milky way Archives

May 12, 2018 by Matt Williams

Evidence for Thousands of Black Holes Buzzing Around the Center of the Milky Way

On September 14th, 2013, astronomers caught the largest X-ray flare ever detected from the supermassive black hole at the center of the Milky Way, known as Sagittarius A* (Sgr A*). Credit: NASA/CXC/Stanford/I. Zhuravleva et al.

Since the 1970s, astronomers have understood that a Supermassive Black Hole (SMBH) resides at the center of the Milky Way Galaxy. Located about 26,000 light-years from Earth between the Sagittarius and Scorpius constellations, this black hole has come to be known as Sagittarius A* (Sgr A*). Measuring 44 million km across, this object is roughly 4 million times as massive as our Sun and exerts a tremendous gravitational pull.

Since that time, astronomers have discovered that most massive galaxies have SMBHs at their core, which is what separates those that have an Active Galactic Nuclei (AGN) from those that don’t. But thanks to a recent survey conducted using NASA’s Chandra X-ray Observatory, astronomers have discovered evidence for hundreds or even thousands of black holes located near the center of the Milky Way Galaxy.

The study which described their findings was recently published in the journal Nature under the title “A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy“. The study was led by Chuck Hailey, the Pupin Professor of Physics and the Co-Director of the Columbia Astrophysics Laboratory (CAL) at Columbia University, and including members from the Instituto de Astrofísica at the Pontificia Universidad Católica de Chile and the Harvard-Smithsonian Center for Astrophysics.

The center of the Milky Way Galaxy, with X-ray binaries circled in red, other X-ray sources circled in yellow, and Sagittarius A* circled in blue at the center. Credit: NASA/CXC/Columbia University/C. Hailey et al.

Using Chandra data, the team searched for X-ray binaries containing black holes that were in the vicinity of Sgr A*. To recap, black holes are not detectable in visible light. However, black holes (or neutron stars) that are locked in close orbits with a star will pull material from their companions, which will then be accreted onto the black holes’ disks and heated up to millions of degrees.

This will result in the release of X-rays which can then be detected, hence why these systems are called “X-ray binaries”. Using Chandra data, the team sought out X-ray of sources that were located within roughly 12 light years of Sgr A*. They then selected sources with X-ray spectra similar to those of known X-ray binaries, which emit relatively large amounts of low-energy X-rays.

Using this method, they detected fourteen X-ray binaries within about three light years of Sgr A*, all of which contained stellar-mass black holes (between 5 and 30 times the mass of our Sun). Two of these sources had been identified by previous studies and were eliminated from the analysis, while the remaining twelve (circled in red in the image above) were newly-discovered.

Other sources which relatively large amounts of high energy X-rays (labeled in yellow) were believed to be binaries containing white dwarfs. Hailey and his colleagues concluded that the majority of the dozen X-ray binaries were likely to contain black holes, based on their variability and the fact that their X-ray emissions over the course of several years was different from what is expected from binaries containing neutron stars.

*Artist”s impression of a black hole binary, consisting of a black hole siphoning material from its companion. Credit: ESO/L. Calçada*

Given that only the brightest X-ray binaries containing black holes are likely to be detectable around Sgr A* (given its distance from Earth), Hailey and his colleagues concluded that this detection implies the existence of a much larger population. By their estimates, there could be at least 300 and as many as one thousand stellar-mass black holes present around Sgr A*.

These findings confirmed what theoretical studies on the dynamics of stars in galaxies have indicated in the past. According to these studies, a large population of stellar mass black holes (as many as 20,000) could drift inward over the course of millions of years and collect around an SMBH. However, the recent analysis conducted by Hailey and his colleagues was the first observational evidence of black holes congregating near Sgr A*.

Naturally, the authors acknowledge that there are other explanations for the X-ray emissions they detected. This includes the possibility that half of the dozen sources they observed are millisecond pulsars – very rapidly rotating neutron stars with strong magnetic fields. However, based on their observations, Hailey and his team strongly favor the black hole explanation.

In addition, a follow-up study conducted by Aleksey Generozov (et al.) of Columbia University – titled “An Overabundance of Black Hole X-Ray Binaries in the Galactic Center from Tidal Captures” – indicated that there could be as many as 10,000 to 40,000 black holes binaries at the center of our galaxy. According to this study, these binaries would be the result of companions being captured by black holes.

In February 2016, LIGO detected gravity waves for the first time. As this artist's illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet. — *Artist’s impression of merging binary black holes. Credit: LIGO/A. Simonnet.*

In addition to revealing much about the dynamics of stars in our galaxy, this study has implications for the emerging field of gravitational wave (GW) research. Essentially, by knowing how many black holes reside at the center of galaxies (which will periodically merge with one another), astronomers will be able to better predict how many gravitational wave events are associated with them.

From this, astronomers could create predictive models about when and how GW events are likely to happen, and well as discerning what role they may play in galactic evolution. And with next-generation instruments – like the James Webb Space Telescope (JWST) and the ESA’s Advanced Telescope for High Energy Astrophysics (ATHENA) – astronomers will be able to determine exactly how many black holes reside near the center of our galaxy.

Further Reading: NASA

February 27, 2018 by Matt Williams

Amazing High Resolution Image of the Core of the Milky Way, a Region with Surprisingly Low Star Formation Compared to Other Galaxies

NASA's Spitzer Space Telescope captured this stunning infrared image of the center of the Milky Way Galaxy, where the black hole Sagitarrius A resides. Credit: NASA/JPL-Caltech

Compared to some other galaxies in our Universe, the Milky Way is a rather subtle character. In fact, there are galaxies that are a thousands times as luminous as the Milky Way, owing to the presence of warm gas in the galaxy’s Central Molecular Zone (CMZ). This gas is heated by massive bursts of star formation that surround the Supermassive Black Hole (SMBH) at the nucleus of the galaxy.

