Astronomers discover Milky Way galaxy’s most-distant stars

Astronomers have detected in the stellar halo that represents the Milky Way’s outer limits a group of stars more distant 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 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 gravitational influence.

The furthest of 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).

These stars, spotted using the Canada-France-Hawaii Telescope on Hawaii’s Mauna Kea mountain, are part of a category of stars called RR Lyrae that are relatively low mass and typically have low abundances of elements heavier than hydrogen and helium.

The most distant one appears to have a mass about 70% that of our sun. No other Milky Way stars have been confidently measured farther away than these.

The stars that populate the outskirts of the galactic halo can be viewed as stellar orphans, probably originating in smaller galaxies that later collided with the larger Milky Way.

Containing an inner and outer layer, the Milky Way’s halo is vastly larger than the galaxy’s main disk and central bulge that are teeming with stars.

The galaxy, with a supermassive black hole at its center about 26,000 light years from Earth, contains perhaps 100 billion–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% of the galaxy’s stars.

Dark matter, which dominates the halo, makes up most of the universe’s mass and is thought to be responsible for its basic structure, with its gravity influencing visible matter to come together and form stars and galaxies.

The halo’s remote outer edge is a poorly understood region of the galaxy.

These newly identified stars are almost half the distance to the Milky Way’s neighboring Andromeda galaxy.