The Giant Significance of A White Dwarf

Have you ever wished upon a star? Although they appear as glints in the nighttime sky to us, stars are complex celestial bodies. While stars are beautiful collections of matter, they actually have a life cycle.

All stars begin as a stellar nebula and transform into different phases over time, depending on their location and composition.

Stars will often collapse into their white dwarf stage near the end of their life cycle.

Because stars are located so far away from Earth, they hold many mysteries. It’s up to NASA’s astronomers to learn about star life cycles from afar.

The secret of the Small Magellanic Cloud
The Small Magellanic Cloud is a galaxy located 200,000 light years from Earth.

A few months ago, astronomers studying the galaxy made a startling discovery. They found a bright, powerful x-ray burst from a star in the Small Magellanic Cloud.

There are plenty of stars in the universe, but this one is special. In fact, scientists say it’s the fastest-growing white dwarf they’ve ever encountered. This has real implications for our understanding of a star’s life cycle, and even how we understand the expansion of the universe.

What is a white dwarf?
Once a star runs out of fuel, it collapses and shrinks to a white dwarf. A white dwarf is usually the size of Earth, but with immense gravity. The heavy gravitational pull results from compacting and condensing lots of mass into a small area.

Eventually, our Sun will transform into a white dwarf. However, the Earth’s Sun is unusual because it’s unary: it doesn’t have a partner star.

Most stars are binary, in that they exist in pairs. When one star in a binary pair transforms into a white dwarf, its powerful gravity will pull matter away from its partner star.

The significance of ASASSN-16oh
NASA’s Chandra X-Ray Observatory detected low-energy x-rays emitting from the white dwarf star ASASSN-16oh.

This discovery was particularly significant because the star’s x-rays were much brighter than those of other white dwarfs. This put its x-rays into a special category called “supersoft x-rays.”

Scientists have long thought that supersoft x-rays could only form in one way: nuclear fusion. When hydrogen and helium collide, they produce what’s essentially a hydrogen bomb explosion, resulting in x-rays and bright light.

This volatile process leaves remnants that we can easily measure. For example, nuclear fusion is supposed to emit consistent light across the star’s surface.

However, ASASSN-16oh was different.

The optical light on the star wasn’t consistent or bright enough to come from nuclear fusion. Scientists began to wonder if nuclear fusion was indeed at play on the white dwarf, or if another undetected process contributed to the x-ray emissions.

ASASSN-16oh and the future
ASASSN-16oh isn’t generating supersoft x-rays from nuclear fusion. Instead, it’s doing so through a process called accretion.

The white dwarf’s gravity is rapidly pulling gases away from its companion star. As gas collects around the star, it falls to the star’s surface. Once the gas meets with the star’s surface, it generates light and supersoft x-rays.

The problem with ASASSN-16oh is that it’s gaining mass far too quickly. In fact, it’s one of the most rapidly growing white dwarf stars scientists have ever seen. ASASSN-16oh could potentially destroy itself in a supernova explosion because of its rapid growth.

Although the white dwarf likely holds many more mysteries, ASASSN-16oh challenged how scientists understood the life cycle and behavior of stars. We will lean on this knowledge as we continue exploring the heavens beyond Earth. It even has implications for how matter transforms as the universe expands.

The next time you wish on a star, just think of the incredible science happening thousands of light years away.