Month: December 2019

Supernova Classifications

by

The story of Betelgeuse has kicked things off, so it seems appropriate to discuss Supernova.

I am sure if I asked the question;

“What is a Supernova“
Most would answer the obvious,
“An old star exploding at the end of it’s life“
Whilst true, it is also not accurate either.
There was a time when a Supernova was just a Supernova, but then researchers realised that there were different types of Supernova, initially they were Type I and Type II. However, is soon became apparent that things were not as simple as that, so lets take a look at the different types of Supernova, what causes a star to become one, and what type of star becomes what type of Supernova.
©Dr Maryam Modjaz
Type Ia Supernova
Researchers found that when they looked at certain types of Supernova explosion there was a lack of Hydrogen visible in the spectra of the exploded star. Whilst the spectra shows very little hydrogen it does show a lot of carbon; they also show silicon, calcium, and elements up to iron (the result fusion during the intense explosion). It was soon realised that this was a new type of Supernova, one caused not by the collapse of the core of a Supergiant star, but rather by the collapse of a White-Dwarf star, the remnant of a star no too dissimilar to the Sun.

A White Dwarf cannot simply collapse all by itself, it needs to exceed 1.4 solar masses (2.765×10E30 kg), the Chandrasekhar Limit for White Dwarf Stability. All observed Type Ia supernova fall into two categories;

A binary Star system where a giant or Supergiant has expanded as it ages and fills it’s Roche Lobe, material then falls on the white dwarf star, forming an accretion disk at first, which may actually cause a repeating Nova in the first instance, but when enough material coalesces on the White dwarf and it’s mass exceeds the Chandrasekhar limit, collapse of the White dwarf is guaranteed.

When the White Dwarf, which is composed mostly of Carbon, collapses under the force of gravity, the temperature rises dramatically, this results in the fusion of carbon into new elements, however, this carbon detonation is catastrophic for the White Dwarf, the result is a massive explosion that we call a Type Ia Supernova.

Type Ia Supernova can also be the result of the collision of two White Dwarf Stars.
Type Ib Supernova
This type of Supernova is also noted for the lack of Hydrogen in the spectra, but unlike Type Ia above, they show helium. It is believed that this type of Supernova results from the collapse of stars more than 25 times the mass of the Sun.
As the stars evolve along the asymptotic giant branch of the H-R diagram they shed their outer layers, these are the stars we now believe evolve into Red Hypergiants, back into Yellow and Blue Supergiants and later explode in a Type Ib supernova explosion, as happened to Sanduleak-69+202 in 1987, resulting in the Supernova SN1987A.
As the star sheds mass it’s gravitational force reduces, thus allowing it to continue burning fuel at it’s core, however, as with Type II supernova, once the core of the star has an Iron mass more than 1.4 times the mass of the Sun it explodes as a Supernova.
Type Ic Supernova
As with the Type 1b, these show little to no Hydrogen or helium in their spectra, but other aspects of them are different from standard Type Ib, these are little understood at the moment, but researchers believe that they are, like Type Ib, stripped cores of truly massive stars, stars that have frantically shed tens of solar masses to continue fusion, shed their outer layers in repeated expansions, exposing the next layer, which then expands and repeats the process, this continues again until the core reaches a critical mass of Iron, cannot produce any more energy, gravity collapses it, and we see a Type Ic Supernova.
Type II Supernova
Current thinking on Type II supernova is that they are the result of the core collapse of stars between 8 and 25 times the mass of the Sun. These supernova show considerable Hydrogen and Helium in their spectra and it is believed that these stars are at the right mass range to continue the fusion process, without shedding too much of their original mass whilst on the asymptotic giant branch of the H-R diagram. When they core reaches that critical Iron mass, we see a Type II Supernova.

What causes the Core to Collapse?

