Mapping the entirety of our home galaxy is “the most Important thing in astrophysics –Optically, it’s like trying to look through a velvet cloth—black as black can be. In terms of tracing and understanding the spiral structure, essentially half of the Milky Way is terra incognita,” says Thomas Dame, Director of the Radio Telescope Data Center at the Harvard-Smithsonian Center for Astrophysics and Senior Radio Astronomer at the Smithsonian Astrophysical Observatory.
Optically Diluted by a Factor of 100 Million
Our solar system drifts between two spiral arms of the Milky Way, some 27,000 light-years from its center. Optical radiation from the opposite side of the galaxy is diluted by a factor of 100 million due to all the intervening dust and gas.
The ‘Holy Grail’ of Astronomy
The ‘Holy Grail’ of astronomy is to provide a clear perspective of our relationship to the physical universe. The map of our Milky Way galaxy is a part of that, a map that is still incomplete. Like the ancient sea-faring mariners, no spacecraft has ever traveled beyond the opaque central disk to turn back and take its picture.
“How important is it, really, for us to be able to see clear across to the other side of our own galaxy?” asks Tom Bania, a radio astronomer at Boston University. “It is the most important thing in all of astrophysics. It took humankind thousands of years to map the Earth accurately; a map of the galaxy will constrain about a dozen or so models of the structure and evolution of the Milky Way.”
A Four-Armed Spiral
For astronomers trying to map it, suggests Scientific American, “the effort is a bit like learning the anatomy of a human body from the perspective of a single skin cell somewhere on a forearm. How many spiral arms does the Milky Way have, and how do those spiral arms branch and curl around the galaxy? How many stars does the Milky Way really contain? How much does it weigh? What does our cosmic home actually look like, viewed from another nearby galaxy?”
“Radio measurements of distances to hundreds of young and massive stars clearly indicate that the Milky Way is a 4-armed spiral,” Mark Reid, Senior Radio Astronomer at the Smithsonian Astrophysical Observatory, part of the Harvard-Smithsonian Center for Astrophysics, wrote to The Daily Galaxy in an email.
Very Long Baseline Array (VLBA) Measurement
Using the National Science Foundation’s Very Long Baseline Array (VLBA), an interlinked system of 10 radio telescopes stretching across Hawaii, North America and the Caribbean, an international team of astronomers have directly measured the distance to an object called G007.47+00.05, a star-forming region located on the opposite side of the galaxy from our solar system.
Unlike optical light, radio waves travel through the intervening dust in our Milky Way Galaxy relatively unimpeded. Molecular gas clouds and massive young stars, both of which typically reside in spiral arms, are significantly bright sources of radio emission. The measurement showed G007.47+00.05 to be some 66,000 light-years away—nearly 40,000 light-years beyond the galactic center, and roughly double the distance of the previous record-holding direct measurement of distance in the Milky Way.
“Equivalent to Seeing a Baseball on the Surface of the Moon”
The VLBA’s measurement is “equivalent to seeing a baseball on the surface of the moon,” says lead author Alberto Sanna, a postdoctoral researcher at the Max Planck Institute for Radio Astronomy in Germany. The feat, Sanna says, shows “we can measure the whole extent of our galaxy, to accurately number and map the Milky Way’s spiral arms and know their true shapes, so that we can learn what the Milky Way really looks like.”
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 spiral arms, 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.
“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 distance 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.
Maser Mileposts –”Looking Through the Entire Milky Way
“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 director of The Max Planck Institute for Radio Astronomy, 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?
This image above of the Milky Way is based on 200 trigonometric parallaxes for masers in massive star forming regions from two large radio astronomy projects, the Bar and Spiral Structure Legacy (BeSSeL) Survey and the Japanese VLBI Exploration of Radio Astrometry (VERA) to survey the Milky Way from the inside out. These parallaxes allow scientists to directly measure the forms of spiral arms across roughly one-third of the Milky Way, and have extended the spiral arm traces into the portion of the Milky Way seen from the Southern Hemisphere using tangencies along some arms based on carbon monoxide emission (Reid et al. 2019).
The image clearly presents the Milky Way as a barred spiral galaxy with four major arms and some extra arm segments and spurs. It is currently the most scientifically accurate visualization of what the Milky Way looks like.
The Complete Picture
“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 Dame of the CfA and Andreas Brunthaler of MPIfR. The team reported their findings in the 13 October, 2018 issue of the journal Science.
In 2018, an international team of astronomers discovered that the Milky Way’s disc of stars becomes increasingly ‘warped’ and twisted the further away the stars are from the galaxy’s center. “We usually think of spiral galaxies as being quite flat, like Andromeda which you can easily see through a telescope,” says Richard de Grijs, a co-author and astronomer from Australia’s Macquarie University.
The first accurate 3D image of our galaxy shown at the top of the page reveals its true shape: warped and twisted. Astronomers from Macquarie University and the Chinese Academy of Sciences have used 1339 ‘standard’ stars to map the real shape of our home galaxy in a paper published in Nature Astronomy today. Artist’s impression above of the warped and twisted Milky Way disk. (Chen Xiaodian)