The core of the Milky Way also has a SMBH (Sagittarius A*) and all the gas it needs to form new stars. But for some reason, star formation in our galaxy’s CMZ is less than the average. To address this ongoing mystery, an international team of astronomers conducted a large and comprehensive study of the CMZ to search for answers as to why this might be.

The study, titled “Star formation in a high-pressure environment: an SMA view of the Galactic Centre dust ridge” recently appeared in the Monthly Notices of the Royal Astronomical Society. The study was led by Daniel Walker of the Joint ALMA Observatory and the National Astronomical Observatory of Japan, and included members from multiple observatories, universities and research institutes.

*A false color Spitzer infrared image of the Milky Way’s Central Molecular Zone (CMZ). Credit: Spitzer/NASA/CfA*

For the sake of their study, the team relied on the Submillimeter Array (SMA) radio interferometer, which is located atop Maunakea in Hawaii. What they found was a sample of thirteen high-mass cores in the CMZ’s “dust ridge” that could be young stars in the initial phase of development. These cores ranged in mass from 50 to 2150 Solar Masses and have radii of 0.1 – 0.25 parsecs (0.326 – 0.815 light-years).

They also noted the presence of two objects that appeared to be previously unknown young, high-mass protostars. As they state in their study, all of this indicated that stars in CMZ had about the same rate of formation as those in the galactic disc, despite their being vast pressure differences:

“All appear to be young (pre-UCHII), meaning that they are prime candidates for representing the initial conditions of high-mass stars and sub-clusters. We compare all of the detected cores with high-mass cores and clouds in the Galactic disc and find that they are broadly similar in terms of their masses and sizes, despite being subjected to external pressures that are several orders of magnitude greater.”

To determine that the external pressure in the CMZ was greater, the team observed spectral lines of the molecules formaldehyde and methyl cyanide to measure the temperature of the gas and its kinetics. These indicated that the gas environment was highly turbulent, which led them to the conclusion that the turbulent environment of the CMZ is responsible for inhibiting star formation there.

*A radio image from the NSF’s Karl G. Jansky Very Large Array showing the center of our galaxy. Credit: NSF/VLA/UCLA/M. Morris et al.*

As they state in their study, these results were consistent with their previous hypothesis:

“The fact that >80 percent of these cores do not show any signs of star-forming activity in such a high-pressure environment leads us to conclude that this is further evidence for an increased critical density threshold for star formation in the CMZ due to turbulence.”

So in the end, the rate of star formation in a CMZ is not only dependent on their being a lot of gas and dust, but on the nature of the gas environment itself. These results could inform future studies of not only the Milky Way, but of other galaxies as well – particularly when it comes to the relationship that exists between Supermassive Black Holes (SMBHs), star formation, and the evolution of galaxies.

For decades, astronomers have studied the central regions of galaxies in the hopes of determining how this relationship works. And in recent years, astronomers have come up with conflicting results, some of which indicate that star formation is arrested by the presence of SMBHs while others show no correlation.

In addition, further examinations of SMBHs and Active Galactic Nuclei (AGNs) have shown that there may be no correlation between the mass of a galaxy and the mass of its central black hole – another theory that astronomers previously subscribed to.

As such, understanding how and why star formation appears to be different in galaxies like the Milky Way could help us to unravel these other mysteries. From that, a better understanding of how stars and galaxies evolved over the course of cosmic history is sure to emerge.

Further Reading: CfA, MNRAS

February 1, 2018 by Matt Williams

Astronomers Find One of the Oldest Stars in the Milky Way

A recent survey has discovered the first stars of the Milky Way. Credit: Gabriel Pérez, SMM (IAC)

According to modern cosmological models, the Universe began in a cataclysm event known as the Big Bang. This took place roughly 13.8 billion years ago, and was followed by a period of expansion and cooling. During that time, the first hydrogen atoms formed as protons and electrons combined and the fundamental forces of physics were born. Then, about 100 million years after the Big Bang, that the first stars and galaxies began to form.

The formation of the first stars was also what allowed for the creation of heavier elements, and therefore the formation of planets and all life as we know it. However, until now, how and when this process took place has been largely theoretical since astronomers did not know where the oldest stars in our galaxy were to be found. But thanks to a new study by a team of Spanish astronomers, we may have just found the oldest star in the Milky Way!

The study, titled “J0815+4729: A chemically primitive dwarf star in the Galactic Halo observed with Gran Telescopio Canarias“, recently appeared in The Astrophysical Journal Letters. Led by David S. Aguado of the Instituto de Astrofisica de Canarias (IAC), the team included members from the University of La Laguna and the Spanish National Research Council (CSIC).

*Artist’s impression of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)*

This star is located roughly 7,500 light years from the Sun, and was found in the halo of the Milky Way along the line of sight to the Lynx constellation. Known as J0815+4729, this star is still in its main sequence and has a low mass, (around 0.7 Solar Masses), though the research team estimates that it has a surface temperature that is about 400 degrees hotter – 6,215 K (5942 °C; 10,727 °F) compared to 5778 K (5505 °C; 9940 °F).

For the sake of their study, the team was looking for a star that showed signs of being metal-poor, which would indicate that it has been in its main sequence for a very long time. The team first selected J0815+4729 from the Sloan Digital Sky Survey-III Baryon Oscillation Spectroscopic Survey (SDSS-III/BOSS) and then conducted follow-up spectroscopic investigations to determine its composition (and hence its age).

This was done using the Intermediate dispersion Spectrograph and Imaging System (ISIS) at the William Herschel Telescope (WHT) and the Optical System for Imaging and low-intermediate-Resolution Integrated Spectroscopy (OSIRIS) at Gran Telescopio de Canarias (GTC), both of which are located at the Observatorio del Roque de los Muchachos on the island of La Palma.

