When distant stars explode, they emit bursts of energy called gamma-ray bursts that are so bright that telescopes on Earth can detect them. Studying these pulses, which can also come from mergers of some exotic astronomical objects such as black holes and neutron stars, can help astronomers like me understand the history of the universe.
Space telescopes detect an average of one gamma-ray burst per day, adding to the thousands of bursts detected over the years, and a community of volunteers makes research into these bursts possible.
On November 20, 2004, NASA launched the Neil Gehrels Swift Observatory, also known as Swift. Swift is a multi-wavelength space telescope that scientists use to learn more about these mysterious gamma-ray bursts from the universe.
Gamma ray bursts usually last only a very short time, from a few seconds to a few minutes, and most of their emission is in the form of gamma rays, which is part of the light spectrum that our eyes cannot see. Gamma radiation contains a lot of energy and can damage human tissues and DNA.
Fortunately, Earth’s atmosphere blocks most gamma rays from space, but that also means the only way to observe gamma rays is through a space telescope like Swift. During the 19 years of his observations, Swift observed more than 1,600 gamma-ray bursts. The information it gathers from these outbursts helps astronomers on the ground measure the distances to these objects.
Looking back in time
Data from Swift and other observatories have taught astronomers that gamma-ray bursts are among the most powerful explosions in the universe. They are so bright that space telescopes like Swift can detect them from across the universe.
In fact, gamma-ray bursts are among the most distant astrophysical objects observed by telescopes.
Because light travels at a finite speed, astronomers are essentially looking back in time as they look further into the universe.
The farthest gamma-ray burst ever observed occurred so far away that the light took 13 billion years to reach Earth. So when telescopes took pictures of that gamma-ray burst, they observed the event as it looked 13 billion years ago.
Gamma-ray bursts allow astronomers to learn more about the history of the universe, including how the birth rate and mass of stars change over time.
Types of gamma-ray bursts
Astronomers now know that there are actually two types of gamma-ray bursts: long and short. They are classified based on how long their pulses last. The long gamma-ray bursts have pulses longer than two seconds, and at least some of these events are associated with supernovae – exploding stars.
When a massive star, or one at least eight times as massive as our Sun, runs out of fuel, it will explode as a supernova and collapse into a neutron star or black hole.
Both neutron stars and black holes are extremely compact. If you were to shrink the entire Sun to a diameter of about 12 miles, or the size of Manhattan, it would be as dense as a neutron star.
Some particularly massive stars can also release beams of light when they explode. These jets are concentrated beams of light driven by structured magnetic fields and charged particles. When these jets are aimed at Earth, telescopes like Swift will detect a gamma-ray burst.
On the other hand, short gamma-ray bursts have pulses of less than two seconds. Astronomers suspect that most of these short bursts occur when two neutron stars or a neutron star and a black hole merge.
When a neutron star gets too close to another neutron star or a black hole, the two objects will orbit each other and creep closer as they lose some of their energy through gravitational waves.
These objects eventually converge and emit short jets. When the short jets are aimed at Earth, space telescopes can detect them as short gamma-ray bursts.
Classifying gamma-ray bursts
Classifying bursts as short or long is not always that simple. In the past few years, instead of the expected mergers, astronomers have discovered some curious short gamma-ray bursts associated with supernovae. And they found some long gamma-ray bursts associated with mergers rather than supernovae.
These confusing cases show that astronomers do not fully understand how gamma-ray bursts arise. They suggest that astronomers need a better understanding of the shapes of gamma-ray pulses to better connect the pulses to their origins.
But it is difficult to systematically classify the pulse shape, which is different from the pulse duration. Pulse shapes can be very diverse and complex. Until now, even machine learning algorithms have not been able to correctly recognize all the detailed pulse structures that astronomers are interested in.
Community Science
My colleagues and I enlisted the help of volunteers through NASA to identify pulse structures. Volunteers learn to identify the pulse structures, then look at images on their own computers and classify them.
Our preliminary results suggest that these volunteers – also called citizen scientists – can quickly learn and recognize the complex structures of gamma-ray pulses. By analyzing this data, astronomers can better understand how these mysterious eruptions arise.
Our team hopes to find out whether more gamma-ray bursts in the sample challenge the previous short and long classification. We will use the data to more closely investigate the history of the universe through gamma-ray burst observations.
This citizen science project, called Burst Chaser, has grown since our preliminary results and we are actively recruiting new volunteers to join our quest to study the mysterious origins behind these bursts.
This article is republished from The Conversation, an independent nonprofit organization providing facts and analysis to help you understand our complex world.
It was written by: Amy Lien, University of Tampa.
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Amy Lien receives funding from the NASA Citizen Science Seed Funding Program.