Astronomers discover Milky Way galaxy’s most-distant stars

By Will Dunham

WASHINGTON, Jan 12 (Reuters) – Astronomers have detected
in the stellar halo that represents the Milky Way’s outer limits
a group of stars more distant from Earth than any known within
our own galaxy – almost halfway to a neighboring galaxy.

The researchers said these 208 stars inhabit the most remote
reaches of the Milky Way’s halo, a spherical stellar cloud
dominated by the mysterious invisible substance called dark
matter that makes itself known only through its gravitational
influence. The furthest of them is 1.08 million light years from
Earth. A light year is the distance light travels in a year, 5.9
trillion miles (9.5 trillion km).

These stars, spotted using the Canada-France-Hawaii
Telescope on Hawaii’s Mauna Kea mountain, are part of a category
of stars called RR Lyrae that are relatively low mass and
typically have low abundances of elements heavier than hydrogen
and helium. The most distant one appears to have a mass about
70% that of our sun. No other Milky Way stars have been
confidently measured farther away than these.

The stars that populate the outskirts of the galactic halo
can be viewed as stellar orphans, probably originating in
smaller galaxies that later collided with the larger Milky Way.

“Our interpretation about the origin of these distant stars
is that they are most likely born in the halos of dwarf galaxies
and star clusters which were later merged – or more
straightforwardly, cannibalized – by the Milky Way,” said Yuting
Feng, an astronomy doctoral student at the University of
California, Santa Cruz, who led the study, presented this week
at an American Astronomical Society meeting in Seattle.

“Their host galaxies have been gravitationally shredded and
digested, but these stars are left at that large distance as
debris of the merger event,” Feng added.

The Milky Way has grown over time through such calamities.

“The larger galaxy grows by eating smaller galaxies – by
eating its own kind,” said study co-author Raja GuhaThakurta, UC
Santa Cruz’s chair of astronomy and astrophysics.

Containing an inner and outer layer, the Milky Way’s halo is
vastly larger than the galaxy’s main disk and central bulge that
are teeming with stars. The galaxy, with a supermassive black
hole at its center about 26,000 light years from Earth, contains
perhaps 100 billion–400 billion stars including our sun, which
resides in one of the four primary spiral arms that make up the
Milky Way’s disk. The halo contains about 5% of the galaxy’s
stars.

Dark matter, which dominates the halo, makes up most of the
universe’s mass and is thought to be responsible for its basic
structure, with its gravity influencing visible matter to come
together and form stars and galaxies.

The halo’s remote outer edge is a poorly understood region
of the galaxy. These newly identified stars are almost half the
distance to the Milky Way’s neighboring Andromeda galaxy.

“We can see that the suburbs of the Andromeda halo and the
Milky Way halo are really extended – and are almost
‘back-to-back,'” Feng said.

The search for life beyond the Earth focuses on rocky
planets akin to Earth orbiting in what is called the “habitable
zone” around stars. More than 5,000 planets beyond our solar
system, called exoplanets, already have been discovered.

“We don’t know for sure, but each of these outer halo stars
should be about as likely to have planets orbiting them as the
sun and other sun-like stars in the Milky Way,” GuhaThakurta
said.

(Reporting by Will Dunham; Editing by Lisa Shumaker)