Consistent with what modern theory predicts, the star was found in the Galactic halo – the extended component of our galaxy that reaches beyond the galactic disk (the visible portion). It is in this region that the oldest and most metal-poor stars are believed to be found in galaxies, hence why the team was confident that a star dating back to the early Universe would be found here.

*The William Herschel Telescope, part of the Isaac Newton group of telescopes, located on Canary Island. Credit: ing.iac.es*

As Jonay González Hernández – a professor from the University of La Laguna, a member of the IAC and a co-author on the paper – explained in an IAC press release:

“Theory predicts that these stars could use material from the first supernovae, whose progenitors were the first massive stars in the galaxy, around 300 million years after the Big Bang. In spite of its age, and its distance away from us, we can still observe it.”

Spectra obtained by both the ISIS and OSIRIS instruments confirmed that the star was poor in metals, indicating that J0815+4729 has only one-millionth of the calcium and iron that the Sun contains. In addition, the team also noticed that the star has a higher carbon content than our Sun, accounting for almost 15% percent of its solar abundance (i.e. the relative abundance of its elements).

In short, J0815+4729 may be the most iron-poor and carbon-rich star currently known to astronomers. Moreover, finding it was rather difficult since the star is both weak in luminosity and was buried within a massive amount of SDSS/BOSS archival data. As Carlos Allende Prieto, another IAC researcher and a co-author on the paper, indicated:

“This star was tucked away in the database of the BOSS project, among a million stellar spectra which we have analysed, requiring a considerable observational and computational effort. It requires high-resolution spectroscopy on large telescopes to detect the chemical elements in the star, which can help us to understand the first supernovae and their progenitors.”

In the near future, the team predicts that next-generation spectrographs could allow for further research that would reveal more about the star’s chemical abundances. Such instruments include the HORS high-resolution spectrograph, which is presently in a trial phase on the Gran Telescopio Canarias (GTC).

“Detecting lithium gives us crucial information related to Big Bang nucleosynthesis,” said Rafael Rebolo, the director of the IAC and a coauthor of the paper. “We are working on a spectrograph of high-resolution and wide spectral range in order to measure the detailed chemical composition of stars with unique properties such as J0815+4719.”

These future studies are sure to be a boon for astronomers and cosmologists. In addition to being a chance to study stars that formed when the Universe was still in its infancy, they could provide new insight into the early stages of the universe, the formation of the first stars, and the properties of the first supernovae. In other words, they would put us a step closer to know how the Universe as we know it formed and evolved.

Further Reading: IAC, The Astrophysical Journal Letters

January 29, 2018 by Evan Gough

The Most Detailed Map Ever Made of the Milky Way in Radio Waves

The FUGIN project used the 45 meter Nobeyama radio telescope in Japan to produce the most detailed radio wave map yet of the Milky Way. Image: NAOJ/NASA/JPL-Caltech — The FUGIN project used the 45 meter Nobeyama radio telescope in Japan to produce the most detailed radio wave map yet of the Milky Way. Top: Three color (false color) radio map of the Milky Way (l=10-50 deg) obtained by the FUGIN Project. Red, green, and blue represent the radio intensities of 12CO, 13CO, and C18O, respectively. Second Line: Infrared image of the same region obtained by the Spitzer Space Telescope. Red, green, and blue represent the intensities of 24?m, 8?m, and 5.8?m radio waves respectively. Top Zoom-In: Three color radio map of the Milky Way (l=12-22 deg) obtained by the FUGIN Project. The colors are the same as the top image. Lower-Left Zoom-In: Enlarged view of the W51 region. The colors are the same as the top image.Lower-Right Zoom-In: Enlarged view of the M17 region. The colors are the same as the top image. Image: NAOJ/NASA/JPL-Caltech

A Japanese telescope has produced our most detailed radio wave image yet of the Milky Way galaxy. Over a 3-year time period, the Nobeyama 45 meter telescope observed the Milky Way for 1100 hours to produce the map. The image is part of a project called FUGIN (FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope.) The multi-institutional research group behind FUGIN explained the project in the Publications of the Astronomical Society of Japan and at arXiv.

The Nobeyama 45 meter telescope is located at the Nobeyama Radio Observatory, near Minamimaki, Japan. The telescope has been in operation there since 1982, and has made many contributions to millimeter-wave radio astronomy in its life. This map was made using the new FOREST receiver installed on the telescope.

When we look up at the Milky Way, an abundance of stars and gas and dust is visible. But there are also dark spots, which look like voids. But they’re not voids; they’re cold clouds of molecular gas that don’t emit visible light. To see what’s happening in these dark clouds requires radio telescopes like the Nobeyama.

The Nobeyama 45m radio telescope at the Nobeyama Radio Observatory in Japan. Image:NAOJ

The Nobeyama was the largest millimeter-wave radio telescope in the world when it began operation, and it has always had great resolution. But the new FOREST receiver has improved the telescope’s spatial resolution ten-fold. The increased power of the new receiver allowed astronomers to create this new map.

The new map covers an area of the night sky as wide as 520 full Moons. The detail of this new map will allow astronomers to study both large-scale and small-scale structures in new detail. FUGIN will provide new data on large structures like the spiral arms—and even the entire Milky Way itself—down to smaller structures like individual molecular cloud cores.

FUGIN is one of the legacy projects for the Nobeyama. These projects are designed to collect fundamental data for next-generation studies. To collect this data, FUGIN observed an area covering 130 square degrees, which is over 80% of the area between galactic latitudes -1 and +1 degrees and galactic longitudes from 10 to 50 degrees and from 198 to 236 degrees. Basically, the map tried to cover the 1st and 3rd quadrants of the galaxy, to capture the spiral arms, bar structure, and the molecular gas ring.

