"Long before it's in the papers"
March 23, 2015


Colliding stars could explain 17th-century mystery explosion

March 23, 2015
Courtesy of ESO
and World Science staff

A “s­tar” that Eu­ro­pe­an as­tro­no­mers saw ap­pear out of the blue in 1670 was ac­tu­ally a rare, vi­o­lent breed of stel­lar col­li­sion, a new study pro­poses.

The burst was spec­tac­u­lar enough to be easily seen with the na­ked eye in­i­tial­ly, but the traces it left were so faint that it took care­ful anal­y­sis with so-called sub­mil­lime­ter tele­scopes to un­rav­el the mys­tery cen­turies lat­er, as­tro­no­mers claim.

This chart of the po­si­tion of a "nova" (marked in red) that ap­peared in 1670 was recorded by the fa­mous as­tron­o­mer He­ve­lius and was pub­lished by the Roy­al So­ci­e­ty in Eng­land in their jour­nal Phil­o­soph­i­cal Trans­ac­tions. (Cred­it: Roy­al So­ci­e­ty)

The findings ap­pear on­line in the jour­nal Na­ture on March 23.

Some of sev­en­teenth cen­tu­ry’s great­est as­tro­no­mers, in­clud­ing He­ve­lius, the fa­ther of lu­nar map­mak­ing, and Cas­si­ni, who dis­cov­ered four of Sat­urn’s moons, care­fully doc­u­mented the ap­pearance of a “new star” in 1670. He­ve­lius de­scribed it as “no­va sub capite Cyg­ni”—a “new star be­low the head of the Swan.” As­tro­no­mers now know it as “No­va Vul­pec­u­lae 1670.” 

No­va Vul 1670 was lat­er said to be both the old­est recorded no­va—a par­tic­u­lar type of stel­lar out­burst—and the one that left the faintest traces for lat­er stu­dy.

“For many years this ob­ject was thought to be a no­va, but the more it was stud­ied the less it looked like an or­di­nary no­va—or in­deed any oth­er kind of ex­plod­ing star,” said the stu­dy’s lead au­thor, Tom­asz Kamin­ski.

Af­ter its first ap­pearance, No­va Vul 1670 var­ied in bright­ness over two years, then van­ished and reap­peared twice be­fore twin­kling out for good.

In the last cen­tu­ry, as­tro­no­mers learned that most no­vae oc­cur when a par­tic­u­lar type of star pulls away ma­te­ri­al from a com­pan­ion star, lead­ing the ma­te­ri­al to even­tu­ally ex­plode. But No­va Vul 1670 did­n’t fit this pic­ture and re­mained a mys­tery.

Even with ever-increasing tel­e­scop­ic pow­er, the event was be­lieved for a long time to have left no trace. Only in the 1980s did as­tro­no­mers de­tected a faint cloud where the re­mains should be. The new study probed that ar­ea by in­ves­ti­gat­ing light waves of a par­tic­u­lar “wave­length­”—the length of light waves de­ter­mine their en­er­gy and col­or. This re­vealed a “very un­usu­al chem­i­cal com­po­si­tion” in the gas, said Kamin­ski, of the Eu­ro­pe­an South­ern Ob­serv­a­to­ry and of the Max Planck In­sti­tute for Ra­di­o As­tron­o­my in Bonn, Germany.

“We have now probed the ar­ea with sub­mil­lime­ter and ra­di­o wave­lengths. We have found that the sur­round­ings of the rem­nant are bathed in a cool gas rich in molecules,” he ex­plained.

The as­tro­no­mers used tele­scopes in­clud­ing the Eu­ro­pe­an South­ern Ob­serv­a­to­ry’s At­a­cama Path­find­er Ex­pe­ri­ment tel­e­scope in Chil­e to an­a­lyze the chem­i­cal make­up and meas­ure the ra­ti­os of dif­fer­ent iso­topes, or vari­ants of par­tic­u­lar el­e­ments, in the gas. The in­ves­ti­ga­tors con­clud­ed that there was too much of the cool ma­te­ri­al to at­trib­ute it to a no­va, and the iso­tope ra­ti­os were al­so wrong. But if it was­n’t a no­va, then what was it?

