Our Galaxy- Recent Findings.
Scientists have spent the past 20 years trying to figure out what the milky way looks like.
The work is not easy because we are inside the galaxy, and cannot see it from the outside.
But scientists have ingenious workarounds.
What they know so far is a dense,
barred centred embedded in a layered disk of gas and stars,
some of which pile up into arms that spiral through the disk,
on encased in a sparse spherical hallow of stars.
Piecing together this much of a milky way map has been so difficult,
that scientists often cite the story of the blind men and the elephant.
People who cannot see each touch a elephant’s trunk,
ear or leg and respectively describe a snake, a fan, or a tree trunk.
They miss the whole elephant entirely.
At least astronomers knew what they didn’t know.
They knew stars in different parts of the galaxy where different ages,
but they couldn’t account for why.
They knew stars formed a gigantic clouds of gas, but the clouds were all but unmappable.
They had seen the other galaxies merge with one and another,
but didn’t know whether an earlier milky way might have done the same.
Earlier scientists thought that our galaxy was in equilibrium, stable since birth,
orderly and elegant.
But that picture has changed in recent years,
as scientists have been systematically mapping stars wholesale.
The bounty of data comes from new surveys,
notably one by European Space Agency (ESA) observatory Gaia,
that are collecting stupefying amounts of information.
As of 1993, ESA’s previous star mapping satellite, Hipparcos, had mapped 2.5 million stars.
By 2023, Gaia had mapped 1.8 billion of them.
Many other telescopes and surveys, have gathered images and spectra
- star’s light spread out into its individual wavelength - for millions of stars.
Notable among them is the Sloan Digital Sky Surveys, or SDSS.
With all these data astronomers are making the first exact maps of the milky way,
which will have the location of the stars in three dimensions,
that’s a record of their motions made by repeatedly imaging over time.
The result is a deep, high resolution movie of a few billion swirling stars.
This helps to reveal not only the galaxies structure,
but also its surprisingly tumultuous history.
It also reveals the histories of its stars, and the galaxy’s means of making more stars.
It is the single largest increase in astronomical knowledge.
The maps show not the Milky way in static equilibrium, as scientists expected,
but rather the galaxies departure from it.
Mapping the stars is nothing new.
4000 years ago, ancient Mesopotamians watched the Sun, Moon,
and wandering planets move against the stars of the zodiac’s constellations.
These constellations were called True shepherd of Heaven, the Old man, the Goat fish,
the Hired man, etc,.
They believed that these constellations were laid out by the great god Marduk,
so people could arrange their lives and organise the year.
If a baby was born in the month when the moon was in the Bull of heaven,
and the moon then moved night by night to the rest of the constellations of the zodiac,
until it came full circle back to the bull, they the baby would be one month old.
Mesopotamians astronomers looked at specific stars, called normal stars,
and held up fingers to measure the daily distances between the normal stars and the moon, Sun and planets.
By 120BCE Greek astronomer Hipparchus have replaced fingers held against the sky,
with a universal grid of longitude and latitude on which stars could be located.
Beginning in the early 1600s astronomers invented telescopes,
then bigger telescopes that could see fainter things.
They later added cameras and spectrographs that collected and dissected starlight.
They later refined the cameras’ focus by flying satellites above Earth’s distorting atmosphere.
The technological outcome, like the number of stars, is also stupefying.
The Mesopotamians may have been off by a finger or so held at arms length,
may a degree of an arc.
Hipparchus was off by about half a degree, or 30 arc minutes.
The Gaia satellite, however is off by no more than 24 millionth of an arc second.
This kind of precision means astronomers can find structures in the milky way,
that are not only features of its map, but also evidence of its history.
Among the first structure form their conformations of stars arcing through the hallow in
streams that were born together and still travel as one.
Scientists started to look for structures in the hallow of stars that surrounds.
and is bound to the Milky way.
The hallow, at the galaxy’s farthest reaches, was known to be made of old stars,
and assume to be feature less.
