GRBs are most violent explosions in universe


By Tom Burns - Stargazing



Editor’s note: This is part one of a two-part series on gamma-ray bursts.

They last for but a second or two. For that brief instant, they can outshine the entire energy output of the perhaps one trillion stars in the cosmos. And telescopes in orbit around Earth detect such supremely energetic outbursts on an average of one every day.

Such a gargantuan event is called a gamma-ray burst, or GRB for short. They are usually detected in galaxies at extreme distances from our Earth and sun, and their home galaxy, the Milky Way. The galaxies in which they occur are primarily young ones, not like our more mature Milky Way.

Astronomers have never detected a single GRB in our home galaxy, thank goodness. A burst within a few thousand light-years of our planet would have cataclysmic consequences for life on Earth.

A GRB detected on December 14, 1997, is a case in point. GRB971214, as astronomers call it, produced a brief release of gamma-ray energy so great that it rivaled the power of the Big Bang, the creation of the universe. Caltech astronomer George Djorgovski said, “For about one or two seconds, this burst was as luminous as all the rest of the entire universe.”

And GRB971214 is not just far away. It’s really, really far away at about 12 billion light-years. Since the light had to travel 12 billion light-years to get to Earth, the explosion must have happened near the universe’s beginning as galaxies were forming.

Thousands of GRBs have been detected by the Dutch-Italian Beppo-SAX satellite and NASA’s space-based Compton Gamma Ray Observatory, Swift Observatory, and Fermi Gamma-ray Space Telescope.

Gamma-ray astronomy is among the most exotic scientific pursuits. To understand its importance, we must ask the most profound question of all: What the heck is a gamma ray?

Energy comes in a variety of flavors. Visible light is a form of energy, but light is, in reality, a mixture of different energy forms.

When scientists first broke visible light into components, they discovered a rainbow of colors — from blue through yellow and down to red. Each color represents a different kind of energy. For example, blue is more energetic than yellow, and both are more energetic than red.

But those bands of energy are only the colors we can see. The colors of the rainbow, collectively called the visible spectrum of light, are a small part of the larger electromagnetic spectrum.

Visible light sits more or less in the middle of this spectrum. The invisible bands of energy to each side are either more or less energetic than visible light.

Infrared radiation is slightly less energetic than red light. Still, it will cook you if enough of it strikes your body. We can’t see it, but we can feel it as heat. Radio waves are even less powerful, but they pass nicely through Earth’s atmosphere. That quality makes them quite handy for communication.

If you lie out in the sun too long, you will discover firsthand that ultraviolet energy is more energetic than blue light. The ultraviolet range of the electromagnetic spectrum causes our skin to burn if we are bathed in sunlight too long.

Even-more-powerful X-rays can pass right through your body and form an image on film.

At the highest end of the EM spectrum, gamma rays are the most powerful energy of all. They are generated in enormous quantities by mind-meltingly massive explosions — as thermonuclear bombs explode, as a star’s dense core creates energy, and as stars collide.

Gamma rays are common as dirt in the universe, but our deepening understanding of them is new, in large part because of the filtering effect of Earth’s atmosphere. In fact, the sun’s radiation of those high-powered energies would deep-fry us all like chicken nuggets except for one vital saving grace.

Earth’s blanket of air lets in the gentle, low-energy parts of the EM spectrum, such as visible light and radio waves. However, it blocks out most of the nasty high-energy stuff such as X-rays and gamma rays.

As a result, until the 20th century, humans were blissfully unaware of the high-energy universe surrounding us.

In the 1960s, rocketeers could finally launch probes outside the atmosphere, and it was only then that the invisible became visible.

It isn’t easy to overstate the value of satellites. So much of what we know about the universe has come from studying the parts of the electromagnetic spectrum blocked by Earth’s atmosphere.

The first hint of gamma-rays’ existence arose from the most deadly of human inventions, nuclear weapons. A surge of gamma rays is a tell-tale sign that atomic bombs have ignited.

To help enforce 1963’s Atmospheric Test Ban Treaty on those devices, the United States launched a series of Earth-orbiting satellites collectively called Vela, specifically designed to detect gamma rays from nuclear detonations.

On July 2, 1967, Vela detected a six-second burst coming not from Earth but from space. Unfortunately, the US government did not declassify the event until 1973, when astronomers finally found out that such bursts even existed.

Of course, no one had any idea what caused them. Vela was capable of determining their exact location in the sky. However, by the time astronomers could point a telescope in their general direction, the burst was gone. Since astronomers didn’t know their specific sources, they couldn’t tell how far away they are.

Speculation abounded. At the time, most astronomers believed that neutron stars must have something to do with them.

Neutron stars are the result of the death of a star more massive than our sun. When stars die, they collapse into much smaller versions of themselves, and they don’t lose much of the stuff they were initially composed of.

Imagine a star several million miles wide collapsing to just a few miles wide. The result is a tiny, incredibly dense object with intensely concentrated gravitational pull.

Anything colliding with a neutron star would be accelerated to mind-boggling speed as it fell in. The resulting collision would generate quite a lot of energy.

Two decades ago, I asked Professor Gerald Newsom, now retired from the OSU Department of Astronomy, about the process. He said that “a marshmallow dropped onto a neutron star would release about the same energy as the explosion of a hydrogen bomb.”

If the explosions were in our Milky Way galaxy (and therefore relatively close to us), they could be caused by something as loosely packed and small as a comet just a few miles wide falling into a neutron star. Luckily for life on our planet, no such GRB has been detected.

Since the explosions are happening in other galaxies (and thus far away), they would have to be generated by much more massive collisions, say two neutron stars falling into each other. An even more enormous explosion of gamma rays would happen if a neutron star fell into an even denser object like a black hole.

And there matters stood for years. However, astronomers are a cautious lot. Other progenitors of such GRB events might exist. As Professor Newsom pointed out to me years ago, “Nature may have a lot of ways of doing this.”

We’ll discuss those other ways next week.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.