Waiting, and watching, for the “new” star

.The world is on alert, for T Corona Borealis

Any day now, a “new” star could flare up in the northern sky, and become visible to the unaided eye. But astronomers can’t say exactly when. So, Nigeria and the world are waiting, and watching: Hoping to get a glimpse of T. Corona Borealis (T CrB), as it erupts in a thermonuclear conflagration.

It won’t be the first, or the last, time. But few Nigerians living today, will be around when it erupts again. T CrB is a type of cataclysmic variable star, that explodes periodically. Light from these explosions, creates what astronomers call a “nova”. (Not to be confused with “supernova”).

“Nova, means “new,” in Latin. But T CrB is not really a new star. It’s been pulsating, beyond the visual reach of Earth-bound eyes, for millions of years—becoming visible to humans, for about a week, roughly every 80 years. It then vanishes again, into the dust and distance, that prevails against naked-eye vision.

If predictions hold, the “Blaze Star,” as it is also known, famously, will perform this timeless act again, possibly by year’s end. But scientists offer no guarantees. “Recurrent novae are unpredictable and contrarian,” Dr. Koji Mukai, an astrophysicist at NASA’s Goddard Space Flight Centre, told Jonathan Deal.

“When you think there can’t possibly be a reason they follow a certain set pattern, they do,” he continued, “and as soon as you start to rely on them repeating the same pattern, they deviate from it completely. We’ll see how T CrB behaves.”

The “behavior” Mukai and the world await, is actually that of two stars—not one, as in the case of a supernova. A supernova explosion, obliterates its progenitor star, whose surviving remnants normally form either a neutron star or a black hole. It’s a one-off conflagration, involving a single star. But in nova physics, it takes two to tango. The Blaze Star, is really a cool red giant and a hot white dwarf—binaries, bound to each other, gravitationally, and locked in a synchronous orbit. The red giant circles the white dwarf, every 227 days, at the very close distance of .54 astronomical units (AU).

That’s about half the distance, from Earth to Sun. This proximity, plus differences in their gravitational strength, enables the white dwarf—a small, dense stellar corpse—to pull hydrogen from the giant, into a spiraling accretion disc.

Remember, gravity is proportional to mass. A white dwarf, is an Earth-sized object, with 330,000 solar masses of carbon and oxygen, compacted into its core. It is so dense, NASA estimates, that if a mere teaspoon-full could be brought to Earth, it would weigh 5.5 tonnes!

By contrast, T CrB’s binary mate (spectral class M3 III), has expanded to 120 times the Sun’s size and cooled. Having left the main (hydrogen-burning) sequence (on the HR Diagram), it is now fusing helium into carbon—with a stellar wind, that blows gas into the surrounding nebula.

The outer edges of this nebula define what—in astronomical parlance—is called a “Roche lobe,” consisting of gas that has been trapped in the giant’s gravitational field. But the gravity of the white dwarf is stronger: 350,000 times that of Earth!

The close-orbiting dwarf, thus pulls hydrogen out through the apex of its teardrop-shaped lobe (Lagrange point-1, to be technical!) into a swirling accretion disc of gas and dust. Hydrogen gas spirals around the white dwarf, descends to its surface, and keeps piling up.

R. K. Zamanov, et.al. thus postulate three interacting dynamics: (1) The cool giant’s wind and comparatively weak gravitational field; (2) a nebula and accretion disc formed from material lost by the giant; and (3) a hot white dwarf of 100,000 K, or above. which ionizes hydrogen in the nebula.

The dwarf’s powerful gravity, compacts and heats the hydrogen. “Eventually,” The Smithsonian Magazine offers, “the white dwarf grows hot enough on its surface—a temperature of about 10 million Kelvin—to produce a nuclear explosion”.

It is the flash from this thermonuclear explosion, that we will see as a “nova”. T CrB is expected to shine, at optical wavelengths, 1,500 times brighter than in its quiescent (normal) state—although, Wikipedia says, it’ll still be dimer than 120 other stars in the night sky.

“When the system is quiescent,” Wikipedia continues, “the red giant dominates the visible light output and the system appears as an M3. The hot component contributes some emission and dominates the ultraviolet output”. Hence, the star gets bluer, as eruption approaches.

