Astronomy

ASTRONEWS

5°

PUPPIS

VELA

CARINA

PICTOR

DORADUS

RETICULUM

Canopus

_

South

Celestial

Pole

FAST

FAC T

10 ASTRONOMY • JULY 2018

SPECIAL DELIVERY. About one-third of the organic material on Mars was delivered by asteroid and comet strikes,

an international team of astronomers has calculated.

T

he Large and Small Magellanic Clouds

are the Milky Way’s largest satellite

galaxies — and they’re locked in a

cosmic game of tug-of-war. Though

these dwarf galaxies orbit the larger Milky

Way, they also orbit each other, exerting

gravitational inf luence that has ousted a

huge cloud of gas from one of the two.

Using the Hubble Space Telescope (HST),

astronomers have finally determined that

the Small Magellanic Cloud is the losing

party in this battle.

The results, published February 21

in The Astrophysical Journal, examine a

structure called the Leading Arm. The

Magellanic Stream, which is an extended

“tail” of gas, visible at radio wavelengths,

that trails the two dwarf galaxies, was found

in 2013 to contain gas mixed from both the

Large and Small Magellanic Clouds. But,

astronomers had wondered, what about the

composition of the Leading Arm? Roughly

half the size of our galaxy, this arc of gas

connecting the Magellanic Clouds to the

Milky Way is 1 billion to 2 billion years old,

and it provides additional fuel for new star

formation within our galaxy.

As its name suggests, the structure leads

the Magellanic Clouds in their orbits

around the Milky Way, and astronomers

have long known it was pulled from at least

one of the clouds. Which dwarf galaxy spe-

cifically had lost the gas was unknown until

recent ultraviolet observations made with

HST were combined with radio observa-

tions from the Robert C. Byrd Green Bank

Telescope at the Green Bank Observatory in

West Virginia, and other observatories.

SMALL MAGELLANIC CLOUD IS LOSING COSMIC FIGHT

MERCURY’S SOUTHERN EXPOSURE

Andrew Fox of the Space Telescope

Science Institute in Baltimore and his col-

leagues found the answer by studying how

light from seven distant quasars — the

bright cores of massive galaxies — filtered

through the gas of the Leading Arm. As the

quasars’ light travels through the gas, it

excites atoms along the way. By breaking

the light that arrives at Earth apart by

wavelength using a spectrograph, the group

was able to measure the composition of the

gas. In particular, they looked at features

produced by oxygen and sulfur, which trace

the amount of heavy elements (those above

hydrogen in the periodic table) in the gas.

They also combined their measurements

with observations of hydrogen at radio

wavelengths to ultimately determine both

the composition and velocity of the gas.

These “fingerprints” were then compared

with the composition and velocity of the

Large and Small Magellanic Clouds to

determine the Leading Arm’s origin.

“We’ve found that the gas matches the

Small Magellanic Cloud,” Fox said in a

press release. “That indicates the Large

Magellanic Cloud is winning the tug-of-

war, because it has pulled so much gas out

of its smaller neighbor.”

HST’s ultraviolet capabilities were vital to

the measurements because the features the

group needed to measure from oxygen and

sulfur aren’t visible at other wavelengths.

Now that the origin of the gas has been

determined, astronomers are hoping to bet-

ter map the structure’s full size. As the

Leading Arm streams from the Magellanic

Clouds onto the Milky Way, it becomes

fragmented, making the structure harder to

study. Learning more about the structure

will help astronomers better trace the fuel

this gas provides for generations of future

stars and planets in the Milky Way. — A.K.

The sky’s second-brightest star,

magnitude –0.7 Canopus, lies just 9°

from Mercury’s South Celestial Pole.

ORIGIN STORY. A recent study observed light from distant quasars passing through the Leading Arm,

a fragmented cloud of gas from the Milky Way’s largest satellites. Astronomers found the gas has been pulled

from the Small Magellanic Cloud by the Large Magellanic Cloud. In contrast, the Magellanic Stream trailing

the two galaxies contains gas from both satellites mixed together. NIDEVER ET AL./NRAO/AUI/NSF/MELLINGER/LEIDEN-ARGENTINE-BONN/

LAB SURVEY/PARKES OBS/WESTERBORK OBS/ARECIBO OBS/FEILD/STSCI/NASA/ESA/A. FOX/STSCI

PICTURESQUE POLE STAR.

If you stood in one of the

permanently shadowed craters

at Mercury’s south pole and

looked overhead, these

are the stars you would

see. The innermost

planet’s axis points

toward a rather bland

part of the sky in

the constellation

Pictor the Painter.

The only naked-

eye star that lies

within 1° of the

pole is magnitude

3.3 Alpha (α)

Pictoris.

— Richard Talcott

Magellanic Stream

LMC

SMC

Leading

Arm

A

B

C

QUASARS

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