In 2006, SDSS and other surveys began verifying halo stars with the same colours,
and brightness that seem to move together in long streams, called Triangulum.
Astronomers suspected the streams came from outside the galaxy.
Their stars had been born together in some little, nearby galaxy.
They were pulled into a stream when that galaxy came too close to the gravitational tides,
of the much larger Milky way.
The picture made sense, but verifying it was complicated.
To believe stars were in the stream, astronomers needed to see, that the stars were related - that they had been born together with in the same gas cloud,
and they bear the distinct chemical signatures of the elements present in the cloud.
As stars age, they turned light elements into heavier ones, which astronomers call ‘metals’.
They then die in explosions that scatter the metals back into the gas around them.
The more generation of stars that have lived and died in the galaxy’s gas,
the more metal rich are the new stars that are born inside it.
The more metal rich, the younger the star.
Stars in a stream, form the gas clouds in the same galaxy,
should have the same chemistry and ages.
Stars in a stream should share the same motions.
Motions towards or away from us are easy to calculate, from stars’ spectra.
But measurements of the so called proper motions across the sky have been imprecise.
In 2016, Gaia began releasing its wealth of data - chemical compositions,
ages and precise 3 dimensional locations and motions, including proper motions,
for billions of stars.
Gaia’s data and other surveys, astronomers were able to identify which stars were born
outside the milky way and had migrated in, and which had been born here.
They could not only verify foreign star streams,
but could also track each stream’s orbit back to its own galaxy.
By 2021, astronomers had found 60 streams in the hallow.
23 of them had likely birth place in dwarf galaxies, or in the milky way’s globular clusters.
Globular clusters are mysterious bound balls that orbit our galaxy.
The stars are generally around 10 billion years old.
The ages of the streams are harder to estimate, but they are probably a few billion years old.
Astronomers expect that they will eventually find around 100 streams.
The streams running through the halo were some of the first signs,
of the galaxy’s departure from stability.
Then scientists began uncovering other grouping of stars,
that didn’t follow the expected pattern.
They found a batch of milky way stars in the wrong place.
They were in the old metal-poor and had the orbits of old halo stars,
but they had the metal rich chemistry of youngest stars from the milky way disk.
They wondered whether they were stars that had some how wondered into the halo.
One group of scientists found an entirely different batch of stars,
in the halo that were going unusually fast and in the opposite direction,
from the rest of the halo.
They named this bean shaped batch, as the wrong-way batch.
Another group of scientists found that the bean’s stars were also old and metal-poor.
The called the bean Gaia-Enceladus, for the Earth goddess Gaia’s son, Enceladus.
In 2023, scientists found a stream of stars with the same old, metal-poor chemistry,
and wrong-way motion as the bean, and thought this stream,
was probably tracing the bean’s fall into the milky way.
Astronomical community settled on a comprise name for bean,
the Gaia-Enceladus Sausage, or GES.
The generic noun for a GES type entity is ‘blob’.
Scientists discovered that in midst of a foreign blob of metal poor stars,
was a group of metal with stars, native to the milky way.
Scientists suggested that when the GES collided with our galaxy,
it splashed the native’s stars out of their normal orbits in the disk,
and up into the halo.
They call the star group Splash.
Putting their blobs, streams and splashes together,
astronomers concluded that between 8 billion and 10 billion years ago,
Enceladus - about a quarter of the size of the milky way -
struck our galaxy head on and merged into it as a blob.
GES stars now make up most of the milky way’s halo, and the merger thickened its disk.
Scientists call it the most transformative event in milky way history.
Older less violent transformations had happened not in the halo,
but in the body of the galaxy itself.
In 2022, Scientists found signs of a proto galaxy apparently turning into a galaxy.
Verification was complicated and hinged on knowing which stars are native to the milky way.
Scientists measured in situ stars’ chemistry and found two populations :
one group was ancient, metal poor, moving chaotically and forming stars slowly;
the other was younger, metal-rich, moving coherently and forming stars 10 times faster.
