Sky and Telescope - December 2015 - 12

News Notes

STELLAR I Twins Simplify Distance Calcs
Astronomers have devised a new ruler
to measure the distance to stars in our
galaxy: the spectra of stellar twins.
The most accurate galactic yardstick
used today relies on a star's parallax, the
apparent shift in the star's position as
Earth loops around the Sun. The closer
the star is to Earth, the more pronounced
its shift. This method only works for
nearby stars; even the ESA's Gaia satellite
(S&T: Apr. 2014, p. 10) will measure the
parallaxes of only 1% of the stars in the
Milky Way Galaxy.
Paula Jofré (University of Cambridge,
UK) recently stumbled upon a novel
alternative using stellar spectra. Stars
with identical spectra have other identical
characteristics, including their intrinsic
brightnesses. If two stars have the same
absolute brightness, but one is twice as
far away, it appears one-fourth as bright
as the nearby one - a relationship known
as the inverse-square law. So if astrono-

mers know the closer star's distance, they
can accurately estimate how far away the
dimmer star lies.
Jofré and her colleagues analyzed 536
stable, Sun-like stars for which high-resolution spectra were available. Within this
sample, the researchers found 175 pairs
of spectroscopic twins. And for each set
of twins, one star had a reliable parallax
measurement. With that in hand, they
could easily calculate the distance to the
other with the inverse-square method.
As the team reports in the October
21st Monthly Notices of the Royal Astronomical Society, its technique showed just
a 7.5% diﬀerence from parallax measurements from the Hipparcos satellite, which
in turn have an uncertainty of about
3%. But the twin method's uncertainty
doesn't increase for more distant stars -
a nagging problem with parallax-based
determinations.
■ SHANNON HALL

MISSIONS I Next
Target for New Horizons
A small body known as 2014 MU69 will
be the next destination for NASA's New
Horizons spacecraft. Astronomers found
it in June 2014 with the Hubble Space
Telescope, during a dedicated Kuiper Belt
search. This object lies 43.3 astronomical
units (6.49 billion km) from the Sun and is
incredibly dim, 25.6 in magnitude.
Assuming the surface is 20% reﬂective, its diameter might be about 45 km
(30 miles) across - roughly 10 times the
size of a typical comet. The team picked
2014 MU69 over a slightly larger candidate,
2014 PN70, because the spacecraft can
reach it more quickly and use less fuel getting there. Its near-circular, low-inclination
orbit also implies that this body has not
been strongly perturbed or altered since
the solar system's formation. Four trajectory corrections in late October and early
November should set New Horizons on
course for a rendezvous on January 1, 2019
(assuming NASA extends the mission).
■ J. KELLY BEATTY

ESO / M. KORNMESSER

EARLY UNIVERSE I Light Detected from the First Stars?

This illustration depicts the early galaxy
CR7. The galaxy's center (bottom) might
contain members of the ﬁrst generation of
stars. Two other clusters (top) are tainted
with elements heavier than helium.

December 2015 sky & telescope

Astronomers have come upon a tantalizing signal that might be from some of
the universe's ﬁrst stars.
The ﬁrst stars appeared more than
13 billion years ago, in swirling clouds
of pristine hydrogen and helium. These
ﬁrst-generation stars, called Population
III, would have been responsible for
churning out the heavier elements that
shaped the evolution of second-generation
stars and the galaxies they lived in.
Until now, this ﬁrst, vital generation
has been largely hypothetical. Now David
Sobral (University of Lisbon, Portugal)
and colleagues have discovered that the
primordial galaxy CR7 has a spectrum
with exactly two emission lines: the
Lyman-alpha line from excited hydrogen
atoms and another emission line from
ionized helium.
"If this is Population-III star formation, then that is very exciting," says Jonathan Tan (University of Florida). "It would

be the ﬁrst example of such a process."
But he cautions that marginally contaminated, second-generation stars might also
produce the observed spectrum.
Whatever the stars are, they are not
alone. The team reports in the August
1st Astrophysical Journal that two clusters of redder, second-generation stars
lie roughly 10,000 light-years away from
the bluish, pristine clump at the galaxy's
center. The red stars irradiate their bluer
siblings, altering their chemistry. The
presence of other stars means that, even
if the blue ones are ﬁrst generation, they
are not purely primordial.
The galaxy shines at us from 12.9 billion years ago. It's unclear why one part
of the galaxy would already be forging the
second generation of stars while another
part is just getting started on the ﬁrst. It
could be that we are witnessing a wave of
star formation sweep through the galaxy.
■ MONICA YOUNG

Sky and Telescope - December 2015

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