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The Milky Way’s shape — a peculiar disk

John Healy

Popular media is abuzz today with news regarding research by NAOC (National Astronomical Observatories of Chinese Academy of Sciences) on the shape of our galaxy, the Milky Way, published in Nature Astronomy today. A peculiar disk. A new 3D model.

Original article on Nature Astronomy: “An intuitive 3D map of the Galactic warp’s precession traced by classical Cepheids” (February 4, 2019).

Here’re two examples about that article: ABC Science News and Phys.org. The Phys.org article contains visualizations.

Milky Way is ‘S-shaped’, 3D map reveals, in a new way of looking at our celestial home

Key points:

  • We have long thought the Milky Way was warped, but it is also twisted, according to a new study of pulsating stars.
  • The new map is the most accurate representation of the Milky Way so far, say astronomers.
  • The Gaia telescope will give us a much better picture in the future.

The map provides an “unprecedented view of the Milky Way”, said study co-author Richard de Grijs of Macquarie University.

To create the map, Professor de Grijs and colleagues from the Chinese Academy of Sciences focused on 1,339 pulsating beacons known as Cepheid stars.

To cut through the dust, Professor de Grijs and colleagues based their calculations on observations of Cepheid stars made using the Wide-Field Infrared Survey Explorer, which detects longer wavelengths than visible light.

Joss Bland-Hawthorn of the University of Sydney agreed the new study was a first step, which will be followed up in more detail by observatories such as the Gaia space telescope.

“This is probably the cleanest demonstration of the warp in stars and it looks different from the warp in gas,” said Professor Bland-Hawthorn, who was also not involved in the new study.

But we don’t really know what causes these warps, he added.

The Milky Way is warped

The Milky Way galaxy’s disk of stars is anything but stable and flat. Instead, it becomes increasingly warped and twisted far away from the Milky Way’s center, according to astronomers from National Astronomical Observatories of Chinese Academy of Sciences (NAOC).

“Somewhat to our surprise, we found that in 3-D, our collection of 1339 Cepheid stars and the Milky Way’s gas disk follow each other closely. This offers new insights into the formation of our home galaxy,” says Prof. Richard de Grijs from Macquarie University in Sydney, Australia, and senior co-author of the paper. “Perhaps more importantly, in the Milky Way’s outer regions, we found that the S-like stellar disk is warped in a progressively twisted spiral pattern.”

“This new morphology provides a crucial updated map for studies of our galaxy’s stellar motions and the origins of the Milky Way’s disk,” says Dr. DENG Licai, senior researcher at NAOC and co-author of the paper.

We’ve come a long way from “perfect” celestial spheres, eh.


16 thoughts on “The Milky Way’s shape — a peculiar disk”

  1. Milky Way Collided with Dark-Matter Dominated Galaxy Less Than Billion Years Ago (June 18, 2019)

    According to new research led by Rochester Institute of Technology (RIT), the collision of the recently-discovered dwarf galaxy Antlia 2 with our Milky Way Galaxy hundreds of millions of years ago is responsible for ripples in the Milky Way’s outer gas disk.

    Dr. Chakrabarti and her colleagues also explored other potential causes for the ripples in the Milky Way’s outer disk, but ruled out the other candidates.

    The team’s paper will be published in the Astrophysical Journal Letters.

  2. This Space.com article “This 3D Map of the Milky Way Is the Best View Yet of Our Galaxy’s Warped, Twisted Shape” (August 1, 2019) by Charles Q. Choi discusses recent research on the Milky Way’s shape using Cepheid variable stars.

    These new findings shed light on the warped, twisted shape of the galaxy’s disk, researchers added.

    Using the Optical Gravitational Lensing Experiment, which monitors the brightness of nearly 2 billion stars, the scientists charted the distance between the sun and more than 2,400 Cepheids throughout the Milky Way. “This took six years but it was worth it,” study lead author Dorota Skowron, an astrophysicist at the University of Warsaw in Poland, told Space.com.

    Better 3D maps of the Milky Way can help scientists better understand its shape. For instance, “the number of the main spiral arms is still debated; also, the severity of the spiraling of the arms,” Skowron said.

  3. Lots of media coverage recently about research on “bubbles” of gamma and X-ray emissions about 25 thousand light-years distant above and below the Milky Way’s galactic center – Fermi bubbles. This Space.com artcle by Paul Sutter on September 4, 2019 is an example: “Something Strange Is Happening in the Fermi Bubbles.”

