Astronomy

ASTRONEWS

Satellite galaxies

Spiral galaxy

Spiral galaxy

Satellite galaxy

Globular clusters

Bulge

Disk (arms)

Open

cluster

Elliptical galaxy

Elliptical galaxy

Dense

inner core

Stellar halo

Dark matter halo

Dark matter halo

Outer

regions

Globular

clusters

WWW.ASTRONOMY.COM 17

NASA’s Kepler space telescope’s

nine-year hunt for exoplanets has

not only assisted in the search

for extraterrestrial life, it has also

provided clues about the behav-

ior of stars and the formation of

solar systems in our galaxy. Sadly,

though, the spacecraft is expected

to run out of fuel within the next

few months.

Without a fuel gauge, the exact

date of Kepler’s demise is still

unknown, and the spacecraft hasn’t

shown signs of slowing down yet.

NASA is continually monitoring the

craft for signs of low fuel, such as

changes in thruster performance

and fuel tank pressure, but no

warnings have arisen so far.

Kepler will continue to carry out

research campaigns and send sci-

entific data back to Earth until its

thrusters, which are needed to aim

the spacecraft to take and transmit

data, show signs of fuel depletion.

Spacecraft typically need to

reserve fuel for a final orbital

adjustment so they don’t collide

with other satellites, come crashing

down to Earth unexpectedly, or, in

the case of craft sent to other plan-

ets, contaminate extraterrestrial

environments. Kepler, however, is

in an isolated area 94 million miles

(151 million kilometers) from Earth,

so every last bit of fuel can be

used to continue observing and

collecting data.

Kepler was initially hobbled

when the second of four reaction

wheels, which stabilize the craft,

broke in 2013. However, NASA

engineers managed to use the

solar wind to steady the telescope

for short periods, resulting in

80-day observing campaigns

carried out as a second mission,

dubbed K2. As of March 9, the orig-

inal Kepler mission had confirmed

2,342 exoplanets, with K2 adding

307. With a few months of opera-
     tion remaining, the craft will likely
     add even more worlds to its list
     of discoveries. — A.J.

Kepler telescope has

only months of fuel left

TRANSIT TELESCOPE. NASA’s Kepler space telescope spots extrasolar planets

by watching for the dip that occurs when a planet transits, or crosses, the face of

its star. Because it stares at the same stars for extended periods of time, Kepler

can also be used to better understand some types of stellar characteristics, such

as flares and starspots. NASA AMES/W. STENZEL

HOW DO YOU BUILD A GALAXY?

Spiral galaxies gain their distinctive shape from two main

components: the bulge and the disk. The central bulge is usually

shaped like a flattened or elongated sphere, densely packed

with older stars but little gas and dust. The disk is made up of

arms containing a high fraction of gas and dust, from which new

stars are formed in open clusters. Spiral galaxies are surrounded

by a spherical, sparsely populated stellar halo, which also

contains globular clusters of ancient stars. Surrounding the

entire galaxy is a larger spherical halo of dark matter, and these

galaxies are often circled by smaller satellite galaxies attracted

through gravity.

Elliptical galaxies have very little gas and dust. They are

composed mainly of old, red stars and contain no open clusters.

These galaxies are shaped like the bulge of a spiral galaxy,

with a dense inner core and a less dense outer region, but lack

a disk or arms. Because elliptical galaxies form via mergers,

they often have accumulated many more globular clusters and

satellite galaxies in a spherical halo than spirals. They, too, are

encompassed by a larger dark matter halo. — A.K.

GALACTIC ROAD MAP. Galaxies fall into two broad categories: disklike

(also called late-type) galaxies and bulgelike (early-type) galaxies. Their

different components and structures provide information about their

evolution, teaching astronomers how galaxies are built over time.

MEET THE NEIGHBORS. The EXtreme PREcision Spectrometer (EXPRES), an instrument on the 4.3-meter Discovery

Channel Telescope, is collecting data to identify Earth-sized planets around neighboring stars.

FAST

FAC T

M87 is one of the most massive elliptical

galaxies in the local universe, weighing

an estimated 2.7 trillion solar masses and

hosting about 12,000 globular clusters.

ASTRONOMY

: ROEN KELLY