The Milky
Way might not have formed through the merger of several smaller galaxies as previously thought, but by
some other unknown process, a new study suggests.
Home to our
solar system and viewable in our own backyards, this crowd of stars called the Milky Way offers
astronomers one of the best chances for understanding how a galaxy forms.
"The Milky
Way is the only galaxy in the universe that we can study in detail. Still, we
haven't yet understood how it did form," Manuela Zoccali of the Department
of Astronomy and Astrophysics at the Pontifical Catholic University of Chile
told SPACE.com. "Shedding light on its formation is fundamental to
understand how all the galaxies in the universe have formed."
Parts of
our galaxy
The Milky
Way, often seen from Earth as a hazy halo of
stars in the night sky, is a spiral
galaxy with several arms of
gas, dust and stars, coiling out from a spherical nucleus in the shape of
a flattened disk. The starry center is called a bulge because it protrudes
from the flattened disk.
Until now,
the best theoretical models predicted dwarf galaxies beget larger and larger
galaxies, as multiple star packs clumped together or a heftier galaxy started gobbling
up its neighbors. If this were the case for the Milky Way, Zoccali said, the
stars in the galactic bulge should have once been part of the disk. Over eons,
as more galactic mergers occurred, some of the stars should be tugged toward
the center to form the bulge.
"We have
proved that this is not the case," Zoccali said.
Using the
European Southern Observatory's Very Large Telescope (VLT) array in Paranal, Chile, an international team of astronomers, led by Zoccali,
examined the chemical makeup of 50 giant stars in the direction of the galactic
bulge. They discovered the stars at the center of
the Milky Way showed distinct element amounts compared to the disk stars, a
sign that the two galaxy components formed separately.
"In other
words, bulge stars did not originate in the disk and then migrate inward to
build up the bulge but rather formed independently of the disk," Zoccali said.
They
detailed their findings in the current issue of the journal Astronomy and
Astrophysics.
Making
stars
In essence,
by cracking these chemical codes, the astronomers were able to peer back in
time at the stars'
births.
Just before
a star is born, its dusty neighborhood in space is swirling with interstellar
matter. The chemical elements within the matter vary over time and location. So
stars
born from one batch of dust and gas would hold a different chemical make-up
than stars born in another cosmic cloud.
The
chemical codes also hold other clues. "What you're really seeing when you look
at these chemical fingerprints is a star formation rate, or a star formation
history," Verne Smith, at the University of Texas at El Paso, said in a telephone interview.
Two key
chemical markers are oxygen and iron. Oxygen is predominantly produced during
the explosion of massive, short-lived stars called Type II supernovae,
while iron originates in the explosion of longer-lived stars called Type Ia supernovae.
As these stars are blown to pieces, they spew their innards into interstellar
space where it becomes the seeds for other stars.
Basically, if
a star is loaded with oxygen with minimal iron the star may have developed at a
lightning-fast rate, scientists explain.
Bulge
forms fast
The
astronomers found that the stars within the bulge contain more oxygen relative
to iron than their counterparts out in the disk, where we reside.
By
comparing the chemical compositions of the stars with computer models, the
astronomers suggest the galactic bulge formed in less than a billion years,
most likely as a result of a series of starbursts when the universe was young.
How did the
independent star gangs hook up? "We astronomers really haven't figured out this
part yet," Zoccali said.
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