In Hollywood blockbusters, explosions are often among the stars of the show. In space, explosions of actual stars are a focus for scientists who hope to better understand their births, lives, and deaths and how they interact with their surroundings. Using NASA’s Chandra X-ray Observatory, astronomers have studied one particular explosion that may provide clues to the dynamics of other, much larger stellar eruptions. A team of researchers pointed the telescope at GK Persei, an object that became a sensation in the astronomical world in 1901 when it suddenly appeared as one of the brightest stars in the sky for a few days, before gradually fading away in brightness. Today, astronomers cite GK Persei as an example of a “classical nova,” an outburst produced by a thermonuclear explosion on the surface of a white dwarf star, the dense remnant of a Sun-like star. A nova can occur if the strong gravity of a white dwarf pulls material from its orbiting companion star. If enough material, mostly in the form of hydrogen gas, accumulates on the surface of the white dwarf, nuclear fusion reactions can occur and intensify, culminating into a cosmic-sized hydrogen bomb blast. The outer layers of the white dwarf are blown away, producing a nova outburst that can be observed for a period of months to years as the material expands into space. Classical novas can be considered to be “miniature” versions of supernova explosions. Supernovas signal the destruction of an entire star and can be so bright that they outshine the whole galaxy where they are found. Supernovas are extremely important for cosmic ecology because they inject huge amounts of energy into the interstellar gas, and are responsible for dispersing elements such as iron, calcium and oxygen into space where they may be incorporated into future generations of stars and planets. Although the remnants of supernovas are much more massive and energetic than classical novas, some of the fundamental physics is the same. Both involve an explosion and creation of a shock wave that travels at supersonic speeds through the surrounding gas. The more modest energies and masses associated with classical novas means that the remnants evolve more quickly. This, plus the much higher frequency of their occurrence compared to supenovas, makes classical novas important targets for studying cosmic explosions. Chandra first observed GK Persei in February 2000 and then again in November 2013. This 13-year baseline provides astronomers with enough time to notice important differences in the X-ray emission and its properties. This new image of GK Persei contains X-rays from Chandra (blue), optical data from NASA’s Hubble Space Telescope (yellow), and radio data from the National Science Foundation’s Very Large Array (pink). The X-ray data show hot gas and the radio data show emission from electrons that have been accelerated to high energies by the nova shock wave. The optical data reveal clumps of material that were ejected in the explosion. The nature of the point-like source on the lower left is unknown. Over the years that the Chandra data span, the nova debris expanded at a speed of about 700,000 miles per hour. This translates to the blast wave moving about 90 billion miles during that period. One intriguing discovery illustrates how the study of nova remnants can provide important clues about the environment of the explosion. The X-ray luminosity of the GK Persei remnant decreased by about 40% over the 13 years between the Chandra observations, whereas the temperature of the gas in the remnant has essentially remained constant, at about one million degrees Celsius. As the shock wave expanded and heated an increasing amount of matter, the temperature behind the wave of energy should have decreased. The observed fading and constant temperature suggests that the wave of energy has swept up a negligible amount of gas in the environment around the star over the past 13 years. This suggests that the wave must currently be expanding into a region of much lower density than before, gi

Photo by: Chandra X-ray Observatory Center

Chandra X-ray Observatory Center

You Love Supernova, So How About Micronova?

In space, even the smallest explosions are insanely powerful. Take for instance the newly discovered “micronova,” which sounds cute and cuddly and not at all deadly…except for the fact that it’s the explosive equivalent of a nuclear bomb a million times bigger than Mount Everest.

May 26, 2022

Here’s one way to build a nuclear bomb in space. You start with a pair of stars, which is the most common way to find stars, so this is the easy part. Then you have one star age faster than the other. This is also pretty easy, since more massive stars burn through their material at a higher rate, and it’s likely that one of your stars will simply be more massive than the other.

And then you just wait, and time and gravity will do their job.

Okay, so it turns out it’s all easy parts.

