Determining the Mass of the Milky Way Using Hypervelocity Stars

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

Planets Could Travel Along with Rogue ‘Hypervelocity’ Stars, Spreading Life Throughout the Universe

Back in 1988, astronomer Jack Hills predicted a type of “rogue”star might exist that is not bound to any particular galaxy. These stars, he reasoned, were periodically ejected from their host galaxy by some sort of mechanism to begin traveling through interstellar space.

Since that time, astronomers have made numerous discoveries that indicate these rogue, traveling stars indeed do exist, and far from being an occasional phenomenon, they are actually quite common. What’s more, some of these stars were found to be traveling at extremely high speeds, leading to the designation of hypervelocity stars (HVS).

And now, in a series of papers that published in arXiv Astrophysics, two Harvard researchers have argued that some of these stars may be traveling close to the speed of light. Known as semi-relativistic hypervelocity stars (SHS), these fast-movers are apparently caused by galactic mergers, where the gravitational effect is so strong that it fling stars out of a galaxy entirely. These stars, the researchers say, may have the potential to spread life throughout the Universe.

This finding comes on the heels of two other major announcements. The first occurred in early November when a paper published in the Astrophysical Journal reported that as many as 200 billion rogue stars have been detected in a cluster of galaxies some 4 billion light years away. These observations were made by the Hubble Space Telescope’s Frontier Fields program, which made ultra-deep multiwavelength observations of the Abell 2744 galaxy cluster.

This was followed by a study published in Science, where an international team of astronomers claimed that as many as half the stars in the entire universe live outside of galaxies.

Using ESO's Very Large Telescope, astronomers have recorded a massive star moving at more than 2.6 million kilometres per hour. Stars are not born with such large velocities. Its position in the sky leads to the suggestion that the star was kicked out from the Large Magellanic Cloud, providing indirect evidence for a massive black hole in the Milky Way's closest neighbour. Credit: ESO
Image of a moving star captured by the ESO Very Large Telescope, believed to have been ejected from the Large Magellanic Cloud. Credit: ESO

However, the recent observations made by Abraham Loeb and James Guillochon of Harvard University are arguably the most significant yet concerning these rogue celestial bodies. According to their research papers, these stars may also play a role in spreading life beyond the boundaries of their host galaxies.

In their first paper, the researchers trace these stars to galaxy mergers, which presumably lead to the formation of massive black hole binaries in their centers. According to their calculations, these supermassive black holes (SMBH) will occasionally slingshot stars to semi-relativistic speeds.

“We predict the existence of a new population of stars coasting through the Universe at nearly the speed of light,” Loeb told Universe Today via email. “The stars are ejected by slingshots made of pairs of massive black holes which form during mergers of galaxies.”

These findings have further reinforced that massive compact bodies, widely known as a supermassive black holes (SMBH), exist at the center of galaxies. Here, the fastest known stars exist, orbiting the SMBH and accelerating up to speeds of 10,000 km per second (3 percent the speed of light).

According to Leob and Guillochon, however, those that are ejected as a result of galactic mergers are accelerated to anywhere from one-tenth to one-third the speed of light (roughly 30,000 – 100,000 km per second).

Image of a hypervelocity star found in data from the Sloan Digital Sky Survey. Credit: Vanderbilt University
Image of a hypervelocity star found in data from the Sloan Digital Sky Survey. Credit: Vanderbilt University

Observing these semi-relativistic stars could tell us much about the distant cosmos, according to the Harvard researchers. Compared to conventional research, which relied on subatomic particles like photons, neutrinos, and cosmic rays from distant galaxies, studying ejected stars offers numerous advantages.

“Traditionally, cosmologists used light to study the Universe but objects moving less than the speed of light offer new possibilities,” said Loeb. “For example, stars moving at different speeds allow us to probe a distant source galaxy at different look-back times (since they must have been ejected at different times in order to reach us today), in difference from photons that give us just one snapshot of the galaxy.”

In their second paper, the researchers calculate that there are roughly a trillion of these stars out there to be studied. And given that these stars were detected thanks to the Spitzer Space Telescope, it is likely that future generations will be able to study them using more advanced equipment.

All-sky infrared surveys could locate thousands of these stars speeding through the cosmos. And spectrographic analysis could tell us much about the galaxies they came from.

But how could these fast moving stars be capable of spreading life throughout the cosmos?

Could an alien spore really travel light years between different star systems? Well, as long as your theory doesn't require it to still be alive when it arrives - sure it can.
The Theory of Panspermia argues that life is distributed throughout the universe by celestial objects. Credit: NASA/Jenny Mottar

“Tightly bound planets can join the stars for the ride,” said Loeb. “The fastest stars traverse billions of light years through the universe, offering a thrilling cosmic journey for extra-terrestrial civilizations. In the past, astronomers considered the possibility of transferring life between planets within the solar system and maybe through our Milky Way galaxy. But this newly predicted population of stars can transport life between galaxies across the entire universe.”

The possibility that traveling stars and planets could have been responsible for the spread of life throughout the universe is likely to have implications as a potential addition to the Theory of Panspermia, which states that life exists throughout the universe and is spread by meteorites, comets, asteroids.

But Loeb told Universe Today that a traveling planetary system could have potential uses for our species someday.

“Our descendants might contemplate boarding a related planetary system once the Milky Way will merge with its sister galaxy, Andromeda, in a few billion years,” he said.

Further Reading: arxiv.org/1411.5022, arxiv.org/1411.5030