# Do hypervelocity stars have Oort clouds?

If you had a hypervelocity star travelling through space incredibly quickly, would the hypervelocity star's Oort cloud remain intact? Would the high speed of a hypervelocity star shed the comets and dwarf planets in its Oort cloud due to the star's high speeds, or would the Oort cloud get stretched into a teardrop shape? Could the Oort cloud remain unaffected?

There is no problem with orbiting a star that moves very fast. Remember that space is mostly empty, so there is nothing like air resistance or friction that would tend to deform the orbit.

Stars that become hypervelocity stars likely tend to lose their Oort clouds. They gain their velocity by passing very close to a massive body and getting a velocity increment, for example by having a secondary star or planet ripped away by tidal forces. That already doesn't bode well for the stability of any Oort cloud.

Simulations show that planets tend to be ejected from hypervelocity systems unless they are very tightly bound, since they have lower mass and respond more strongly to the velocity changes. This suggests that the lightly bound and small bodies in the Oort cloud will be scattered - likely at even higher velocities.

Hypervelocity stars are moving at up to 1000 $km/s$ relative to the general movement of stars in their part of the galaxy (which is typically 100 $km/s$ relative to one another or the galactic centre). The Oort cloud of a star is a collection of rocky and icy bodies orbiting it up to 1 light year or so away.

Those bodies are too dense and massive to be deflected much by passing through the interstellar gas at these kinds of speeds and they are so widely spread that they could pass through the Oort cloud of another star without much risk of collisions, There are two things that might affect them, though:

1. Gravitational interactions with stars they pass could strip some Oort cloud bodies, or send then into the system as comets, or whatever

2. Whatever caused this star to become a hypervelocity star in the first place (eg a close encounter with a supermassive black hole) might well have disrupted the Oort cloud, possibly fatally.

## Secondhand Science

Note (12/2015): Hi there! I'm taking some time off here to focus on other projects for a bit. As of October 2016, those other projects include a science book series for kids titled Things That Make You Go Yuck! -- available at Barnes and Noble, Amazon and (hopefully) a bookstore near you!

Co-author Jenn Dlugos and I are also doing some extremely ridiculous things over at Drinkstorm Studios, including our award-winning webseries, Magicland.

There are also a full 100 posts right here in the archives, and feel free to drop me a line at [email protected] with comments, suggestions or wacky cold fusion ideas. Cheers!

## Hypervelocity Stars

Scroll down to the bottom of the page for links to more images and a podcast with Warren Brown.

The center of the Milky Way galaxy is a crowded, busy neighborhood: clusters upon clusters of pulsating young stars, giant clouds of gas, dying stars exploding, and, in the middle of it all, a massive black hole. So powerful is the gravitational pull of the black hole that stars in closest orbit have been measured circling it at 26.8 million miles per hour--far faster than the sun, which pokes around the galaxy at a mere 500,000 miles per hour.

According to Warren Brown, an astronomer at the Harvard-Smithsonian Center for Astrophysics, this black hole has an effect that extends beyond its close neighbors. In rare cases, certain star clusters have come too close to the black hole and been slung free on a trajectory that sends one of the stars hurtling out of the galaxy--leaving in its wake clues about both the history and the structure of the universe.

Brown discovered the first of these “hypervelocity” stars in 2005 by sheer serendipity. After finishing his Ph.D. in astronomy at Harvard in 2002, he set out to study the movement of older blue stars on the fringes of the Milky Way. (Their increased end-of-life luminosity and puffed size makes them stand out more clearly.) In the midst of his surveys, he discovered a much younger blue star inexplicably speeding toward the edge of the galaxy--on an escape trajectory--at 1.6 million miles per hour. “It was an outlier,” says Brown. “It was so fast that you couldn’t explain it with normal mechanisms.”

He theorized that the star, to generate so much speed, must initially have been part of a trio: a close pair of stars and a third that orbited the two. “We think this third, outer, star was sort of trapped into an orbit around the black hole because it was farther out,” says Brown. As the third star was pulled off, the remaining pair gained gravitational energy from the black hole and were propelled away at an accelerated speed, eventually merging into one.

His most recent paper on the subject, published in July by Astrophysical Journal Letters, confirms what Brown had long suspected: that the resulting star’s origin was the center of the Milky Way. Using NASA’s Hubble Space Telescope, he and a team of researchers pinpointed the exact location of the star in July 2006, and again last December. The difference between the two images was nothing more than a fraction of a pixel, but it was enough to indicate where the star’s path began. “It was pretty exciting,” says Brown. “The proper motion pointed directly back to the center of the Milky Way galaxy.” The discovery challenged previous suggestions from other astronomers that hypervelocity stars could have come from outside the galaxy--specifically from a nearby satellite galaxy known as the Large Magellanic Cloud.