The an­swer is a spec­tac­u­lar col­li­sion be­tween two stars—more bril­liant than a no­va, but less so than a so-called “superno­va,” they ar­gue. This col­li­sion pro­duces some­thing called a red tran­sient, a very rare event in which stars ex­plode due to a merg­er with anoth­er star. They spew ma­te­ri­al from the stel­lar in­te­ri­ors in­to space, even­tu­ally leav­ing be­hind only a faint rem­nant em­bed­ded in a cool en­vi­ron­ment, rich in mo­le­cules and dust. 

“This kind of disco­very is the most fun: some­thing that is com­pletely un­ ex­pect­ed!” said co-au­thor Karl Men­ten of the Max Planck In­sti­tute.

* * *

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A “star” that European astronomers saw appear out of the blue in the sky in 1670 was actual ly a rare, violent breed of stellar collision, a new study proposes. The burst was spectacular enough to be easi ly seen with the naked eye initially, but the traces it left were so faint that it took careful analysis with so-called submillimeter telescopes to un ravel the mystery centuries later, astronomers claim. The results appear online in the journal Nature on March 23. Some of seventeenth century’s greatest astronomers, including Hevelius, the father of lunar mapmaking, and Cassini, who discovered four of Saturn’s moons, careful ly documented the appearance of a “new star” in 1670. Hevelius described it as “nova sub capite Cygni”—a “new star below the head of the Swan.” Astronomers now know it as “Nova Vulpeculae 1670.” Nova Vul 1670 was later said to be both the oldest recorded nova—a particular type of stellar outburst—and the one that left the faintest traces for later study. “For many years this object was thought to be a nova, but the more it was studied the less it looked like an ordinary nova—or indeed any other kind of exploding star,” said the study’s lead author, Tomasz Kaminski of the European Southern Observatory and the Max Planck Institute for Radio Astronomy, Bonn, Germany. After its first appearance, Nova Vul 1670 varied in brightness over two years, then vanished and reappeared twice before twinkling out for good. The intrepid astronomers of the day lacked the equipment needed to solve the riddle of the apparent nova’s peculiar performance. In the last century, astronomers came to un derstand that most novae occur when a particular type of star pulls away material from a companion star, leading the material to eventual ly explode. But Nova Vul 1670 didn’t fit this picture and remained a mystery. Even with ever-increasing telescopic power, the event was believed for a long time to have left no trace. On ly in the 1980s did astronomers detected a faint cloud where the remains should be. The new study probed that area by invest igating light waves of a particular “wavelength”—the length of light waves determine their energy and color. This revealed a “very un usual chemical composition” in the gas, Kaminski said. “We have now probed the area with submillimeter and radio wavelengths. We have found that the surroundings of the remnant are bathed in a cool gas rich in molecules,” he explained. The astronomers used telescopes including the European Southern Observatory’s Atacama Pathfinder Experi ment telescope in Chile to analyze the chemical makeup and measure the ratios of different isotopes, or variants of particular elements, in the gas. The invest igators concluded that there was too much of the cool material to attribute it to a nova, and the isotope ratios were also wrong. But if it wasn’t a nova, then what was it? The answer is a spectacular collision between two stars—more brilliant than a nova, but less so than a so-called “supernova,” they argue. This collision produces something called a red transient, a very rare event in which stars explode due to a merger with another star. They spew material from the stellar interiors into space, eventual ly leaving behind on ly a faint remnant embedded in a cool environment, rich in molecules and dust. “This kind of discovery is the most fun: something that is complete ly un expected!” said co-author Karl Menten of the Max Planck Institute. n