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Starts With A Bang

Will These Two Stars Merge Into A Luminous Red Nova In 2022?

The contact binary system KIC 9832227 is worth another look.

Astronomers have seen some incredible occurrences across space and time.

The nova of the star GK Persei, shown here in an X-ray (blue), radio (pink), and optical (yellow) composite, is a great example of what we can see using the best telescopes of our current generation. When a white dwarf accretes enough matter, nuclear fusion can spike on its surface, creating a temporary brilliant flare known as a nova. (X-RAY: NASA/CXC/RIKEN/D.TAKEI ET AL; OPTICAL: NASA/STSCI; RADIO: NRAO/VLA)

There are novae: fusion flares on white dwarf stars’ surfaces.

At peak brightness, a supernova can shine nearly as brightly as the rest of the stars in a galaxy combined. This 1994 image shows a typical example of a core-collapse supernova relative to its host galaxy. Although many cosmic events are even more powerful, most are less bright and luminous than supernovae. (NASA/ESA, THE HUBBLE KEY PROJECT TEAM AND THE HIGH-Z SUPERNOVA SEARCH TEAM)

There are supernovae: explosive stellar cataclysms.

Two images of NGC 6946: one from 2011 and a similar one from May 14, 2017, which shows the new and brightening supernova, SN 2017eaw. Note how the supernova occurred in the spiral arms of this galaxy: typical of a core-collapse supernova, which usually occur in the regions where new stars are just forming. (GIANLUCA MASI / VIRTUAL TELESCOPE PROJECT / TENAGRA OBSERVATORIES, LTD)

In between those two are a novel phenomenon: luminous red novae.

The remnant of the star CK Vulpeculae, recently recognized as the result of a luminous red nova. A central star still exists, but the surrounding material is consistent neither with a classical nova nor a supernova, but as the aftermath of a mergeburst between two normal, contacting stars. (ESO / T. KAMIŃSKI)

These outbursts persist for months, glowing and echoing in visible and infrared light.

6-image sequence of the luminous red nova eruption of V838 Monocerotis. After the initial flash, the light and ejecta both spread outwards, creating a spectacular blooming ‘cosmic rose’ from star V838 Monocerotis. Additionally, the light echoed and reflected off of the surrounding material, enabling astronomers to see the early stages again on “replay.” (NASA / ESA / Z. LEVAY (STSCI))

V838 Monocerotis emitted a spectacular one in 2002.

This sequence shows the evolution of the star V838 Monocerotis from 1989 to 2006. What appeared to be a nondescript contact binary star suddenly erupted, making an enormous outburst and ejecting a fair amount of material. It brightened significantly: brighter than a nova but not as bright as a supernova. (LASUNNCTY / WIKIMEDIA COMMONS / CCA-SA-4.0)

The leading explanation is a mergeburst: when two stars, already in contact, merge into one.

When two stars touch one another as they orbit each other, they compose a contact binary. In general, contact binaries cannot be resolved by a telescope’s imaging system, but the brightness over time, as well as spectroscopic analyses, can reveal the binary nature of such systems. (ESO/L. CALÇADA)

The merging contact binaries V4332 Sagittarii and V1309 Scorpii created luminous red novae.

The light curve over time of V1309 Scorpii, showing the luminous red nova (8–9) outburst. Along with V838 Monocerotis and a few other stars, V1309 Scorpii helped astronomers understand how luminous red novae could be created: through the merger of stars that are members of a contact binary system. (TYLENDA, R. ET AL. ASTRON.ASTROPHYS. 528 (2011) A114)

Such identifications led to a question, “can we predict the next luminous red nova?”

In addition to exoplanets, many binary stars were found with NASA’s Kepler mission. Here, an illustration of the star Kepler-16 shows its binary nature, as eclipsing binaries are sensitive to the transit method. A contact binary is a more extreme example of an eclipsing binary, as the two stars actually touch. (NASA)

Perhaps: by observing contact binaries with quickening orbital periods.

A contact binary model of the stars that compose the system KIC 9832227. If the orbital period of these stars shortens further, the stars will come into greater contact with one another, and among the possible fates that can ensue is the possibility of a luminous red nova. (CALVIN COLLEGE/CARA ALEXANDER, DANIEL VAN NOORD, CHRIS SPEDDEN, AND LARRY MOLNAR)

As the period shortens, the two stars approach a merger.

Gravitationally and through friction, astrophysical objects can inspiral, touch, and merge. Two stars can persist in a contact binary phase for some time, but when their overlap is significant enough, they can undergo a critical event where a mergeburst occurs, creating a luminous red nova and leaving just a single star behind. (NASA)

When a critical “closeness” is reached, it triggers a luminous red nova.

The location of the faint binary star system KIC 9832227 in the constellation of Cygnus. In 2017, this system was identified as being a fascinating candidate system for a luminous red nova in the year 2022. (LARRY MOLNAR/CALVIN COLLEGE/CALVIN.EDU)

The binary system KIC 9832227 looked perfect: a contact binary whose period is shortening.

The contact binary, KIC 9832227, as imaged with Calvin College’s telescope in Rehoboth, NM. The bright star circled in red is itself a binary that cannot be resolved by telescopes, but the smaller, fainter star inside the red circle is an unrelated background star. (EVAN COOK/CALVIN COLLEGE)

A 2017 prediction, using new and archival data, indicated an upcoming merger in 2022.

An incorrect calibration of the NSVS (green) data, as shown correctly here, led to the prediction of Molnar et al. (black square) to indicate that a merger was imminent. With the NSVS and Vulcan (blue) data folded in, it is now far less clear what the fate of this star system is, and when, if ever, a luminous red nova will ensue. (Q.J. SOCIA ET AL. (2018), APJL 864, L32)

However, a timing error was swiftly uncovered and corrected, negating the 2022 prediction.

This illustration of an exotic binary star system shows interactions, but a substantial separation. In reality, the full suite of data concerning the star KIC 9832227 showed that the separation distance between the two stars in the system increased a few decades ago before decreasing again recently. It is not known why this is occurring or what this implies. (M. GARLICK/UNIVERSITY OF WARWICK/ESO)

Instead, the period first lengthened before recently shortening, presenting yet another mystery.

This ultraviolet view of the gas in the constellation of Cygnus shows knots, gaps, and dense regions. Ultraviolet, infrared, and optical views, combined, can help reveal the history of star formation and stellar evolution in a region of space. Where and when the next luminous red nova will arise is largely unknown at present. (NASA/IPAC/MSX)

KIC 9832227 may eventually make a luminous red nova, but probably not in 2022.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.


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