Reading a Chapter of Galactic History from a Single Star

The globular cluster Terzan 5, one of the oldest and most massive globular clusters in the Milky Way
Credit: ESO/F. Ferraro; CC BY 4.0

The Milky Way, like the galaxy NGC 1300 shown here in an image from the Hubble Space Telescope, has a central bar of stars. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); Acknowledgment: P. Knezek (WIYN)

The star SOS1 is not like its neighbors. Using chemical and dynamical data, stellar sleuths have tracked this star from its current home in the Milky Way’s central bar back to its likely origin in one of the most massive globular clusters in our galaxy.

A Star in a Bar

Today, the Milky Way has an intricate and interlocking structure: thin and thick disks of stars surrounded by an extended halo, with a bulge of old stars at the center. A bar of stars cuts across the center of our galaxy, and globular clusters — ancient collections of thousands to millions of stars — dot the galactic bulge and halo. These structures didn’t always exist, and a major goal for galactic research is understanding when and how the many components of our galaxy were assembled.

One piece of the puzzle might be provided by the star 2M17454705-2639109, also known as SOS1. This star is located in the busy galactic downtown of the Milky Way’s center, orbiting within the central bar of stars. Data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) show that SOS1 has a curious chemical composition that sets it apart from its neighbors. Now, researchers have shown that these chemical differences may be evidence that SOS1 originated far from its current location — and that many other stars in the galactic bar might have completed similar journeys.

The reddish stars of the globular cluster Liller 1 glow behind bright blue stars in the foregroun
Credit: ESA/Hubble & NASA, F. Ferraro; CC BY 4.0

SOS1: Far from Home?

A team led by Stefano Souza (Leibniz Institute for Astrophysics Potsdam; University of São Paolo; Max Planck Institute for Astronomy) investigated SOS1’s origins by first comparing its chemical abundance pattern to those of different populations of stars in the Milky Way. The observed pattern of low carbon, high nitrogen, and high aluminum matches expectations for second-generation stars in globular clusters: densely packed, roughly spherical collections of thousands to millions of stars.

Chemical (left) and age (right) comparison between SOS1 and stars in the globular cluster Terzan 5.
Credit: Souza et al. 2024

But how would a star born in a globular cluster end up in the Milky Way’s central bar? Souza’s team highlighted two possible scenarios: SOS1 might have been ejected from its home cluster by a gravitational interaction with a binary star system, or — deemed more likely — it could have been stolen from its home cluster by the tidal forces of the Milky Way.

Candidate Clusters

Souza’s team used N-body simulations to determine if SOS1 once called one of the existing globular clusters home. (The team notes that it’s possible that SOS1’s parent star cluster no longer exists, having been pulled apart by the Milky Way’s powerful tidal forces.) The likeliest candidate is Terzan 5, which is among the most massive and most ancient globular clusters in the Milky Way. The simulations suggest that SOS1 might have been bound to this cluster 353 million years ago.

The chemical abundances of SOS1 support this hypothesis, since SOS1’s curious chemical makeup is consistent with that of the oldest and most metal-poor stars in the cluster. The final clue would be a comparison of the ages of Terzan 5 and SOS1. Though the data did not allow for a precise determination of the star’s age, the preliminary analysis suggests that it is of a similar age to the cluster.

The chemical similarities and dynamical properties make it likely that SOS1 once resided in a globular cluster, possibly Terzan 5. Its current residence in the Milky Way’s central bar supports the idea that ancient globular clusters contributed stars to the bar through tidal stripping.

By Kerry Hensley

Citation

“Tracing Back a Second-Generation Star Stripped from Terzan 5 by the Galactic Bar,” Stefano O. Souza et al 2024 ApJL 977 L33.
doi:10.3847/2041-8213/ad91af

Source: American Astronomical Society/AAS-Nova

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Sagittarius A*: Telescopes Show the Milky Way's Black Hole is Ready for a Kick

Illustration of Sagittarius A*
Credit: NASA/CXC/M.Weiss

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A Tour of Telescopes Show the Milky Way's Black Hole is Ready for a Kick - More Videos

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This artist’s illustration depicts the findings of a new study about the supermassive black hole at the center of our galaxy called Sagittarius A* (abbreviated as Sgr A*). As reported in our latest press release, this result found that Sgr A* is spinning so quickly that it is warping spacetime — that is, time and the three dimensions of space — so that it can look more like an American football.

These results were made with NASA’s Chandra X-ray Observatory and the NSF’s Karl G. Jansky Very Large Array (VLA). A team of researchers applied a new method that uses X-ray and radio data to determine how quickly Sgr A* is spinning based on how material is flowing towards and away from the black hole. They found Sgr A* is spinning with an angular velocity that is about 60% of the maximum possible value, and with an angular momentum of about 90% of the maximum possible value.

