Sky & Telescope - USA (2019-11)

NEWS NOTES

12 NOVEMBER 2019 • SKY & TELESCOPE

A SURVEY OF MORE THAN 2,

Cepheid variable stars has revealed the

Milky Way’s warped disk in new detail,

Dorota Skowron (University of Warsaw,

Poland) and colleagues report in the

August 2nd Science.

Cepheids are giant stars that breathe

in and out at a rate proportional to

their intrinsic brightness, a relation-

ship that makes them superb distance

markers. Skowron used the Optical

Gravitational Lensing Experiment

(OGLE), as well as data from fi ve other

surveys and catalogs, to map the loca-

MILKY WAY

Astronomers Map

Our Galaxy’s Warp

tions of Cepheids within a few tens of

thousands of light-years of the Sun.

The project confi rms the existence of a

severe warp in our galaxy’s disk, remi-

niscent of pizza dough bent in mid-toss.

The warp has also shown up in maps

using neutral hydrogen gas, stars, dust,

and stellar motions, as well as a recent

infrared study that used roughly half as

many Cepheids as Skowron’s team did

(S&T: June 2019, p. 12).

When they plotted the variable stars’

locations looking down at our galaxy’s

disk, the astronomers noticed that

the Cepheids clump together in space.

Curious, the team took the three most

prominent clumps and calculated the

ages of the stars in them. They found

that the stars in each group had a

similar age to one another — the three

groups are approximately 64, 113, and

175 million years old, respectively. The

youngest clump’s stars cluster tightly

together, whereas the oldest clump’s

stars are the most spread out.

The team speculates that these

Cepheid populations were born in three

recent bursts of star formation. As

time passed, stars that formed together

would have naturally gone their sepa-

qAn artist’s illustration shows how the Ce-

pheids (green points) trace out the Milky Way’s

warp. A yellow circle marks the Sun’s position.

12 NOVEMBER 2019• SKY & TELESCOPE

IN BRIEF

Alpha Centauri Planet Hunt

A new instrument installed on the Very

Large Telescope in Chile has completed

a 100-hour campaign looking for planets

in the Alpha Centauri system. While a

planet has already been found around the

red dwarf Proxima Centauri, it is probably

dessicated (S&T: May 2017, p. 10) due to

the small star’s outsize magnetic activity.

Alpha Centauri A and B, however, are

larger Sun-like stars whose lower activity

levels give planets a better shot at holding

onto their atmospheres. Astronomers

conducted an observing campaign of

the system that ended June 22nd using

the Near Earths in the AlphaCen Region

(NEAR) instrument. NEAR could detect

the presence of planets twice Earth’s

size or bigger. The astronomers expect

to announce the presence — or absence

— of large Earth-like planets in the Alpha

Centauri system by October.

■ MONICA YOUNG

Galactic Center Gravity

Test Confirmed

A team of astronomers has confi rmed that

the light from a star passing near our galaxy’s

central black hole behaved as predicted by

Einstein’s theory of gravity. The star made

its closest approach in May 2018, whizzing

within 120 astronomical units of the black

hole (S&T: Sept. 2018, p. 22). Tuan Do, An-

drea Ghez (both at the University of California,

Los Angeles), and colleagues used the Keck

telescopes and other facilities to measure the

star’s redshift during its close pass, testing

Einstein’s general theory of relativity. They

report in the August 16th Science a relativ-

istic redshift of about 200 km/s, confi rming

an earlier result from a team led by Reinhard

Genzel (Max Planck Institute for Extraterres-

trial Physics, Germany). The teams’ next goal

is to measure the star’s orbital precession. Ex-

plaining how much Mercury’s orbit precesses

around the Sun was one of the original selling

points for general relativity. Now, astronomers

want to see if the physics works the same

way in the most extreme gravitational environ-

ments we can probe. The teams may have

results on that front within a year or so.

■ CAMILLE M. CARLISLE

Incoming Asteroid Spotted

Two asteroid hunters, the Asteroid Terrestrial-

impact Last Alert System (ATLAS) and the

second Pan-STARRS telescope (PS2), worked

together to fi nd and calculate the entry path

of a tiny, 4-meter space rock that ultimately

burned up in Earth’s atmosphere. ATLAS is

designed to provide early warning of incoming

asteroids. The newest discovery, designated

2019 MO, came in the early morning hours

of June 22nd. An automated system initially

gave the space rock a modest impact rating

of 2, but that was upgraded to a “likely” im-

pact rating of 4 when astronomers located the

asteroid using additional observations with

PS2, which sits atop Haleakala− in Hawai‘i.

Twelve hours after the asteroid’s discovery,

Nexrad weather radar in San Juan, Puerto

Rico, detected the asteroid as it burned up in

the atmosphere over the ocean.

■ MONICA YOUNG

rate ways, explaining why the oldest

stars are the most spread out. Computer

simulations confi rm that the starbirth

episodes would have stretched into the

pattern that the team’s map reveals in

the Milky Way.

■ CAMILLE M. CARLISLE

• View a video that shows the 3D
  model of the Milky Way at https://is.gd/
  warpedMilkyWay.

pThe Cepheids, overplotted on a Milky Way

map, range from 30 million (blue) to 400 million

(red) years old. The black circle marks the Sun.

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