Brown has now identified 13 more of the 16 known hypervelocity stars, and intends to locate more of these rare celestial bodies, which account for just one in every 100 million stars. All his previous surveys of the sky have focused on the Northern Hemisphere, but he plans to begin working with a team of astronomers at Australian National University to examine the southern sky. “I think there’s every reason to expect we’ll have a sample of 40 to 50 stars in five years,” he says.

Building a cache of data from the stars will help Brown and other astronomers begin to answer larger questions about the universe. “Hypervelocity stars can provide a measure of what’s been happening down in the galactic center in the past few hundred million years,” he says.

They could also offer a better understanding of dark matter--the mysterious, unseen substance throughout the universe that has long been one of astronomy’s greatest mysteries. As the stars continue their galactic exit, Brown will be watching for any deviation in their trajectories caused by gravitational influences. “How they arc out betrays the distribution of mass in the Milky Way--both the stars we can see and the dark matter that we can’t see,” he explains. “So we may actually have our first measure of the distribution of dark matter around the Milky Way.”

Scroll down to the bottom of the page for links to more images and a podcast with Warren Brown.

## Runaway Stars

The runaway star AE Aurigae, above and right of center, embedded in the Flaming Star Nebula (IC 405) in the constellation Auriga.

The Flaming Star Nebula (IC405) ranks as one of the showpiece sights in the northern constellation Auriga. This glowing emission nebula gains its energy from the star AE Aurigae, a 6 th -magnitude massive blue-white star about 1,500 light years away. This brilliant star, which outshines our Sun by some 30,000 times, blasts out ultraviolet light that ionizes the cloud of hydrogen gas around the star. As the hydrogen atoms reassemble, they emit light at signature wavelengths of red and green light that make these nebulae so beautiful. Most such nebulae are energized by stars that formed within their densest and most opaque regions. But that’s not the case with the Flaming Star Nebula. AE Aurigae did not originate here it’s just passing by chance through a cold cloud of hydrogen as it hurtles through the Milky Way, far removed from the place it was born .

AE Aurigae is an example of a runaway star, one that gained kinetic energy from a chance interaction with another star long ago. By measuring its speed and direction, astronomers have determined this star is on a beeline through the Milky Way at a speed of 100 km/s, and its traveling directly away from another famous showpiece object, the Orion Nebula, and likely formed within this nebula about 2.5 million years ago.

So how did AE Aurigae get from Orion to Auriga?

The answer, it seems, is that it had a chance gravitational encounter with a pair of other massive stars shortly after it was formed. AE Aurigae likely had a companion star before this encounter, and both stars passed close to another binary or multiple star system. In a complex gravitational dosey doe, these multiple star systems swapped and traded gravitational energy such that two stars became bound to each other in a long elliptical orbit and remained in place, while at least two others were slingshotted out of the Orion Nebulae. One was AE Aurigae, and the other was the runaway star Mu Columbae in the southern constellation Columba. The paths of both these stars trace back to the Trapezium Cluster, a pack of massive blue-white stars embedded in the Orion Nebula.

At its current speed, AE Aurigae will pass through the gas cloud that it’s currently energizing in about 20,000 years, which means the lovely Flaming Star Nebula will fade from view. In the meantime, this part of the sky remains a favorite of astrophotographers. Even visual observers can see this nebula and AE Aur itself with a modest telescope in dark sky. It’s well worth the effort to see the nebula and the runaway star that was born in a completely different part of the sky.

### A Game of Chance – and Gravity

The interactions that propelled AE Aurigae and Mu Columbae across the sky are felt to a lesser degree by most stars sometime in their lives. Our own Sun was born in a big galactic star cluster about 5 billion years ago. Every star in our ‘home cluster’ was eventually nudged by gravity out into the wider Milky Way. We owe our existence, in part, to dozens of chance encounters between the Sun and other stars, any one of which might have sent our solar system into a different part of the galaxy, possibly into the neighborhood of massive stars destined to blow up as supernovae and irradiate our planet with gamma rays that make it impossible for life to form. So we count ourselves lucky.

Also during the Sun’s long life, other stars likely passed through or near the Oort Cloud at the outer edge of our solar system, likely sending a blizzard of comets into the inner solar system, some of which may have hit the Earth and seeded it with the building blocks of life. This is not something we expect (or want) to happen again in the near future, but such events may have accelerated the development of life on Earth.

