VLBA measurement promises complete picture of Milky Way

Eastern end of the Very Long Baseline Array (VLBA), St.
Croix, U.S. Virgin Islands. Credit: VLBA

Astronomers using the National Science Foundation’s Very Long
Baseline Array (VLBA) have directly measured the distance to
a star-forming region on the opposite side of our Milky Way
Galaxy from the Sun. Their achievement nearly doubles the
previous record for distance measurement within our Galaxy.

“This means that, using the VLBA, we now can accurately map the
whole extent of our Galaxy,” said Alberto Sanna, of the
Max-Planck Institute for Radio Astronomy (MPIfR) in Germany.

Distance measurements are crucial for understanding the
structure of the Milky Way. Most of our Galaxy’s material,
consisting principally of stars, gas, and dust, lies within a
flattened disk, in which our Solar System is embedded. Because
we can’t see our Galaxy face-on, its structure, including the
shape of its , can only be mapped by
measuring distances to objects elsewhere in the Galaxy.

The astronomers used a technique called trigonometric parallax,
first used in 1838 to measure the distance to a star. This
technique measures the apparent shift in the sky position of a
celestial object as seen from opposite sides of the Earth’s
orbit around the Sun. This effect can be demonstrated by
holding a finger in front of one’s nose and alternately closing
each eye—the finger appears to jump from side to side.

Measuring the angle of an object’s apparent shift in position
this way allows astronomers to use simple trigonometry to
directly calculate the distance to that object. The smaller the
angle, the greater the distance. The VLBA, a continent-wide
radio telescope system with ten dish antennas distributed
across North America, Hawaii, and the Caribbean, can measure
the minuscule angles associated with great distances. In this
case, the measurement was roughly equal to the angular size of
a baseball on the Moon.

The new VLBA observations, made in 2014 and 2015, measured a
distance of more than 66,000 light-years to a star-forming
region called G007.47+00.05 on the opposite side of the Milky
Way from the Sun, well past the Galaxy’s center, some 27,000
light-years distant. The previous record for a parallax
measurement was about 36,000 light-years.

“Most of the stars and gas in our Galaxy are within this
newly-measured distance from the Sun. With the VLBA, we now
have the capability to measure enough distances to accurately
trace the Galaxy’s spiral arms and learn their true shapes,”
Sanna said.

The VLBA observations measured the to a region where new stars are being
formed. Such regions include areas where molecules of water and
methanol act as natural amplifiers of radio signals—masers, the
radio-wave equivalent of lasers for light waves. This effect
makes the radio signals bright and readily observable with
radio telescopes.

“The Milky Way has hundreds of such star-forming regions that
include masers, so we have plenty of ‘mileposts’ to use for our
mapping project, but this one is special. We’re looking all the
way through the Milky Way, past its center, way out into the
other side,” said the MPIfR’s Karl Menten.

The astronomers’ goal is to finally reveal what our own Galaxy
looks like if we could leave it, travel outward perhaps a
million light-years, and view it face-on, rather than along the
plane of its disk. This task will require many more
observations and much painstaking work, but, the scientists
say, the tools for the job now are in hand. How long will it
take?

“Within the next 10 years, we should have a fairly complete
picture,” Mark Reid of the Harvard-Smithsonian Center for
Astrophysics (CFA) predicted.

Sanna, Menten, and Reid worked with Thomas Dame of the CfA and
Andreas Brunthaler of MPIfR. The team reported their findings
in the 13 October issue of the journal Science.

Explore further:

Earth’s Milky Way neighborhood gets more respect

More information: A. Sanna el al., “Mapping spiral
structure on the far side of the Milky Way,” Science
(2017). science.sciencemag.org/cgi/doi …
1126/science.aan5452

Journal reference: Science

Provided by:
National Radio Astronomy Observatory

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