In the stellar halo, which represents the Milky Way’s outer limits, astronomers have discovered a group of stars farther from Earth than any other known within our own galaxy – nearly halfway to a neighboring galaxy.
The researchers said these 208 stars inhabit the most distant reaches of the Milky Way’s halo, a spherical stellar cloud dominated by the mysterious invisible substance called dark matter that makes itself known only through its gravity. The furthest from them is 1.08 million light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).
Spotted by the Canada-France-Hawaii Telescope on Mount Mauna Kea in Hawaii, these stars are part of a class of stars called RR Lyrae that are relatively low in mass and typically contain few elements heavier than hydrogen and helium. The farthest one appears to have a mass about 70 percent that of our Sun. No other Milky Way star has been measured with certainty further away than this one.
The stars populating the outskirts of the galactic halo can be seen as stellar orphans, likely from smaller galaxies that 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 doctoral student in astronomy. student at the University of California, Santa Cruz, who led the study, presented this week at a meeting of the American Astronomical Society in Seattle.
“Their host galaxies are gravitationally shredded and consumed, but these stars are left behind as debris from the merger at that great distance,” Feng added.
Due to such calamities, the Milky Way has grown over time.
“The larger galaxy grows by eating smaller galaxies — by eating its own kind,” said co-author Raja GuhaThakurta, UC Santa Cruz’s chair of astronomy and astrophysics.
The halo of the Milky Way consists of an inner and outer layer and is much larger than the main disk and central bulge of the galaxy teeming with stars. The galaxy, with a supermassive black hole at its center about 26,000 light-years from Earth, contains perhaps 100 billion to 400 billion stars, including our sun, which resides in one of the four primary spiral arms that make up the Milky Way’s disk. The halo contains about 5 percent of the galaxy’s stars.
Dark matter, which dominates the halo, makes up most of the universe’s mass and is believed to be responsible for its basic structure, with gravity influencing visible matter to come together to form stars and galaxies.
The far outer edge of the halo is a poorly understood region of the galaxy. These newly identified stars are nearly half the distance to the Milky Way’s neighboring Andromeda galaxy.
“We can see that the outskirts of the Andromeda halo and the Milky Way halo are really extended — and almost back-to-back,” Feng said.
The search for life beyond Earth focuses on rocky planets related to Earth that orbit the stars called the “habitable zone”. More than 5,000 planets outside our solar system have already been discovered, the so-called exoplanets.
“We’re not sure, but each of these outer halo stars should be about as likely to have planets as the sun and other sun-like stars in the Milky Way,” GuhaThakurta said.
© Thomson Reuters 2023