Astronomy

ASTRONEWS

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CLOSING TIME. The University of Chicago plans to cease on-site

operations of the historic Yerkes Observatory on October 1, 2018.

Most of our galaxy’s stars reside in the Milky

Way’s bulge and disk, with a much smaller

population scattered in a spherical galactic halo.

However, a few large clusters exist outside the

disk. Astronomers previously believed these

outliers were traces of smaller galaxies absorbed

by the Milky Way, but researchers recently found

that these stars were likely born in the galactic

disk and later ejected by invading galaxies.

In a paper published February 26 in the jour-

nal Nature, a team of researchers led by the Max

Planck Institute for Astronomy examined 14

stars in the Triangulum-Andromeda and A

regions. The two dense star clusters lie on

opposing sides of the Milky Way’s disk, about

14,000 light-years below and above it, respec-

tively. To find how the clusters ended up in

these unlikely locations, the researchers first

measured the spectra of the stars to determine

what they are made of.

“The analysis of chemical abundances is a

very powerful test, which allows [us], in a way

similar to the DNA matching, to identify the par-

ent population of the star. Different parent pop-

ulations, such as the Milky Way disk or halo,

dwarf satellite galaxies or globular clusters, are

known to have radically different chemical com-

positions. So once we know what the stars are

made of, we can immediately link them to their

parent populations,” said the paper’s lead

author Maria Bergemann of the Max Planck

Institute for Astronomy, in a press release.

The team used the Keck Observatory’s HIgh

Resolution Echelle Spectrometer (HIRES) in

Hawaii to gather spectra from the stars. The data

showed that both groups were abundant in

M giants — stars in which elements heavier than

helium are plentiful, consistent with the chemical

compositions of stars in the outer galactic disk.

The dwarf galaxies that invade the Milky Way, on

the other hand, have hardly any M giants.

With near-identical compositions to the stars

in the outer disk, researchers concluded that

these clusters are likely former disk residents.

“This phenomenon is called galactic eviction,”

said co-author Judy Cohen of Caltech. “These

structures are pushed off the plane of the Milky

Way when a massive dwarf galaxy passes

through the galactic disk. This passage causes

oscillations, or waves, that eject stars from the

disk, either above or below it depending on the

direction that the perturbing mass is moving.”

An additional study conducted by Allyson

Sheffield at Columbia University and research-

ers at Fermilab found similarities in the two

groups’ speed and structure. This suggests that

the same event could have landed them in their

current positions.

Such a discovery not only gives insight into

past events that shaped our galaxy, but also

hints at how the Milky Way will handle future

invaders. — A.J.

Outlying stars traced to our galaxy’s disk

A circle of cyclones

MAKING WAVES. The passage of a dwarf galaxy through the disk of the Milky Way causes oscillations that oust

stars from the disk to regions above and below it. A single past event may be responsible for the current positions

and densities of the Triangulum-Andromeda and A13 star clusters. T. MUELLER/NASA/JPL-CALTECH

SPIRAL PATTERNS. Recent data from Jupiter’s north

pole reveal a massive central cyclone encircled by eight

smaller, close-knit cyclones. The eight surrounding

cyclones, which appear in this composite infrared

image taken by the Jovian Infrared Auroral Mapper on

NASA’s Juno mission, range from 2,500 to 2,900 miles

(4,000 to 4,600 kilometers) in diameter and have violent

winds that can gust up to 220 mph (350 km/h). They’re

packed together so tightly that their spiral arms touch

each other, but for reasons unknown to researchers,

the cyclones don’t merge. These strong storms are part

of the weather layer of Jupiter’s atmosphere, which

reaches 1,900 miles (3,000 km) deep and contains about

1 percent of the planet’s total mass. By comparison, less

than one-millionth of Earth’s mass is contained in its

NASA/JPL-CALTECH/SWRI/ASI/INAF/JIRAM atmosphere. — A.J.

Sun

Triangulum-

Andromeda

A