Our own Milky Way galaxy, long considered a “little sister” to the larger Andromeda Galaxy, is all grown-up, according to new research presented today that shows the Milky Way to be bigger and faster than previously thought.
The findings, presented at a meeting of the American Astronomical Society in Long Beach, Calif., by Harvard-Smithsonian Center for Astrophysics (CfA) researchers, show that the galaxy has about 50 percent more mass — about the same as Andromeda — and is rotating about 100,000 mph faster than previously thought.
Our solar system is located on an arm of the Milky Way about 28,000 light-years from its center.
“No longer will we think of the Milky Way as the little sister of the Andromeda Galaxy in our Local Group family,” said Mark Reid of the CfA.
The larger stature has a downside, however. With greater mass, our galaxy exerts greater gravitational pull than previously thought, which increases the likelihood of collisions with the Andromeda galaxy or smaller nearby galaxies.
New findings on planet and star formation
Harvard-Smithsonian researchers also presented new findings on star and planet formation at the meeting. New research led by Thayne Currie of the CfA shows that giant planets like Jupiter must form relatively quickly out of the material surrounding a new star because that material is all but exhausted in 5 million years.
Other findings on star formation, presented by the CfA’s Elizabeth Humphreys, show that stars can form near the Milky Way’s center despite the presence of a black hole equal to the mass of 4 million of our suns. Researchers had been puzzled at the presence of young stars near the galaxy’s center because they had thought the black hole’s intense gravitational fields would shred the molecular clouds from which stars form. By identifying two protostars just a few light-years from the galaxy’s center, Humphreys and colleagues at the CfA and the Max Planck Institute for Radio Astronomy in Germany confirmed that stars do in fact form there.
“We literally caught these stars in the act of forming,” Humphreys said.
A bigger, faster Milky Way
Scientists made the discoveries about the Milky Way’s mass and speed using the National Science Foundation’s Very Long Baseline Array (VLBA) radio telescope. Taking advantage of the VLBA’s unparalleled ability to make extremely detailed images, the team is conducting a long-term program to measure distances and motions in our galaxy.
The scientists focused their attention on regions of prolific star formation across the galaxy. In areas within these regions, gas molecules are strengthening naturally occurring radio emissions in the same way that lasers strengthen light beams. These areas, called cosmic masers, serve as bright landmarks for the sharp radio vision of the VLBA. By observing these regions repeatedly at times when the Earth is at opposite sides of its orbit around the sun, the astronomers can measure the slight apparent shift of the object’s position against the background of more distant objects.
“The new VLBA observations of the Milky Way are producing highly accurate direct measurements of distances and motions,” said Karl Menten of the Max Planck Institute for Radio Astronomy, a member of the team. “These measurements use the traditional surveyor’s method of triangulation and do not depend on any assumptions based on other properties, such as brightness, unlike earlier studies.”
The astronomers found that their direct distance measurements differed from earlier, indirect measurements, sometimes by as much as a factor of two. The star-forming regions harboring the cosmic masers “define the spiral arms of the galaxy,” Reid explained. Measuring the distances to these regions thus provides a yardstick for mapping the galaxy’s spiral structure.
“These direct measurements are revising our understanding of the structure and motions of our galaxy,” Menten said. “Because we’re inside it, it’s difficult for us to determine the Milky Way’s structure. For other galaxies, we can simply look at them and see their structure but we can’t do this to get an overall image of the Milky Way. We have to deduce its structure by measuring and mapping,” he added.
The VLBA can fix positions in the sky so accurately that the actual motion of the objects can be detected as they orbit the Milky Way’s center. Adding in measurements of motion along the line of sight, determined from shifts in the frequency of the masers’ radio emissions, the astronomers are able to determine the full 3-D motions of the star-forming regions. Using this information, Reid reported that “most star-forming regions do not follow a circular path as they orbit the galaxy; instead we find them moving more slowly than other regions and on elliptical, not circular, orbits.”
