“One evening, when I was contemplating as usual the celestial vault, whose aspect was so familiar to me, I saw, with inexpressible estonishment, near the zenith, in Cassiopeia, a radiant star of extraordinary magnitude. Struck with surprise, I could hardly believe my eyes…”
Tycho Brahe, November 1572
Imagine that while looking at the familiar patterns of constellations in the sky, you suddenly notice a new star in one of them. And not just any star, but an extremely bright one, as brilliant as the massive giant Jupiter or even Venus! This is exactly what a supernova explosion looked like to our distant ancestors. But what do modern scientists know about this amazing phenomenon? And can such an explosion harm us?
An uninvited guest
Records of new stars suddenly appearing in the sky and being observed for months go back to the distant past. Until the Renaissance, they were extremely surprising and disrupted people’s belief in the constancy and immutability of the sky, although they were generally considered a good omen.
The earliest record of such a phenomenon is preserved thanks to the astronomers of ancient China and dates back to 185 AD: a new star, now known as SN 185, appeared in the constellation of Centaurus and was observed for 8 months. In the chronicles, such new stars were referred to as ‘guest stars’, thus emphasising the temporary nature of the phenomenon. And the famous 16th-century Danish astronomer Tycho Brahe called them nova, suggesting that this is what the birth of a new star looks like.
Until the twentieth century, all ‘new stars’ were considered to be the same or almost the same. This perception changed radically when Edwin Hubble convincingly showed that the Andromeda Nebula was actually another distant galaxy, and thus the nova S Andromedae discovered in it in 1885 must in fact be extremely bright. Such powerful phenomena became known as supernovae, thus separating them from classical novae, which had a lower peak luminosity.
Like some other astronomical terms, the concepts of ‘nova’ and ‘supernova’ are of phenomenological origin, namely they reflect the nature of the observed phenomenon, not its essence. Modern scientists know that novae and supernovae outbursts actually occur in very old stars or their systems. But astronomy is a rather conservative science in this regard: the terms that have already taken root are not changed even when the real physical nature of the phenomenon has already been revealed.
The explosion of a nova increases the brightness of a star by about 10,000 times, while during a supernova flare, the luminosity increases by billions of times. If we are talking about a supernova in another galaxy, the radiation flux at the time of the explosion is often comparable to that of an entire galaxy.
Over the past 5 years, an average of about 21,000 supernovae have been discovered annually, both by automatic observations (such as the ZTF – Zwicky Transient Facility) and by amateur astronomers. But they were all located in other galaxies.
Explosive binary systems – type Ia supernovae
To date, quite a few types of supernovae have been identified, based mainly on their spectra, but there are two main mechanisms: an explosion in a binary stellar system and an explosion of a single massive star at the final stage of its evolution.
If one of the components of a binary star is a massive white dwarf consisting mainly of carbon and oxygen, the system can be considered a good candidate for supernova. The second companion can be almost any star: a red giant, a red dwarf, an ‘ordinary’ star like our Sun, or even another white dwarf. The main prerequisite is their close interaction.
A massive white dwarf gradually pulls matter from its stellar companion, accumulating it on the surface, and at some point, its mass exceeds the maximum allowable. This limit is known as the Chandrasekhar limit, and is about 1.4 solar masses. As the matter accumulates, the temperature and pressure in the white dwarf’s nucleus increase, uncontrolled thermonuclear fusion begins, and within seconds the star explodes, completely destroying itself.
Another mechanism involves the merger of two less massive white dwarfs. If their total mass exceeds the Chandrasekhar limit, this will again lead to an increase in temperature, the onset of thermonuclear reactions and a supernova explosion.
At the moment of the explosion, in one scenario or another, the real magic of the Universe happens – heavy elements are born. The substance is scattered into the surrounding space at a breakneck speed – up to several percent of the speed of light. And scientists on Earth record the explosion of a type Ia supernova.
For astronomers, Type Ia supernovae are a real ‘Holy Grail’. Because they all have roughly the same maximum luminosity and are bright enough to detect an explosion in a distant galaxy, scientists use them as standard candles. In other words, it is thanks to type Ia supernovae that the distances to a large number of galaxies have become known.
