Milky Way Grew From the Inside Out
G. Stinson (MPIA)
This image shows the latest results as colored dots superimposed on an artist’s conception of the Milky Way. Red dots show stars that formed when the Milky Way was young and small, while blue shows stars that formed more recently, when the Milky Way was big and mature. The color scale shows how many billion years have passed since those stars formed.
, this week, astronomers using data from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer (WISE) missions presented their findings of a special class of star and their associated "bow shocks." Here are a few of the stunning examples of "runaway stars" and their impact on interstellar gases.
Image: As seen by Spitzer, the star Kappa Cassiopeiae (HD 2905) is generating a dazzling infrared bow shock. The star's stellar wind and magnetic field are hitting the thin interstellar gases, highlighting the star's direction of travel (toward the lower right of the image).
MORE: Top 10 Spitzer Nebula Stunners
As a boat cruises through the ocean, water at the bow of the vessel will be pushed against water flowing in the opposite direction. The resulting wave is descriptively known as a "bow wave," which trails away from the boat as it continues on its way. Now imagine a star, powering through the interstellar medium. That star is itself pumping out stellar gases. In the direction the star is travelling, the stellar winds buffer against the interstellar gases and a huge bow shock -- not too dissimilar to our boat analogy -- is generated. Depending on the conditions in the interstellar medium and the speed at which the star is moving, these shocks can be detected from Earth -- the heated gases are spotted via their infrared signature. This stunning Spitzer observation shows the striking detail that can be revealed in the bow shocks of some of the speediest stars. This particular example shows the star Zeta Ophiuchi (Zeta Oph) traveling at around 54,000 mph (24 kilometers
) relative to its surroundings. Runaways are a special type of star that is alone and the nature of their bow shock can reveal some information about their origins.MORE: Runaway Stars Die Lonely Deaths in Cosmic No Man's Land
This observation is also of Zeta Oph but imaged by the WISE mission. The infrared signature as seen by WISE is more diffuse than the Spitzer view as cooler dust and gas is being detected by the space telescope's filters. The size and shape of a particular star's bow shock also reveals some information about the star's mass and speed. Zeta Oph is around 20 times more massive than our sun, generating more powerful stellar winds and is traveling faster. Its bow shock will therefore be more dramatic than anything our sun can generate. "Some stars get the boot when their companion star explodes in a supernova, and others can get kicked out of crowded star clusters," said William Chick from the University of Wyoming in Laramie, at the AAS meeting. "The gravitational boost increases a star's speed relative to other stars."MORE: Hubble Spies Trailblazing Star Ripped from Stellar Nursery
NASA/JPL-Caltech/University of Wyoming
Interestingly, the researchers used archival data from both WISE and Spitzer to identify the presence of bow shocks throughout the galaxy, finding 200 candidate signals. Although some of the signals were in fact glowing star-formation nebulae, ground observatories confirmed that most were indeed caused by runaway stars. "We are using the bow shocks to find massive and/or runaway stars," said Henry "Chip" Kobulnicky, also from the University of Wyoming. "The bow shocks are new laboratories for studying massive stars and answering questions about the fate and evolution of these stars."MORE: Renegade Star Rips Through Space
NASA/JPL-Caltech/University of Wyoming
Another group of researchers, who also presented their results this week, are going about the "bow shock hunt" in a different way. "WISE and Spitzer have given us the best images of bow shocks so far," said Cintia Peri of the Argentine Institute of Radio Astronomy. "In many cases, bow shocks that looked very diffuse before, can now be resolved, and, moreover, we can see some new details of the structures." Peri's team is seeking out the speedy stars first and then finding their associate bow shocks after.MORE: Hubble Spots Speeding Rogue Star
NASA/JPL-Caltech/University of Wyoming
The diffuse glow of a star's bow shock can also be seen in this Spitzer observation. Of all the runaway stars identified in this new study, all ranged in mass from 8 to 30 solar masses.MORE: Tiny and Speedy: 'Homeless' Galaxies Ejected From Clusters
Bow shocks haven't only been detected by WISE and Spitzer. WISE's predecessor, NASA's Infrared Astronomical Satellite (IRAS), scanned the whole sky in 1983 and identified the first glowing bow shocks emanating from runaway stars. Other missions, including the Hubble Space Telescope, have also spied the phenomenon, but not just from the speedy stars. In this stunning observation by Hubble, a very young star called LL Ori was spied with a very pronounced bow shock. As young stars evolve furiously, they generate very powerful stellar winds that ram into the surrounding gases inside their star forming nebulae -- in this case inside the star factory in the heart of the Orion nebula. The result of the collision of supersonic gases powering through the nebula can also create bow shocks, sans runaway star.Hubble at 25: Space Telescope's Top Science Discoveries
Scientists have made a cosmic growth chart of the Milky Way galaxy, an innovative blending of data collected by the ongoing Sloan Digital Sky Survey and a new technique to determine the ages of stars.
As expected, the analysis shows the galaxy’s central disk formed from the inside out, with red giant stars as old as about 13 billion years clustered toward the center and younger stars about 1 billion years old closer to the disk’s edge, astronomer Melissa Ness, with the Max Planck Institute for Astronomy in Heidelberg, Germany, told reporters at the American Astronomical Society meeting in Kissimmee, Florida.
“What we’re able to do … is understand how our galaxy has formed in detail, looking at the dispersion of ages, the gradient of the ages, how the ages change as a function of both the height from the (disk’s) plane and the radius," Ness said. "It’s understanding the details of this inside-out formation that is now possible."
Unique to the survey is its age-dating technique, which is based on a star’s size. Ness and colleagues used high-quality Sloan survey spectra, which reveals a star’s chemistry, with optical data collected by NASA’s Kepler space telescope to develop a model that can be used to pinpoint a star’s age.
“This is somewhat revolutionary because ages have previously been considered very hard to get, particularly from stellar spectra. They’re important, but they’re difficult,” Ness said.
The key was a newly discovered relationship between a star’s age and its ratio of carbon-to-nitrogen, concentrations of which can be ferreted out by analyzing a star’s spectra.
Older red giant stars have the highest carbon-to-nitrogen ratios and younger stars the lowest, Ness said.
Working with a sample of 2,000 stars, scientists studied how the ratios change based on a star’s mass. They then incorporated the results into a computer model and used it to calculate masses and ages of all 70,000 red giant stars observed in Sloan’s APOGEE (Apache Point Observatory Galaxy Evolution Experiment) survey.
“We hope there will be many follow-up studies,” Ness told Discovery News. “We’ve only just determined the age catalog ourselves.”