- 4133 - EARLY UNIVERSE - is the same? Scientists have made amazing progress in uncovering more and more information on how the Universe began and what conditions were like all those billions of years ago. Powerful infrared telescopes, especially the ground-breaking James Webb Space Telescope, have let astronomers study the ancient light from the early Universe.
-------------- 4133 - EARLY UNIVERSE - is the same?
- One of the mysteries astronomers want to
untangle concerns star formation. Has it changed much since the Universe’s
early days? The Universe has evolved a
lot since the first stars formed, and astrophysicists want to know if stars
form differently now than they did billions of years ago.
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- The primary difference between stars that
formed billions of years ago and stars
that form now concerns the material available during the process. Not
only the amount of material but also how enriched it is. This is referred to as
“stellar metallicity”. Elements heavier
than hydrogen and helium are called “metals”. These metals only come from one
place: the stars themselves.
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- Stars create elements heavier than hydrogen
and helium through “nucleosynthesis”. The intense conditions inside massive
stars create elements like oxygen, iron, carbon, and everything in the periodic
table beneath H and He. When these stars explode as supernovae or shed their
outer layers as red giants, these elements are cast out into space, available
for the next round of star formation.
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- We live in a Universe where countless stars
have lived and died, synthesized heavy elements and spread them out into space.
Consequently, stars in the modern Universe are formed from a variety of
elements. Those elements that make up
life on Earth.
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- But the Universe’s first stars didn’t have
metals available to them and consisted of only hydrogen and helium. Any planets that formed around these original
stars couldn’t have been anything like Earth. Without previous generations of
stars and the heavier elements they synthesized, rocky planets wouldn’t exist,
and neither would we.
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- Researchers located a star-forming region in
the Milky Way exhibiting low metallicity that’s similar to the primitive
Universe, compared to the higher metallicity in the contemporary Universe.
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- Recaping the broad strokes of the star
formation process as science understands it. Things begin in a massive
structure in space called a giant molecular cloud. These clouds contain
everything available to a nascent star.
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- The clouds are dominated by hydrogen, the
most abundant element in the Universe by far, and also the simplest. They’re
called “molecular clouds” because individual hydrogen atoms don’t want to be
alone. They bond together as a molecule containing two atoms of hydrogen.
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- These clouds are not perfectly uniform in
their structure. There are irregularities, regions where the gas is more or
less dense, and exhibits different temperatures, motions, and velocities.
Eventually, small knots or clumps of gas form in these clouds, and over time
their gravity draws more gas into them.
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- Eventually, enough material is drawn in
to form a “protostar”, which gives off some energy but hasn’t begun fusion yet.
Only when enough mass has gathered to drive the core pressure and temperature
to extremes does fusion begin. Then we have a “star”, beginning its long
journey along the main sequence.
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- But how do differing levels of metallicity
affect the stars that result from this detailed process?
-
- Higher metallicity can result in stars with
lower temperatures. Stars with higher metallicities can be cooler while they’re
on the main sequence and on the giant branch. They can also appear redder.
-
- None of this is particularly surprising.
Even though metals, meaning elements heavier than hydrogen and helium, make up
only about 2% of the Universe’s baryons by mass, they have a powerful effect on
heating and cooling during star formation. Metallicity is one of the most
critical factors in star formation.
-
- A star’s future is determined by its mass.
Huge stars, many times more massive than our Sun, burn hot and don’t last long.
These can end as supernovae. Stars much less massive than our Sun are called
low-mass stars, and they vastly outnumber high-mass stars. These stars,
including the plentiful red dwarf stars, can live for trillions of years.
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- Stars seldom form in isolation. Typically,
they form in clusters, and in these clusters, the masses of individual stars
are similar. Would stars forming in the early low-metallicity Universe maintain
this similarity in masses?
-
- Star labelled “S209” can help answer that
question because it has another thing going for it other than its low
metallicity. It’s close, only about 2.5 parsecs (8.1 light-years) away. This is
close enough to enable us to resolve cluster members clearly (~1000 au
separation) down to a mass-detection limit of ~ 0.1 solar masses and we have
identified two star-forming clusters in S209, with individual cluster scales ~1
pc.
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- S209 holds two separate star clusters. One
is large, and one is small, and the larger one contains up to 1,500 stars. This
work marks the first time that astronomers have identified a cluster with this
many identifiable members in the Milky Way’s outer regions.
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- It’s critical because other, similar
regions contain only about 100 stars, a sample too small to draw reliable
conclusions from. The Subaru
Telescope’s power identified stars as small as 1/10 the Sun’s mass, up to stars
20 times the Sun’s mass. It all comes
down to what’s called the Initial Mass Function (IMF), and its utility in
understanding stars, solar systems, and even galaxies.
-
- The IMF and the stellar metallicity help
dictate how stars will form in a given cluster. As stars form, they regulate
their own mass through self-radiative feedback. The effect of self-radiative
feedback is more pronounced in a low-metallicity environment, so these
environments should produce more high-mass stars.
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- S209 contains a few more stars that are
more massive than other star-forming regions in the neighbourhood, but only
slightly more. S209 also has slightly more stars that are less massive than the
Sun.
-
- The outer part of the Milky Way is known
to have properties similar to those of the early Universe. Results suggest that although a relatively
large number of massive stars formed in the early Universe, the number is not
dramatically different from that of typical star clusters in the present day.
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- Weirdness didn’t pervade everything. Stars
are the Universe’s basic building blocks, and this research indicates that they
formed much the same way back then as they do now.
-
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August 31, 2023 EARLY UNIVERSE - is the same? 4133
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