Astronomy - 06.2019

The Moon

400,000 300,000 200,000 100,000 0

Jupiter

Earth

Miles

86,882 miles

(139,823 km)

7,918 miles

(12,742 km)

2,158 miles

(3,475 km)

The Sun’s diameter

864,340 miles (1,391,020 km)

ASTRONEWS

FAST

FAC T

WWW.ASTRONOMY.COM 11

HOW BIG

IS THE SUN?

Astronomers have discovered that a

star in the Andromeda Galaxy (M31)

has been regularly erupting for the

past million years, leaving behind one

of the biggest shells of ejected mate-

rial ever seen. The new research, pub-

lished January 9 in the journal Nature,

not only marks the first discovery of

such a super-remnant in another gal-

axy; it also paves the way for detect-

ing a potentially huge population

of repeatedly exploding stars called

recurrent novae.

SWING YOUR PARTNER

The star responsible for this expan-

sive remnant, which stretches more

than 400 light-years across, is actu-

ally a diminutive white dwarf, the

Earth-sized remnant of a Sun-like

star. But in the case of this rem-

nant, which bears the catchy name

M31N 2008–12a, the culprit is not an

ordinary white dwarf: This star has a

dance partner.

As the tiny white dwarf and its

nearby companion star orbit each

other, the white dwarf rapidly

siphons hydrogen from its buddy.

When this material reaches the sur-

face, it’s heated and compressed

thanks to the white dwarf’s intense

gravitational pull. Eventually, the

hydrogen reaches a breaking point

and spontaneously fuses to create

helium, resulting in a powerful sur-

face explosion called a nova.

This burst of fusion causes the

white dwarf to temporarily brighten

up to a millionfold as it ejects material

outward at up to 3 percent the speed

of light. In the case of M31N 2008–12a,

over time these repeated explosions

have created an extensive and ever-

expanding cocoon of gas and dust

around the white dwarf. According

to the paper, “Larger than almost all

known remnants of even supernova

explosions, the existence of this shell

demonstrates that the nova M31N

2008–12a has erupted with high

frequency for millions of years.”

IT KEEPS GOING AND GOING

The massive size of the remnant is

not its only claim to fame. Indeed,

M31N 2008–12a also now holds the

title of most frequently recurring

nova, as it erupts at least once a year.

“When we first discovered that

M31N 2008–12a erupted every year,

we were very surprised,” said

co-author Allen Shafter of San Diego

State University in a press release.

Most recurrent novae explode only

about once a decade.

Even though the white dwarf has

spent the past million years or so

exploding, researchers don’t think it

will last forever. The study concludes

that in less than 40,000 years, the

white dwarf will either explode one

last time as a type Ia supernova or

collapse under the weight of the

accreted material into a neutron

star. — Jake Parks

QUICK TAKES

GROWING DARKER

Dark energy’s repulsive force

may be increasing over time,

suggests a new study that

used nearly 1,600 quasars to

track the universe’s expansion.

• GAUGING GRAVITY
  Scientists have repurposed a
  movement-tracking device on
  the Curiosity rover to measure
  tiny variations in Mars’
  gravitational field, revealing
  new clues about the formation
  of Mount Sharp, which the
  rover is currently climbing.


• 
  

  FEELING SPACE
  Astronomers teamed up with
  teachers to design a workshop
  where those with hearing loss
  can feel vibrations from
  cosmic sources like stars,
  galaxies, and supernovae.

  
  


• 
  

  SHORTER TIMELINE
  According to data from the
  Gaia satellite, the Milky Way
  will collide with the
  Andromeda Galaxy in some
  3.9 billion years, not 4.5 billion
  years as previously thought.

  
  


• 
  

  SALTY STAR
  New ALMA observations
  revealed the first detection
  of sodium chloride (table salt)
  in the disk surrounding a
  young, massive star in the
  Orion Nebula (M42).

  
  


• 
  

  ROCK ON
  Asteroid Day, which aims to
  raise awareness about
  asteroids and the search to
  find them, will celebrate its
  fifth anniversary June 30 with
  thousands of events hosted
  around the world.

  
  


• 
  

  IN PLAIN SIGHT
  An international team of
  researchers uncovered
  hundreds of thousands of
  previously unseen galaxies
  in radio data from the Low
  Frequency Array telescope.

  
  


• 
  

  SYNTHETIC LIFE
  Research funded by NASA has
  created a DNA-like molecule
  that can store and transmit
  information, which may force
  scientists to rethink life
  beyond Earth.

  
  


• 
  

  SHOT IN THE DARK
  Japan’s Hayabusa2 spacecraft
  fired a tantalum bullet into the
  surface of asteroid Ryugu to
  dislodge and collect samples
  that will be brought to Earth
  in December 2020. — J.P.

  
  

KEEP COOL. Research suggests the insulating crust of dwarf planet Ceres could keep salty water (called cryomagma)

in liquid form beneath the surface for up to 10 million years, explaining the various ages of its surface deposits.

In Andromeda,

a star explodes

like clockwork

This nova has erupted once

a year for millions of years,

leaving behind the largest

known cloud of stellar debris

around a white dwarf.

Some 1.3 million Earths

could fit inside the Sun.

ASTRONOMY

: ROEN KELLY AFTER SOHO/EIT CONSORTIUM/MDI TEAM

SUN WORSHIP. Our solar system is so

named because it’s dominated by Sol, the

Sun. This image, taken July 19, 2000, includes a

vast array of sunspots. But it’s hard to imagine how

just how big those sunspots — and the Sun itself —

are without a visual comparison. We’ve depicted

Jupiter, Earth, and the Moon to scale here, so you can

see how they stack up against our local star.

— Michael E. Bakich

NOT DEAD YET. Like M31N 2008–12a, GK Persei (pictured) is a prime example of

a nova remnant.

X-RAY: NASA/CXC/RIKEN/D. TAKEI ET AL.; OPTICAL: NASA/STS

CI; RADIO: NRAO/VLA