New Milky Way map reveals stars in our galaxy move far from home

July 31, 2015, Pennsylvania State University

Scientists with the Sloan Digital Sky Survey-III (SDSS) have created a new map of the Milky Way that provides the first clear evidence of migration of stars throughout our galaxy. The study, which determined that 30 percent of stars have traveled across the galaxy, is bringing a new understanding of how stars are formed and travel throughout the Milky Way.

"We were able to measure the properties of nearly 70,000 in our galaxy for this particular study using the innovative SDSS infrared spectrograph," said Donald Schneider, Distinguished Professor of Astronomy and Astrophysics at Penn State and a coauthor of the study. "This exercise can be described as galactic archeology. These data reveal the locations, motions, and compositions of the stars, which provide insights into their formation and their history." Schneider is the SDSS-III survey coordinator and the project's scientific publications coordinator.

To build a map of the Milky Way, the scientists used the SDSS Apache Point Observatory Galactic Evolution Explorer (APOGEE) spectrograph to observe 100,000 stars during a 4-year campaign.

"In our modern world, many people move far away from their birthplaces, sometimes halfway around the world," said Michael Hayden of New Mexico State University (NMSU), the lead author of the new study. "Now we're finding the same is true of stars in our galaxy. About 30 percent of the stars in our galaxy have traveled a long way from where they were born." The key to creating and interpreting this new map is measuring the elements in the atmosphere of each star. "From the chemical composition of a star, we can learn its ancestry and life history," Hayden said.

The chemical information comes from spectra, detailed measurements of how much light the star gives off at different wavelengths. Spectra show prominent lines that correspond to elements and compounds. Reading the spectral lines of a star can tell astronomers the chemicals the star is made of.

"Stellar spectra show us that the chemical makeup of our galaxy is constantly changing," said Jon Holtzman, an astronomer at NMSU who was involved in the study. "Stars create heavier elements in their cores, and when the stars die, those heavier elements go back into the gas from which the next stars form."

As a result of this process of "chemical enrichment," each generation of stars has a higher percentage of heavier elements than the previous generation had. In some regions of the galaxy, star formation has proceeded more vigorously than in other regions—and in these more vigorous regions, more generations of new stars have formed. Thus, the average amount of heavier elements in stars varies among different parts of the galaxy. Astronomers then use the amount of heavy elements in a star to determine in what part of the galaxy the star was born.

Hayden and colleagues used APOGEE data to map the relative amounts of 15 separate elements, including carbon, silicon, and iron, for stars all over the galaxy. What they found surprised them—up to 30 percent of stars had compositions indicating that they were formed in parts of the galaxy far from their current positions.

"While on average the stars in the outer disk of the Milky Way have less heavy-element enrichment, there is a small fraction of stars in the outer disk that have heavier element abundances that are more typical of stars in the inner disk," said Jo Bovy of the Institute for Advanced Study and the University of Toronto, another member of the research team.

When the team looked at the pattern of element abundances in detail, they found that much of the data could be explained by a model in which stars migrate radially, moving closer or farther from the galactic center with time. These random in-and-out motions are referred to as "migration," and likely are caused by irregularities in the galactic disk, such as the Milky Way's famous spiral arms. Evidence of stellar migration previously had been seen in stars near the Sun, but the new study is the first clear evidence that migration occurs throughout the galaxy.

Future studies by astronomers using data from SDSS promise even more discoveries. "These latest results take advantage of only a small fraction of the available APOGEE data," said Steven Majewski, the Principal Investigator of APOGEE. "Once we unlock the full information content of APOGEE, we will understand the chemistry and shape of our galaxy much more clearly."

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1.3 / 5 (3) Jul 31, 2015
Evidence of stellar migration previously had been seen in stars near the Sun, but the new study is the first clear evidence that migration occurs throughout the galaxy.

This would likely work the same way as planetary migration, gravity assist ejecting a star from a more central position to an outer position, or throwing another star inward, but also have some additional contributing factors.

A supernova changes the local gravitational environment on time scales a thousands of years (see Crab Nebula,) by distributing 1/3 to 2/3rds of the progenitor star's mass throughout a vast volume tens of light-years across.This can provide both a mass change and a momentum change to stars which interact with the nebula. A change of a couple percent in momenta could easily cause a star to migrate inward or outward in the disk over timescales of hundred-millions or billions of years.

Another factor is the MW is interacting asymmetrically with other (dwarf)galaxies
Enthusiastic Fool
2.8 / 5 (6) Aug 01, 2015
Coincidental Convergence here with:
or (iprefer the full PDF)

Seems like both the PSU group using SDSS and the Gaia-ESO group should exchange notes.

This would likely work the same way as planetary migration, gravity assist ejecting...

While I'm sure that occasionally contributes to migration I think based on the paper I linked above that there must be a good mechanism for the ordered structures in the MW than random ejection. When kinematics and metallicity line up so well with structural distribution there must be a process we don't yet fully comprehend. If 4/5ths of the stars in the MW are multiple stars or binary that's a lot of double star ejections going on. (googling I can find refs for 4/5ths, 2/3rd, 85%, an odd 30% from Smithsonian in 2k6)

Enthusiastic Fool
2.8 / 5 (6) Aug 01, 2015

...tens of light-years across...

The Crab is ~5.5 ly across. At what distance is that effectively a point mass for gravitational interaction? Also it's proper motion was conserved so it's not like it exploded and stopped moving as a galactic banana peel for incoming stars. Planetary nebula don't last forever either as I'm sure you are aware. I think you've pointed out things than can perturb stars locally in close groups and even account for the rogue bullets zipping about but still doesn't account for total structure or migration. I also think you are underestimating the distance between stars. Until one of us can do the math to prove the other retarded I guess we'll have to leave it at "I think" "You think".
1 / 5 (5) Aug 01, 2015
Our Sun moves through the MW at about 240 km/s. These rogues must be moving an awfully lot faster, maybe up to 1000 km/s? Or faster? How do these rogues get away with this in violation of gravitational fields by which DM supposedly rules the Universe gluing everything together.
5 / 5 (4) Aug 03, 2015
... in violation of gravitational fields by which DM supposedly ...

The gravitational field in principle does not set a limit on speed.
There is no violation. Wrong again.
Now show how you solve a Differential Equation !
1 / 5 (2) Aug 05, 2015
When the team looked at the pattern of element abundances in detail, they found that much of the data could be explained by a model in which stars migrate radially, moving closer or farther from the galactic center with time.

Since Physorg did not bother referencing this key reference:

Seems that the stars move mostly radially outward, rather than inward as favored by the merger maniacs.

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