Red Giant Blows a Bubble

Red Giant Blows a Bubble

A bright star is surrounded by a tenuous shell of gas in this unusual image from the NASA/ESA Hubble Space Telescope. U Camelopardalis, or U Cam for short, is a star nearing the end of its life. As it begins to run low on fuel, it is becoming unstable. Every few thousand years, it coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse. The gas ejected in the star’s latest eruption is clearly visible in this picture as a faint bubble of gas surrounding the star. U Cam is an example of a carbon star. This is a rare type of star whose atmosphere contains more carbon than oxygen.

Photo by: ESA/NASA

ESA/NASA

A bright star is surrounded by a tenuous shell of gas in this unusual image from the NASA/ESA Hubble Space Telescope. U Camelopardalis, or U Cam for short, is a star nearing the end of its life. As it begins to run low on fuel, it is becoming unstable. Every few thousand years, it coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse. The gas ejected in the star’s latest eruption is clearly visible in this picture as a faint bubble of gas surrounding the star. U Cam is an example of a carbon star. This is a rare type of star whose atmosphere contains more carbon than oxygen.

The larger star will burn through its available supply of hydrogen and helium and die, leaving behind a core of carbon and oxygen, known as a white dwarf. Eventually, the smaller star will reach the end of its life cycle, swelling to become a red giant.

Occasionally some of the atmosphere of that red giant will swirl onto the white dwarf. If too much gas falls onto the surface, the densities will go critical and the entire white dwarf will undergo a runaway nuclear fusion event, also known as a giant bomb – a supernova.

If just some of the piled-up material ignites without cracking the white dwarf, you get a smaller, weaker explosion, known as a nova. The power behind a nova comes from the rapid fusion of hydrogen into helium, and while not as bright as their supernova cousins, they’re still pretty impressive.

A new look at the debris from an exploded star in our galaxy has astronomers re-examining when the supernova actually happened. Recent observations of the supernova remnant called G11.2-0.3 with NASA’s Chandra X-ray Observatory have stripped away its connection to an event recorded by the Chinese in 386 CE. Historical supernovas and their remnants can be tied to both current astronomical observations as well as historical records of the event. Since it can be difficult to determine from present observations of their remnant exactly when a supernova occurred, historical supernovas provide important information on stellar timelines.

Photo by: X-ray: NASA/CXC/NCSU/K. Borkowski et al; Optical: DSS

X-ray: NASA/CXC/NCSU/K. Borkowski et al; Optical: DSS

A new look at the debris from an exploded star in our galaxy has astronomers re-examining when the supernova actually happened. Recent observations of the supernova remnant called G11.2-0.3 with NASA’s Chandra X-ray Observatory have stripped away its connection to an event recorded by the Chinese in 386 CE. Historical supernovas and their remnants can be tied to both current astronomical observations as well as historical records of the event. Since it can be difficult to determine from present observations of their remnant exactly when a supernova occurred, historical supernovas provide important information on stellar timelines.

For a long time, astronomers believed that these were your only two options when it came to these kinds of explosions. Either the entire white dwarf goes off (a supernova) or just the gas that piled onto it (a nova).

But NASA’s telescope TESS found something interesting when it was doing its job to hunt for planets orbiting other stars: a bright flash lasting only a few hours. Their interest piqued, some astronomers began searching for similar kinds of flashes, until they found what they were looking for using the European Southern Observatory’s Very Large Telescope.

They found a micronova, which has the same setup as a supernova or regular nova, just…not as powerful. They believe that powerful magnetic fields swirling around the white dwarf prevent gas from the companion red giant from just falling wherever it wants. Instead, it forces the gas to flow onto the poles of the white dwarf. There the gas piles up until it reaches the critical densities required to go nuclear. But since there isn’t an entire atmosphere’s worth of material to blow up, the explosion isn’t as intense as a regular nova.

Still, I wouldn’t go anywhere near it.

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Paul M. Sutter

Paul M. Sutter is an astrophysicist at Stony Brook University and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space.

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