There’s something poignant and haunting about ancient astronomers documenting things in the sky whose nature they could only guess at. It’s true in the case of Père Dom Anthelme, who in 1670 saw a star suddenly burst into view near the head of the constellation Cygnus, the Swan. The object was visible with the naked eye for two years, as it flared in the sky repeatedly. Then it went dark. We call that object CK Vulpeculae.
Anthelme couldn’t have known what the object was, and modern astronomers have struggled to understand its nature too. Modern astronomers labelled it as a nova—a star that flares brightly as it ejects material. But a new study suggests that CK Vulpeculae is in fact a very rare object; the remnant of a collision between a white dwarf and a brown dwarf. And Anthelme was the first person to see one.
An international team of astronomers made this discovery using the Atacama Large Millimeter/submillimeter Array (ALMA) of telescopes in Chile. The study was led by astrophysicists at Keele University (England), and is published in the Monthly Notices of the Royal Astronomical Society. The team included two Professors of Physics and Astronomy at the University of Minnesota: Charles Woodward and Robert Gehrz.
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A white dwarf is the end state of a star like our Sun. Once its fuel is gone, the white dwarf shines due to stored thermal energy. No more fusion takes place. About 97% of the stars in the Milky Way will end as white dwarfs.
A brown dwarf is also known as a failed star. It’s an object that never gained enough mass to trigger fusion. They’re between about 15 to 75 times the mass of Jupiter.
In the case of CK Vulpeculae, the two dwarf stars were binary companions that likely orbited each other for billions of years. This binary configuration is normal for stars and astronomers think that most stars start out this way. But binary stars are seldom identical twins, and in this case, the white dwarf was larger; ten times larger. It was a gravitational bully.
“It was shredded, and its remains spun out in two jets.” – Prof. Charles Woodward, College of Science and Engineering, University of Minnesota.
Eventually, the two stars collided, and the brown dwarf was destroyed. Professor Charles Woodward of the University of Minnesota described it like this: “It was as if you put salsa fixings into a blender and forgot to put the lid on. The white dwarf was like the blades at the bottom and the brown dwarf was the edibles. It was shredded, and its remains spun out in two jets—like a jet of goop shooting from the top of your blender as you searched frantically for the lid.”
The brown dwarf was torn apart by its larger white sibling. Its remains collided with the surface of the white dwarf, and the white dwarf’s intense gravity superheated the material from the brown dwarf. This caused thermonuclear “burning” of the material, and the ejection of molecules and isotopes. This is the nature of the brightness that Anthelme saw 348 years ago, though he could never have guessed at it.
“Collisions like this could contribute to the chemical evolution of our galaxy and universe.” – Professor Robert Gehrz, University of Minnesota.
The ejected material is what gives CK Vulpecula it hourglass shape. In the telescope image, the compact, bright central shape is the white dwarf, and the hourglass is the remnants of the brown dwarf. The object, also known as CK Vul, is still ejecting material to this day.
The Evidence for CK Vulpeculae
One clue that this is a collision remnant is in the organic molecules like formaldehyde and methyl alcohol that are present in the hourglass. Those molecules could never have survived in the interiors of a star and must have been produced in the collision.
The amount of dust in the debris is another clue. The dust amounted to about one percent the mass of the Sun, which is much too high for a nova. “That’s too high for a classical nova outburst and too low for mergers of more massive stars, as had been proposed earlier,” said Sumner Starrfield, a professor at Arizona State University who was involved in the study.
“Collisions like this could contribute to the chemical evolution of our galaxy and universe,” noted Minnesota’s Gehrz. “The ejected material travels out into space, where it gets incorporated into new generations of stars.”
Many times throughout history, astronomers observed things which they couldn’t possibly hope to understand. It’s still happening today. In our modern times, we’re still confounded by dark energy, dark matter, and black holes.
What will future generations think of our attempts to understand what we see in the sky today? Though our instruments are much more powerful, and our knowledge much more detailed, we still face a horizon beyond which we are ignorant. Like Per Dom Anthelme, we’re still left guessing about some of the things we see in the sky.
- AAAS Press Release: “Researchers Discover New Type of Stellar Collision”
- Research Paper: “ALMA reveals the aftermath of a white dwarf–brown dwarf merger in CK Vulpeculae“
- ALMA Observatory Press Release: “When Is a Nova Not a ‘Nova’? When a White Dwarf and a Brown Dwarf Collide“
- European Southern Observatory: “Through the Hourglass“