/>The Flaming Star Nebula (IC405) at upper right and the emission nebula IC410 at lower left. Image courtesy Terry Hancock/Grand Mesa Observatory.

### Escaping the Milky Way

Which brings us to the so-called hypervelocity stars.

Runaway stars like AE Aurigae are rare, but stars that gain enough speed to eventually escape from our galaxy are even more unlikely and interesting to consider. In our part of the Milky Way, a star that gets whipsawed by gravity to a speed of more than 500-600 km/s has enough energy to leave the galaxy. But do such stars exist?

The answer, it seems, is yes. That’s the fate of the star US708, for example, that’s moving about 1,200 km/s, more than twice the speed necessary to escape the Milky Way. The ESA’s GAIA satellite, which maps the speed and position of millions of nearby stars, found a few dozen similar stars on their way out of the galaxy. Some of these stars may have wandered too close to a massive black hole near the center of the galaxy and gained enough energy to reach escape velocity. Others may have found themselves in a tight orbit with a companion star that exploded as a supernova. As the star’s companion suddenly lost most of its mass in the explosion, the orbital potential energy of the remaining star was converted to kinetic energy, like a ball being swung around on a string that suddenly breaks, and the remaining star suddenly picks up enough speed to escape the galaxy.

Beyond the Milky Way, these runaway stars are by no means rare it seems, especially in large galaxy clusters. Astronomers have observed hundreds of billions of escaped stars in the Virgo cluster. Perhaps 10% of the mass of the Virgo cluster is composed of stars that are wandering freely in intergalactic space.

In most cases, any planets around these stars will come along for the ride since the interactions that propelled them to escape speed are not sufficiently strong to strip them away. If such a planet harbored biological life, however, it would likely be eradicated by the supernova that propelled its star into intergalactic space.

However, it might be possible for a planet’s biosphere to start over if the raw ingredients for life remained. If such life evolved into complex and intelligent beings over the ensuing billions of years, they would experience a night sky quite unlike our own. They might see other planets in their own star system, and moons if they are present. But they would see no other individual stars, no constellations, no nebulae or star clusters, just dim smudges of distant galaxies. The idea of a sky full of stars, even clouds of stars flecked with clusters and nebulae and lanes of stardust as we see on a dark night, would be unknown to them, conjured perhaps only by alien science fiction writers and artists who to try to image what it would be like to be embedded in the dim and diffuse conglomerations of stars they see so far away.

## Do hypervelocity stars have Oort clouds? - Astronomy

"That's extremely difficult towards boot a star away from the Galaxy. One of the most commonly accepted system for doing this involves communicating along with the supermassive black hole at the Galactic core. That suggests when you trace the superstar back to its place of origin, it stems from the center of our Universe. None of these hypervelocity celebrities arise from the facility, which suggests that there is an unexpected brand-new class of hypervelocity superstar, one along with a different ejection mechanism," Dr. Holley-Bockelmann told the press on January 9, 2014.

More just recently, Palladino and her colleagues have actually spotted an additional TWENTY stars, about the same size as our Sunlight, that they call prospective hypervelocity superstars. "One caveat concerns the popular mistakes in gauging stellar activities. To obtain the velocity of a celebrity, you need to measure the location actually efficiently over many years. If the position is actually measured horribly a handful of times over that number of years interval, this may appear to transfer a whole lot faster in comparison to that actually does. Our company did numerous analytical driving tests to increase the precision of our price quotes. So our team assume that, although a few of our prospects could be quirks, the bulk are actual," Palladino clarified to journalism on January 9, 2014.

The brand-new speed-demons seem to be to sporting activity the same composition as usual superstars originating created by the disk, so the team of stargazers perform certainly not assume that their birth origin found yourself in our Universe's core bulge, the halo that encircles it, or in some other peculiar, unexplainable, and also remote area beyond our Galaxy.

But, precisely how did these celebrities acquire turned approximately such harsh velocities? That is actually the inquiry. "Our company are actually dealing with that," PHYSICIAN Holley-Bockelmann commented to the press on January 9, 2014.

Can't imagine it would make any difference.

Hey everybody, I'm new here and what better way to start then jump right in!! I have a few questions concerning hypervelocity stars for a sci fi book that I want to write.

First of all, what is the agreed upon "average" speed of a hypervelocity star? I have done a bit of googling, but I have seen a lot of different numbers. Would 1000km/s be about right?

Secondly, I have read that some hypervelocity stars are thought to have originated in the Magellanic Clouds. Which one would be more likely to produce hypervelocity stars? The LMC or the SMC?