This type includes Tycho’s Supernova in the constellation Cassiopeia, also known as SN 1572 (after the year of the outburst), which was described in detail by Tycho Brahe, who regularly observed it while it was still visible to the naked eye. The invention of the telescope was still several decades away.
How a lonely massive star dies – supernovae of types Ib and II
In any non-degenerate star (neither a white dwarf nor a neutron star), there is a constant confrontation: gravity tries to compress a ball of hot plasma, and the pressure caused by thermonuclear reactions in its core opposes it.
Although it is counterintuitive, the greater the mass of a star, the shorter its lifespan. It would seem that, given the presence of large reserves of hydrogen that can be converted into helium and heavier elements, high-mass stars should be able to resist gravity longer. In fact, thermonuclear reactions in such stars occur over a much larger volume than in dwarfs like our Sun.
Massive stars are ‘going out for the last time’: they convert hydrogen into helium extremely quickly, burn brightly, and so have been running out of hydrogen fuel for several tens of millions of years. Now helium is being used near the core, which requires higher temperatures to burn. Further thermonuclear reactions synthesise carbon, neon and oxygen, and if the star is sufficiently massive, silicon and iron can form in the centre. The synthesis of heavier elements turns out to be ‘unprofitable’ for the star, as it consumes more energy than it releases, and therefore iron is sometimes called stellar ash. At this stage, the star is like an onion: a hydrogen layer remains on the outside, and the closer to the centre, the heavier elements dominate the chemical composition.
Sometimes very massive stars are stripped of their outer layers of hydrogen and helium in advance. Exposing the core, they blow a spectacular nebula around them. Such objects are called Wolf-Rayet stars. When fusion is no longer able to compensate for gravity, the central part of the star collapses (rapidly and continuously shrinks), scattering the outer layers – a type Ib supernova explodes, with a spectrum that does not contain hydrogen lines.
If a star with a mass of 8-50 solar masses has retained its envelope, hydrogen is detected in the chemical composition of the supernova, and the supernova itself is classified as a type II. Such supernovae are typical of spiral galaxies and are often observed in their arms, where star formation is quite rapid.
What is left after an explosion?
In addition to being a spectacular phenomenon, a supernova leaves behind a beautiful nebula – the so-called supernova remnant. Over time, the nebula dissolves into outer space, enriching the interstellar medium with heavy elements. These elements later form the stars of the next generations and the planets around them. Everything around us, including you and me, is made of them.
Gravitational collapse compresses the central part of the star into either a neutron star or a black hole. The most famous example of such a development is the well-documented supernova outburst SN 1054, which occurred in the constellation Taurus and was recorded in the chronicles of Chinese astronomers of the time. Thanks to their scrupulousness, centuries later scientists were able to identify the outburst with the supernova remnant, the Crab Nebula.
Given the number of supernova remnants found in our Galaxy, such cosmic catastrophes should occur within the Milky Way about three times a century. But in fact, there are only 5 ‘reliable’ supernovae that have been observed over the past 2000 years. These include SN 1006 (in the constellation of Lupus, the brightest of all observed), SN 1054 (in Taurus), SN 1181 (Cassiopeia), SN 1572 (also Cassiopeia) and SN 1604 (the constellation of Ophiuchus). In these cases, the explosion itself was recorded, and the residue after it was definitely detected.
So, the last supernova in our Galaxy was observed in 1604, it is known as the Kepler’s Supernova. But, having analysed the remnant nebulae, scientists suggest that there should have been another supernova in the Cassiopeia constellation around 1680, and another one should have been observed in 1800-1900 in Sagittarius.
The invisible giants
The Crab Nebula has a rather small apparent size – only 6 angular minutes. But hidden from human eyes, our Galaxy is home to truly gigantic supernova remnants. For example, the diameter of the Cygnus Loop reaches 3°, which is 6 times larger than the lunar disc! The nebula is so huge that it has separate structures, such as the ‘Eastern Veil’, ‘Western Veil’ and ‘Witch’s Broom’.
Equally gigantic is the Spaghetti Nebula (also known as Simeis 147) on the border of Auriga and Taurus, discovered in 1952 at the Crimean Astrophysical Observatory. The nebula is estimated to be 40,000 years old and has a pulsar in its centre.