Black holes have two fundamental properties: their mass (how much they weigh) and their spin (how quickly they rotate). Determining either of these two values tells scientists a great deal about any black hole and how it behaves. In the past, astronomers made several other estimates of Sgr A*’s rotation speed using different techniques, with results ranging from Sgr A* not spinning at all to it spinning at almost the maximum rate.

The new study suggests that Sgr A* is, in fact, spinning very rapidly, which causes the spacetime around it to be squashed down. The illustration shows a cross-section of Sgr A* and material swirling around it in a disk. The black sphere in the center represents the so-called event horizon of the black hole, the point of no return from which nothing, not even light, can escape.

Looking at the spinning black hole from the side, as depicted in this illustration, the surrounding spacetime is shaped like an American football. The faster the spin the flatter the football.

The yellow-orange material to either side represents gas swirling around Sgr A*. This material inevitably plunges towards the black hole and crosses the event horizon once it falls inside the football shape. The area inside the football shape but outside the event horizon is therefore depicted as a cavity. The blue blobs show jets firing away from the poles of the spinning black hole. Looking down on the black hole from the top, along the barrel of the jet, spacetime is a circular shape.

A black hole’s spin can act as an important source of energy. Spinning supermassive black holes produce collimated outflows such as jets when their spin energy is extracted, which requires that there is at least some matter in the vicinity of the black hole. Because of limited fuel around Sgr A*, this black hole has been relatively quiet in recent millennia with relatively weak jets. This work, however, shows that this could change if the amount of material in the vicinity of Sgr A* increases.

Chandra X-ray image of Sagittarius A* and the surrounding region
Credit: NASA/CXC/Univ. of Wisconsin/Y.Bai, et al.

To determine the spin of Sgr A*, the authors used an empirically based technique referred to as the “outflow method” that details the relationship between the spin of the black hole and its mass, the properties of the matter near the black hole, and the outflow properties. The collimated outflow produces the radio waves, while the disk of gas surrounding the black hole is responsible for the X-ray emission. Using this method, the researchers combined data from Chandra and the VLA with an independent estimate of the black hole’s mass from other telescopes to constrain the black hole’s spin.

The paper describing these results led by Ruth Daly (Penn State University) is published in the January 2024 issue of the Monthly Notices of the Royal Astronomical Society and appears online at https://ui.adsabs.harvard.edu/abs/2024MNRAS.527..428D/abstract.

The other authors are Biny Sebastian (University of Manitoba, Canada), Megan Donahue (Michigan State University), Christopher O’Dea (University of Manitoba), Daryl Haggard (McGill University) and Anan Lu (McGill University).

NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

Quick Look: Telescopes Show the Milky Way's Black Hole is Ready for a Kick

Source: NASA’s Chandra X-ray Observatory

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Visual Description:

TThis artist's illustration shows a cross-section of Sagittarius A*, pronounced as "SAJ-ee-TARE-ee-us A-star", the supermassive black hole near the center of our Milky Way galaxy.

In the middle of the image, the spinning, circular black hole is presented from the side in black. The swirling gas that surrounds Sagittarius A* is presented in white and yellow on either side of the black hole, within a rectangular-shaped dotted line, indicating the representation is a cross-section view. The shape of the spacetime surrounding Sgr A*, seen as the inner edge of the white and yellow gas on either side of the black hole, has been squashed down. The spacetime around Sgr A* thus resembles the shape of an American football. The background of the image contains a multitude of faint stars, peeking out from within brooding, dark red, indistinct clouds.

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Fast Facts for Sagittarius A*:

Category: Black Holes, Milky Way Galaxy
Coordinates (J2000): RA 17h 45m 40.125s | Dec -29° 00´ 28.24"
Constellation: Sagittarius
Instrument: ACIS
Also Known As: Galactic Center
References: Daly, R. et al., 2024, MNRAS 527, 428–436. arXiv:2310.12108
Distance Estimate: About 26,000 light-years

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Galactic Center: Scientists Take Viewers to the Center of the Milky Way

 Galactic Center

Credit: NASA/CXC/Pontifical Catholic Univ. of Chile /C.Russell et al.

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A new visualization provides an exceptional virtual trip — complete with a 360-degree view — to the center of our home galaxy, the Milky Way. This project, made using data from NASA's Chandra X-ray Observatory and other telescopes, allows viewers to control their own exploration of the fascinating environment of volatile massive stars and powerful gravity around the monster black hole that lies in the center of the Milky Way.

The Earth is located about 26,000 light years, or about 150,000 trillion miles, from the center of the Galaxy. While humans cannot physically travel there, scientists have been able to study this region by using data from powerful telescopes that can detect light in a variety of forms, including X-ray and infrared light.