Thirdly and finally, given the speed and the distance of the LMC/SMC, how long would it take a hypervelocity star to reach Earth? I know this would never happen, but keep in mind it's just for a sci fi novel :P

Thank you all for your help and I look forward to being a member of these forums :) Merry Christmas.

## Astronomers Discover New Type of Hypervelocity Stars

Top and side views of the Milky Way Galaxy show the location of 4 of the new class of HVSs. The general directions from which the stars have come are shown by the colored bands. Image credit: Julie Turner, Vanderbilt University / NASA / ESO.

HVSs – solitary stars moving fast enough to escape the gravitational grasp of our Galaxy – were first proposed by Los Alamos National Laboratory scientist Dr Jack Hills in 1988. Their existence was confirmed 17 years later.

“The original hypervelocity stars are large blue stars and appear to have originated from the Galactic Center,” said Lauren Palladino, the first author on the study published in the Astrophysical Journal (arXiv.org).

HVSs of the new type are very different from the ones that have been discovered previously.

These stars are relatively small – about the size of the Sun – and the surprising part is that none of them appear to come from the Milky Way’s Galactic Center.

The discovery came as the researchers were mapping our Galaxy by calculating the orbits of Sun-like stars in the SDSS Survey.

“It’s very hard to kick a star out of the galaxy. The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our Galaxy. None of these HVSs come from the center, which implies that there is an unexpected new class of HVS, one with a different ejection mechanism,” Dr Holley-Bockelmann said.

Scientists calculate that these stars must move at speeds of more than a million miles per hour relative to the motion of the galaxy to reach escape velocity.

They also estimate that the supermassive black hole at the Milky Way’s Center has a mass equivalent to 4 million suns, large enough to produce a gravitational force strong enough to accelerate stars to hyper velocities.

The typical scenario involves a binary pair of stars that get caught in the black hole’s grip. As one of the stars spirals in toward the black hole, its companion is flung outward at a tremendous velocity.

So far, 18 giant blue hypervelocity stars have been found that could have been produced by such a mechanism.

Now Dr Schneider and his colleagues identified an additional 20 Sun-sized stars that they characterize as HVS candidates.

The new HVSs appear to have the same composition as normal disk stars, so the scientists do not think that their birthplace was in the Galaxy’s central bulge, the halo that surrounds it, or in some other exotic place outside the Milky Way.

“The big question is: what boosted these stars up to such extreme velocities? We are working on that now,” Dr Holley-Bockelmann said.

Palladino LE et al. 2014. Hypervelocity Star Candidates in the SEGUE G and K Dwarf Sample. ApJ 780, 7 doi: 10.1088/0004-637X/780/1/7

## Astronomers Discover New Class of “Hypervelocity Stars”

Top and side views of the Milky Way galaxy show the location of four of the new class of hypervelocity stars. These are sun-like stars that are moving at speeds of more than a million miles per hour relative to the galaxy: fast enough to escape its gravitational grasp. The general directions from which the stars have come are shown by the colored bands. Graphic design by Julie Turner, Vanderbilt University. Top view courtesy of the National Aeronautics and Space Administration. Side view courtesy of the European Southern Observatory.

A newly published study details the discovery of a new class of “hypervelocity stars” – stars that are moving at speeds of more than a million miles per hour relative to the Milky Way and that are fast enough to escape gravitational grasp of the Milky Way.

An international team of astronomers has discovered a surprising new class of “hypervelocity stars” – solitary stars moving fast enough to escape the gravitational grasp of the Milky Way galaxy.

The discovery of this new set of “hypervelocity” stars was described at the annual meeting of the American Astronomical Society this week in Washington, D.C., and is published in the January 1 issue of the Astrophysical Journal.

“These new hypervelocity stars are very different from the ones that have been discovered previously,” said Vanderbilt University graduate student Lauren Palladino, lead author on the study. “The original hypervelocity stars are large blue stars and appear to have originated from the galactic center. Our new stars are relatively small – about the size of the sun – and the surprising part is that none of them appear to come from the galactic core.”

The discovery came as Palladino, working under the supervision of Kelly Holley-Bockelmann, assistant professor of astronomy at Vanderbilt, was mapping the Milky Way by calculating the orbits of Sun-like stars in the Sloan Digital Sky Survey, a massive census of the stars and galaxies in a region covering nearly one quarter of the sky.

“It’s very hard to kick a star out of the galaxy,” said Holley-Bockelmann. “The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our galaxy. None of these hypervelocity stars come from the center, which implies that there is an unexpected new class of hypervelocity star, one with a different ejection mechanism.”