Starscape photograph taken at Nobeyama Radio Observatory by Norikazu Okabe. The FUGIN observation region (l=10-50 deg) is marked. Credit: National Astronomical Observatory of Japan

The aim of FUGIN is to investigate physical properties of diffuse and dense molecular gas in the galaxy. It does this by simultaneously gathering data on three carbon dioxide isotopes: 2CO, 13CO, and 18CO. Researchers were able to study the distribution and the motion of the gas, and also the physical characteristics like temperature and density. And the studying has already paid off.

FUGIN has already revealed things previously hidden. They include entangled filaments that weren’t obvious in previous surveys, as well as both wide-field and detailed structures of molecular clouds. Large scale kinematics of molecular gas such as spiral arms were also observed.

An artist’s image showing the major features of the Milky Way galaxy. Credit: NASA/JPL-Caltech, ESO, J. Hurt

But the main purpose is to provide a rich data-set for future work by other telescopes. These include other radio telescopes like ALMA, but also telescopes operating in the infrared and other wavelengths. This will begin once the FUGIN data is released in June, 2018.

Millimeter wave radio astronomy is powerful because it can “see” things in space that other telescopes can’t. It’s especially useful for studying the large, cold gas clouds where stars form. These clouds are as cold as -262C (-440F.) At temperatures that low, optical scopes can’t see them, unless a bright star is shining behind them.

Even at these extremely low temperatures, there are chemical reactions occurring. This produces molecules like carbon monoxide, which was a focus of the FUGIN project, but also others like formaldehyde, ethyl alcohol, and methyl alcohol. These molecules emit radio waves in the millimeter range, which radio telescopes like the Nobeyama can detect.

The top-level purpose of the FUGIN project, according to the team behind the project, is to “provide crucial information about the transition from atomic gas to molecular gas, formation of molecular clouds and dense gas, interaction between star-forming regions and interstellar gas, and so on. We will also investigate the variation of physical properties and internal structures of molecular clouds in various environments, such as arm/interarm and bar, and evolutionary stage, for example, measured by star-forming activity.”

This new map from the Nobeyama holds a lot of promise. A rich data-set like this will be an important piece of the galactic puzzle for years to come. The details revealed in the map will help astronomers tease out more detail on the structures of gas clouds, how they interact with other structures, and how stars form from these clouds.

January 10, 2018November 7, 2018 by Paul M. Sutter

The Night Sky Magic of the Atacama

night sky above the atacama — Thousands of stars glitter in the black skies above the bone-dry desert of the Atacama in northern Chile. Photo credit: Gerhard Hüdepohl/atacamaphoto.com.

There’s nothing an astronomer – whether professional or amateur – loves more than a clear dark night sky away from the city lights. Outside the glare and glow and cloud cover that most of us experience every day, the night sky comes alive with a life of its own.

Thousands upon countless thousands of glittering jewels – each individual star a pinprick of light set against the velvet-smooth blackness of the deeper void. The arching band of the Milky Way, itself host to billions more stars so far away that we can only see their combined light from our vantage point. The familiar constellations, proudly showing their true character, drawing the eye and the mind to the ancient tales spun about them.

There are few places left in the world to see the sky as our ancestors did; to gaze in wonder at the celestial dome and feel the weight of billions of years of cosmic history hanging above us. Thankfully the International Dark Sky Association is working to preserve what’s left of the true night sky, and they’ve rightfully marked northern Chile to preserve for posterity.

Continue reading “The Night Sky Magic of the Atacama”

October 4, 2017 by Matt Williams

Determining the Mass of the Milky Way Using Hypervelocity Stars

An artist's conception of a hypervelocity star that has escaped the Milky Way. Credit: NASA

For centuries, astronomers have been looking beyond our Solar System to learn more about the Milky Way Galaxy. And yet, there are still many things about it that elude us, such as knowing its precise mass. Determining this is important to understanding the history of galaxy formation and the evolution of our Universe. As such, astronomers have attempted various techniques for measuring the true mass of the Milky Way.

So far, none of these methods have been particularly successful. However, a new study by a team of researchers from the Harvard-Smithsonian Center for Astrophysics proposed a new and interesting way to determine how much mass is in the Milky Way. By using hypervelocity stars (HVSs) that have been ejected from the center of the galaxy as a reference point, they claim that we can constrain the mass of our galaxy.

Their study, titled “Constraining Milky Way Mass with Hypervelocity Stars“, was recently published in the journal Astronomy and Astrophysics. The study was produced by Dr. Giacomo Fragione, an astrophysicist at the University of Rome, and Professor Abraham Loeb – the Frank B. Baird, Jr. Professor of Science, the Chair of the Astronomy Department, and the Director of the Institute for Theory and Computation at Harvard University.

*Stars speeding through the Galaxy. Credit: ESA*

To be clear, determining the mass of the Milky Way Galaxy is no simple task. On the one hand, observations are difficult because the Solar System lies deep within the disk of the galaxy itself. But at the same time, there’s also the mass of our galaxy’s dark matter halo, which is difficult to measure since it is not “luminous”, and therefore invisible to conventional methods of detection.

Current estimates of the galaxy’s total mass are based on the motions of tidal streamers of gas and globular clusters, which are both influenced by the gravitational mass of the galaxy. But so far, these measurements have produced mass estimates that range from one to several trillion solar-masses. As Professor Loeb explained to Universe Today via email, precisely measuring the mass of the Milky Way is of great importance to astronomers:

“The Milky Way provides a laboratory for testing the standard cosmological model. This model predicts that the number of satellite galaxies of the Milky Way depends sensitively on its mass. When comparing the predictions to the census of known satellite galaxies, it is essential to know the Milky Way mass. Moreover, the total mass calibrates the amount of invisible (dark) matter and sets the depth of the gravitational potential well and implies how fast should stars move for them to escape to intergalactic space.”

For the sake of their study, Prof. Loeb and Dr. Fragione therefore chose to take a novel approach, which involved modeling the motions of HVSs to determine the mass of our galaxy. More than 20 HVSs have been discovered within our galaxy so far, which travel at speeds of up to 700 km/s (435 mi/s) and are located at distances of about 100 to 50,000 light-years from the galactic center.