360-Degree Video: An Immersive Visualization of the Galactic Center

This visualization builds on infrared data with the European Southern Observatory's Very Large Telescope of 30 massive stellar giants called Wolf-Rayet stars that orbit within about 1.5 light years of the center of our Galaxy. Powerful winds of gas streaming from the surface of these stars are carrying some of their outer layers into interstellar space.

When the outflowing gas collides with previously ejected gas from other stars, the collisions produce shock waves, similar to sonic booms, which permeate the area. These shock waves heat the gas to millions of degrees, which causes it to glow in X-rays. Extensive observations with Chandra of the central regions of the Milky Way have provided critical data about the temperature and distribution of this multimillion-degree gas.

Sagittarius A*
 Credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI 

Astronomers are interested in better understanding what role these Wolf-Rayet stars play in the cosmic neighborhood at the Milky Way's center. In particular, they would like to know how the stars interact with the Galactic center's most dominant resident: the supermassive black hole known as Sagittarius A* (abbreviated Sgr A*). Pre-eminent yet invisible, Sgr A* has the mass equivalent to some four million Suns.

The Galactic Center visualization is a 360-degree movie that immerses the viewer into a simulation of the center of our Galaxy. The viewer is at the location of Sgr A* and is able to see about 25 Wolf-Rayet stars (white, twinkling objects) orbiting Sgr A* as they continuously eject stellar winds (black to red to yellow color scale). These winds collide with each other, and then some of this material (yellow blobs) spirals towards Sgr A*. The movie shows two simulations, each of which start around 350 years in the past and span 500 years. The first simulation shows Sgr A* in a calm state, while the second contains a more violent Sgr A* that is expelling its own material, thereby turning off the accretion of clumped material (yellow blobs) that is so prominent in the first portion.

Scientists have used the visualization to examine the effects Sgr A* has on its stellar neighbors. As the strong gravity of Sgr A* pulls clumps of material inwards, tidal forces stretch the clumps as they get closer to the black hole. Sgr A* also impacts its surroundings through occasional outbursts from its vicinity that result in the expulsion of material away from the giant black hole, as shown in the later part of the movie. These outbursts can have the effect of clearing away some of the gas produced by the Wolf-Rayet winds.

The researchers, led by Christopher Russell of the Pontifical Catholic University of Chile, used the visualization to understand the presence of previously detected X-rays in the shape of a disk that extend about 0.6 light years outward from Sgr A*. Their work shows that the amount of X-rays generated by these colliding winds depends on the strength of outbursts powered by Sgr A*, and also the amount of time that has elapsed since an eruption occurred. Stronger and more recent outbursts result in weaker X-ray emission.

The information provided by the theoretical modeling and a comparison with the strength of X-ray emission observed with Chandra led Russell and his colleagues to determine that Sgr A* most likely had a relatively powerful outburst that started within the last few centuries. Moreover, their findings suggest the outburst from the supermassive black hole is still affecting the region around Sgr A* even though it ended about one hundred years ago. 

The 360-degree video of the Galactic Center is ideally viewed in virtual reality (VR) goggles, such as Samsung Gear VR or Google Cardboard. The video can also be viewed on smartphones using the YouTube app. Moving the phone around pans to show a different portion of the movie, mimicking the effect in the VR goggles. Finally, most browsers on a computer also allow 360-degree videos to be shown on YouTube. To look around, either click and drag the video, or click the direction pad in the corner. 

Christopher Russell presented this new visualization and the related scientific findings at the 231st meeting of the American Astronomical Society in Washington, DC. Some of the results are based on a paper by Russell et al published in 2017 in the Monthly Notices of the Royal Astronomical Society. An online version is here. The co-authors of this paper are Daniel Wang from University of Massachusetts in Amherst, Mass. and Jorge Cuadra from Pontifical Catholic University of Chile. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

A Quick Look at the Galactic Center

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Fast Facts for Galactic Center:

Category: Normal Galaxies & Starburst Galaxies, Milky Way Galaxy, Black Holes
Coordinates (J2000): RA 17h 45m 40s | Dec -29° 00´ 28.00"
Constellation: Sagittarius
Instrument: ACIS
References: Russell, C. et al. 2017, MNRAS, 464, 4958, arXiv:1607.01562
Distance Estimate: About 26,000 light years

Source: NASA’s Chandra X-ray Observatory

Giant, Magnetized Outflows from our Galactic Center

 A false-color image of our Milky Way as seen in a projection that shows the galactic center at the center of the image, the plane of the galaxy stretching across the central band, and the two arc-shaped radio lobes of emission seen extending north and south of the plane. Several of the newly discovered magnetic structures are labeled.  Credit: Carretti et al., and Nature.  Low Resolution Image (jpg)

Two years ago, CfA astronomers reported the discovery of giant, twin lobes of gamma-ray emission protruding about 50,000 light-years above and below the plane of our Milky Way galaxy, and centered on the supermassive black hole at our galaxy's core. The scientists argued then that the bubbles were produced either by an eruption from the black hole sometime in the past, or else by a burst of star formation in that vicinity. 

