Crab nebula
Crab Nebula.
A little more than 969 years ago - on July 4, 1054 to be precise -
light reached Earth from one of the Universes energetic and violent events :
a supernova or an exploding star.
Although its source was 6500 light years from us, the supernova’s light was so bright,
that it could be seen during daytime for weeks.
Various civilisations around the world documented its appearance.
Hundreds of years later astronomers observing the sky near the constellation Taurus,
noted what looked like a cloud of mist near the tip of the bulls horns.
In the mid 19th century astronomer William Parsons made a drawing of this fuzz ball,
based on his own observations through a 91 centimetre telescope.
He noted that it looked something like a crab.
This name struck : We still call it the Crab Nebula.
Nebula is Latin for fog.
We now know that the Crab is a colossal cloud of debris,
that got blasted away from the explosion site, of the ancient supernova,
at 5 million kilometers per hour.
In the past millennium that material has expanded to a size,
of more than 10 light years across.
It is still so bright that it can be seen with just binoculars, from the dark site.
Astronomers recently aimed the James Webb Space Telescope, JWST, at the Crab,
in hopes of better understanding the nebula’s structure.
What they found might even solve a long standing mystery,
about its origins in the death throes of a bygone star.
The image, also taken by Hubble, reveals almost football shaped cloud of smooth,
vaporous material wrapped in wispy but well defined multicoloured tendrils.
At the clouds centre is a pinpoint of light.
It is a pulsar, the leftover core of the massive star that exploded so long ago.
Hubble observes mainly in visible light.
It’s image reveals mostly shockwaves repelling through the cloud’s material,
and hot gas excited by the central pulsar’s powerful radiation.
JWST in contrast, is sensitive to infrared light, so its image shows different structure.
Rather than revealing shockwaves and hot gas,
the JWST images show features arising from the Crab’s dust and its synchrotron radiation.
The former is composed of tiny grains of silicates,
or complex carbon molecules similar to soot.
It appears primarily in the nebula’s outer tendrils.
The later is the eerie glow emitted by trapped electrons, spiralling at nearly the speed of light, around the pulsar’s intense magnetic field lines.
Synchrotron radiation is best seen in radio and infra red imaging,
so it dominates the smoother inner cloud in JWST’s view.
One of the filters used in these observations is tuned for light from hot iron gas,
tracing the ionised metal’s distribution throughout the tendrils.
These measurements, astronomers hope, might answer a fundamental question about the star, that created this huge, messy nebula nearly a millennium ago.
Stars like our Sun fuse hydrogen into helium in their core.
This thermonuclear reaction creates vast amounts of light and heat, allowing the star to shine.
When the Sun runs out of hydrogen to fuse it will start to die, swelling into a red giant,
before finally fading away.
We have many billions of years before our Sun’s demise is set to begin.
Stars that are more massive than the Sun can fuse heavier elements.
Helium can be turned into carbon.
Carbon can be turned into magnesium, neon, and oxygen,
eventually creating elements such as sulphur and silicon.
If a star has more than 8 times the mass of our Sun, it can squeeze atoms of silicon so hard,
that they fuse into iron.
This spells disaster.
Iron takes away more energy to fuse than they release,
and the star desperately needs the outward push from fusion powered energy,
to support its core against the inward pull of its own gravity.
The star’s core loses that support once iron fusion begins.
This initiates a catastrophic collapse.
A complex series of processes occurs,
and in a split second a truly mind boggling wave of energy is released,
making the star explode.
If the core itself has less than about 2.8 times the mass of our Sun,
it collapses into a superdense, rapidly spinning neutron star.
Its whirling magnetic fields sweep up matter and blast it outwards in two beams,
creating a pulsar.
But if the core is more massive than that, its gravity becomes so strong that it falls in on itself,
becoming a black hole.
The Crab nebula has a pulsar, indicating the core of its Supernova progenitor,
was less than 2.8 times the mass of the Sun.
But the star itself may have been anywhere from 8 - 20 times the Sun’s mass in total.
This presents a problem.
The mass of the crab pulsar is less than twice the Sun’s mass,
and the estimated mass of the entire nebula is as much as 5 times that of the Sun.
But that adds up to only seven solar masses at most.
The star must have been more massive than that to explode.
Where did the rest of the material go?
It is possible there is hidden mass surrounding the pulsar,
embedded in the nebula as yet undetected by telescopes.
The structure of the nebula could provide clues to this material,
or at least point astronomers towards places to look deeper.
Even the star itself is something of an enigma.
How massive was it?
Taking the measure of the nebula might offer answers.
Iron-core collapse is just one way a massive star can explode,
For a star around 8-12 times the Sun’s mass, there is another avenue to annihilation.
The core of such a star is incredibly hot, and there are countless free electrons,
swimming in that dense, searing soup.
A quantum mechanical property called degeneracy pressure,
usually makes the electrons resists compression, adding support to the core.
But during one specific stage of stellar fusion, it possible for those electrons,
to instead be absorbed into atomic nuclei, removing the pressure.
This change can trigger a core collapse before the star has hand a chance to create iron.
Scientists first proposed this supernova triggering electron capture mechanism in 1980.
It wasn’t observed until 2018, via telltale signatures in the light,
from a distant exploding star in another galaxy.
Scientists see hints that crab nebula might have exploded in a similar fashion,
but it is not certain.
Greater clarity would come from JWST’s measurements of how much iron the nebula holds.
The element’s abundance could allow scientists to distinguish between the normal core collapse, and one triggered by electron capture.
This data is still being analysed.
The prospect of solving two different mysteries with one set of observations,
is just the kind of thing scientists love.