Astronomers Measure Black Hole Radius
September 27, 2012

Astronomers Measure Black Hole Radius For The First Time

Lee Rannals for — Your Universe Online

An international team of astronomers was able to measure the radius of a black hole for the first time.

Researchers measured the radius of a black hole at the center of the galaxy M87, which lies about 50 million light-years away from the Milky Way.

M87 has a black hole 6 billion times more massive than the Sun, and the team was able to observe the glow of matter near the edge of the black hole, or the "event horizon," using radio dishes in Hawaii, Arizona and California.

These radio dishes created a telescope array known as the Event Horizon Telescope (EHT) that can depict details 2,000 times finer than what is visible to the Hubble Space Telescope.

Shep Doeleman, assistant director at the MIT Haystack Observatory and research associate at the Smithsonian Astrophysical Observatory, said the event horizon is like an exit door from our universe, which once you walk through, "you're not coming back."

At the edge of a black hole, the gravitational force is so strong that it pulls in everything from its surroundings. However, not everything is able to cross the event horizon to squeeze into the black hole, which results in gas and dust build up. This creates a flat pancake of matter known as an accretion disk.

This disk orbits the black hole at nearly the speed of light, feeds the cosmic object superheated material and can eventually cause the black hole to spin in the same direction as the orbiting material.

High-speed jets can form above the accretion disk, shooting out across the galaxy and extending for hundreds of thousands of light-years. These jets can influence many galactic processes, such as how fast stars form.

Understanding a jet's trajectory could give scientists an idea of the dynamics of black holes in the region where their gravity is the dominant force.

Because M87's jet is magnetically launched from this orbit, astronomers are able to estimate the black hole's spin through careful measurement of its size as it leaves the black hole.

The team used a technique known as Very Long Baseline Interferometry (VLBI) to link data from radio dishes and help view M87 in extremely precise detail.

The astronomers measured the innermost orbit of the accretion disk to be only 5.5 times the size of the black hole event horizon. According to the laws of physics, this size suggests that the accretion disk is spinning in the same direction as the black hole, confirming theories of how black holes power jets from the centers of galaxies.

Next, the team plans to expand its telescope array in order to obtain even more detailed pictures of black holes.

“The basic nature of jets is still mysterious,” Christopher Reynolds, a professor of astronomy at the University of Maryland, said in a statement. “Many astrophysicists suspect that jets are powered by black hole spin ... but right now, these ideas are still entirely in the realm of theory. This measurement is the first step in putting these ideas on a firm observational basis.”

The researchers published their work in the journal Science Express.