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Mapping the Milky Way

October 20, 2016

Cartographers (map makers) in the Age of Discovery made successive approximations to the outline of the New World based on successive explorations. When the observations of Italian explorer, Amerigo Vespucci (1454-1512), indicated that the land mass visited in those early years was not the eastern coast of Asia but a new continent, cartographer, Martin Waldseemüller (1470-1520), commemorated this by naming the continent "America" in a map called the Universalis Cosmographia.

Amerigo Vespucci (1454-1512)

Amerigo Vespucci (1454-1512).

Vespucci sailed far enough south to observe the constellations, Centaurus and Crux (the Southern Cross), as well as the Coalsack Nebula.

These were known to the ancient Greeks, and they were described by Ptolemy, but they had disappeared from northern skies after about 400 AD as a consequence of the axial precession of the Earth.

(Via Wikimedia Commons.)

Just as better knowledge of the New World allowed its maps to become more detailed in time, the same was true for our home galaxy, the Milky Way. Before the 20th century, it wasn't realized that the Milky Way was a galaxy, or that such a thing as a galaxy even existed.

A turning point came in 1920 when two prominent astronomers of their time, Harlow Shapley and Heber Curtis, debated the nature of the Milky Way, the nebulae (as galaxies were then known), and the extent of the universe. Curtis argued that the Andromeda Nebula was actually an external galaxy, but direct evidence for this would come only later with advances in technology.

Edwin Hubble was able to photograph some nebulae in detail to show individual stars. His further observations of Cepheid variables allowed his estimate of nebular distances that were clearly outside the dimension of the Milky Way. His distance estimate for the Andromeda Nebula, now known as the Andromeda Galaxy, was 275,000 parsecs (900,000 light years). The present estimate is 778,000 parsecs (2.54 million light years). The Milky Way is only about 50,000 parsecs (160,000 light years) in diameter.

Composite image of the Andromeda Galaxy in UV and deep IR

Composite image of the Andromeda Galaxy. The image is coded red for 24 μm, green for near-ultraviolet, and blue for far-ultraviolet. I recently viewed the 2016 season premiere of Drunk History. It had a segment about Timothy Leary (1920-1996), and this psychedelic image reminds me of him. As a college student in the 1960s, I heard Leary lecture on two occasions.[2] (NASA/JPL-Caltech/K. Gordon (Univ. of Arizona)/GALEX Science Team image via Wikimedia Commons.)

Knowing the shape of the Milky Way was difficult for two principle reasons. First, much of our galaxy is blocked from view at our vantage point by the massively star-filled galactic center. Second, the galaxy is not just stars separated by void. It contains a lot of obscuring dust. While this inhibits observations in visible light, radio astronomers soon realized that they could image things that their optical astronomy colleagues could not.

In 1954, Dutch astronomers, Jan Hendrik Oort (1900-1992), Hendrik C. van de Hulst (1918-2000), and C.A. Muller, published the spiral structure of the Milky Way Galaxy using observations of the 21 cm wavelength (1.42 GHz) neutral hydrogen emission line. They found that radio emissions at this wavelength passed unabsorbed through regions that obscured optical observation. In an example of swords to ploughshares, they used World War II surplus Radar equipment located at Kootwijk, The Netherlands.

Artist conception of the Milky Way (NASA, ssc2008-10a)

"X" marks the spot.

Earth's location is shown by an "X" in this artist's conception of the spiral structure of the Milky Way Galaxy.

The structure of our spiral has only been elucidated recently.[3]

(Modified NASA/JPL-Caltech/R. Hurt (SSC/Caltech) image ssc2008-10a.)

It's been sixty years since that first mapping of the Milky Way spiral arms. Since then, astronomers have launched space telescopes sensitive to wavelengths of light outside the visible spectrum, and radio astronomers have built larger antennas with much more sensitive radio receivers. All these technologies have improved our knowledge of the Milky Way. The latest map of the Milky Way, based on the first year's observations of the Gaia spacecraft of the European Space Agency, has just been released.[4-7]

The Gaia spacecraft, launched at the end of 2013, is on a five-year mission to measure the positions and proper motions of billions of stars. The Milky Way may contain as many as 400 billion stars. At this time, the positions of 1.1 billion stars have been measured, as well as the proper motions of two million stars.[5]

Detailed portion of the Gaia star map

Detailed portion of the Gaia star map.