Astronomers have identified three main types of novae, along with several subtypes. The classical nova, was originally thought to be a one-off eruption. But it is now believed, that these outbursts are repeated, albeit on timescales of hundreds to hundreds of thousands of years.

Dwarf novae erupt more frequently, and are also dimmer, than classical novae. According to
Eric Weisstein’s World Of Astronomy, brightness can vary from 2 to 5 magnitudes, at intervals of days to decades. The lifetime of an outburst, he says, is 2 to 20 days.

The mechanism involved differs, as well. Both Weisstein and Wikipedia point out, that dwarf novae eruptions arise from an instability in the accretion disk that surrounds the stars, while those of classical novae result from the fusion and detonation of hydrogen on the white dwarf.

Recurrent novae are cataclysmic variable stars, that are known to have two or more eruptions within 100 years, or less. T Corona Borealis belongs to a rare group, called symbiotic recurrent novae—because its close secondary, is a red giant (also a dying star).

Only ten such stars have, so far, been found in our galaxy. “In all,” iopscience.iop.org notes, “we have 37 eruptions from the 10 known galactic [recurrent nova]. Half… have only two known eruptions, although they might have many more in the last century that were missed”.

Beyond the Milky Way, Matthew J. Darnley and Martin Henze report, astronomers have discovered eruptions in some two dozen galaxies—including over 1,100 in our cosmic neighbor, Andromeda. This has yielded ”tremendous insight,” they say, into nova physics and evolution.

But in the days ahead, global attention will be focused on developments closer to home. The world will be watching the 55 or so stars, that make up Corona Borealis, or the Northern Crown, constellation.

A constellation, is a group of stars whose brightest members evince a distinct and recognizable pattern. Various cultures have, since human prehistory, interpreted the patterns of constellations as mythical heroes, tools, landscape, gods and other imagined designs.

Among the Chokwe, of Zambia, for example, stars from three constellations—Bootes, Corona Borealis and Hercules—are, according to Mukandi Siame, used to depict a “running, sweating” messenger. They include, in keeping with the theme being explored, Alphecca and Arcturus.

Visual access to constellations, and their content, are pinpointed by coordinates on the celestial sphere, just as they are for landmarks on the terrestrial globe. “Right Ascension,” for instance, is the equivalent of geographical longitude, and “declination,” is the same as latitude.

The star-group’s name, reflects its mid-northerly location, on the celestial sphere, and the semicircle (backward “C”) pattern of its seven brightest objects—which is reminiscent of a crown. (Its Southern counterpart, Corona Australis, reportedly has similarly arrayed stars).

Corona Borealis is a diminutive constellation, that covers only 0.4 % of the night sky. It ranks 73rd, among the IAU’s 88 constellations. But despite its small size, Borealis is visible to all northern hemisphere observers—plus those south of the equator, from 50 degrees northward.

This bodes well for Nigerians—whose country is just 1,111.95 km north of the equator. It’s not exactly a front row seat, to watch the nova. Yet, the near-equatorial geography, places Nigerians quite close enough, to observe everything that is unfurling.

While waiting for the nova, astronomers advise, you should become familiar with Corona Borealis constellation and the surrounding sky. Learn to find nearby Arcturus (in the constellation Boötes), and Vega (in Lyra)—the first, and second, brightest northern stars.

Borealis lies between them (according to the Chandra X-Ray Observatory, at Right Ascension: 16h; Declination: +30º). With minimal moonlight, and clear skies, you’ll note the crown-like asterism—six middling stars (all 4th magnitude), and the brilliant (2.3 magnitude) Alphecca.

Alphecca, also known as “Gemma,” is the brightest star in the semi-circle pattern which, the Star Facts site indicates, also includes Nusakan (Beta CrB), Theta, Gamma, Delta, Iota and Epsilon CrB. T CrB appears outside the “inverted C,” just beneath Epsilon (magnitude 4.14).

At magnitude 10, T CrB is normally 100 times dimmer than stars we can see, without using binoculars or a telescope. The naked-eye limit, on dark, clear nights, is magnitude 6—keeping in mind, that lower numbers, denote brighter objects and higher numbers, dimmer ones.

The unaided eye, can’t see the Blaze Star until it blows. But the Cloudy Nights site, refers telescope users to right ascension 15h 59m 30.2s, declination 25° 55′ 12.6″. If you don’t have a scope, find the starless area, where T CrB is incubating. This may stand you in good stead, later.