As giant stars age they use up the fuel in their core, just as all stars do, but the larger the star, the faster it uses up it’s fuel. When a Star gets to a certain point, hydrogen fusion in the core slows, this reduces the “gas pressure” in the core that is balancing the force of the enormous gravitational field of the star. When this happens the core begins to slowly contract, this reduces the hold on the outer layers of the star and they cool as they expand, this is how stars turn from dwarf stars, like the Sun, to Red Giants. Depending on the mass of the star, this process will continue all the way through the elements in the bottom of the periodic table until the core is full of Iron. The star will now resemble and onion, with successive layers fusing heavier elements into even heavier elements.
Core Collapse Process.
©Oak Ridge National Laboratory

However, there is a problem with Iron, Iron requires more energy to fuse into heavier elements than the reaction releases. This results in the core slowly contracting until the mass of Iron reaches the Chandrasekhar limit of 1.4 Solar Masses. At this point the core is crushed by the gravity of the star in milliseconds, as the core collapses all the materials making it is crushed even smaller, then it reaches a point called Neutron degeneracy, where all the protons and electrons have been squeezed together to form Neutrons, the core has become a Neutron Star, but as this happens, the outer layers collapse inwards too, the core momentarily rebounds as it reaches the Neutron limit,. emitting huge quantities of Neutrinos in the process.

Core collapse supernova paradigm.
©Oak Ridge National Laboratory

When the Neutrinos and the rebounding core meet the collapsing outer layers, they heat, create a massive shock wave, and the result is a Supernova Explosion.

This photo shows a three colour composite of the well-known Crab Nebula (also known as Messier 1), as observed with the FORS2 instrument in imaging mode in the morning of November 10, 1999. It is the remnant of a supernova explosion at a distance of about 6,000 light-years, observed almost 1,000 years ago, in the year 1054. It contains a neutron star near its centre that spins 30 times per second around its axis (see below). In this picture, the green light is predominantly produced by hydrogen emission from material ejected by the star that exploded. The blue light is predominantly emitted by very high-energy (“relativistic”) electrons that spiral in a large-scale magnetic field (so-called synchrotron emission). It is believed that these electrons are continuously accelerated and ejected by the rapidly spinning neutron star at the centre of the nebula and which is the remnant core of the exploded star. This pulsar has been identified with the lower/right of the two close stars near the geometric centre of the nebula, immediately left of the small arc-like feature, best seen in ESO Press Photo eso9948. Technical information: ESO Press Photo eso9948 is based on a composite of three images taken through three different optical filters: B (429 nm; FWHM 88 nm; 5 min; here rendered as blue), R (657 nm; FWHM 150 nm; 1 min; green) and S II (673 nm; FWHM 6 nm; 5 min; red) during periods of 0.65 arcsec (R, S II) and 0.80 (B) seeing, respectively. The field shown measures 6.8 x 6.8 arcminutes and the images were recorded in frames of 2048 x 2048 pixels, each measuring 0.2 arcseconds. North is up; East is left.
©European Southern Observatory

Kilonova

Only one such event has been positively confirmed, others likely missed or miss identified. This is the name coined for gargantuan explosions caused when two Neutron Stars collide. The first such event was observed in 2016 by multiple observatories and by LIGO, the gravitational wave detector.
Further Reading

Modjaz, M. et al., (2009), “From Shock Breakout to Peak and Beyond: Extensive Panchromatic Observations of the Type Ib Supernova 2008D associated with Swift X-ray Transient 080109”, ApJ, 702, 226 [as pdf ]

Modjaz, Kirshner, Blondin, Challis, & Matheson (2008a), “Double-peaked Oxygen Lines Are not Rare in Nebular Spectra of Core-Collapse Supernovae”, ApJL, 687, L9 [as pdf ]

Tominaga, N. et al. (2005), “The Unique Type Ib Supernova 2005bf: A WN Star Explosion Model for Peculiar Light Curves and Spectra”, ApJ, 633,L97

Modjaz, M. et al., “Optical and NIR Light Curves and Spectra of 30 Type Ib, IIb and Ic Supernovae”, in preparation

The Most likely Naked Eye Supernova

by
ETA Carina. (10h 45m 03.591s / −59° 41′ 04.26″)
It is believed that before Western Sailors ventured south enough to see Eta Carina, once called Eta Argus, that is was the brightest star in the sky as seen from Earth, perhaps as bright as magnitude -4.3, however, since it was discovered by Western astronomers and previously by sailors, it was not this bright, it sat at around magnitude +4.2 right up until the 1830’s.
Western Astronomers had not paid much attention to this not overly bright and seemingly average star, little did they know that this is one of our galaxy’s true giants, a type we now call a Galactic Hypergiant.
This all changed in the spring of 1837 when this apparently unassuming average star underwent a massive brightening event that saw it rival mighty Rigel (β Orionis ) and may have actually achieved a visual brightness of -1.5 for a very short period. This was the start of a period now called “The Great Eruption” and it saw the star rise to become the second brightest star in the entire sky in <arch 1843, before it had faded back below naked eye visibility by 1856.