    [One possibility is that] millions of years ago, Sag A* (the more common name for our giant black hole, because who wants to keep typing or saying “Sagittarius” all the time?) ate a giant meal and got a bad case of indigestion, with the infalling material heating up, twisting around in a complicated dance of electric and magnetic forces, and managing to escape the clutches of the event horizon before falling in. That material, energized beyond belief, raced away from the center of the galaxy, riding on jets of particles accelerated to nearly the speed of light. As they fled to safety, these particles spread and thinned out, but maintained their energetic state to the present day.

  4. There’s a well done animation is this article about galactic evolution.

    ESA > “Galactic crash may have triggered Solar System formation” (May 25, 2020).

    Astronomers have known that Sagittarius [a galaxy discovered in the 1990s to be orbiting the Milky Way] repeatedly smashes through the Milky Way’s disc, as its orbit around the galaxy’s core tightens as a result of gravitational forces. Previous studies suggested that Sagittarius, a so called dwarf galaxy, had had a profound effect on how stars move in the Milky Way. Some even claim that the 10,000 times more massive Milky Way’s trademark spiral structure might be a result of the at least three known crashes with Sagittarius over the past six billion years.

    A new study, based on data gathered by ESA’s galaxy mapping powerhouse Gaia, revealed for the first time that the influence of Sagittarius on the Milky Way may be even more substantial. The ripples caused by the collisions seem to have triggered major star formation episodes, one of which roughly coincided with the time of the formation of the Sun some 4.7 billion years ago.

  5. Beyond visible light, the ultraviolet eyes of NASA’s Hubble Space Telescope continue to provide insights into the history of our very own Milky Way.

    Space.com > “Ancient explosion in Milky Way’s core lit up gas outside the galaxy” by Elizabeth Howell (June 4, 2020).

    Embedded in the center of our galaxy, the Milky Way, a supermassive black hole known as Sagittarius A* (Sgr A*) had unleashed a massive amount of energy. New research shows that the energy was so pervasive that it illuminated gas associated with two satellite galaxies of our own: the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC).

    “This shows us that different regions of the galaxy are linked — what happens in the galactic center makes a difference to what happens out in the Magellanic Stream,” Fox added [1], referring to the massive, ribbon-like gas structure that trails the Magellanic Clouds. “We’re learning about how the black hole impacts the galaxy and its environment.”

    [1] Andrew Fox, the principal investigator of the study and an astronomer at the Space Telescope Science Institute (STScI) in Baltimore.

  6. Our Milky Way was built via mergers of smaller galaxies … computer simulation …

    • Phys.org > “Astronomers discover new ‘fossil galaxy’ buried deep within the Milky Way” by Sloan Digital Sky Survey (Nov 20, 2020)

    Scientists working with data from the Sloan Digital Sky Surveys‘ Apache Point Observatory Galactic Evolution Experiment (APOGEE) have discovered a “fossil galaxy” [aka “Heracles”] hidden in the depths of our own Milky Way.

    This result, published today in Monthly Notices of the Royal Astronomical Society, may shake up our understanding of how the Milky Way grew into the galaxy we see today.

    To separate stars belonging to Heracles from those of the original Milky Way, the team made use of both chemical compositions and velocities of stars measured by the APOGEE instrument.

    See also:

    • Phys.org > “Orbits of ancient stars prompt rethink on Milky Way evolution” by ARC Centre of Excellence for All Sky Astrophysics in 3D (ASTRO 3D) (Nov 16, 2020)

    Theories on how the Milky Way formed are set to be rewritten following discoveries about the behavior of some of its oldest stars.

    An investigation into the orbits of the Galaxy’s metal-poor stars – assumed to be among the most ancient in existence – has found that some of them travel in previously unpredicted patterns.

  7. ESA’s Gaia space observatory maps our Milky Way in much better detail.

    • Cnet > “Most detailed 3D map ever of Milky Way includes nearly 2 billion stars” by Gael Fashingbauer Cooper (Dec 3, 2020) – The new data it offers provides measurements that are “orders of magnitude” more exact than prior information, scientists say.

    “Gaia has been staring at the heavens for the past seven years, mapping the positions and velocities of stars,” said Caroline Harper, head of space science at the UK Space Agency. “Thanks to its telescopes, we have in our possession today the most detailed billion-star 3D atlas ever assembled.”

    This week’s release is the first of two parts, with the second expected in 2022. Gaia’s “stellar census” began in 2013.

