An Epic Nova Exploded in the milky way Is gone. And now?
Sunday, August 8, while in Tokyo the Olympic brazier was extinguished and the end of the
games was celebrated with a fireworks display, a new light lit up the night sky. Or rather,
it was rekindled: after 15 years from the last explosion, recorded on February 12,
2006, RS Ophiuchi - a so-called recurring Nova, that is, a star that increases its
brightness at more or less regular intervals - came back to shine with overbearing power.
Many people, once they heard the news, tried to look for the star, which appeared in the
constellation Ophiuchus. And someone, more timely than others has also managed to capture,
with the help of maps (and binoculars), the vision of a fairly weak and insignificant star,
immersed like many others in the middle of the great river of the Milky Way.
A bright dot that experts gave "magnitude +4.8", and then just four times brighter
than the faintest stars visible to the naked eye, which is attributed a magnitude of +6.5.
From the point of view of the show, let's admit it, there was not much to see...
(The real beauty of the phenomenon, as we will explore in a moment,
is in fact to observe it with the awareness of what it really is, that is: a cosmic catastrophe),
but someone seems not to have been able to see even that little... How come? Well, the answer is
simple: they waited too long! Five days after the explosion, in fact, the star had already returned
to the limit of visibility to the naked eye and in mid-August it was already weaker than magnitude
+8... It was necessary to look for it immediately, without basking in the idea that the star would
remain visible for a long time... for weeks or months! In fact, novae stars are quick to
appear "out of nowhere", but just as quick to return to the shadows. Remember that next time!
Many centuries ago, on a July night in the year 134 B.C., mankind was struck for the first time
by a strange and unexpected sight: a new star had lit up in the sky. The star shone with a bright
splendor for some time, then slowly faded and disappeared. As Pliny, the famous Roman historian
and naturalist wrote, the strange apparition induced the Greek astronomer Hipparchus to compile
a catalog of all known stars "so that posterity could know if there were really changes in
the sky". And posterity did in fact observe numerous changes similar to those of 134 BC.
Until today we can count the appearance of a thousand stars in our galaxy,
not all of them equally bright. The ancients, who when they witnessed
the phenomenon mistakenly thought of the birth of a new star, gave these stars the name of "Novae".
But today we know that in reality those stars are not "born" at the moment they appear to our eyes,
but it simply happens that in a few hours they increase thousands of times their brightness,
passing from the condition of star observable only with large telescopes,
to that of star easily visible to the naked eye. It is not a rare phenomenon: it has been
calculated that in the Milky Way alone about 30 novae are lit every year,
but many of them are not observable because too far away or hidden by clouds of interstellar gas.
Currently, a dozen or so are discovered each year. Increases in brightness so large and so fast can
only be justified by a colossal release of energy. But caused by what? For what
mysterious mechanism a small and weak star comes to emit energy with such violence?
Even today astronomers are squabbling over the details, but it seems now
established that the scenario capable of explaining this incredible phenomenon is
that of two stars that by mutual gravitational attraction form a so-called binary system.
It must be a particular system, however, formed by a white dwarf and a star with a mass slightly
greater than the solar one that is in the red giant phase. White dwarfs have a diameter similar
to that of the Earth but a quantity of matter equal to that of the Sun. Therefore they have
a great density and a gravitational force tens of thousands of times higher than Earth's one.
And thanks to the intense gravitational field the white dwarf attracts hydrogen and helium from the
outer layers of the red giant and attracts them to its surface, increasing mass and volume.
Gradually the attracted gas, heating in its fall, thickens on the Dwarf to form a spiraling
"accretion disk" that continuously transfers new material from the Red Giant to the Dwarf.
The incandescent hydrogen that continuously pours on the star compresses the underlying
matter until it mixes with it, triggering a reaction that results in an unstable condition.
An explosion ensues, following which the star's luminosity can increase
150,000 times and the surface temperature can rise to 100 million degrees.
The white dwarf returns to the initial stage in a period of time between a dozen days and a few
months, but then immediately resumes the process of accretion that will lead in due course to a
subsequent explosion. The process is repetitive but not strictly periodic. For these variables
is valid, at least qualitatively, the law of Kukarkin and Parenago, according to which the
time between two explosions is the longer the greater the increase in brightness.
We speak in this case of recurrent novae, but it is widely believed among astronomers that
all novae repeat, although some do so on such a large time scale as to have made them virtually
undetectable to man throughout history. But, beware. Although the Novae of all types
recorded in our galaxy amount to a few hundred, the recurring ones are very rare, just a dozen.
They are physical systems with properties intermediate between normal novae - stars
that show enormous variations of luminosity from 8 to 15 magnitudes with great rapidity,
then slowly decay to normal luminosity - and dwarf novae, known to present more moderate variations
of luminosity - from 2 to 6 magnitudes - but much more frequent, with periodicity from 10
to 1000 days. The recurrent novae behave in an intermediate way, showing brightness variations
from 4 to 9 magnitudes, in periods from 10 to 100 years: they are therefore events that cannot
be considered "rare" in astronomical terms, but certainly difficult to observe systematically.