This peeked interest in the star, so it was keenly observed by a number of astronomers of the day. In 1892 it had another smaller eruption, rising to the 6th magnitude before it once again faded.

Astronomers proposed a number for idea for the outbursts, although they knew too little about the inner working of stars to have an credible theories.

Sometime in the 1940’s Eta Carina began to increase in brightness again, when it actually happened in unsure, there was a war on you know, but since then it has steadily rose in brightness to its current magnitude 4.5.
So what is going on?
That is a very good question that has been asked consistently since the late 1830’s. Today we have a distinct advantage over our long lost forbears, not only do we have truly colossal telescopes compared to them, we have orbital missions that can probe the star in everything from gamma rays through UV, visible light, IR and even microwave, and what have we learnt in all that time…?
Eta Carina is embedded in a twin area of nebulosity know as The Humunculus and the Little Humunculus. the former caused by the eruption in 1837-1843 and the latter most likely resulting from the outburst in 1892.
Eta Carina is a member of Trumpler 16 (Tr16), a massive OII star formation region some 7500ly from Earth, Eta Carina is in fact a double star system, consisting of two true giants, Eta Carina A is a colossal 100 times the mass of the Sun, whereas Eta Carina B is estimated to have a mass in the range of 30-80 times the mass of the Sun, no slouch in it’s own right.
Eta Carina B appears to be in a 5.54 year orbit of the primary based on modern observation which show in increase in X-ray emissions every 5.54 years, it may indicate that the star is passing through the extended outer envelope of Eta Carina A. See the images for a theoretical orbit of the pair.
Now, studying light from the outburst that rebounded, or “echoed,” off interstellar dust that has only recently reached Earth, researchers have found the original explosion created an astounding ~12 solar mass cloud of debris expanding more than 20 times faster than anyone ever expected, some 8890 km/s, or more than 32 million km/hr. That is fast enough to cross our entire solar system in a matter of days.
So far, velocities that high have only been seen in the aftermath of supernova explosions, not in events that leave a star intact, not even the smaller Nova eruptions of stars see velocities that high.
So did Eta Carina actually undergo a Supernova event in 1837? Clearly the answer is appears to be NO because the stars are still there, so what is happening?
“We see these really high velocities in a star that seems to have had a powerful explosion, but somehow the star survived,” said Nathan Smith of the University of Arizona. “The easiest way to do this is with a shock wave that exits the star and accelerates material to very high speeds.”
Smith and Rest, of the Space Telescope Science Institute in Baltimore, wrote a pair of papers in The Monthly Notices of the Royal Astronomical Society describing the recent observations and offering a possible explanation.
Researchers first detected the light echoes from Eta Carina back in 2003, using telescopes at the Cerro Tololo Observatory in Chile. Later they secured observing time on the larger Magellan telescopes and Gemini South Observatory, also in Chile, the team collected spectra to allow them to determine the velocity of the expanding debris.. the data they collected simply did not fit with accepted ideas about stellar evolution.
As we have discussed in earlier posts, massive stars normally die when their cores run out of fuel so the outward pressure of fusion generated energy suddenly stops, gravity takes over and we see a core collapses, resulting in the tremendous shock wave that blows the outer layers of the star into space. Current theory holds that, depending on the mass of the progenitor star, it either creates a neutron star, or possibly a stellar mass black hole..
In the case of Eta Carina’s eruption, an unknown process must have produced event that resulted in supernova like shockwave, but came it must have been short of enough energy needed to destroy the star.