It now appears that these giant bubbles of hot gas can be seen at radio wavelengths as well. Writing in the new issue of the journal Nature, CfA astronomer Gianni Bernardi and eight of his colleagues describe finding humongous lobes of radio emission emanating from the Galactic Center. Moreover, the emission is polarized, a general property that electromagnetic radiation can have; some sunglasses take advantage of the fact that reflected sunlight becomes polarized. In the case of radio wavelengths, the explanation for polarization is the presence of strong magnetic fields. 

The scientists calculate that the radio lobes, which closely match the gamma-ray lobes in overall dimensions but which contain three ridge-like substructures, are probably polarized by the presence of strong magnetic fields that extend out of the galactic plane in both directions for tens of thousands of light-years, and which contain an energy roughly equivalent to the total current output of the Sun for a time equal to the lifetime of the universe. They argue that the activity is driven by star-formation activity, rather than black-hole activity, and that it originates in a region around the Galactic Center about 650 light-years in size. Not least, the scientists argue that the ridges seen in the magnetically-shaped outflow are the result of several episodes of star-formation that constitute a phonograph-like record of star formation in this region over at least the past ten million years. 

Source: Smithsonian Astrophysical Observatory

Galactic Center: New Vista of Milky Way Center Unveiled

Credit: NASA/CXC/UMass/D. Wang et al.

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of the Galactic Center

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Chandra Mosaic of Galactic Center
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A dramatic new vista of the center of the Milky Way galaxy from NASA's Chandra X-ray Observatory exposes new levels of the complexity and intrigue in the Galactic center. The mosaic of 88 Chandra pointings represents a freeze-frame of the spectacle of stellar evolution, from bright young stars to black holes, in a crowded, hostile environment dominated by a central, supermassive black hole.

Permeating the region is a diffuse haze of X-ray light from gas that has been heated to millions of degrees by winds from massive young stars - which appear to form more frequently here than elsewhere in the Galaxy - explosions of dying stars, and outflows powered by the supermassive black hole - known as Sagittarius A* (Sgr A*). Data from Chandra and other X-ray telescopes suggest that giant X-ray flares from this black hole occurred about 50 and about 300 years earlier.

The area around Sgr A* also contains several mysterious X-ray filaments.
Some of these likely represent huge magnetic structures interacting with streams of very energetic electrons produced by rapidly spinning neutron stars or perhaps by a gigantic analog of a solar flare.

Scattered throughout the region are thousands of point-like X-ray sources. These are produced by normal stars feeding material onto the compact, dense remains of stars that have reached the end of their evolutionary trail - white dwarfs, neutron stars and black holes.

Because X-rays penetrate the gas and dust that blocks optical light coming from the center of the galaxy, Chandra is a powerful tool for studying the Galactic Center. This image combines low energy X-rays (colored red), intermediate energy X-rays (green) and high energy X-rays (blue).

The image is being released at the beginning of the "Chandra's First Decade of Discovery" symposium being held in Boston, Mass. This four-day conference will celebrate the great science Chandra has uncovered in its first ten years of operations. To help commemorate this event, several of the astronauts who were onboard the Space Shuttle Columbia - including Commander Eileen Collins - that launched Chandra on July 23, 1999, will be in attendance.

Fast Facts for Galactic Center:

Scale: Image is 120 by 48 arcmin
Category: Normal Galaxies & Starburst Galaxies, Milky Way Galaxy
Coordinates: (J2000) RA 17h 45m 23s | Dec -29° 01' 17
Constellation: Sagittarius
Observation Date: 88 pointings between 03/29/2000 - 07/19/2007
Observation Time: 26 days 3 hours
Obs. ID: 658,944-945, 1561, 2267-2296, 2943, 2951-2954, 3392-3393, 3549, 3663, 3665, 4500, 4683-4684, 5360, 5892, 5950-5954, 6113, 6363, 6639, 6640-6646, 7034-7048, 7345-7346, 7554-7557, 8214, 8459, 8567
Color Code Energy: Red (1-3 keV); Green (3-5 keV); Blue (5-8 keV)
Instrument: ACIS
References: M. P. Muno, et al., 2009 ApJS 181 110-128
Distance Estimate: About 26,000 light years