Origins of the Milky Way

Mar 19, 2010
An optical image of the dwarf galaxy in Sculptor. Astronomers have found a very old star in this galaxy, in support of the idea that some of the Milky Way's old stars were once residents of neighboring galaxies.

(PhysOrg.com) -- According to current astronomical models, the Milky Way and other large galaxies formed over billions of years in a process that involved interactions between smaller galaxies, and in particular the gradual capture of many stars from nearby dwarf galaxies (small galaxies with hundreds or thousands of times fewer stars than the Milky Way).

Our current galactic neighborhood hosts one other comparably large galaxy, , and several dozen dwarf galaxies of various types, including the so-called Magellanic Clouds and a dwarf galaxy called the Sculptor Dwarf Galaxy. piecing together the history of the Milky Way, including its genetic heritage from neighbors, recognize that our story also very likely reflects the cosmic story of how galaxies everywhere are assembled.

The Sculptor is about 300,000 light-years away, and is notable because among other things it contains many that are lacking in heavy elements like iron. Since iron is produced very gradually inside stellar furnaces, a dearth of iron shows that the system (including its stars) is old, and dates from a time before these elements became abundant.

In the case of Sculptor, its stars on average have only a few percent of the heavy elements that we see in the sun. The outer halo of the Milky Way has some much older stars though, some with only one-ten-thousandth of the iron in the sun, or even less.

The problem is this: if the Milky Way did form in part by capturing stars from its neighboring dwarf galaxies, those dwarfs should have stars at least as old as ours, if not older. Why, wondered astronomers, have none been seen?

Writing in the latest issue of Nature, CfA astronomer Anna Frebel and two colleagues describe their discovery of a very old, metal-poor star in Sculptor. The scientists were concerned that previous conclusions were based either on studies of too few stars, or on incomplete diagnostics that used a misleading heavy element. Using a new method, they identified a set of stars in Sculptor that looked like possible old candidates, and then observed them with detailed optical spectroscopy.

They discovered that one of these stars was indeed extremely old - it has an iron abundance less than 0.25% of the sun's, making it a close analog of the old stars in the halo and comparable in age to the age of the universe. The results are interesting in themselves, but also because they reinforce the notion that our galaxy has stars that did once belong to our neighbors.

Explore further: Young binary star system may form planets with weird and wild orbits

More information: Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor, Nature 464, 72-75 (4 March 2010), doi:10.1038/nature08772

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User comments : 10

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Question
not rated yet Mar 19, 2010
Can something as large and majestic as a rotating galaxy like our Milky Way really be formed in the 13.7 billion years since the Big Bang?
brant
3.5 / 5 (4) Mar 19, 2010
Can something as large and majestic as a rotating galaxy like our Milky Way really be formed in the 13.7 billion years since the Big Bang?


Sure! Just throw in some dark matter, some dark energy, a few black holes, and the God particle!!!
XQZME
3.4 / 5 (5) Mar 20, 2010
Please explain, "Since iron is produced very gradually inside stellar furnaces, a dearth of iron shows that the system (including its stars) is old". If iron forms gradually and there is a dearth of it, should that not mean that the system is young?
Quantum_Conundrum
3.4 / 5 (5) Mar 20, 2010
Please explain, "Since iron is produced very gradually inside stellar furnaces, a dearth of iron shows that the system (including its stars) is old". If iron forms gradually and there is a dearth of it, should that not mean that the system is young?


I said the same thing. This is backwards, because the standard claim from astronomers regarding stars is that they fuse lighter elements to heavier and heavier elements. Thus, the older a star is, the MORE iron it should contain....

A very old star SHOULD contain almost entirely iron and heavier elements in it's core....while a very young star should contain little or no iron.

Even if this were a "first generation star" that has somehow managed to keep burning all ths time, it would be composed almost entirely of iron and heavier elements by now, because it clearly would have fused all of it's lighter elements by now.
Quantum_Conundrum
3.5 / 5 (2) Mar 20, 2010
Since second generation stars start with some iron in them to begin with...but are still nearly 90-99% hydrogen, one would expect second generation stars to actually have LESS iron than any "Old" first generation stars that still exist...

On the other hand, if this is a "new" first generation star, then it makes sense that it has very little iron...

Again, a second generation star has some iron.

An old first generation star has mostly iron.

A new first generation star, formed in a pure hydrogen and helium environment, has little or no iron. Since this has little or no iron, it must be an all new first generation star, and not an old star at all...
seneca
not rated yet Mar 20, 2010
.Why, wondered astronomers, have none been seen? ..
There is still high number of brown and red dwarfs hidden. Infrared telescopes like WISE would say more for us.
..way really be formed in the 13.7 billion years since the Big Bang? ..
We should be prepared for fundamental change of the whole cosmology, because it seems, the origin of many galaxies goes deeper, then the age of most distant parts of Universe.
seneca
not rated yet Mar 20, 2010
. Since this has little or no iron, it must be an all new first generation star, and not an old star at all..
But the iron can get trapped into old metallic stars and planets, thus avoiding recycling in the new generation of stars.
RealScience
5 / 5 (4) Mar 20, 2010
Large stars do not live long enough to become 'very old stars'. Stars small enough to live a long time do not produce significant amounts of iron except near the very ends of their lives, and so their iron content for most of their lives is the iron that they were born with.

The article is does not clear make it clear that it is NOT the individual starts that increase gradually in iron content.
It is on a galactic scale that iron content increases gradually. Thus the iron content in the clouds that new stars are formed from increases gradually, as dying stars add iron.
Thus older stars that formed before there was much iron have low iron, and newer stars the formed from gas clouds enriched in iron by dying stars have more iron through out their lives.

LKD
not rated yet Mar 22, 2010
Wasn't it stated in previous articles that dwarf galaxies had a hard time keeping their matter due to the lack of mass? So they would have a much harder time producing stars, produce generally smaller stars, and produce far few heavy elements, making dating it a lot more speculative?
yyz
not rated yet Mar 24, 2010
@LKD,

Looking far back into the history of the Universe, the dwarf galaxies that will later go on to merge into ever-larger stellar structures are still gas-rich, tightly bound systems (remember that dwarf galaxies have high mass-to-light ratios [ie dark matter dominated] to hold on to primordial gas, dust & stars). Of course they're metal poor systems, but not until encountering a much larger galaxy (with its' attendant DM halo) are the gas, dust & stellar components of the dwarf galaxy stripped in successive passes. These 'stellar streams' have been seen in our galaxy and several other nearby galaxies, giving more credence to this formation scenario. Also, epochs of star formation in dwarf galaxies can be inferred by careful spectral analysis, to see if periods of star formation coincide with close approaches by the dwarf galaxy.