Star death gives insight into black holes

Supermassive black holes at the heart of almost every galaxy exert a great gravitational force that holds the galaxy together. Occasionally, an unfortunate star winds up wandering too close to the black hole and is ripped apart, producing stellar displays of intense light and high energy radiation.

Using images from the Hubble Space Telescope, NASA’s Swift satellite and the Chandra X-Ray Observatory, astronomers at the Space Telescope Science Institute have detected such a display in a galaxy 3.8 billion light years away from Earth.

Initially detected by the Swift’s Burst Alert Telescope at the end of March in the Draco Constellation, the explosion was subsequently imaged by dozens of telescopes and Swift alerted astronomers worldwide of its position. Hubble identified the explosion’s origin at the center of a galaxy and imaged it while Chandra conducted a four-hour X-ray exposure that localized its origin to the center of the galaxy with greater precision.

What sets this explosion apart from other displays of stellar doom is its intensity, especially since the black hole in question is less massive than the black hole at the center of our galaxy. Previous displays were inadequate in strength and provided little information to astronomers seeking to compare their theoretical models of stellar destruction to empirical data.

“For a number of years astronomers have thought seriously about what we might observe if a star got too close to a supermassive black hole,” Andrew Fruchter, astronomer at the Space Telescope Science Institute, wrote in an e-mail to The News-Letter. “But until now, we have never had an observation that many astronomers thought was an example of this in action. This may be our first good look at what happens when a star is destroyed by a black hole.”

As a star approaches a black hole, its matter begins to get ripped apart and dragged in. Matter is drawn towards the black hole by the intense gravitational force of the singularity at the center of the black hole, where the mass of the black hole is concentrated into an infinitely dense form. The singularity is a single point in smaller black holes, while it takes up the geometry of a ring in supermassive black holes.

It is unclear how the star matter interacts with or gets incorporated into the singularity itself. “We don’t ever get to observe the singularity. The best we can do is watch what happens as matter plunges toward the event horizon, also known as the Schwarzschild radius, of the black hole,” Fruchter wrote. Relativity dictates that we will continue to observe the object get infinitely closer to the singularity as it falls into the event horizon. The final descent into the singularity is impossible to observe.

These distant displays of stellar material do have a time limit though, as the energy emitted from the matter is limited in its duration as it swirls around the black hole. “As matter approaches the event horizon the light that it emits becomes more and more redshifted, until the matter completely fades away from view,” Fruchter wrote. Redshifting occurs when the source of an electromagnetic wave, such as star material emitting light, moves in a way that alters the wavelength an observer such as our telescopes perceives.

What made it possible for astronomers to observe this specific explosion was its orientation. Fruchter notes that this black hole oriented the stellar materials in a way that expels enough of their radiation in the direction of our telescopes, making them visible.

“We think that some of the radiation we are observing is coming from an accretion disk — a vortex of infalling matter circling the black hole,” Fruchter wrote. “Our observations suggest that the accretion disk has in turn produced a relativistic jet — like those coming out of quasars — which is pointed directly towards us.”

These sorts of displays are difficult to spot because they are very rare. Fruchter notes that while such an event has not been observed in our galaxy, the Swift telescope has been scanning millions of galaxies in search of these kinds of exciting displays. “This is expected to be a rare event in the life of any one galaxy, but we can always hope!”