The researchers attribute this to what they call spiral density-wave shocks, which can take gas in a circular orbit, compress it to form stars, and cause it to go into a new elliptical orbit. This, they explained, helps to reinforce the spiral structure.
Reid and his colleagues found other surprises, too. Measuring the distances to multiple regions in a single spiral arm allowed them to calculate the angle of the arm. “These measurements,” Reid said, “indicate that our galaxy probably has four, not two, spiral arms of gas and dust that are forming stars.” Recent surveys by NASA’s Spitzer Space Telescope suggest that older stars reside mostly in two spiral arms, raising a question of why the older stars don’t appear in all the arms. Answering that question, the astronomers say, will require more measurements and a deeper understanding of how the galaxy works.
The VLBA, a system of 10 radio-telescope antennas stretching from Hawaii to New England to the Caribbean, provides the best ability to see the finest detail, called “resolving power,” of any astronomical tool in the world. The VLBA can routinely produce images hundreds of times more detailed than those produced by the Hubble Space Telescope. The VLBA’s tremendous resolving power, equal to being able to read a newspaper in Los Angeles from the distance of New York, is what permits the astronomers to make precise distance determinations.
The findings on planet formation were made by astronomers examining the 5-million-year-old star cluster NGC 2362. Using the Spitzer Space Telescope, which can detect the signatures of actively forming planets in infrared light, they found that all stars with the mass of the sun or greater have lost their protoplanetary (planet-forming) disks. Only a few stars less massive than the sun retain their protoplanetary disks. These disks provide the raw material for forming gas giants such as Jupiter. Therefore, gas giants have to form in less than 5 million years or they probably won’t form at all.
“Even though astronomers have detected hundreds of Jupiter-mass planets around other stars, our results suggest that such planets must form extremely fast. Whatever process is responsible for forming Jupiters has to be incredibly efficient,” said lead researcher Currie.
Even though nearly all gas giant-forming disks in NGC 2362 have disappeared, several stars in the cluster have “debris disks,” which indicates that smaller rocky or icy bodies such as Earth, Mars, or Pluto may still be forming.
“The Earth got going sooner, but Jupiter finished first, thanks to a big growth spurt,” explained co-author Scott Kenyon.
Kenyon added that while Earth took about 20 to 30 million years to reach its final mass, Jupiter was fully grown in only 2 to 3 million years.
Stars in the center
Astronomers studying star formation in the Milky Way’s center have struggled with a paradox. The young stars must have gotten there somehow, but the powerful gravitational tides stirred by a 4-million-solar-mass black hole should rip apart molecular clouds that act as stellar nurseries. Yet the alternative — stars falling inward after forming elsewhere — should be a rare occurrence.
Using the Very Large Array of radio telescopes, astronomers from the CfA and the Max Planck Institute identified two protostars located only a few light-years from the galactic center. Their discovery shows that stars can, in fact, form very close to the Milky Way’s central black hole.
The center of the Milky Way is a mysterious region hidden behind intervening dust and gas, making it hard to study. Visible light doesn’t make it out, leaving astronomers no choice but to use other wavelengths like infrared and radio, which can penetrate dust more easily.
Humphreys and her colleagues searched for water masers —radio signals that serve as signposts for protostars still embedded in their birth cocoons. They found two protostars located seven and 10 light-years from the galactic center. Combined with one previously identified protostar, the three examples show that star formation is taking place near the Milky Way’s core.
Their finding suggests that molecular gas at the center of our galaxy must be denser than previously believed. A higher density would make it easier for a molecular cloud’s self-gravity to overcome tides from the black hole, allowing it to not only hold together but also collapse and form new stars.
The discovery of these protostars corroborates recent theoretical work in which a supercomputer simulation produced star formation within a few light-years of the Milky Way’s central black hole.
“We don’t understand the environment at the galactic center very well yet,” Humphreys said. “By combining observational studies like ours with theoretical work, we hope to get a better handle on what’s happening at our galaxy’s core. Then, we can extrapolate to more distant galaxies.”
Humphreys’ co-authors are Reid and Menten.