*Artist’s conception of a hyperveloctiy star heading out from a spiral galaxy (similar to the Milky Way) and moving into dark matter nearby. Credit: Ben Bromley, University of Utah*

These stars are thought to have been ejected from the center of our galaxy thanks to the interactions of binary stars with the supermassive black hole (SMBH) at the center of our galaxy – aka. Sagittarius A*. While their exact cause is still the subject of debate, the orbits of HVSs can be calculated since they are completely determined by the gravitational field of the galaxy.

As they explain in their study, the researchers used the asymmetry in the radial velocity distribution of stars in the galactic halo to determine the galaxy’s gravitational potential. The velocity of these halo stars is dependent on the potential escape speed of HVSs, provided that the time it takes for the HVSs to complete a single orbit is shorter than the lifetime of the halo stars.

From this, they were able to discriminate between different models for the Milky Way and the gravitational force it exerts. By adopting the nominal travel time of these observed HVSs – which they calculated to about 330 million years, about the same as the average lifetime of halo stars – they were able to derive gravitational estimates for the Milky Way which allowed for estimates on its overall mass.

“By calibrating the minimum speed of unbound stars, we find that the Milky Way mass is in the range of 1.2-1.9 trillions solar masses,” said Loeb. While still subject to a range, this latest estimate is a significant improvement over previous estimates. What’s more, these estimates are consistent our current cosmological models that attempt to account for all visible matter in the Universe, as well as dark matter and dark energy – the Lambda-CDM model.

Distribution of dark matter when the Universe was about 3 billion years old, obtained from a numerical simulation of galaxy formation. Credit: VIRGO Consortium/Alexandre Amblard/ESA

“The inferred Milky Way mass is in the range expected within the standard cosmological model,” said Leob, “where the amount of dark matter is about five times larger than that of ordinary (luminous) matter.”

Based on this breakdown, it can be said that normal matter in our galaxy – i.e. stars, planets, dust and gas – accounts for between 240 and 380 billion Solar Masses. So not only does this latest study provide more precise mass constraints for our galaxy, it could also help us to determine exactly how many star systems are out there – current estimates say that the Milky Way has between 200 to 400 billion stars and 100 billion planets.

Beyond that, this study is also significant to the study of cosmic formation and evolution. By placing more precise estimates on our galaxy’s mass, ones which are consistent with the current breakdown of normal matter and dark matter, cosmologists will be able to construct more accurate accounts of how our Universe came to be. One step clsoer to understanding the Universe on the grandest of scales!

Further Reading: Harvard Smithsonian CfA, Astronomy and Astrophysics

September 5, 2017September 5, 2017 by Matt Williams

Another Monster Black Hole Found in the Milky Way

Molecular clouds scattered by an intermediate black hole show very wide velocity dispersion in this artist’s impression. This scenario well explains the observational features of a peculiar molecular cloud CO-0.40-0.22. Credit: Keio University

At the center of the Milky Way Galaxy resides the Supermassive Black Hole (SMBH) known as Sagittarius A*. This tremendous black hole measures an estimated 44 million km in diameter, and has the mass of over 4 million Suns. For decades, astronomers have understood that most larger galaxies have an SMBH at their core, and that these range from hundreds of thousands to billions of Solar Masses.

However, new research performed by a team of researchers from Keio University, Japan, has made a startling find. According to their study, the team found evidence of a mid-sized black hole in a gas cluster near the center of the Milky Way Galaxy. This unexpected find could offer clues as to how SMBHs form, which is something that astronomers have been puzzling over for some time.

The study, titled “Millimetre-wave Emission from an Intermediate-mass Black Hole Candidate in the Milky Way“, recently appeared in the journal Nature Astronomy. Led by Tomoharu Oka, a researcher from the Department of Physics and the School of Fundamental Science and Technology at Keio University, the team studied CO–0.40–0.22, a high-velocity compact gas cloud near the center of our galaxy.

*This artist’s concept shows a galaxy with a supermassive black hole at its core. The black hole is shooting out jets of radio waves.Image credit: NASA/JPL-Caltech*

This compact dust cloud, which has been a source of fascination to astronomers for years, measures over 1000 AU in diameter and is located about 200 light-years from the center of our galaxy. The reason for this interest has to do with the fact that gases in this cloud – which include hydrogen cyanide and carbon monoxide – move at vastly different speeds, which is something unusual for a cloud of interstellar gases.

In the hopes of better understanding this strange behavior, the team originally observed CO–0.40–0.22 using the 45-meter radio telescope at the Nobeyama Radio Observatory in Japan. This began in January of 2016, when the team noticed that the cloud had an elliptical shape that consisted of two components. These included a compact but low density component with varying velocities, and a dense component (10 light years long) with little variation.

After conducting their initial observations, the team then followed up with observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. These confirmed the structure of the cloud and the variations in speed that seemed to accord with density. In addition, they observed the presence of radio waves (similar to those generated by Sagittarius A*) next to the dense region. As they state in their study:

“Recently, we discovered a peculiar molecular cloud, CO–0.40–0.22, with an extremely broad velocity width, near the center of our Milky Way galaxy. Based on the careful analysis of gas kinematics, we concluded that a compact object with a mass of about 10⁵ [Solar Masses] is lurking in this cloud.”

Change image showing the area around Sgr A*, where low, medium, and high-energy X-rays are red, green, and blue, respectively. The inset box shows X-ray flares from the region close to Sgr A*. NASA: NASA/SAO/CXC

The team also ran a series of computer models to account for these strange behaviors, which indicated that the most likely cause was a black hole. Given its mass – 100,000 Solar Masses, or roughly 500 times smaller than that of Sagittarius A* – this meant that the black hole was intermediate in size. If confirmed, this discovery will constitute the second-largest black hole to be discovered within the Milky Way.