When T CrB explodes, its brightness will increase from magnitude 10 to around 2.0 or 3.0. Peak naked-eye visibility, astronomers project, is about a half day. The star can still be seen, after this peak, for seven or so days without a viewing aid and up to a month, with binoculars.

Naked-eye observers should find Arcturus, Vahe Peroomian, of the University of Southern California Dornsife, advises, “and look approximately 20 degrees above it. That’s about the span of one hand, from the tip of the thumb to the tip of the pinky, at arm’s length”.

Incidentally, seeing the Blaze Star involves lookback time—the time a photon of light takes to reach your eyes, from its source. The Sky Live website notes, for instance, that T CrB is 2,627.56 (rounded to 3,000) light years away, and moving towards us, at 29 km per second.

This means the explosion itself actually occurred 2,627.56 years ago! “And because it recurs every 80 years or so,” timeanddate.com surmises, “T CrB will have undergone about another 40 outbursts in the 300 centuries since then”!

What everyone is waiting to see, is the light insignia from just one of those eruptions. Researchers have narrowed the timing of its arrival, the Planetary Society reports, down to a 70% chance by September 2024 and a 95% chance of it occurring by the end of the year.

“September,” of course, has already passed—without any eruption. Is there a possibility, NASA asked earlier this year, that September will come and go, without the anticipated nova outburst from T CrB? There are no guarantees, it admitted, “but hope abides”.

That hope lies, essentially, in nova physics—the study of galactic novae, including the last two eruptions of T Corona Borealis, in 1866 and 1946. All predictions, are heavily informed by data pertaining to past behavior of the Blaze Star, particularly in 1946.

Bradley Schaefer, of the Department of Physics and Astronomy, and Professor Emeritus, at Louisiana State University, says his light curves show that eruptions for each star, always come to the same peak brightness, for all ten galactic recurrent novae, and for T CrB in particular.

T CrB entered a “super active” state in 2015, he says, which made it brighter and bluer. A similar active state ensued in 1938, some 8 years before the nova. The end of this state in 1945 was a year before the eruption. T CrB’s own pre-eruption dip, was spotted in March 2023.

Astronomers thus believe, that the star has begun its last phase, leading up to the next outburst. “Given that T CrB has had its high-state turn-on and turn-off on schedule,” Schaefer insists, “we can be confident that a new eruption will blaze away soon in our skies.

Hence, reports NASA’s Jonathan Deal, global astronomical assets have been mobilized and placed on alert. This includes the U.S. space agency’s Fermi Gamma-ray Space Telescope, which is “poised to observe T CrB when the nova eruption is detected”.

He also lists NASA’s James Webb Space Telescope, Neil Gehrels Swift Observatory, IXPE (Imaging X-ray Polarimetry Explorer), NuSTAR (Nuclear Spectroscopic Telescope Array), NICER (Neutron star Interior Composition Explorer), and ESA’s INTEGRAL (Extreme Universe Surveyor).

Deployed, as well, are “numerous ground-based radio telescopes and optical imagers”—including New Mexico’s National Radio Astronomy Observatory’s Very Large Array. Collectively, Deal avers, they will “capture data across the visible and non-visible light spectrum”.

The Chinese Initiative Of T CrB Global Monitoring Network, is inviting amateurs with telescopes whose apertures range from 80mm to 200mm to participate in a global collaboration.
Augmenting the Network, is the T CrB Global Monitoring WeChat group ( [email protected]).

Meanwhile, Bob King, of Sky And Telescope, suggests that those who have binoculars, register for a free account on the American Association of Variable Star Observers website, by clicking “Log In”. Then Check on T CrB regularly; and report any brightening, through the T CrB Time Sensitive Alerts Forum thread.

Astronomers are anxiously awaiting the eruption of T Corona Borealis, not only for its entertainment and educational value, but also for answers to critical scientific questions, about the behaviour of explosive stars.

Symbiotic stars, like T CrB, are of interest to scientists, the iweb.cfa.harvard.edu website avers, “because of specific astrophysical processes that can be studied during their evolution, such as stellar winds, accretion, and the ionization of surrounding dust and gas”.

Concurringly, Wikipedia asserts as well, that “Symbiotic binaries are also vital in the study of stellar wind, ionized nebulae, and accretion because of the unique interstellar dynamics present within the system”.