Just as you can’t see the forest of trees, sometimes you can’t see the galaxy of stars. Because we are inside our Milky Way galaxy, we can’t get a full view of the star forest around us. So this week astronomers around the world were not surprised to learn of a new discovery. Is that one of the spiral arms of our galaxy, located about 4,000 light-years from Earth, protrudes at a strange angle. They call it a rupture in one of the spiral arms of the Milky Way.
This rupture, which extends from the Sagittarius arm of our Milky Way galaxy, is the first large structure identified with such a dramatic orientation. Scientists published their discovery on July 21, 2021 in the peer-reviewed journal Astronomy and astrophysics.
The spiral arm rupture contains famous nebulae
The split region of the Sagittarius spiral arm extends for about 3,000 light-years at an acute angle from the main arm. This region of the sky is the region we look at when we look at the constellations Sagittarius and Serpentine Tail and the Milky Way flowing behind these stars. Some of the stars and nebulae located at the break of the spiral arm are the most famous in the sky. They include the Eagle Nebula, known for the Pillars of Creation photo from the Hubble Space Telescope. And it includes the Omega Nebula and the Trifid Nebula and the Lagoon Nebula. These nebulae are often photographed together.
How scientists found the break
Using data from NASA’s Spitzer Space Telescope, the scientists measured young stars that the infrared view of the telescope could see hiding inside these nebulae. Scientists also used data from ESA’s Gaia mission to measure accurate distances to stars. An author of the study, Alberto Krone-Martins, of the University of California, Irvine, said:
Distances are some of the most difficult things to measure in astronomy. Only Gaia’s recent and direct measurements of distance make the geometry of this new structure so obvious.
The combination of the two data sets revealed that the split segment of the Sagittarius arm contains young stars that move at almost the same speed and in the same direction through space. Caltech lead author Michael Kuhn said:
When we put together the Gaia and Spitzer data and finally look at this detailed, three-dimensional map, we can see that there is quite a bit of complexity in this region that had not been seen until now.
Here comes the playing field
The arms of a spiral galaxy are measured in something called a Brea. If a spiral is so tightly wound that it forms a circle, the angle of passage is zero degrees. When the spiral arm opens, the number indicating the degree of the step angle increases. Kuhn said:
A key property of spiral arms is how strong they meander around a galaxy. Most models of the Milky Way suggest that the Sagittarius Arm forms a spiral that has a step angle of about 12 degrees, but the structure we examined actually stands out at an angle of about 60 degrees.
Scientists have seen cracks in spiral arms in other galaxies before, but this is the first time such a structure has been identified in our own galaxy.
What caused the rupture of the spiral arm of the Milky Way?
Astronomers still don’t know why the Milky Way or any galaxy breaks an arm. But being able to measure the motion of individual stars will help guide them toward new understandings. The stars of the spiral arm are likely to break together. Probably the forces of the Milky Way, such as gravity and the shear of the rotation of the galaxy, influenced it. A co-author, Robert Benjamin of the University of Wisconsin-Whitewater, said:
Ultimately, this is a reminder that there are many uncertainties about the large-scale structure of the Milky Way and that we need to look at the details if we want to understand this broader picture. This structure is a small piece of the Milky Way, but it could tell us something significant about the galaxy as a whole.
Summary: Scientists discovered a spiral arm rupture in our Milky Way galaxy, located about 4,000 to 7,000 light-years from Earth. This region contains some of the most famous nebulae in the sky.
Source: a high-pitch angle structure in the Sagittarius arm
Via JPL-Caltech