Study Shows the Milky Way is Out to Lunch

Chicken Little was right. The sky is falling.

Thousands of stars stripped from the nearby Sagittarius dwarf
galaxy are streaming through our vicinity of the Milky Way galaxy,
according to a new view of the local universe constructed by a team
of astronomers from the University of Virginia and the University of

Using volumes of data from the Two-Micron All Sky Survey
(2MASS), a major project to survey the sky in infrared light led by
the University of Massachusetts, the astronomers are answering
questions that have baffled scientists for decades and proving that
our own Milky Way is consuming one of its neighbors in a dramatic
display of ongoing galactic cannibalism.

The study, to be published in the Dec. 20 issue of the
Astrophysical Journal, is the first to map the full extent of the
Sagittarius galaxy and show in visually vivid detail how its debris
wraps around and passes through our Milky Way. Sagittarius is 10,000
times smaller in mass than the Milky Way, so it is getting stretched
out, torn apart and gobbled up by the bigger Milky Way.

“It’s clear who’s the bully in the interaction,” said Steven
Majewski, U.Va. professor of astronomy and lead author on the paper
describing the results.

In model images made to show the interaction in 3-D,
available at, the Milky Way
appears as a flattened disk with spiral arms, while Sagittarius is
visible as a long flourish of stars swirling first under and then
over and onto the Milky Way disk.

“If people had infrared-sensitive eyes, the entrails of
Sagittarius would be a prominent fixture sweeping across our sky,”
Majewski said. “But at human, visual wavelengths, they become buried
among countless intervening stars and obscuring dust. The great
expanse of the Sagittarius system has been hidden from view.”

Not any more. By using infrared maps, the astronomers
filtered away millions of foreground stars to focus on a type of star
called an M giant. These large, infrared-bright stars are populous in
the Sagittarius galaxy but uncommon in the outer Milky Way. The 2MASS
infrared map of M giant stars analyzed by Majewski and collaborators
is the first to give a complete view of our galaxy’s meal of
Sagittarius stars, now wrapping like a spaghetti noodle around the
Milky Way. Prior to this work, astronomers had detected only a few
scattered pieces of the disrupted Sagittarius dwarf. Even the
existence of Sagittarius was unknown until the heart of this nearest
satellite galaxy of the Milky Way was discovered by a British team of
astronomers in 1994.

“We sifted several thousand interesting stars from a catalog
of half a billion,” said co-author Michael Skrutskie, U.Va. professor
of astronomy and principal investigator for the 2MASS project. “By
tuning our maps of the sky to the ‘right’ kind of star, the
Sagittarius system jumped into view.”

“This first full-sky map of Sagittarius shows its extensive
interaction with the Milky Way,” Majewski said. “Both stars and star
clusters now in the outer parts of the Milky Way have been ‘stolen’
from Sagittarius as the gravitational forces of the Milky Way nibbled
away at its dwarf companion. This one vivid example shows that the
Milky Way grows by eating its smaller neighbors.”

“Astronomers used to view galaxy formation as an event that
happened in the distant past,” noted David Spergel, a professor of
astrophysics at Princeton University after viewing the new finding.
“These observations reinforce the idea that galaxy formation is not
an event, but an ongoing process.”

The study’s map of M giants depicts 2 billion years of
Sagittarius stripping by the Milky Way, and suggests that Sagittarius
has reached a critical phase in what had been a slow dance of death.

“After slow, continuous gnawing by the Milky Way, Sagittarius
has been whittled down to the point that it cannot hold itself
together much longer,” said 2MASS Science Team member and study
co-author Martin Weinberg of the University of Massachusetts. “We are
seeing Sagittarius at the very end of its life as an intact system.”

Does this mean we are at a unique moment in the life of our
galaxy? Yes and no.

“Whenever possible, astronomers appeal to the principle that
we are not at a special time or place in the universe,” Majewski
said. “Because over the 14 billion-year history of the Milky Way it
is unlikely that we would just happen to catch a brief event like the
death of Sagittarius, we infer that such events must be common in the
life of big spiral galaxies like our own. The Milky Way probably
dined on a number of dwarf galaxy snacks in the past.”

On the other hand, Majewski and his colleagues have been
surprised by the Earth’s proximity to a portion of the Sagittarius

“For only a few percent of its 240 million-year orbit around
the Milky Way galaxy does our Solar System pass through the path of
Sagittarius debris,” Majewski said. “Remarkably, stars from
Sagittarius are now raining down onto our present position in the
Milky Way. Stars from an alien galaxy are relatively near us. We have
to re-think our assumptions about the Milky Way galaxy to account for
this contamination.”

The new findings will help astronomers measure the total mass
of the Milky Way and Sagittarius galaxies, and probe the quantity and
distribution of the invisible dark matter in these systems.

“The shape of the Sagittarius debris trail shows us that the
Milky Way’s unseen dark matter is in a spherical distribution, a
result that is quite unexpected,” Weinberg said.

“The observations provide new insights into the nature of the
mysterious dark matter,” said Princeton’s Spergel. “Either our galaxy
is unusual or the dark matter has richer properties than postulated
by conventional models.”

2MASS was a joint project of the University of Massachusetts
and the Infrared Processing and Analysis Center/California Institute
of Technology. The National Aeronautics and Space Administration, and
the National Science Foundation funded the project. Additional
funding for the Sagittarius study with 2MASS came from the David and
Lucile Packard Foundation and the Research Corporation.

High-resolution color images of the Milky Way’s interaction with
Sagittarius are available at


  • Dr. Steven Majewski, (434) 924 4893,
  • Dr. Michael Skrutskie, (434) 924 4328,
  • Dr. Martin Weinberg, (413) 545 3821,