Astronomers thought that the populations represented different stages of galactic history,
and called their stages, ’simmering’ and ‘boiling’ respectively.
They also measured the in situ stars’ orbits and found two epochs.
The early one with metal poor stars’ orbits going all over the place, and the other one with metal rich stars orbiting more coherently.
The transition was from hot mess, to relatively cold spinning disk.
They called the hot mess Aurora, after the ancient Greek Goddess of dawn.
Scientists studied the chemistry of two million in situ stars across the sky,
and found a gravitationally bound group of ancient,
metal poor stars in the centre of the galaxy.
They called it ‘the poor old heart’ of the milky way.
Names aside, scientists agreed that they are probably studying the same transformation.
A chaotic proto galaxy full of old, metal poor stars going in no particular direction,
that then spun into a disk and began to form new stars like fireworks.
Stars tell us only part of the story, because the Milky Way is only partly stars,
and the rest is mostly gas.
Stars are born from gas clouds, so that the two are intimately related.
Astronomers who study stars and those who study gas,
work and largely overlapping communities.
Because stars are born in gas, and later enrich that gas with the elements they produce,
gas astronomers are interested in how the galaxy stays alive, and therefore its present.
And because stars retain the orbits and chemistry of their origins,
star people tend to be interested in how the galaxy evolved, and therefore its past.
Astronomers have been able to map gas clouds for only the past hundred years are so.
This is because the clouds are large, diffuse and dim, and are hard to study.
Scientists could outline their positions in the sky,
but could only approximate their distances and shapes.
Gaia data allow scientists to deduct gas clouds through their stars, but the method is indirect.
It is done via a proxy of a proxy.
Gas clouds are 99% gas.
The other 1% is dust, a fine suit mixed with the gas so throughly that the map of the dust,
is more or less the map of the gas.
Dust can be identified by its effect on starlight.
Stars shining through dust look redder and dimmer.
By mapping reddened, dimmed stars, scientists can trace an outline of the dust,
and therefore the gas.
The dust filled gas clouds are also peppered with well known precisely located stars,
and astronomers can connect these stellar dots to map out the clouds.
Still, a measurement this indirect, is like describing the elephant,
by touching the hair on its tail.
It is like looking at one part of a million of the elephant.
Scientists found a dozen or more long, thread like clouds of gas, scattered like toothpicks,
throughout the galaxies spiral arms, that might serve as birth places,
for the arms wealth of new stars.
They called the clouds ‘bones’.
They also discovered a similarly long and narrow gas cloud, and called it the split.
Scientists mapped the gas clouds that had clusters of new born stars.
It was the local stellar nursery.
The shock was that the nurseries are all alive in a narrow line.
Seeing from the side, this alignment looks like a wave, that like a split but larger,
undulates through the plane of the galaxy.
They named it Radcliffe Way.
The Radcliffe ways is 10 times longer and 100 times wider than the bones.
One reason these filaments of gas are interesting is that they - the bones especially -
probably coincide with the galaxies spiral arms.
No one yet knows how many arms, the galaxy has.
So far the arms look less like coherence structures than like arms plus branching feathers.
This makes the counting of their numbers dicey.
If we could look at our galaxy from the outside,
we will probably see it as having something between the disorganised,
blotchy arms of these flocculent spiral, and the elegant orderly arms of a grand design spiral.
The consensus is that spiral arms are best studied in galaxies we don’t live in.
Scientists mapped what is known as the Local Bubble,
a nearly empty region around the solar system, made of hot, rarefied gas,
and found the bubble outlined by groups of young stars, all moving outward.
Scientists proposed that the bubble was created 14 million years ago,
when a cluster of stars exploded as supernovae.
This swept up the ambient gas and carried it into a large sphere,
on whose surface the gas cooled into clouds, and began forming its own stars.
Scientists wondered whether the gas structures, the Bones, the Split, the Radcliffe Wave,
and the Local Bubble, are variants of the same thing.
That is they are long filaments of gas inside which smaller clouds are compressed into stars.
