Astronomers have discovered a group of stars farther from Earth than any known in our own galaxy almost halfway to a neighboring galaxy in the stellar halo that represents the outer limits of the Milky Way. The researchers said these 208 stars inhabit the outermost regions of the Milky Way’s halo, a spherical cloud of stars dominated by a mysterious invisible substance called dark matter that makes itself known only through its gravitational influence.
The most distant of these 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). The most distant star discovered is 1.08 million light-years from Earth.
co-author Raja GuhaThakurta, professor of astronomy and astrophysics at UC Santa Cruz said “This study redefines what constitutes the outer boundary of our galaxy, our galaxy and Andromeda are both so large that there is almost no space between the two galaxies”.
The stars, which were observed using the Canada-France-Hawaii Telescope on Hawaii’s Mauna Kea, are part of a category of stars called RR Lyrae, which are relatively low-mass and typically have a low abundance of elements heavier than hydrogen and helium.
The most distant one appears to have a mass of about 70 percent that of our Sun. No other stars in the Milky Way have been reliably measured farther than these. The stars that inhabit the fringes of the galactic hall can be considered orphan stars, probably originating from smaller galaxies that later collided with the larger Milky Way.
Yuting Feng, a doctoral student in astronomy said “Our interpretation of the origin of these distant stars is that they were most likely born in haloes of dwarf galaxies and star clusters that were later merged or more directly cannibalized—by the Milky Way”.
Their host galaxies have been gravitationally crushed and consumed, but these stars remain at this great distance as merger debris. The Milky Way has grown over time thanks to such disasters.
The Milky Way’s halo, which contains the inner and outer layers, is much larger than the main disk of the galaxy and the central bulge, which is teeming with stars. The galaxy, with a supermassive black hole at its center about 26,000 light-years from Earth, contains about 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 stars in the galaxy.
Dark matter, which dominates the halo, makes up most of the mass of the universe and is thought to be responsible for its basic structure, with its gravity influencing the visible matter to coalesce 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 from the Milky Way’s neighboring galaxy Andromeda.
Feng said “We see that the suburbs of the Andromeda halo and the Milky Way halo are really extended — and they’re almost ‘back to back”.
