Stars, Universe

A Duo Of Dancing Stars

Small stars like our Sun die with great beauty, surrounded by beautiful shrouds of multicolored gasses that once formed their outer layers — leaving only their relic cores behind as a silent testimony to the Universe that they once were. Our Sun, like other small stars, will first become a bloated red giant that will swell in size to the ghastly point that its flames will swallow the Mercury, Venus, and possibly Earth planets within. Then it will wither into a thin, compact white dwarf — the former heart. A multinational team headed by astronomers from the University of Warwick (U.K.) announced in March 2020 that they had found a curious anomaly concerning a tightly dancing pair of such dead stars. The scientists detected a massive white dwarf star with a strange carbon-rich atmosphere that could actually be two white dwarfs that merged together as they performed their bizarre macabre dance in the space between stars — narrowly escaping explosive destruction.

The astronomers spotted an unusual ultra-massive white dwarf with an atmospheric composition that had never been seen before, located about 450 light-years from Earth. This significant discovery represented the first time that astronomers had observed a fused dancing pair of white dwarfs using the atmospheric structure as a guide to cracking the mystery of their true identities.

The discovery, published in the March 2, 2020 issue of the journal Nature Astronomy, could shed new light on the enduring question of how massive white dwarf stars are evolving, as well as the number of supernovae residing in our barred-spiral Milky Way Galaxy.

A survey of data derived from the Gaia telescope of the European Space Agency ( ESA) detected the ultra-massive white dwarf, named WD J0551 + 4135,. The researchers used spectroscopy performed using the William Herschel Telescope to follow up on their observation. The scientists focused on those white dwarfs that were identified as being particularly massive — an achievement made possible by the Gaia mission. The scientists were able to assess the chemical makeup of their environment by splitting apart the light released by the mysterious star and discovered that it held an extremely large carbon content.

On March 2, 2020, the University of Warwick Press Release, the lead investigator Dr. Mark Hollands from the University of Warwick’s Department of Physics clarified that “This star stands out as something we’d never seen before. You might expect to see an additional layer of hydrogen, often combined with helium or even a combination of helium and carbon.

The majority of white dwarfs are comparatively small, weighing-in around 0.6 times our Sun’s height. But WD J0551 + 4135 measures in at an astounding 1.14 times solar mass, rendering it almost twice the total mass of other white dwarfs. Despite being more enormous than our Planet, it is compressed into a small compact sphere which is only two-thirds of Earth’s diameter.

To overcome the enigmatic puzzle, astronomer-detectives have agreed to discover the real source of the light. WD age J0551 + 4135 also provided an important clue to them. Older stars circle our Milky Way Galaxies even more rapidly than younger ones, so this peculiar white dwarf zipped further than 99 percent of the other similar white dwarfs of the same cooling age in our Galaxies. This means this dead star is much older than what it looks like.

On March 2, 2020, University of Warwick Press Release, Dr. Hollands continues to clarify that “We have a configuration that we can’t understand through natural stellar development, a mass twice the average for a white dwarf, and a cinematic period older than that suggested through cooling. We ‘re fairly sure how one star becomes a white dwarf and it shouldn’t.

The Passing Of A Single Star

White dwarfs are all that remains of stars, including our own Planet, once they have stopped consuming their whole supply of fuel that is required for nuclear fusion. At this lethal stage, the dying little star has spilled into space the outer gaseous membranes. The grand finale of a small star contrasts with the noisy and destructive death of more massive stars that die in violent and devastating blasts of a supernova. Small stars like our Sun “go gently into the good night” and fall with exquisite beauty and love. Indeed, their lovely multicolored gaseous shrouds inspired astronomers to call them the “butterflies of the Universe,” as a tribute to their celestial beauty.

Solitary, small stars like our Sun are gently perishing. However, if the drama contains a different stellar actor, ghastly complications develop. If a small star resides with another star in a binary system, then a wild party will inevitably take place. When the duo’s first “dies” leaves behind its massive white dwarf heart, the celestial body gravitationally sips substance from its still-living star — and victim — star. If the vampire-like dwarf continues taking more and more material from his unfortunate friend, he can eventually consume enough material to gain enough mass to “go serious.” At this stage, the white dwarf pays for his crime and explodes — just like the big guys.

The suggestion that WD J0551 + 4135 is actually an entity that evolved as a result of the fusion of a pair of white dwarfs is focused on a hypothesis of its origin, connected but not similar. In this scenario, when one of the two stars exploded into a bloated red giant at the end of its existence, it absorbed its companion star, pulling its orbit ever closer as the first star withered through its white dwarf level. And there was an encore show as the other star was a massive red giant. Over the span of billions of years, gravitational wave absorption gradually compressed the system, to the extent that the waltzing celestial pair fused to create a single body.

The Singers And The Music

Even though it was predicted that white dwarf mergers would occur, the one involving the unusual WD J0551 + 4135 is stranger than expected. This is because most of the fusions occur between stars sporting different masses in our Milky Way, whereas this odd fusion probably occurred between a duo of similarly-sized stars. There is also a cap on how big the resultant single white dwarf cam is. This is because in a Type Ia supernova explosion, if the resulting stellar corpse weighs-in at more than 1.4 times solar masses, it will “go critical” and blow itself to smithereens. It is possible, however, that such fatal stellar explosions can be triggered at slightly lower masses, and so this odd white dwarf is particularly useful because it demonstrates how massive a white dwarf can get to tell the story and still “live.”

Since the fusion restarts the cooling cycle of the body, it is impossible for astronomers to determine the real age of the object. The stellar corpse probably merged about 1.3 billion years ago — but there may have been a duo of original dead stars for many billions of years before that event.

WD J0551 + 4135 is important because it is one of only a handful of blended white dwarfs to be identified — and it is the only one to be identified by its composition so far.

On March 2, 2020, University of Warwick Press Release, Dr. Hollands explained that “There aren’t so many white dwarfs this massive, though there are more than you would expect to see implying that some of them were probably formed by fusions.”

“In the future, we may be able to use a technique called astroseismology to learn about the white dwarf’s core composition from its stellar pulsations which would be an independent method confirming this star formed from a merger. Maybe the most exciting aspect of this star is that it must have just about failed to explode as a supernova–these gargantuan explosions are really important in mapping the structure of the Universe as they can be detected out to very large distances. However, there remains much uncertainty about what kind of stellar systems make it to the supernova stage,” he added.

“Amazing as it might sound to calculate the properties of this ‘lost’ supernova and look similar in the future teaches us a lot regarding thermonuclear self-annihilation pathways,” Dr. Hollands continues to reflect.

This work is reported under the title An Ultra-massive white dwarf with a combined hydrogen-carbon atmosphere in the March 2, 2020 issue of the journal Nature Astronomy as a possible merger vestige.

Star Dances Around Black Hole