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Astronomers discover a new way that stars die

As with everything else in the universe, stars die. Usually in spectacular fashion.

When we look up at the night sky, we marvel at the beauty of the countless stars visible to the naked eye. However, there’s much more to these twinkling lights than meets the eye.

Every star has its own life story, from birth to death. Usually, this journey unfolds in very predictable ways.

Take our sun, for instance. It’s a relatively small star. As it ages, it gradually sheds its outer layers, dims, and eventually turns into a white dwarf star.

But bigger stars live a different kind of life. They burn brighter and die younger, ending their lives in spectacular supernova explosions. These supernovas create extremely dense remnants, like neutron stars and black holes.

Sometimes, two of these remnants get together and can even collide. But recent research suggests there’s a fourth scenario for how stars can die and end their lives. This method has long been hypothesized, but never seen until now.

Observing a star die in a previously unknown way

Scientists have been investigating a strange gamma-ray burst (GRB). These events are a sudden and intense flash of gamma rays.

Using the Gemini South telescope in Chile, along with other telescopes, astronomers found evidence of what seems like a massive cosmic car crash involving stars or their remnants. But here’s the twist – this collision took place near a supermassive black hole in an ancient galaxy.

“Stars can die in some of the densest regions of the Universe where they can be driven to collide,” explains Andrew Levan. He is a Dutch astronomer from Radboud University and the leading author of this groundbreaking study published in Nature Astronomy.

“This is exciting for understanding how stars die and for answering other questions, such as what unexpected sources might create gravitational waves that we could detect on Earth.”

Importance of ancient galaxies for learning how stars die

Now, you may wonder, why are ancient galaxies important? Ancient galaxies are past their prime in star-making. They may not have many giant stars left, which are usually responsible for long GRBs. But they do have a busy center. These centers are filled with stars and ultra-dense remnants like white dwarfs, neutron stars, and black holes.

Scientists have thought for a long time that given the chaotic nature of a black hole’s neighborhood, a collision leading to a GRB was just a matter of time. But finding solid evidence of this has been challenging.

Then, on October 19, 2019, NASA’s Neil Gehrels Swift Observatory spotted a bright flash of gamma rays that lasted over a minute. This was the first hint of such an event.

Astronomers consider any GRB that lasts more than two seconds to be “long”. These usually occur when massive stars, at least ten times the size of our sun, die. But not always.

The study involved studying a GRB

To investigate further, scientists used Gemini South for long-term observations of the fading glow after the GRB. This allowed them to pinpoint the GRB’s location to within 100 light-years of an ancient galaxy’s nucleus. The location was very close to its supermassive black hole. There were no signs of a supernova either, which would typically be expected.

“Rather than being a massive star collapsing, the burst was most likely caused by the merger of two compact objects,” said Levan. “Pinpointing its location to the center of a previously identified ancient galaxy, we had the first tantalizing evidence of a new pathway for stars to meet their demise.”

The core of an ancient galaxy is packed with stars and their remnants. It’s so densely populated that a collision is not as unlikely as it would be in more normal galactic environments. Especially near a supermassive black hole with its intense gravitational pull. These collisions can lead to massive explosions visible even from great cosmic distances.

These kinds of events may be happening all over the Universe in similarly crowded areas. We just haven’t noticed them yet. This is partly because the galactic centers are full of dust and gas, which can obscure the GRBs and their afterglow. But this particular GRB, identified as GRB 191019A, was an exception.

Astronomers are now searching for more of these events

Scientists are eager to find more such events. The ultimate goal is to link a GRB detection with a gravitational wave detection. An observation of this kind would reveal more about these events and confirm their origins, even in murky environments. The Vera C. Rubin Observatory, set to launch in 2025, will be key to this research.

“Studying gamma-ray bursts like these is a great example of how the field is really advanced by many facilities working together,” said Levan. “These observations add to Gemini’s rich heritage developing our understanding of stellar evolution,” adds Martin Still. He is the program director for the International Gemini Observatory.

These celestial events are a stark reminder that the cosmos is a dynamic, ever-evolving place. Each star has its own unique journey, and sometimes, these journeys can take unexpected, spectacular turns.

More about star death

Stars, much like living organisms, have a life cycle — they’re born, live for a while, and eventually die. But unlike living organisms, the lifetime of a star can stretch billions of years. The way a star dies depends largely on its mass. Let’s take a look at the life cycle of stars and their eventual demise.

Low-Mass and Medium-Mass Stars (like our Sun)

Main Sequence

Stars spend the majority of their life in a stage called the main sequence, where they’re burning hydrogen to produce helium in their cores. This is a process called nuclear fusion, and it releases an incredible amount of energy in the form of light and heat, which is why stars shine.

Red Giant Phase

As a star like our Sun consumes all of its hydrogen fuel, the balance between gravity pulling inward and pressure from fusion pushing outward is lost. The core contracts under gravity and heats up. This causes the outer layers of the star to expand and cool, turning the star into a red giant.

Helium Fusion and Planetary Nebula

In the core of the red giant, helium atoms fuse together to form carbon. When the helium fuel runs out, the core contracts again, and the outer layers are ejected. The ejection of the outer gaseous layers forms a planetary nebula, and what’s left of the star is the hot, dense core.

White Dwarf

This core cools and contracts over billions of years, creating a white dwarf. When it stops emitting significant heat or light, it becomes a black dwarf. However, the universe is not old enough for any black dwarfs to exist yet.

High-Mass Stars

Main Sequence

Like smaller stars, high-mass stars spend most of their life in the main sequence. However, they burn through their hydrogen fuel much faster due to higher pressure and temperature in their cores.

Supergiant Phase

When the hydrogen fuel is exhausted, high-mass stars also become red giants. But due to their larger mass, they continue to contract and heat up, enabling fusion of heavier elements like carbon, oxygen, and iron. This causes the star to expand even more, becoming a supergiant.

Supernova Explosion

The fusion process stops at iron since iron doesn’t release energy when fused. With no outward pressure from fusion to counteract gravity, the iron core collapses under its own gravity, which causes a massive explosion known as a supernova.

Neutron Star or Black Hole

The remnant of this explosion can form a neutron star, which is an incredibly dense object made mostly of neutrons. If the original star was large enough (typically more than about 20 times the mass of the Sun), the core’s gravity would be so strong that it would form a black hole. These are objects with gravity so intense that not even light can escape it.

This is a broad overview of how stars die. Keep in mind that there’s a lot of variation in the specifics depending on many factors. These include the star’s exact size, its chemical composition, and its distance from other stars. Also, our understanding of stellar evolution is constantly being refined as we make new observations and develop more sophisticated models.

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