Astrophysicists calculate that a star must get a million-plus mile-per-hour kick relative to the motion of the galaxy to reach escape velocity. They also estimate that the Milky Way’s central black hole has a mass equivalent to four million suns, large enough to produce a gravitational force strong enough to accelerate stars to hyper velocities. The typical scenario involves a binary pair of stars that get caught in the black hole’s grip. As one of the stars spirals in toward the black hole, its companion is flung outward at a tremendous velocity. So far, 18 giant blue hypervelocity stars have been found that could have been produced by such a mechanism.

Now Palladino and her colleagues have discovered an additional 20 sun-sized stars that they characterize as possible hypervelocity stars. “One caveat concerns the known errors in measuring stellar motions,” she said. “To get the speed of a star, you have to measure the position really accurately over decades. If the position is measured badly a few times over that long time interval, it can seem to move a lot faster than it really does. We did several statistical tests to increase the accuracy of our estimates. So we think that, although some of our candidates may be flukes, the majority are real.”

The astronomers are following up with additional observations.

The new rogues appear to have the same composition as normal disk stars, so the astronomers do not think that their birthplace was in the galaxy’s central bulge, the halo that surrounds it, or in some other exotic place outside the galaxy.

“The big question is: what boosted these stars up to such extreme velocities? We are working on that now,” said Holley-Bockelmann.

Katharine Schlesinger from the Australian National University, Carlos Allende Prieto from the Universidad de La Laguna in Spain, Timothy Beers from the National Optical Astronomy Observatory in Tucson, Young Sun Lee from New Mexico State University and Donald Schneider from Pennsylvania State University also contributed to the discovery.

The research was supported by funds from the Graduate Assistance in Areas of National Need program, National Science Foundation grants AST 0847696, AST 0607482, Physics Frontier Center grants PHY 0216783, the Aspen Center for Physics, the Alfred P. Sloan Foundation and the U.S. Department of Energy Office of Science.

Publication: Lauren E. Palladino, et al., “Hypervelocity Star Candidates in the SEGUE G & K Dwarf Sample,” 2014, ApJ, 780, 7 doi:10.1088/0004-637X/780/1/7

Image: Julie Turner, Vanderbilt University. Top view courtesy of the National Aeronautics and Space Administration. Side view courtesy of the European Southern Observatory.

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In: Astrophysical Journal , Vol. 878, No. 1, 17, 10.06.2019.

Research output : Contribution to journal › Article › peer-review

T1 - Hypervelocity Stars from a Supermassive Black Hole-Intermediate-mass Black Hole Binary

N2 - In this paper we consider a scenario in which the currently observed hypervelocity stars in our Galaxy have been ejected from the Galactic center as a result of dynamical interactions with an intermediate-mass black hole (IMBH) orbiting the central supermassive black hole (SMBH). By performing three-body scattering experiments, we calculate the distribution of the ejected stars' velocities given various parameters of the IMBH-SMBH binary: IMBH mass, semimajor axis, and eccentricity. We also calculate the rates of change of the BH binary orbital elements due to those stellar ejections. One of our new findings is that the ejection rate depends (although mildly) on the rotation of the stellar nucleus (its total angular momentum). We also compare the ejection velocity distribution with that produced by the Hills mechanism (stellar binary disruption) and find that the latter produces faster stars on average. Also, the IMBH mechanism produces an ejection velocity distribution that is flattened toward the BH binary plane, while the Hills mechanism produces a spherically symmetric one. The results of this paper will allow us in the future to model the ejection of stars by an evolving BH binary and compare both models with Gaia observations, for a wide variety of environments (galactic nuclei, globular clusters, the Large Magellanic Clouds, etc.).

AB - In this paper we consider a scenario in which the currently observed hypervelocity stars in our Galaxy have been ejected from the Galactic center as a result of dynamical interactions with an intermediate-mass black hole (IMBH) orbiting the central supermassive black hole (SMBH). By performing three-body scattering experiments, we calculate the distribution of the ejected stars' velocities given various parameters of the IMBH-SMBH binary: IMBH mass, semimajor axis, and eccentricity. We also calculate the rates of change of the BH binary orbital elements due to those stellar ejections. One of our new findings is that the ejection rate depends (although mildly) on the rotation of the stellar nucleus (its total angular momentum). We also compare the ejection velocity distribution with that produced by the Hills mechanism (stellar binary disruption) and find that the latter produces faster stars on average. Also, the IMBH mechanism produces an ejection velocity distribution that is flattened toward the BH binary plane, while the Hills mechanism produces a spherically symmetric one. The results of this paper will allow us in the future to model the ejection of stars by an evolving BH binary and compare both models with Gaia observations, for a wide variety of environments (galactic nuclei, globular clusters, the Large Magellanic Clouds, etc.).

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