What might have happened?
Smith and Armin posit that Eta Carina may have been a triple star system with two massive stars orbiting close together as a binary system with the third star, possibly a few solar masses, orbiting the binary pair.
As the more massive of the binary pair aged, it expanded, filled it’s Roche Lobe and started to exchange it’s outer layers with the other star within the binary.
This second star took a huge amount from the expanded giant, swelling to around 100 times the mass of the Sun, in the process it striped away the dying star’s outer atmosphere, thus leaving an exposed helium core estimated to be around 30 times more massive than the Sun.
“From stellar evolution, there’s a pretty firm understanding that more massive stars live their lives more quickly and less massive stars have longer lifetimes, So the hot companion star seems to be further along in its evolution, even though it is now a much less massive star than the one it is orbiting. That doesn’t make sense without a transfer of mass.” Rest, 2015 Interview.
The transfer of mass from one star to the other would dramatically alter the gravitational balance and architecture of this unusual stellar system, allowing the helium core star to move away from its younger, but now significantly larger, partner, so far, in fact, that gravitational perturbations with the proposed Eta Carina C caused it to spiral inwards toward the binary pair.
The fate of Eta Carina C was sealed, nothing could stop the inevitable collision with the now supermassive star at the heart of the system, resulting in an explosion of gargantuan proportions, reminiscent of a Type II supernova.
In its initial stages, ejected material would have moved relatively slowly as the stars spiralled closer and closer together. When the they finally merged, debris was blown away more than 100 times faster than previous ejecta, catching up and ramming into the slower moving material, generating the colossal output seen ion the 1837-1843 eruption.
The helium-core star, meanwhile, ended up in a short period elliptical orbit that carries it through the giant central star’s outer atmosphere every 5.4 years, generating X-rays and shock waves.
The now binary system shines more than 5 million times brighter than out Sun, inside the massive ejection cloud we see as the Humunculus nebula.
The Future
The future of both stars is sealed, both are so massive that both will end their lives as Supernova. Eta Carina B is now the less massive star, but it is highly evolved, the helium shell is undoubtedly burning heavier elements beneath it’s layers, so when this will erupt as a type Ia supernova is pure speculation.
Eta Carina A is doomed, as a Galactic Hypergiant it now has no escape route to it’s ultimate fate, as it burns hydrogen as a prodigious rate the core will fill up with helium ash, the star will expand to even more colossal proportions, will it also absorb it’s older family member, who knows, but at an average orbital separation of around 15.5 AU (2,320,000,000km), or about half way between Saturn and Uranus in our Solar system, there is good reason to think that it too will be absorbed as it already appears to pass through the outer atmosphere already, perhaps this is already the start of it’s death dive.
Of course any star around naked eye visibility erupting as a supernova would be a pot of gold for researchers and amateurs alike. Not only would it be a spectacular visual site, but information gleaned from such a nearby event would be, literally, astronomical. The fact that such a huge star would be the culprit would make it even more special, not only would we learn a huge amount about the immediate events following such a core collapse event close up, but we would get valuable information about how such an event would impact the other star in this binary system and the massive nebula already surrounding this binary pair.
Just an end note, the absolute magnitude of Eta Carina is -8.6, to put that in perspective, Venus at it’s closest can reach -4.4 magnitude and the full Moon is magnitude -12.7. This would make Eta Carina some 80 times brighter than Venus is it were only 32.6 light years away, what a spectacle that would be.