There is a debate about the grand Barnard’s Loop, a nebula that stretches 10° and covers about half of the constellation Orion. Some scientists attribute it to the molecular complex of Orion, while others believe it to be the remnant of a supernova that exploded about 2 million years ago. Estimates of the distance to the nebula vary significantly: from about 520 to 1430 light years. The upper limit is consistent, for example, with the distance to the well-known Orion Nebula (M42), whose light has been travelling to us for 1340 years. And the ‘explosive’ origin of the Barnard’s Loop is supported by several stars (Epsilon Aurigae, μ Columbae, and 53 Arietis) that radially disperse rather quickly from the area of the arc’s geometric centre. The latter circumstance hints at a supernova explosion in a multiple stellar system.
Some may ‘French leave’
Does a massive star always end its life with a spectacular supernova explosion, spreading out matter in the form of an impressive nebula? For the most part, yes, but astronomers know of several cases of ‘failed supernovae’. However, none of them are in our Galaxy.
In 2009, a red supergiant with a mass of 18-25 solar masses disappeared without a trace in the galaxy NGC 6946. This was the first reliably established case of a ‘failed supernova’. And only recently, we reported a similar disappearance of the huge star M31-2014-DS1 in the Andromeda Galaxy.
The mechanism of such a silent collapse is not fully studied, primarily because of the small number of observations of such phenomena: in addition to the two mentioned above, there is only one other candidate. However, it is believed that the phenomenon may be caused by the trapping of neutrinos in an extremely dense nucleus, the release of which usually provokes the formation of a shock wave. If the release does not occur, all or almost all of the star’s matter falls into the centre, giving rise to a completely new object – a black hole.
Beforehand warning
For our remote ancestors, the appearance of what we now call a supernova in the sky was a complete surprise. But if a similar event occurs today, modern astronomers will receive a certain warning. And the point here is precisely the release of neutrinos.
During the compression of the stellar core, which has a mass 8 to 50 times that of the Sun, neutronisation of the substance occurs: neutrons and neutrinos are actively produced. Neutrinos are small particles that interact very weakly with matter, so they leave the crash site in just 10 seconds.
Although extremely light, the released neutrinos are surprisingly fast, in fact, their speed approaches the speed of light. The loss of neutrinos causes the shock wave directed inside the star to turn outward and destroy its outer layers – now a supernova explosion occurs.
Thus, neutrinos get a certain head start, and various types of radiation rush to catch up with them. Despite the fact that these particles are slightly slower than quanta, the difference in release time may be enough for the neutrinos to reach the observer first. Thus, if a supernova explodes in our Galaxy, neutrino detectors are likely to be the first to detect its explosion.
It is interesting that such an event did occur in the history of astronomy. When a supernova flared in the Large Magellanic Cloud in 1987, now known as SN 1987A, detectors recorded an excess of neutrinos a few hours before the flare was registered in the optical range.
Could supernovae be dangerous for us?
Certain panic moods are occasionally raised in connection with the potential imminent explosion of the red supergiant Betelgeuse. This raises natural questions about the impact of a supernova on the Earth and its biosphere. During this cosmic catastrophe, a number of events do indeed occur that could potentially pose a threat.
For example, an explosion is accompanied by a shock wave, but its power decreases rapidly. Neutrinos, which carry away most of the energy, hardly interact with matter – they will pass through the Earth, and only a tiny part of them will be trapped in detectors. But cosmic rays – high-energy particles accelerated by a supernova to almost the speed of light – can really cause damage. X-rays and gamma rays can also have a negative impact on the atmosphere: by destroying the ozone layer, they make the biosphere vulnerable to the Sun’s ultraviolet radiation.
Nevertheless, the distance is a decisive factor in the question of a real threat from supernovae. For a supernova to really cause the negative impact described, it must be 25-30 light years away. At present, it is believed that an explosion that occurs at least 160 light years away will not cause any significant damage to the Earth’s biosphere. The amount of hard radiation of all types will be so small that it will be detected only by special instruments.
The closest candidate for supernovae to us is, surprisingly enough, not Betelgeuse at all, but the IK Pegasi binary star. It consists of a fairly massive white dwarf and a hot white star of spectral class A, which shows small pulsations. This configuration can indeed cause a type Ia supernova explosion. In addition, IK Pegasi is 154 light-years away, which is close to the “danger line”. However, even in this system, an explosion is not expected at least for the next 1.9 billion years.