This represents something of a first for astronomers, since the vast majority of black holes discovered to date have been either small or massive. Studies that have sought to locate Intermediate Black Holes (IMBHs), on the other hand, have found very little evidence of them. Moreover, these findings could account for how SMBHs form at the center of larger galaxies.

In the past, astronomers have conjectured that SMBHs are formed by the merger of smaller black holes, which implied the existence of intermediate ones. As such, the discovery of an IMBH would constitute the first piece of evidence for this hypothesis. As Brooke Simmons, a professor at the University of California in San Diego, explained in an interview with The Guardian:

“We know that smaller black holes form when some stars die, which makes them fairly common. We think some of those black holes are the seeds from which the much larger supermassive black holes grow to at least a million times more massive. That growth should happen in part by mergers with other black holes and in part by accretion of material from the part of the galaxy that surrounds the black hole.

“Astrophysicists have been collecting observational evidence for both stellar mass black holes and supermassive black holes for decades, but even though we think the largest ones grow from the smallest ones, we’ve never really had clear evidence for a black hole with a mass in between those extremes.”

*Artist’s impression of two merging black holes, which has been theorized to be a source of gravitational waves. Credit: Bohn, Throwe, Hébert, Henriksson, Bunandar, Taylor, Scheel/SXS*

Further studies will be needed to confirm the presence of an IMBH at the center of CO–0.40–0.22. Assuming they succeed, we can expect that astrophyiscists will be monitoring it for some time to determine how it formed, and what it’s ultimate fate will be. For instance, it is possible that it is slowly drifting towards Sagittarius A* and will eventually merge with it, thus creating an even more massive SMBH at the center of our galaxy!

Assuming human beings are around to detect that merger, its fair to say that it won’t go unnoticed. The gravitational waves alone are sure to be impressive!

Further Reading: Nature Astronomy

July 28, 2017July 28, 2017 by Matt Williams

What Is the Name Of Our Galaxy?

The band of light (the Milky Way) that is visible in the night sky, showing the stellar disk of our galaxy. Credit: Bob King

Since prehistoric times, human beings have looked up at at the night sky and pondered the mystery of the band of light that stretches across the heavens. And while theories have been advanced since the days of Ancient Greece as to what it could be, it was only with the birth of modern astronomy that scholars have come come to know precisely what it is – i.e. countless stars at considerable distances from Earth.

The term “Milky Way”, a term which emerged in Classical Antiquity to describe the band of light in the night sky, has since gone on to become the name for our galaxy. Like many others in the known Universe, the Milky Way is a barred, spiral galaxy that is part of the Local Group – a collection of 54 galaxies. Measuring 100,000 – 180,000 light-years in diameter, the Milky Way consists of between 100 and 400 billion stars.

Structure:

The Milky Way consists of a Galactic Center that is shaped like a bar and a Galactic Disk made up of spiral arms, all of which is surrounded by the Halo – which is made up of old stars and globular clusters. The Center, also known as “the bulge”, is a dense concentration of mostly old stars that measures about 10,000 light years in radius. This region is also the rotational center of the Milky Way.

*Illustration of the supermassive black hole at the center of the Milky Way. Credit: NRAO/AUI/NSF*

The Galactic Center is also home to an intense radio source named Sagittarius A*, which is believed to have a supermassive black hole (SMBH) at its center. The presence of this black hole has been discerned due to the apparent gravitational influence it has on surrounding stars. Astronomers estimate that it has a mass of between 4.1. and 4.5 million Solar masses.

Outside the barred bulge at the Galactic Center is the Galactic Disk of the Milky Way. This consists of stars, gas and dust which is organized into four spiral arms. These arms typically contain a higher density of interstellar gas and dust than the Galactic average, as well as a greater concentration of star formation. While there is no consensus on the exact structure or extent of these spiral arms, they are commonly grouped into two or four different arms.

In the case of four arms, this is based on the traced paths of gas and younger stars in our galaxy, which corresponds to the Perseus Arm, the Norma and Outer Arm, the Scutum-Centaurum Arm, and the Carina-Sagittarius Arm. There are also at least two smaller arms, which include the Cygnus Arm and the Orion Arm. Meanwhile, surveys based on the presence of older stars show only two major spirals arms – the Perseus arm and the Scutum–Centaurus arm.

Beyond the Galactic Disk is the Halo, which is made up of old stars and globular clusters – 90% of which lie within 100,000 light-years (30,000 parsecs) from the Galactic Center. Recent evidence provided by X-ray observatories indicates that in addition to this stellar halo, the Milky way also has a halo of hot gas that extends for hundreds of thousands of light years.

*Artist’s conception of the spiral structure of the Milky Way with two major stellar arms and a bar. Credit: NASA/JPL-Caltech/ESO/R. Hurt*

Size and Mass:

The Galactic Disk of the Milky Way Galaxy is approximately 100,000 light years in diameter and about 1,000 light years thick. It is estimated to contain between 100 and 400 billion stars, though the exact figure depends on the number of very low-mass M-type (aka. red dwarf) stars. This is difficult to determine because these stars also have low-luminosity compared to other class.

The distance from the Sun to the Galactic Center is estimated to be between 25,000 to 28,000 light years (7,600 to 8,700 parsecs). The Galactic Center’s bar (aka. its “bulge”) is thought to be about 27,000 light-years in length and is composed primarily of red stars, all of which are thought to be ancient. The bar is surrounded by the ‘5-kpc ring’, a region that contains much of the galaxy’s molecular hydrogen and where star-formation is most intense.

The Galactic Disk has a diameter of between 70,000 and 100,000 light-years. It does not have a sharp edge, a radius beyond which there are no stars. However, the number of stars drops slowly with distance from the center. Beyond a radius of roughly 40,000 light years, the number of stars drops much faster the farther you get from the center.