Betelgeuse, and the story of a Supernova

by
There has been quite a bit of discussion of late regarding the dimming of the star α Orionis (Alpha Orionis) or commonly known as Betelgeuse.

Betelgeuse is a semi-regular variable Supergiant Star that is located to the upper right of the attached Hertzsprung-Russel diagram. It is classified as spectral class M1-M 2-1a to ab. The classification depends on where, in the variability cycle, the star happens to be when observed.

The star is evolved, hence it being a Red Supergiant, but don’t be lulled into thinking a star is red due to its age, or in fact its size, although these are factors in Betelgeuse’s case. Stars of all sizes can be any of the spectral classes (O-B-F-A-G-K-M).

The colour of a star is indicative of the temperature of the photosphere, or visible surface. The age is only a secondary indicator. Red giants, or red galactic hyper giants are red simply because they are very bloated and their outer layers cool down. Red dwarves are compact, but produce little energy, compared to larger stars, thus their surfaces are cooler than our Sun and appear Redder. (Again, see the H-R diagram attached)

Red stars have surface temperatures between 3200°K and 4600°K. Stars are classified by their colour temperature using the spectral classes;

 

O = 40,000°K plus
B = 15,000 to 30,000°K
A = 7500 to 9500°K
F = 6500 to 7500°K
G = 4800 to 6400°K
K = 3300 to 4600°K
M = 2800 to 3200°K

 

The OBAFGKM spectral sequence has recently been extended to include class L (objects with temperature around 2000 Kelvin) and class T (with temperature less than 1300 Kelvin). Objects of spectral type L and T are not (technically speaking) stars at all, since they are not hot enough for fusion to occur in their cores. These are the brown dwarfs that are sometimes referred to as failed stars.

 

Betelgeuse is likely variable due to pulsations, caused by convection in the photosphere,visible surface, these variations can cause the star to vary from +0.0 to +1.5 visual magnitude. Currently the star is around 1.2 magnitude (+/-0.25 mag). (Thanks to Tom Polakis for the corrected light curve data)

 

The mass of Betelgeuse, and the distance to the star, have been a hot topic for a long time. It was once believed to be physically associated with the Orion OB1 association and at a distance of around 1500 light years (460 parsecs), this would put the mass of the star as high as 50 solar masses, a true Galactic giant. However, in recent decades both these figures have been corrected downwards thanks to more accurate parallax measurements.

Data from both the Hyparcos and Gaia missions indicates it to have been ejected from the Orion OB1 association and is rushing though space at a speed of +30km/s relative to Earth, creating an almost 4 light year wide bow shock in the surrounding space and that Betelgeuse to be around 640 (=/-15) light years (196 parsecs), which would indicate a mass of range of 17 to 25 solar masses, however a new approach to mass calculation for single stars has indicated a mass of 11(+/-3) solar masses. The mass determination is a critical factor is calculating the life span of the star.

 

The star is believed to be approximately 10 million years old and moved off the main sequence between 500,000 years and 2 million years ago, this uncertainty is a result of the mass uncertainty. Stars of 10 solar masses will live up to 20 million years, but stars of 20 solar masses may only live for a maximum of 10 million years. Without knowing the exact mass of Betelgeuse, it is hard to estimate how long it is before it undergoes the core collapse phase.

 

Most researchers believe that Betelgeuse will one day end it’s days in a spectacular type II supernova, and that this can be expected no sooner than 10,000 years but no later than 1 million years in the future, however, this is not certain either, some researchers believe that some stars below around 20 solar masses may be able to shed enough material in their Red Supergiant phase to avoid the catastrophic core collapse that is required for a supernova to occur – this is dependent on the amount of mass the star is able to shed via the stellar wind and other processes, if the star still has a final mass <8 solar masses, then core collapse is inevitable and so is a supernova explosion.

 

Whilst Betelgeuse may become a supernova, it is not the only star in the sky capable of this, Antares is an M1.5-1a to 1b Red Supergiant that is also around 10 million years old and some 550 light years (168 parsecs), with a mass ranging from 11-14.3 solar masses, it is also approaching the end of it’s life and would be a supernova even brighter than Betelgeuse due to the proximity of the star.

 

These stars are so far away they present no danger to live on Earth, Yes, after a few million years, there may be a slightly higher influx of charged particles from their direction, but unless you were naked in space, they would not present any greater hazard than the normal background radiation – and if you were naked in space, you would be dead so it’s an irrelevance.

 

There are around 37 stars that are visible to amateur equipment, some even the naked eye, that are believed to be supernova candidates, in fact, Rigel A, also in Orion, is one of them, a B8-1a Blue Supergiant with a mass at least 21 times that of the Sun. It was once believed that only Red Supergiants exploded as supernova, then in 1986 Sanduleak-69+202, a Blue Supergiant star in the Large Magellanic cloud exploded without any warning, it became Supernova SN1987A, thus changing how we understood which stars will or can erupt as supernova.

 

I hope this post clears up any concerns anyone has about Betelgeuse and the recent hype about the star becoming a Supernova, and any threat it may pose to life on Earth.

EDIT:
Tom Polakis, of The Lowell Observatory, supplied an updated data set from the AAVSO which give an accurate figure for the visual magnitude of Betelgeuse.

You cannot copy content of this page