Shown are the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC).

(ESA/Gaia/DPAC copyrighted image.)

This preliminary map was built from data collect by Gaia from July, 2014, to September, 2015.[5] Gaia's mission was to extend the observations of the Hipparcos satellite, launched on a four year mission in August of 1989.[6] This name of this satellite is based on that of the Greek astronomer, Hipparchus of Nicaea, who compiled the first stellar catalog from observation with the unaided eye.[7] Hipparcos was able to determine the brightness, position, and proper motion of about 100,000 stars, after correcting for a significant problem with its orbit.[6]

Gaia has a billion pixel detector capable of seven micro-arcseconds angular resolution.[6] This detector is composed of 106 individual charge-coupled devices (CCD).[7] As can be imagined, the data stream from such a detector is considerable, and it amounts to 40 gigabytes per day.[4,7] Just as Hipparcos, Gaia has had its own problems. Stray light getting past its 10 meter diameter sun shade has posed a challenge for imaging the fainter stars.[4,6] Also, there's been an icing problem on its telescope mirrors.[4]

At this point in its mission, Gaia has deduced the brightness, position and proper motion of 1,142 million stars.[7] The position measurements will allow for a three-dimensional map of our galaxy.[4] The proper motions were derived with comparison to the datasets from Hipparcos and Tycho-2, a star catalog that lists the 2.5 million brightest stars.[5]

The Gaia star map (first year)

The Gaia star map after the first year of observations. This map doesn't show the stars themselves, but the density of stars.[5] Lighter regions indicate a greater number of stars. The mottled appearance is a data artifact, and it will disappear when all data are compiled. (ESA/Gaia/DPAC copyrighted image. Click for a larger, annotated image.)

Since Gaia's limiting magnitude is 20, it's not expected to discover any unknown Solar System objects.[5] It can, however, detect exoplanets by an analysis of stellar motions. About 20,000 exoplanets are expected to be discovered this way to add to the present total of 3,000.[5-6] One area in which Gaia excels is the discovery of variable stars. At this point 3,194 variable stars have been detected, 386 of which are new discoveries.[7]

An updated map is expected in late 2017, and this will push the position accuracy of some stars to 10 micro-arcseconds, very close to the design limit of seven.[4] Spacecraft use star positions for navigation, and the Gaia catalog would be useful for navigation of the New Horizons spacecraft as it heads to the Kuiper belt to encounter 2014 MU69.[5] Gaia's mission will end in mid-2019, but there's a possibility of an extension to 2024 to increase the accuracy of its catalog.[4] One possible follow-up is an infrared version of Gaia, which would detect the dim red dwarf and brown dwarf stars near our Solar System.[4]


  1. H. C. van de Hulst, C. A, Muller, and J.H. Oort, "The spiral structure of the outer part of the Galactic System derived from the hydrogen emission at 21 cm wavelength," Bulletin of the Astronomical Institutes of the Netherlands, vol. 12, no. 452 (May 14, 1954), pp. 117-149.
  2. Legend of a Mind was a 1968 song by The Moody Blues. Its opening lines are, "Timothy Leary's dead - No, n-n-no he's outside, looking in." Leary died in 1996, not 1968, so this is an allusion to something else. You can see this song performed in a YouTube video.
  3. Ye Xu, Mark Reid, Thomas Dame, Karl Menten, Nobuyuki Sakai, Jingjing Li, Andreas Brunthaler, Luca Moscadelli, Bo Zhang and Xingwu Zheng, "The local spiral structure of the Milky Way," Science Advances, vol. 2, no. 9 (September 28, 2016), article no. e1600878, DOI: 10.1126/sciadv.1600878.
  4. Govert Schilling, "Star-mapping mission shows Milky Way to be larger than thought," Science, September 14, 2016, DOI: 10.1126/science.aah7296.
  5. Emily Lakdawalla, "Gaia's first galaxy map," The Planetary Society, September 14, 2016.
  6. Jonathan Amos, "Gaia space telescope plots a billion stars," BBC News, September 14, 2016 .
  7. Katyanna Quach, "Map to the stars: Gaia's first data dump a piece of 3D Milky Way puzzle," The Register (UK), September 14, 2016.

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