Introduction

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  2. Introduction
  3. About VERA

About VERA

VERA is a project aimed at creating a three-dimensional map of the Milky Way Galaxy. Construction of its observation network was completed in 2002, and observations for the project continued until 2022. Since then, VERA's radio telescopes have remained active, contributing to international observations, including those conducted by the East Asian VLBI Network (EAVN).

The goal of the VERA project is to use a radio interferometry technique called VLBI to measure the distances and motions of radio-emitting objects in the Milky Way Galaxy with unprecedented precision, ultimately revealing the true structure of the Galaxy. Led by the National Astronomical Observatory of Japan, researchers from various fields and numerous universities and research institutes participated in the project, conducting observations from 2003 to 2022.

*VERA stands for “VLBI Exploration of Radio Astrometry” and means “truth” in Latin.

Measuring Distances

To directly determine the distances of celestial objects, we measure a quantity known as “annual parallax.” Because the Earth orbits the Sun over the course of a year, the apparent positions of celestial objects-meaning their direction as seen from Earth-shift slightly between seasons, such as between summer and winter. This small shift is called annual parallax. Naturally, it is an extremely tiny displacement, imperceptible to the human eye.

In the VERA project, the positions of many radio-emitting celestial objects in the Milky Way are measured with ultra-high precision throughout the year. By analyzing their annual parallax, researchers can precisely determine their locations within the Galaxy.

Triangulation and Annual Parallax

Triangulation is a method of measuring an object’s distance by observing it from two different points and calculating the distance based on the difference in its apparent direction. The angular difference between the two viewpoints is called parallax (or triangular parallax). Triangular parallax is commonly used in daily life, with the most familiar example being human vision. Humans perceive depth and distance by utilizing the parallax created between their two eyes.

When measuring the distances of celestial objects, triangulation is the most accurate method that does not rely on assumptions. (By contrast, other methods of determining astronomical distances are indirect and depend on certain assumptions, such as the brightness of stars and galaxies or the expansion law of the universe.)

As the Earth orbits the Sun over the course of a year, the apparent positions of celestial objects-meaning their direction as seen from Earth-measure celestial positions with ultra-high precisionshift slightly between seasons, such as between summer and winter. This small shift is known as annual parallax.

Annual parallax and the distance of a celestial object are inversely proportional. This means that by measuring annual parallax, we can determine the object's distance directly, without relying on any assumptions. However, annual parallax is an extremely small effect, imperceptible to the human eye. For example, when observing a celestial object near the center of the Milky Way, the annual parallax is an astonishingly tiny 1/30 millionth of a degree.

VERA, however, utilizes cutting-edge observational technology to measure celestial positions with ultra-high precision-down to 10 microarcseconds (1/360 millionth of a degree). By tracking celestial positions with this level of accuracy throughout the year and measuring their annual parallax, VERA can precisely determine their distances.

To put VERA’s precision into perspective, 10 microarcseconds is equivalent to the apparent size of a 1-yen coin placed on the surface of the Moon as seen from Earth.

*One microarcsecond is one-millionth of an arcsecond. One arcsecond is 1/3,600 of a degree.

>>The History of Annual Parallax Measurement

As the Earth orbits the Sun, the positions of celestial objects shift slightly with the seasons (annual parallax). VERA detects these changes by precisely measuring the positions of celestial objects and calculates their distance based on the magnitude of the shift.

VERA Results

VERA has measured the annual parallax and motion of more than 100 celestial objects. In addition, similar observations have been carried out by the U.S. VLBA, and the combined results are summarized below, showing the distribution of star-forming regions in the Milky Way, which are ideal for depicting its spiral structure. As shown in the figure, the Milky Way’s rotational motion and spiral structure are visible. These measurements have also enabled the precise determination of important constants, such as the distance to the center of the Milky Way and the Milky Way’s rotation speed near the solar system.

Achievements of the 20-Year VERA Project:
− Advancing High-Precision Astrometry in the 120-Year History of the National Astronomical Observatory of Japan, Mizusawa −
https://www.miz.nao.ac.jp/veraserver/hilight/20201125_catalog/

Here is the diagram.

Objects represented by arrows of the same color belong to the same arm. It has been revealed that the distribution and rotational motion of objects along the spiral arms, as previously imagined (shown in the artistic image with the overlaid black curves), align closely with the direct observations made in this study.

Special Feature: “Two-Beam Observation”

VERA's most distinctive feature is its “two-beam” telescope, which allows for the simultaneous observation of two celestial objects.

In traditional single-beam observations, atmospheric turbulence made it difficult to precisely measure the positions of celestial objects. However, by observing two adjacent celestial objects at the same time, VERA cancels out the effects of atmospheric turbulence, enabling highly precise positional measurements. (This observational technique is known as relative VLBI.)

*VERA is the world’s only radio telescope designed for astronomical observations that can receive two beams simultaneously.

In conventional VLBI observations, atmospheric fluctuations make precise position determination difficult. However, VERA simultaneously observes two nearby celestial objects (mainly a maser source and a quasar), allowing it to cancel out shared atmospheric fluctuations and achieve high-precision position measurements.

Target Objects

VERA primarily observes maser sources within our galaxy.

Maser sources are celestial objects that emit extremely strong radio waves at specific frequencies, many of which are either newly formed young stars or aging stars.

Among masers, VERA focuses on the brightest types, such as water masers and silicon monoxide masers. Currently, around 1,000 maser objects have been identified in the Milky Way.

Additionally, VERA observes extragalactic radio galaxies and QSOs as positional reference objects for its dual-beam observations.

Blue represents star-forming regions (areas where young stars are being born), while red represents late-type stars (old stars).

The lower-right area corresponds to the southern sky, which VERA cannot observe, resulting in a lack of detected objects in that region.

Array and Observation Stations

VERA's observation array consists of four stations: Mizusawa, Iriki, Ogasawara, and Ishigaki-jima.

By combining observations from these four stations, VERA achieves the performance equivalent to a telescope with a diameter of 2,300 km.

Domestic Radio Telescopes
  • Ibaraki University Takahagi 32m & Hitachi 32m radio telescopes
  • National Astronomical Observatory of Japan Nobeyama Radio Observatory
  • Japan Aerospace Exploration Agency Usuda Deep Space Center 64-m parabolic antenna
  • Gifu University 11-meter radio telescope
  • Yamaguchi University 32-m radio telescope

Observation Equipment

The system configuration of the VERA observation stations is shown in the diagram below.

Click on each section to view a detailed explanation of the corresponding equipment.

Diagram of VERA Observation Station System Configuration