Astronomy

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Mars reaches opposition

24 times during this 50-year

period, with the most (four)

coming within the borders

of Leo the Lion.

WWW.ASTRONOMY.COM 19

MARS IN MOTION

T

he Milky Way and the Andromeda

Galaxy (M31) are giant spiral galax-

ies. And in about 4 billion years,

they will collide in a gravitational

sumo match that will ultimately

bind them forever. Previously, astronomers

believed that Andromeda would dominate

with a mass up to three times that of the

Milky Way. But new research suggests we’ve

overestimated our opponent.

In a study published February 14 in the

Monthly Notices of the Royal Astronomical

Society, a team of Australian astronomers

announced that Andromeda is not actually

the heavyweight we once thought it was.

Instead, they found that our nearest large

galactic neighbor is more or less the same

mass as the Milky Way — some 800 billion

times the mass of the Sun.

To determine Andromeda’s mass, the

team studied the orbits of high-velocity

planetary nebulae, which contain aging

stars moving at high speeds. They coupled

their observations with a technique that

calculates the speed required for a quick-

moving star to escape the gravitational pull

of its host galaxy. The speed needed for

ejection is known as escape velocity.

“When a rocket is launched into space, it

is thrown out with a speed of 11 kilometers

per second [6.8 miles per second] to over-

come the Earth’s gravitational pull,” Prajwal

Kaf le of the University of Western Australia

branch of the International Centre for

Radio Astronomy Research said in a press

release. “Our home galaxy, the Milky Way,

is over a trillion times heavier than our tiny

planet Earth, so to escape its gravitational

pull, we have to launch with a speed of 550

kilometers per second [342 miles per sec-

ond]. We used this technique to tie down

SIZING UP

ANDROMEDA

the mass of Andromeda.”

This is not the first time a galaxy’s mass

has been recalculated based on its escape

velocity. In 2014, Kaf le used a similar tech-

nique to revise the mass of the Milky Way,

showing that our galaxy has much less dark

matter — a mysterious form of matter that

has gravity but does not interact with light

— than previously thought.

Much like the 2014 study, this paper sug-

gests that previous research has overesti-

mated the amount of dark matter present in

the Andromeda Galaxy. “By examining the

orbits of high-speed stars, we discovered

that [Andromeda] has far less dark matter

than previously thought,” said Kafle.

Although revising down Andromeda’s

overall mass may seem like it should help

the Milky Way out during our eventual

collision, the researchers say that new simu-

lations are first needed to determine exactly

how the galaxies’ eventual meeting will

go down. But no matter what happens in

4 billion years, Kaf le says the new finding

“completely transforms our understanding

of the Local Group” of galaxies, which is

dominated gravitationally by Andromeda

and the Milky Way.

For now, it appears we can take solace in

the newfound knowledge that the Milky

Way is not nearly as overpowered by

Andromeda as we once thought. As

University of Sydney astrophysicist Geraint

Lewis said, “We can put this gravitational

arms race to rest.” — J.P.

LONE STAR. S2, a star that will test general relativity when it swings by our galaxy’s supermassive black hole this year,

does not have a significant binary companion. If such a companion had existed, it may have complicated measurements.

A ZODIACAL JOURNEY. The

Red Planet takes 1.88 years to circle

the Sun. During a typical orbit,

Mars spends the most time in the

constellation where it executes its

retrograde loop — the apparent

backward motion that occurs when

Earth overtakes the outer planet

around the time of opposition. To

average out this effect, here we

chart the percentage of time Mars

spends in each constellation along

the zodiac during the 21st century’s

first half. — Richard Talcott

ASTRONOMY

: ROEN KELLY

MATCHING MASS. The Andromeda Galaxy, shown here in an ultraviolet image from NASA’s Galaxy Evolution

Explorer, is roughly the same mass as the Milky Way, not three times as massive, as previously thought.

NASA/JPL-CALTECH