Maybe the Split, Radcliffe Wave and Local Bubble are historically related.
The Local Bubble lies between the Split and the Radcliffe Wave.
We live in a bubble between a big snake and a smaller one.
Scientists speculate that if we could rewind time to see the locations and motions of the Split,
and the Radcliffe Wave, 15 million years ago,
we would find that the two were close enough to intersect.
Right at their presumed crossing point, where gas would have been densest,
and most likely to produce new stars, astronomers see a lively crowd of young stars.
This group of clusters is called the Scorpius-Centaurus association, are Sco-Cen for short.
Moreover this Split Wave intersection and Sco-Cen,
happen to be at the centre of the Local Bubble, and therefore possibly the bubble’s origin.
But this is still not for sure.
If the story told by the stars is the galaxy’s history of assembling itself,
and if the story told by the gas is the galaxy’s cycles of star formation,
then the stars and gas together should show the galaxy’s past and present.
It is like a movie that reveals the evolving equilibrium.
Following is the summary of the elephant we know so far.
13 billion years ago, in a universe that was less than a billion years old,
the milky way was born as a shapeless cloud of gas and dust.
It was forming metal-poor stars and rotating incoherently.
The stars’ orbit was also haphazard.
For the first billion years are so, smaller clouds and dwarf galaxies,
crashed into the baby milky way.
It sent up sprays of both immigrant and native stars into a halo.
Gas carried by incoming colliders also set off more star formations in the milky way.
By about 12.5 billion years ago the galaxy was rotating more coherently.
One to two billion years later it had spun up into a disk,
in which stars orbit were tidily circular.
Stars now formed at a quiet simmer.
They burnt quickly through their lives and died explosively.
This enriched the gas out of which the next generations,
of increasingly metal-rich stars would be born.
10 million years ago, the Enceladus galaxy collided with the milky way.
Over the next two billion years, it dissolved into it.
The Gaia Enceladus Sausage took over the halo.
Its sped up the stars in the Milky Way’s thick disk, and poured in gas.
This added to the Milky Way’s gas, and increased its star formation.
Gradually over the next two billion years inside the thick disk,
gas and stars settled into a denser, thin disk and collected into spiral arms.
Six billion years ago, a dwarf galaxy called Sagittarius sideswiped the Milky Way,
and swung around it.
Every few hundred million years after that, it brushed past the Milky Way again.
Each time it was leaking stars in a trail.
It created streams that curved through the Milky Way’s halo, wrapping around it twice.
During the next five billion years, other incoming objects did the same,
until the entire Milky Way was surrounded with streamers.
By then the spiral arms of the concentrated thin disk, gas had gathered into long threads.
These were the bones, splits, filaments, along which stars lit up in clusters.
Closer to the Sun, starting about 15 million years ago,
massive stars in the Sco-Cen association formed.
There lived fast lives and blew up, creating the Local Bubble.
On its dense surface more stars formed.
The 37 clusters that now make the Sco-Cen association,
have fired of in burst every five million years.
Carving out more bubbles with more dense surfaces forming more stars.
This ensured that the galactic neighbour hood foams with new sparklers.
The filamentary clouds don’t survive the floods of radiation from star birth,
and after 5 to 20 million years they have sheared apart back into the galaxy.
There the gas will eventually cool, and under the influence of gravity and rotation,
re-condense into filaments, and then again into stars.
4000 years ago, people in Mesopotamia knew the names of some stars and constellations,
as though they were family or Gods.
They used to write them down in stone and use them to plant crops, measure time,
and predict lives.
We have renamed some of them, and still use them as Taurus, Scorpio,
Sagittarius and Orion.
We still use this to locate ourselves in the galaxy.
With new surveys, we can see how constellations and stars,
have warped and shifted with time.
The galaxy will continue to change.
The gas and the star maps are complete near the Sun, but get hazy further out.
By 2023, astronomers have mapped only two billion of the Milky Ways 100 million stars.
Astronomers are still working to map the rest.