    See also:

    • Space.com > “A ‘tsunami’ for astrophysics: New Gaia data reveals the best map of our galaxy yet” by Megan Gannon (Dec 3, 2020)

    Data from Gaia has already been used across a wide range of applications over the past four years. The mission has helped researchers find the corpse of a galaxy that the Milky Way cannibalized 10 billion years ago, spot 20 hypervelocity stars unexpectedly zooming toward the galactic center, and identify about 1,000 nearby stars where hypothetical extraterrestrials would be able to see signs of life on Earth.

    Closer to home, the spacecraft has allowed scientists to find previously unknown asteroids, and its precise data even allowed NASA to make a crucial, last-minute adjustment to the path of its New Horizons probe in 2018 to successfully swing past the icy rock Arrokoth, the most distant and primitive object in the solar system ever visited by a spacecraft.

    See also:

    A much more dynamic history and state of our Milky Way …

    • ESA > Science & Exploration > “Gaia’s Milky Way discoveries” (Nov 26, 2020)

    ESA’s Gaia mission is surveying more than a billion stars in our cosmic neighborhood to chart the history and evolution of our home galaxy, the Milky Way. This video highlights some of the mission’s discoveries based on the first 22 months of scanning the sky.

    • YouTube > ESA > “Gaia’s Milky Way discoveries” (Nov 25, 2020)

  8. Another recap of recent research on galactic formation. The role of new data and digital simulations in crafting a more complex origin story and ongoing saga. Disk, halo, dwarf galaxies, globular clusters. The Gaia spacecraft. Stars of various metallicities. Galactic wreckage. Galactic interlopers.

    • Quanta Magazine > Astrophysics > “The New History of the Milky Way” by Charlie Wood (December 15, 2020) – Over the past two years, astronomers have rewritten the story of our galaxy.

    Michael Petersen, an astronomer at the University of Edinburgh: “The theme is that the Milky Way is not a static object. Things are changing rapidly everywhere.”

  10. A jar of stars?

    When I was a kid, there was a family day event at the local YMCA. The activities included a “how many jelly beans in the jar” contest. The prize was a stuffed animal. Well, I looked at a drawing of the jar in the contest poster and at the real (large) jar, which was sort of cube like. I decided to guess the count by estimating the number of beans along each dimension and multiplying. Obviously not considering some factors, I think I fudged that mental calculation by adding a number. I don’t remember how close I was to the actual count; but my guess was the closest one.

    This Space.com article on estimating the number of stars in our galaxy hints at the complexity of any such effort. A galaxy’s not just a bunch of stars packed closely together within a well-defined spatial “jar.”

    • Space.com > “How many stars are in the Milky Way?” by Elizabeth Howell (June 9, 2021)

    (quote) “It’s a surprisingly difficult question to answer. You can’t just sit around and count stars, generally, in a galaxy,” said David Kornreich, an assistant professor at Ithaca College in New York. He was the founder of the “Ask An Astronomer” service at Cornell University.

    Even in the Andromeda Galaxy — which is bright, large and relatively close by Earth, at 2.3 million light-years away — only the largest stars and a few variable stars (notably Cepheid variables) are bright enough to shine in telescopes from that distance. A sun-size star would be too difficult for us to see. So astronomers estimate, using some of the techniques below.

    Galaxy’s mass – solar mass vs. number of stars
    Spectroscopy, star types
    Dark matter %

    Answer: Billions … how many billions, eh.

    Related posts

    How many stars?

  11. An overview of how astronomers know the distance to a star or a distant galaxy. The cosmic distance ladder.

    • Space.com > “Some celestial yardsticks are visible in the night sky” by Joe Rao (Nov 7, 2021) – Here’s where to look to see some key landmark stars.

    (quote) In our sky this week, there are three stars that have been used to demonstrate how to measure stellar distances: Vega, 61 Cygni and Delta Cephei [a Cepheid variable].

    Vega and 61 Cygni, along with the southern star Alpha Centauri, were the first three stars whose distances were successfully measured by the trigonometric parallax method in the late 1830s.

    Delta Cephei served as astronomy’s “Rosetta stone,” providing a clue for how to solve this problem [i.e., parallaxes too small to get meaningful measurements for stars over 100 light-years away]. John Goodricke, an English amateur astronomer, discovered in 1784 and 1785 that this star appeared to brighten and fade over a roughly five-day cycle.

  12. Available for viewing via PBS’ smartphone app, a helpful overview of our galaxy’s history & dynamics, 3D visualizations, and future interactions.

    Particularly, an overview of how Gaia, a spacecraft in an orbit ~1 million miles from Earth [Sun–Earth Lagrange point L2 located approximately 1.5 million kilometres from Earth], uses parallax [with an accuracy of 10 microarcseconds] to measure positions & distances of ~1 billion stars – astrometry.