Well, something very similar to what we have described so far must have happened on
RS Ophiuchi last August 8. Or rather, something must have happened more than 5000 years ago,
since this is the distance that separates us, in light years, from the binary star that
bears this name. Evidently, 5000 years ago the white dwarf of that system has been freed in an
instant of all the material stolen from the red giant with which it lives for millions of years,
transforming everything into energy and light radiation. A catastrophe of planetary proportions.
Fortunately, such turbulent binary systems are not able to form a planetary system, so there is no
embarrassing risk for us earthlings to applaud a cosmic event that in the meantime is wiping
out millions of creatures... In short ... rest assured, certainly no one was hurt on RS Ophiuchi!
The 2021 outburst was its ninth resurrection in 123 years of observations.
Previous outbursts had occurred in 1898, 1907, 1933, 1945, 1958, 1967, 1985, and 2006.
Of course, there is no way to know if there may have been others before 1898.
But it's safe to assume that this streak of recurring explosions has been repeating for
hundreds of thousands of years. Or even longer. Who knows.
The event of last August 8 was however quite special since the brightness reached
was one of the highest ever recorded by this Nova, which - let's remember - in the
exclusive club of recurrent Novae the second absolute for brightness after T Corona Borealis,
which in its appearances of 1866 and 1946 reached the magnitude +2.5! Incidentally, none of the
other "recurring" reaches naked-eye visibility, not even during the brightest appearances.
But let's take a closer look at the behavior of RS Ophiuchi from its discovery until today. OK?
"Hey, guys, just a moment before we continue... BE sure to join the Insane Curiosity Channel...
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The story begins with the 1898 explosion, but it was not directly observed.
Only in 1904, examining some photographic plates, the American astronomer Williamina Fleming,
noticed a nebular residue that she suspected might have been released by a Nova. This was
later confirmed by the position of subsequent explosions, identical to that of 1898.
The 1907 appearance occurred early in the year, when the star was unobservable due to
conjunction with the Sun. However, the flare is considered highly probable because just
after the conjunction the brightness was lower than that recorded just before the conjunction.
The outburst of 1933 that reached magnitude +6.4, at the limit of visibility to the naked eye, was
the one that convinced astronomers that the star was a very rare Nova recurring at short period.
Also in 1945 RS Ophiuchi appeared in conjunction with the Sun.
In that case, however, there was a more prolonged observation of the last phases of the flare,
which made its attribution more certain.
In 1958, the Nova returned to manifest itself reaching in mid-July the magnitude +6.
The outburst of 1967 occurred on October 27 and on that occasion the star reached magnitude +5.
In 1985, 27 years after the previous eruption, the star turned again on
January 26, reaching magnitude +5.4. After that it was necessary to arrive on
February 12, 2006, to see RS Ophiuchi break the wall of the fifth magnitude and get up to +4.5.
Record almost equaled last August 8 with a magnitude of 4.8.
At this point, one is left to wonder, "But these cyclical
resurrections, will they continue forever?" Actually, no. To hear most astrophysicists
tell it, RS Ophiuchi's fate is to explode as a Type Ia supernova within 100,000 years.
Confused for a long time with Novae, Supernovae represent a far more energetic explosive event.
At its peak brightness, a supernova can increase by 20 magnitudes (100 million times)
and exceed the Sun's brightness by 10 billion times. A phenomenon completely destructive,
revealed, as well as by the enormous amount of energy emitted, by the observed speed of
expansion that is around 10,000 km for second. Type Ia Supernovae occur starting from scenarios
very similar to that of recurrent Novae, in binary systems formed by a White Dwarf and a Red Giant.
The only difference is in the mass of the Dwarf. Below the limit of 1.4 solar masses (the so-called
Chandrasekhar Limit), the Dwarf does not have the strength to trigger thermonuclear reactions
even in its core and therefore continues its life as a recurrent Novae. But if the Dwarf during its
explosions is not able to get rid completely of hydrogen stolen from the Red Giant, then its mass
can progressively increase and exceed the Limit, until the sudden destruction of the whole System.
Well, apparently, the RS Ophiuchi White Dwarf seems to be going on this way...
it is increasing in mass, and very "soon", within maybe 100.000 years,
it will overtake the Chandrasekhar Limit. Our great-grandchildren, in that distant future,
could thus witness the ignition in Ophiuchus of a star perhaps brighter than the Moon.
But keeping our feet on the ground... will those who missed this summer's Nova have to wait another
15 years? Isn't there some other recurring Nova that may be reactivating soon, in our galaxy?
Yes, we think so... Keep an eye out for U Scorpii... and also T coronae Borealis…
Few hopes instead for the explosion of a Galactic Supernova, that in the terrestrial skies is
missing now more than 400 years. Although, sooner or later...