Astronomy

Relative

number

of

galaxies

ASTRONEWS

14 ASTRONOMY • APRIL 2018

Making

sense of how

galaxies are

classified

Elliptical galaxies are the oldest,

largest galaxies in the universe.

They look like fuzzy, relatively

uniform balls of stars and con-

tain a high percentage of older,

redder stars and very little gas

and dust. These galaxies can be

further broken into categories

by their ellipticity, or roundness,

with flatter structures approach-

ing the next class of galaxies.

Lenticular galaxies are a

morphological step between

ellipticals and spirals. These gal-

axies have flatter shapes than

ellipticals when viewed edge-on.

They are relatively devoid of star

formation and star-forming mate-

rial, but still retain a clear central

bulge structure reminiscent of

spiral galaxies.

Spiral galaxies have a bright

center (called the bulge) and two

or more winding arms. These

galaxies are younger and bluer

than ellipticals, with several areas

of star formation and plentiful

gas and dust. Spiral galaxies are

broken down into further catego-

ries based on the shape of their

bulge and arms: round versus

elongated (barred) bulges, and

spiral-shaped versus ring-shaped

arms. In this system, the Milky

Way is a barred spiral galaxy.

Irregular galaxies have no

clearly defined structure. These

galaxies instead have a variety of

shapes, and many may represent

an “in-between” stage resulting

from the mergers or interactions

of two or more galaxies. — A.K.

Scientists have confirmed that the old-

est fossils ever discovered — found in a

nearly 3.5 billion-year-old rock from Western

Australia — contain 11 complex microbes that

are members of five distinct species.

The findings, published December 18 in

the Proceedings of the National Academy of

Sciences, not only suggest that life on our

planet originated some 4 billion years ago,

but also help support the increasingly wide-

spread theory that life in the universe is much

more common than previously thought.

“By 3.465 billion years ago, life was already

diverse on Earth; that’s clear,” said J. William

Schopf, a professor of paleobiology at the

University of California, Los Angeles, and the

study’s lead author, in a press release. “This

tells us life had to have begun substantially

earlier, and it confirms that it was not difficult

for primitive life to form and to evolve into

more advanced microorganisms.”

To analyze the microorganisms, the

researchers used an instrument called a sec-

ondary ion mass spectrometer (SIMS), one of

only a few in the world. By measuring the

ratio of carbon-12 to carbon-13 isotopes, SIMS

helped the scientists determine the microbes’

anatomies, and also revealed how they lived.

“The difference in carbon isotope ratios

correlates with their shapes,” said John Valley,

professor of geoscience at the University of

Wisconsin–Madison and co-author of the

s tudy. “ T heir C-13 -to - C-12 ratios are charac ter-

istic of biology and metabolic function.”

Based on the chemical analysis, the

researchers concluded that the 11 fossilized

microbes spanned five different taxonomical

groups. Some of the microbes were a type of

now-extinct bacteria from the domain

Archaea, while others were more similar to

microbial species still around today.

The results show that “these are a primitive

but diverse group of organisms,” Schopf said.

Their complex and varied structures at such

an early point in Earth’s history demonstrate

that life can take root and evolve much more

rapidly than suggested by earlier research.

When combined with the fact that there

are trillions of stars in the universe — and the

growing consensus among astronomers that

exoplanets are commonplace — the case for

life existing elsewhere in the universe has

never been stronger. “If the conditions are

right, it looks like life in the universe should

be widespread,” Schopf said.

Schopf initially described the fossils in

Science in 1993 and confirmed their biological

origin in Nature in 2002. However, this is the

first study to reveal both how complex the fos-

sils are and to describe exactly what they are.

“This study was 10 times more time

consuming and more difficult than I first

imagined,” Valley said, “but it came to fruition

because of many dedicated people who have

been excited about this since day one. ... I

think a lot more microfossil analyses will be

made on samples of Earth and possibly from

other planetary bodies.” — J.P.

Oldest fossils suggest life

in the universe is common

The Milky Way’s central

bar is estimated at some

27,000 light-years long.

SKYE HIGH. Geologists discovered a 60 million-year-old meteorite

beneath a lava flow in Scotland’s Isle of Skye.

WHERE DID

YOU DIG UP

THAT OLD

FOSSIL?

These

fossilized

microbes —

discovered in

a rock sample

collected from

Apex Chert

in Western

Australia —

are 3.5 billion

years old.

The microbes’

significant

complexity

at such an

early point in

Earth’s history

suggests life

can begin and

evolve more

quickly than

once believed.

J. WILLIAM SCHOPF/UCLA

FAST

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