In the stellar halo that marks the Milky Way’s outer limits, astronomers have spotted a cluster of stars farther from Earth than any known within our own galaxy – almost halfway to a neighboring galaxy.
The researchers said these 208 stars inhabit the most remote areas of the world Milky Way‘s halo, a spherical star cloud dominated by the mysterious invisible substance called dark matter, which is only noticeable through its gravitational influence. The farthest of them is 1.08 million light-years away Earth. A light year is the distance that light travels in one year, 5.9 trillion miles (9.5 trillion km).
This Stars, discovered with the Canada-France-Hawaii Telescope on Mount Mauna Kea, Hawaii, belong to a category of stars called RR Lyrae, which have relatively low mass and typically low abundances of elements heavier than hydrogen and helium are. The most distant appears to have a mass about 70 percent of ours Sun. No other Milky Way stars have been reliably measured further away than this one.
The stars populating the outskirts of the galactic halo can be viewed as stellar orphans, likely descended from smaller ones galaxies which later collided with the larger Milky Way.
“Our interpretation of the origin of these distant stars is that they were most likely born in the halos of dwarf galaxies and star clusters that were later merged — or more simply, cannibalized — by the Milky Way,” said Yuting Feng, a PhD astronomer student at the University of California, Santa Cruz, which led the study, presented at a meeting of the American Astronomical Society in Seattle this week.
“Their host galaxies have been shredded and digested by gravity, but these stars remain at this great distance as debris from the merger event,” Feng added.
The Milky Way has grown through such catastrophes over time.
“The larger galaxy grows by eating smaller galaxies — by eating its own kind,” said Raja GuhaThakurta, study co-author, Chair of Astronomy and Astrophysics at UC Santa Cruz.
The Milky Way’s halo, which contains an inner and outer layer, is considerably larger than the galaxy’s main disc and central bulge, which is teeming with stars. With a supermassive black hole at its center, about 26,000 light-years from Earth, the galaxy contains perhaps 100 to 400 billion stars, including our Sun, which is in one of the four primary spiral arms that make up the Milky Way’s disc. The halo contains about 5 percent of the galaxy’s stars.
Dark matter, dominating the halo, accounts for most of the mass of the universe and is believed to be responsible for its basic structure, with its gravity affecting visible matter to come together to form stars and galaxies.
The halo’s remote outer edge is a poorly explored region of the galaxy. These newly identified stars are nearly half as distant as the Milky Way’s neighboring Andromeda galaxy.
“We can see that the suburbs of the Andromeda halo and the Milky Way halo are really spread out — almost ‘back to back,'” Feng said.
The search for life beyond Earth focuses on Earth-like rocky planets orbiting stars in the so-called “habitable zone.” More than 5,000 planets outside our solar system, so-called exoplanets, have already been discovered.
“We don’t know for sure, but each of these outer halo stars should be about as likely to be orbited by planets as the Sun and other Sun-like stars in the Milky Way,” GuhaThakurta said.
© Thomson Reuters 2023
