The Week in Space and Physics: Waiting for a Nova
On a nova about to burst, the strength of the Carrington Flare, Euclid's ice problem and the impending demise of Chandra
Once every eighty years the star T Coronae Borealis blows up. We know this because we’ve seen it happen at least twice - the first time in 1866, when the star was officially discovered, and another in 1946, when telescopes studied the exploding star in more detail. It has probably happened many more times: records from 1787 hint at a sighting, as do writings by a German abbot in the year 1217.
That marks T Coronae Borealis out as an oddity. Most stars explode only once, destroying themselves in the process. A handful - such as the giant Eta Carinae - have been seen to detonate again and again, but rarely on such a strict schedule as T Coronae Borealis keeps.
Such is the curiosity around this star that astronomers have been keeping a close eye on it for decades. In the almost eighty years since it last exploded, the star has faded, rendering it invisible to the naked eye. Around 2015, however, it started to brighten once more, becoming bluer in colour as it became more visible to observers.
This brightening replicated the pattern seen in the years before 1946. And, just as it did in 1945, the star last year darkened dramatically. Another explosion seems imminent. T Coronae Borealis, astronomers now believe, will blow up sometime in the next few months.
The eruptions of this star are classified as nova, a kind of small stellar explosion that suddenly bursts into being and then fades away over a few weeks. Nova are not uncommon. Roughly ten happen in the Milky Way every year, and most attract little attention. A handful are even close enough to be seen with the naked eye - though only on the scale of one per decade.
T Coronae Borealis distinguishes itself, however, with the repeating nature of its nova. Not only are they easily visible to the naked eye - T Coronae Borealis will soon be as bright as Polaris, the North Star - but they seem to be extremely rare in the Milky Way. Just ten examples of such recurring nova are known.
In the case of T Coronae Borealis, the novae are probably being caused by the interaction of a white dwarf and a red giant. These two stars are circling each other, and as they do the smaller and denser dwarf is stripping hydrogen from the red giant. Over time the hydrogen builds up around the dwarf star until, roughly once every eight decades, it ignites in a thermonuclear explosion.
Though this is the theory, many questions remain about exactly how this works, how the stars survive the explosions and why they show a predictable pattern of dimming and brightening in the years before the nova takes place. Answers may well come with its next explosion, now due as soon as April.
Rethinking the Carrington Flare
An intense wave of solar particles crashed over the Earth last weekend, sparking spectacular displays of the aurora. The event, known as a coronal mass ejection, followed the eruption of a powerful flare from the Sun’s surface, and comes as our star’s activity nears the peak of its eleven year long cycle.
Though intense, the particles presented little danger to the Earth. Our planet’s magnetic field deflected and channeled them safely away, preventing harmful bursts of radiation from hitting the surface. More powerful waves, however, can cause harm - and are especially risky for satellites, astronauts and modern electrical grids.
In a worst case scenario, a big solar storm could knock out vital communications and navigation satellites, knock power stations offline across entire continents and place astronauts in deadly danger. Yet astronomers are uncertain about how likely such a scenario is, and how often we can expect our star to throw out such dangerous flares.
The largest flare known with certainty hit the Earth in 1859. It was named the Carrington Flare, after the astronomer that recorded it, and it was strong enough to spark fires in early telegraph systems across the world. If such a flare were to hit today, the impact on our technologically dependent civilization could be catastrophic.
A new analysis of measurements taken in 1859, however, suggests the storm was rarer and more powerful than previously thought. The study is based on magnetic measurements taken at two observatories in London. Both show a large jump in activity as the storm hit, with extreme fluctuations appearing soon after.
That would be expected from such a big storm. The speed of the fluctuations, however, looks much larger than earlier studies had found, which suggests the storm was bigger than thought. If so, the authors say, it could have been an event unparalleled in the past thousand years. Certainly nothing like it has been seen since.
Hints from tree ring records show that big storms hit once or twice per millennium. But the data is patchy and sometimes hard to read. And no matter how rare they are, we can be certain that another one will one day hit the Earth. Whenever that day comes, we should make sure we are ready.
Euclid’s Icy Mirrors
The work of Europe’s Euclid space telescope was recently interrupted by a build-up of ice on the observatory’s mirrors. Though the layer of ice was thin - no more than a few billionths of a meter thick - it proved enough to disrupt the sensitive measurements the telescope is making.
Euclid was launched last year to hunt for the fingerprints of dark matter and dark energy on the visible universe. Over the next six years the telescope is expected to conduct one of the largest cosmic surveys of all time, an effort that will see it map out the precise positions of billions of galaxies. That, researchers hope, will then allow them to trace the presence of the dark matter lurking throughout the cosmos.
The build-up of ice, however, threatened that effort. The thin layer it formed on Euclid’s mirrors had the effect of dimming the light seen by its sensors. That, in turn, reduced the quality of the data the telescope was returning.
Removing ice, of course, can be done by simply heating up the telescope. Yet that would have upset the delicate balance of its optics, and that would have meant weeks of work to recalibrate everything. Instead the spacecraft’s engineers devised a way of heating up individual components inside the telescope. That, according to ESA, seems to have fixed the ice problem without disrupting Euclid’s work for more than a few days.
End of the Road for Chandra?
During the 1990s, NASA launched a series of four big space telescopes collectively known as the Great Observatories. Each was dedicated to a different wavelength of light: Compton to gamma rays, Spitzer to infrared, Chandra to X-rays and Hubble to visible and ultraviolet light.
Of those four, only two are still functional. Compton fell back to Earth in 2000 after a gyroscope failed, and Spitzer ran low on vital coolant in 2009. Hubble and Chandra, however, have each notched up more than a quarter-century in space.
Time may now be running short for Chandra. Though the telescope continues to function well, NASA is facing difficult funding choices in the years ahead. Chandra, it seems, is one area where they have decided to cut back. Over the next few years, NASA’s budget proposal says, the telescope's operations budget will fall to a minimal level.
The loss of Chandra will leave few good X-ray telescopes in orbit. Although a possible successor, Athena, is planned in Europe, it will not be launched until 2035 at the earliest.