    • PBS > NOVA > S46 > E16 > “Universe Revealed: Milky Way”

  13. Research on our galaxy’s evolution finds signs of another merger.

    • Universe Today > “Gaia Finds Ancient Satellite Galaxy Pontus Embedded in Milky Way” by David Dickinson (February 23, 2022) – A recent study looked at stellar streams hidden in Gaia data, to uncover evidence of an ancient remnant dubbed Pontus.

    The study used the very latest astrometric data from Gaia to look at the distance and motion of 170 globular clusters, 41 star streams and 46 satellite galaxies bound to the Milky Way.

    The five known streams were Sagittarius, Cetus, Gaia-Sausage/Enceladus, LMS-1/Wukong, and Arjuna/Sequoia/I’itoi.

    The sixth new stream is dubbed ‘Pontus,’ meaning ‘sea’ in Greek.

    Terms

    Astrometry

    Parallax

    Astronomical forensics

  14. Edge on view of the Milky Way with several structures indicated
    Image credit: Edge on view of the Milky Way with several structures indicated, 22 February 2013, (as is) by Gaba p, licensed under Creative Commons.

    While “the age of a star is one of the most difficult parameters to determine,” analysis of a galactic set of particular stars’ brightness and position and spectroscopic data indicates an older age for a part of our Milky Way.

    • ESA > “Gaia finds parts of the Milky Way much older than expected” (Mar 23, 2022)

    Using data from ESA’s Gaia mission, astronomers [at the Max-Planck Institute for Astronomy, Heidelberg, Germany] have shown that a part of the Milky Way known as the ‘thick disc‘ began forming 13 billion years ago, around 2 billion years earlier than expected, …

    They chose to look at sub giant stars. In these stars, energy has stopped being generated in the star’s core and has moved into a shell around the core. The star itself is transforming into a red giant star. Because the sub giant phase is a relatively brief evolutionary phase in a star’s life, it permits its age to be determined with great accuracy, but it’s still a tricky calculation [inferred by comparing a star’s characteristics with computer models of stellar evolution].

    Suspected Milky Way timeline:

    Phase 1

    • (0.8 billion years after Big Bang)
    • formation of stars in nascent thick disc
    • possible formation of inner parts of halo
    • …
    • (2 billion years later)
    • merger with dwarf galaxy Gaia-Sausage-Enceladus
    • population of halo with stars
    • major population of thick disc with stars (ongoing formation until ~6 billion years after the Big Bang)

    Phase 2

    • major population of thin disc with stars (including our Sun)

    Terms

    • Metallicity

  16. Research on galactic evolution abounds with yet another Gaia data dump: “Gaia is currently the scientific mission that generates the most scientific papers …”

    • Space.com > “4 big Milky Way mysteries the next Gaia mission data dump may solve” by Tereza Pultarova (May 31, 2022)

    [Image caption] This image shows the paths of 40,000 stars located within 326 light-years of the solar system over the next 400,000 years, based on measurements and projections from the European Space Agency’s Gaia spacecraft. (Image credit: ESA/Gaia/DPAC)

    1. Where do stars come from?

    By combining the information about chemical composition with the modeling of the stellar trajectories, astronomers will be able to track groups of stars to their birthplaces inside (but also outside) the Milky Way.

    2. How does the galaxy “hang” together?

    “All the stuff in the galaxy is exerting gravity at every star, and this gravity determines how fast the star moves,” [Gaia project scientist] De Bruijne said. “So by measuring motions of stars, you also probe the matter distribution in the Milky Way. And that is really important for figuring out how the galaxy hangs together.”

    3. What happened in the Milky Way’s “childhood”?

    Gaia measurements enable astronomers to perform what they call “galactic archaeology.” By reconstructing the trajectories of millions of stars, they can learn about events that happened in the distant past, billions of years ago. These events involve cataclysmic collisions with other galaxies, the ripples of which can be observed in the galaxy to this day.

    4. What’s wrong with the Milky Way’s disk?

    Brown hopes the new data will shed more light on this disturbance [warping of our galactic disk] and its origins. The upcoming data release will contain information about the motion of more than 30 million stars in Gaia’s line of sight — that is, how fast they move toward or away from the telescope, or the so-called radial velocity. The prior releases contained radial velocities of only 7 million stars. The more information astronomers have, the finer the details their analysis tools can reveal about the galaxy.

    Entangled verse

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