Location of the Solar System:

The Solar System is located near the inner rim of the Orion Arm, a minor spiral arm located between the Carina–Sagittarius Arm and the Perseus Arm. This arm measures some 3,500 light-years (1,100 parsecs) across, approximately 10,000 light-years (3,100 parsecs) in length, and is at a distance of about 25,400 to 27,400 light years (7.78 to 8.4 thousand parsecs) from the Galactic Center.

History of Observation:

Our galaxy was named because of the way the haze it casts in the night sky resembled spilled milk. This name is also quite ancient. It is translation from the Latin “Via Lactea“, which in turn was translated from the Greek for Galaxias, referring to the pale band of light formed by stars in the galactic plane as seen from Earth.

Persian astronomer Nasir al-Din al-Tusi (1201–1274) even spelled it out in his book Tadhkira: “The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color.”

Astronomers had long suspected the Milky Way was made up of stars, but it wasn’t proven until 1610, when Galileo Galilei turned his rudimentary telescope towards the heavens and resolved individual stars in the band across the sky. With the help of telescopes, astronomers realized that there were many, many more stars in the sky, and that all of the ones that we can see are a part of the Milky Way.

In 1755, Immanuel Kant proposed that the Milky Way was a large collection of stars held together by mutual gravity. Just like the Solar System, this collection would be rotating and flattened out as a disk, with the Solar System embedded within it. Astronomer William Herschel (discoverer of Uranus) tried to map its shape in 1785, but he didn’t realize that large portions of the galaxy are obscured by gas and dust, which hide its true shape.

It wasn’t until the 1920s, when Edwin Hubble provided conclusive evidence that the spiral nebulae in the sky were actually whole other galaxies, that the true shape of our galaxy was known. Thenceforth, astronomers came to understand that the Milky Way is a barred, spiral galaxy, and also came to appreciate how big the Universe truly is.

The Milky Way is appropriately named, being the vast and cloudy mass of stars, dust and gas it is. Like all galaxies, ours is believed to have formed from many smaller galaxies colliding and combining in the past. And in 3 to 4 billion years, it will collide with the Andromeda Galaxy to form an even larger mass of stars, gas and dust. Assuming humanity still exists by then (and survives the process) it should make for some interesting viewing!

We have written many interesting articles about the Milky Way here at Universe Today. Here’s 10 Interesting Facts About the Milky Way, How Big is the Milky Way?, Why is our Galaxy Called the Milky Way?, What is the Closest Galaxy to the Milky Way?, Where is the Earth in the Milky Way?, The Milky Way has Only Two Spiral Arms, and It’s Inevitable: Milky Way, Andromeda Galaxy Heading for Collision.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We’ve also recorded an episode of Astronomy Cast about the Milky Way. Listen here, Episode 99: The Milky Way.

Sources:

June 22, 2017 by Bob King

Saturn Rides Bareback On The Galactic Dark Horse

The bright dot is Saturn and it shines on the back of the Galactic Dark Horse, a collection of dark nebulae in the constellation Ophiuchus that resembles a prancing horse. The head is to the right with a wisp of a tail to the left. The photo, taken on June 20, 2017, has been turned 90° to the right, so the horse stands upright. Credit: Bob King

I didn’t notice it with the naked eye, but as soon as the time exposure ended and I looked at the camera’s back display, there it was — Saturn riding barebacked on the Galactic Dark Horse! The horse, more of a prancing pony, is a collection of dark nebulae in the southern sky beautifully placed for viewing on late June evenings. The Dark Horse is part of the Great Rift, a dark gap that splits the band of the Milky Way in half, starting at the Northern Cross and extending all the way down to the “Teapot” of Sagittarius in the south.

The Great Rift appears to unzip the summer Milky Way right down the middle. Saturn and the Dark Horse are seen at lower right. Credit: Bob King

While appearing to be little more than empty, starless space, in reality the Rift consists of enormous clouds of cosmic dust and gas in the plane of the galaxy called dark nebulae that blot out the light of more distant stars. If you could suck it all up with a monster vacuum cleaner and expose the billions of stars otherwise hidden, the Milky Way would cast obvious shadows — even suburban skywatchers would routinely see it.

Saturn dominates the scene at left center in this photo taken on June 20. To its right you can see the prancing pony standing on its tail with legs sticking out to the right. Several bright Milky Way star clouds are also visible including the Small Sagittarius Star Cloud (left) and the Large Sagittarius Star Cloud below and left of Saturn. Antares in Scorpius is at upper right. Can you find the firefly that flashed during the exposure? Credit: Bob King

Tiny dust particles spewed by older, evolved stars and exploding supernovas have been settling in the plane of the galaxy since its birth 13.2 billion years ago. While the dust is sparse, it adds up over the light years to form a thick, dark band silhouetted against the more distant stars. Gravity has been at work on the dust since the earliest days, compressing the denser clumps into new stars and star clusters. But much raw material remains. Within the curdles of dark nebulae, astronomers use dust-penetrating infrared and radio telescopes to watch new stars in the process of incubation.

Dense cores of dust within the Pipe Nebula are collapsing to form new stars. We can’t see them yet because of obscuring dust. The left end of the Pipe forms the long back leg and rump of the Dark Horse. The much smaller Snake Nebula (shaped like the letter “S”) is visible at top center. Credit and copyright: Yuri Beletsky

There are more obvious parts of the Rift to the naked eye but few conjure up as striking an image as the Dark Horse, located about one outstretched fist to the left of the Scorpius’ brightest star, Antares. Saturn sits astride the horse’s back or eastern side. While it’s fun to see the horse as a single figure, astronomers catalog the various body parts as individual dark nebulae with separate numbers and even names. The largest part of the horse, the hind leg, is nicknamed the Pipe Nebula and lies 600-700 light years away. The Pipe is further subdivided into B59, B72, B77 and B78, from a survey of dark nebulae by early 20th century American astronomer E.E. Barnard.

