- 4183 - SUPERNOVAE - the birth of the Universe? Bursts of star formation can not explain the mysterious brightness at this early universe beginning? When scientists viewed the James Webb Space Telescope’s (JWST) first images of the universe’s earliest galaxies, they were shocked. The young galaxies appeared too bright, too massive and too mature to have formed so soon after the Big Bang. It would be like an infant growing into an adult within just a couple years.
--------------------- 4183 - SUPERNOVAE - the birth of the Universe?
- The startling
discovery even caused some physicists to question the standard model of
cosmology, wondering whether or not it should be upended.
-
- Using new
simulations astrophysicists now have discovered that these galaxies likely are
not so massive after all. Although a galaxy’s brightness is typically
determined by its mass, the new findings suggest that less massive galaxies can
glow just as brightly from irregular, brilliant bursts of star formation.
-
- Not only does this
finding explain why young galaxies appear deceptively massive, it also fits
within the standard model of cosmology.
Typically, a galaxy is bright because it’s big. But because these
galaxies formed at cosmic dawn, not enough time has passed since the Big Bang.
How could these massive galaxies assemble so quickly? Our simulations show that
galaxies have no problem forming this brightness by this cosmic dawn.
-
- A period that
lasted from roughly 100 million years to 1 billion years after the Big Bang,
“cosmic dawn” is marked by the formation of the universe’s first stars and
galaxies. Before the JWST launched into space, astronomers knew very little
about this ancient time period.
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- Prior to JWST,
most of our knowledge about the early universe was speculation based on data
from very few sources. With the huge increase in observing power, we can see
physical details about the galaxies and use that solid observational evidence
to study the physics to understand what’s happening.
-
- Astronomers are
using advanced computer simulations to model how galaxies formed right after
the Big Bang. The simulations produced cosmic dawn galaxies that were just as
bright as those observed by the JWST.
-
- The simulations
are part of the Feedback of Relativistic Environments(FIRE) project. They combine astrophysical theory and
advanced algorithms to model galaxy formation. The models enable researchers to
probe how galaxies form, grow and change shape, while accounting for energy,
mass, momentum and chemical elements returned from stars.
-
- They discovered
that stars formed in bursts. A concept
known as “bursty star formation.” In massive galaxies like the Milky Way, stars
form at a steady rate, with the numbers of stars gradually increasing over
time. But so-called bursty star formation occurs when stars form in an
alternating pattern, many stars at once, followed by millions of years of very
few new stars and then many stars again.
-
- Bursty star
formation is especially common in low-mass galaxies. The details of why this happens are still
the subject of ongoing research. But what we think happens is that a burst of
stars form, then a few million years later, those stars explode as supernovae.
The gas gets kicked out and then falls back in to form new stars, driving the
cycle of star formation.
-
- But when galaxies
get massive enough, they have much stronger gravity. When supernovae explode,
they are not strong enough to eject gas from the system. The gravity holds the
galaxy together and brings it into a steady state.
-
- Most of the light
in a galaxy comes from the most massive stars.
Because more massive stars burn at a higher speed, they are shorter
lived. They rapidly use up their fuel in nuclear reactions. So, the brightness
of a galaxy is more directly related to how many stars it has formed in the
last few million years than the mass of the galaxy as a whole.
-
- The supernova,
pinpointed by amateur astronomers, could prove to be a lynchpin in our understanding
of massive star deaths. A massive star
that exploded in the Pinwheel Galaxy in May, 2023, appears to have unexpectedly
lost approximately one sun's worth of ejected mass during the final years of
its life before going supernova, new observations have shown.
-
- This star that had exploded in the nearby Pinwheel Galaxy
(Messier 101), is just 20 million light-years away in the constellation of Ursa
Major, the Great Bear. However, haste
is key when it comes to supernova
observations: Astronomers are keen to understand exactly what is happening in
the moments immediately after a star goes supernova. Yet all too often, a
supernova is spotted several days after the explosion took place, so they don’t
get to see its earliest stages.
-
- Considering how
close, relatively speaking, SN 2023ixf was to us and how early it was
identified, it was a prime candidate for close study. They measured the supernova's light spectrum,
and how that light changed over the coming days and weeks. When plotted on a
graph, this kind of data forms a "light curve."
-
- The spectrum from
SN 2023ixf showed that it was a type II supernova. This is a category of supernova explosion
involving a star with more than eight times the mass of the sun. In the case of
SN 2023ixf, searches in archival images of the Pinwheel suggested the exploded
star may have had a mass between 8 and
10 times that of our sun. The spectrum was also very red, indicating the
presence of lots of dust near the supernova that absorbed bluer wavelengths but
let redder wavelengths pass. This was all fairly typical, but what was
especially extraordinary was the shape of the light curve.
-
- Normally, a type
II supernova experiences what astronomers call a 'shock breakout' very early in
the supernova's evolution, as the blast wave expands outwards from the interior
of the star and breaks through the star's surface. Yet a bump in the light
curve from the usual flash of light stemming from this shock breakout was
missing. It didn’t turn up for several days.
-
- New observations
revealed a significant and unexpected amount of mass loss, close to the mass of
the sun, in the final year prior to explosion.
Imagine an unstable star puffing off huge amounts of material from its
surface. This creates a dusty cloud of ejected stellar material all around the
doomed star.
-
- The supernova
shock wave not only has to break out through the star, blowing it apart, but
also has to pass through all this ejected material before it becomes visible.
This took several days for the supernova in question.
-
- Massive stars
often shed mass. Betelgeuse’s took over
late 2019 and early 2020, belching out a cloud of matter with ten times the
mass of Earth’s moon that blocked some of Betelgeuse’s light, causing it to appear
dim.
-
- However,
Betelgeuse isn’t ready to go supernova just yet, and by the time it does, the
ejected cloud will have moved far enough away from the star for the shock
breakout to be immediately visible. In the case of SN 2023ixf, the ejected
material was still very close to the star, meaning that it had only recently
been ejected, and astronomers were not expecting that.
-
- The only way to
understand how massive stars behave in the final years of their lives up to the
point of explosion is to discover supernovae when they are very young, and
preferably nearby, and then to study them across multiple wavelengths. Using both optical and millimeter telescopes
astronomers effectively turned SN 2023ixf into a time machine to reconstruct what
its progenitor star was doing up to the moment of its death.
-
- We can think of an
evolved massive star as being like an onion, with different layers. Each layer
is made from a different element, produced by sequential nuclear burning in the
star's respective layers as the stellar object ages and its core contracts and
grows hotter. The outermost layer is hydrogen, then you get to helium.
-
- Then, you go
through carbon, oxygen, neon and magnesium in succession until you reach all
the way to silicon in the core. That silicon is able to undergo nuclear fusion
reactions to form iron, and this is where nuclear fusion in a massive star’s
core stops. Iron requires more energy to be put into the reaction than comes
out of it, which is not efficient for the star.
-
- Thus the core
switches off, the star collapses onto it and then rebounds and explodes
outwards.
-
- One possibility is
that the final stages of burning high-mass elements inside the star, such as
silicon (which is used up in the space of about a day), is disruptive, causing
pulses of energy that shudder through the star and lift material off its
surface. What the story of SN 2023ixf does tell us is, at the very least, that
despite all the professional surveys hunting for transient objects like
supernovas, amateur astronomers can still make a difference.
-
-
October 10, 2023 SUPERNOVAE - the
birth of the Universe? 4183
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