Discovery of a Faint Red Source Speeding at a Million Miles Per Hour Towards the Galactic Center
The vast majority of stars near the Sun have random velocities of order tens of kilometers per second in the local standard of rest (LSR), an order of magnitude lower than the LSR rotation speed of 240 kilometers per second around the center of the Milky-Way galaxy. As a result, the vertical excursions of stars up and down from the Galactic plane are an order-of-magnitude smaller than their path around the Galactic center. This shapes the Galactic disk like a pancake with a thickness that is ten times smaller than its radius.
Given this organized rotational motion, it is easy to notice a small population of rare stars which are moving relative to the Galactic center at a much larger speed, beyond the local threshold of 500 kilometers per second needed to escape from the gravitational pull of the entire Milky-Way galaxy. These are labeled “hypervelocity stars.” How are they produced?
A natural process for generating fast-moving stars is gravitational slingshot from the center of the Milky-Way, home to the supermassive black hole Sgr A*. This beast weighs 4 million suns and its event horizon is half the size of Mercury’s orbit around the Sun. When a pair of stars bound by gravity approaches the black hole, the tide from the black hole could be strong enough to separate the two stars, bringing one closer to the black hole and ejecting the other out at a hypervelocity speed. This process was proposed in a 1988 paper by Jack Hills, and elaborated upon by other astrophysicists, including 17 papers that I co-authored over the years.
The first hypervelocity star was discovered in 2005 by Warren Brown and collaborators. It was a massive star at a distance of 300,000 light years moving out at twice the escape speed. Subsequent examples were consistent with stars speeding out of the Galactic center as expected from the Hills mechanism, although recent data suggests that some hypervelocity stars were born in the Milky-Way halo.
Recently, a new hypervelocity star with anomalous properties was announced in a paper by Adam Burgasser and collaborators. The star, named CWISE J124909.08+362116.0, has a very low mass and was identified by citizen scientists as a faint red source with a high proper motion of 0.9 arcsecond per year. It was found to move within the Galactic plane towards the Galactic center at a speed of 456 kilometers per second — equivalent to a million miles per hour. The magnitude of this speed is just below the local escape speed, but the direction of motion is opposite to that of an escaping star. Its estimated distance of about 400 light-years from the Sun makes it the nearest hypervelocity star ever reported. Low mass stars of this type constitute the most abundant population of stars. The faintness and proximity of this fast-moving source suggests that there may be a substantial population of high velocity, low-mass stars like it at larger distances.
The first anomalous property of this low-mass star involves its low abundance of heavy elements, making it unusual relative to most stars in the Galactic center which are rich in heavy elements. The second anomaly is that this hypervelocity star is moving towards the Galactic center and not away from it. The third anomaly is that its speed is just below the local escape speed from the Milky-Way. The fourth anomaly is that its trajectory lies in the plane of the Milky-Way disk.
Is there a simple explanation to these four anomalies?
An obvious possibility is that this star was ejected from the Galactic center by the Hills mechanism. With an ejection just short of the escape speed, the star reached a large distance in the Galactic halo, turned around like a tennis ball thrown up in the air, and is now on its return path to the Galactic center. This interpretation is natural if the Hills mechanism results in a broad range of ejection speeds. The stars ejected below the escape speed return back after billions of years. The alignment of the ejection velocity with the Milky-Way disk might result from the original pair of stars orbiting the center along the Galactic plane. The low abundance of heavy elements may reflect a population of stars that formed out of fresh gas with low enrichment of heavy elements.
However, stars with low enrichment of heavy elements are more common in the halo of the Milky-Way as relics of early generations of galaxies that merged with the Milky-Way. This raises a second possible interpretation. Perhaps the Milky-Way halo had a shell of stars around it that rained down from a large distance, of order hundreds of thousands of light years, towards the Galactic center. Such a shell would appear to an outside observer as a ring of stars. Indeed, there is a class of “ring galaxies” which appear this way. They are interpreted to be the outcome of galaxy mergers, in which some stars are kicked out of the center, reach a maximum distance and return back. In this context, the new hypervelocity star is a relic from a past merger of the Milky-Way with another galaxy.
Interstellar tourist agencies should charge the highest price for roundtrip tickets on a habitable planet around a returning star of this type. The interstellar journey, taking a few billion years, explores the entire landscape of the Milky-Way galaxy, from the dense environment near the supermassive black hole to the rarefied outskirts of the Milky-Way halo, a million light-years away. Low mass stars are slow burners and can support life for trillions of years. Advertisements of the trip might state: “The trip offers a magnificent view of the Milky-Way disk from a distance; an opportunity to escape from what you hate, and return to what you love a few billion years later.”
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