You’ll need dark skies and averted vision to spot the Dark Horse. Let Saturn and Antares be your guides. The nebula is highest in the sky around 12:30 a.m. in late June as shown in the map above. Latitude shown is 40° North. Created with Stellarium

While the dark horse shows up well in time-exposure photos, you’ll need dark, rural skies to view it with the naked eye. It’s only a couple fists high for those of us living in the northern U.S. and southern Canada, but considerably higher up from the southern states and points south. The figure is large but faint, about 10° long by 7° wide, and stands due south and highest in the sky around 12:30 a.m. in late June. Allow your eyes time to fully dark adapt beforehand. Try for the dark rump and hind leg first then work from there to fill in the rest of the horse.

If we could see the Milky Way galaxy edge-on from afar, it would look similar to NGC 891 in Andromeda. Both have long bands of interstellar dust along their equators that appear dark against the bright, starry backdrop. Credit: Hunter Wilson

Once I knew what to look for, I could fleetingly see the entire horse with its various protrusions as a subtle darkness against the brighter Milky Way. Averted vision, the technique of playing your eye around the subject rather than staring directly at it, helped make it happen. Wide-field binoculars will show it easily and in greater detail against a fabulously rich star field.

The best time to horse around under the Milky Way happens from now till the end of the month, when the bright Moon sends the critter into hiding.

April 30, 2017May 1, 2017 by Bob King

Star-travel 5 Million Years Into The Milky Way’s Future

Two Million Stars on the Move

Gaze into Gaia’s crystal ball and you will see the future. This video shows the motion of 2,057,050 stars in the coming 5 million years from the Tycho-Gaia Astrometric Solution sample, part of the first data release of European Space Agency’s Gaia mission.

Gaia is a space observatory parked at the L2 Lagrange Point, a stable place in space a million miles behind Earth as viewed from the Sun. Its mission is astrometry: measuring the precise positions, distances and motion of 1 billion astronomical objects (primarily stars) to create a three-dimensional map of the Milky Way galaxy. Gaia’s radial velocity measurements — the motion of stars toward or away from us — will provide astronomers with a stereoscopic and moving-parts picture of about 1% of the galaxy’s stars.

Think about how slowly stars move from the human perspective. Generations of people have lived and died since the days of ancient Greece and yet the constellations outlines and naked eye stars appear nearly identical today as they did then. Only a few stars — Arcturus, Sirius, Aldebaran — have moved enough for a sharp-eyed observer of yore to perceive their motion.

Given enough time, stars do change position, distorting the outlines of the their constellations. This view shows the sky looking north in 91,000 A.D. Both Lyra and the Big Dipper are clearly bent out of shape! Created with Stellarium

We know that stars are constantly on the move around the galactic center. The Sun and stars in its vicinity orbit the core at some half-million miles an hour, but nearly all are so far away that their apparent motion has barely moved the needle over the time span of civilization as we know it.

This video shows more than 2 million stars from the TGAS sample, with the addition of 24,320 bright stars from the Hipparcos Catalogue that weren’t included in Gaia’s first data release back in September 2016. The video starts from the positions of stars as measured by Gaia between 2014 and 2015, and shows how these positions are expected to evolve in the future, based on the stars’ proper motions or direction of travel across space.

This frame will help you get your footing as you watch the video. Orion (at right) and the Alpha Persei stellar association and Pleiades (at left) are shown. Credit: ESA/Gaia/DPAC

Watching the show

The frames in the video are separated by 750 years, and the overall sequence covers 5 million years. The dark stripes visible in the early frames reflect the way Gaia scans the sky (in strips) and the early, less complete database. The artifacts are gradually washed out as stars move across the sky.

Using the map above to get oriented, it’s fun to watch Orion change across the millennia. Betelgeuse departs the constellation heading north fairly quickly, but Orion’s Belt hangs in there for nearly 2 million years even if it soon develops sag! The Pleiades drift together to the left and off frame and then reappear at right.

Stars seem to move with a wide range of velocities in the video, with stars in the galactic plane moving quite slow and faster ones speeding across the view. This is a perspective effect: most of the stars we see in the plane are much farther from us, and thus seem to be moving slower than the nearby stars, which are visible across the entire sky.

Artist’s impression of The Milky Way Galaxy to provide context for the video. The Sun and solar system are located in the flat plane of the galaxy, so when we look into the Milky Way (either toward the center or toward the edge), the stars pile up across the light years to form a band in the sky. If we could rise above the disk and see the galaxy from the halo, we’d be able to look down (or up) and see the galaxy as a disk with winding spiral arms. Credit: NASA

Some of the stars that appear to zip in and out of view quickly are passing close to the Sun. But motion of those that trace arcs from one side of the sky to the other while passing close to the galactic poles (top and bottom of the frame) as they speed up and slow down, is spurious. These stars move with a constant velocity through space.

Stars located in the Milky Way’s halo, a roughly spherical structure centered on the galaxy’s spiral disk, also appear to move quite fast because they slice through the galactic plane with respect to the Sun. In reality, halo stars move very slowly with respect to the center of the galaxy.

Early in the the visualization, we see clouds of interstellar gas and dust that occupy vast spaces within the galaxy and block the view of more distant suns. That these dark clouds seem to disappear over time is also a spurious effect.

After a few million years, the plane of the Milky Way appears to have shifted towards the right as a consequence of the motion of the Sun with respect to that of nearby stars in the Milky Way. Regions that are depleted of stars in the video will not appear that way to future stargazers but will instead be replenished by stars not currently sampled by Gaia. So yes, there are a few things to keep in mind while watching these positional data converted into stellar motions, but the overall picture is an accurate one.

I find the video as mesmerizing as watching fireflies on a June night. The stars seem alive. Enjoy your ride in the time machine!