years from galactic center.
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The Milky Way is rotating 600,000 miles per hour (254 km/s); which is about 100,000 mph faster than previously thought.
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We’re therefore about 50 percent more massive than originally thought (when incorporating both luminous and dark matter, the Milky Way now weighs in at some 3 trillion solar masses). So, it turns out our galaxy and Andromeda are pretty evenly matched.
Monday was a day full of announcements. And don’t forget about the IYA kickoff! Tune in to ustream.tv at 7:45 p.m. PST Tuesday night to watch the IYA 2009 official opening ceremony.