so I always see people wearing those T-shirts that have a spiral galaxy on them
and then a pin drop that says you are here to mark where we think the location
of the Sun is in our own galaxy the Milky Way of hundreds of billions of
stars the question is though like how do we actually know that that's what the
shape of the Milky Way is and where the Sun is in it because we're trapped
inside it there's no way that we can get out of the Milky Way to take an image of
it because that's billions of trillions of kilometers that you would have to
travel. It'd be like me plonking you in the middle of a forest and saying map
the positions of the trees in this forest oh but you can't actually move to
do it. How we see the Milky Way in the sky is just this band of sort of white-ish
diffuse light along with all the rest of the stars so how do we even know that
the Milky Way is a spiral galaxy?
So all the way back to the 5th century BC Greek
Arabian, Persian philosophers all considered the nature of the Milky Way,
that diffuse band of white across the sky and a lot of them did suggest it
could be made of many many stars that were much further away from us than the
other ones because it didn't move relative to those it was clearly in the
background and it wasn't actually until 1610 that galileo galilei, of dropping
stuff off the Leaning Tower of Pisa fame, actually proved that this was true. He
took a telescope to the Milky Way and showed that it was actually made of many
faint stars the light of which all combined to make that diffused light
across the sky. Fast forward now to 1755 and Thomas Wright and Immanuel Kant (ie
be very famous philosopher)
Chidy: "you just casually cited Immanuel Kant"
both speculated that because you saw that diffuse band of stars in a strip across
the sky that the structure of the Milky Way was kind of like the structure of
the solar system where you had the Sun in the center and the planets going
round it but instead here at the Milky Way you just had a lot of stars all
going around the central object and that those stars were all rotating.
Now that was pure speculation and they had no proof for that they were basically
philosophizing over it but Kant also took one step further and
actually said that the other fuzzy things that we see in the sky that
people called nebula they could also be "island universes" as he dubbed them all
of these systems of rotating stars. Then by 1785 William and Caroline Herschel
are the first people to try and actually map the Milky Way and they did that by
marking the positions of stars along various different lines of sight when we
looked out into space now to do that they assumed that all the stars are the
same brightness so from their apparent brightness you could tell how distant
they were which is not a very good assumption we know now. They also were
very limited by their telescopes you know back then they weren't great and
they also didn't understand that dust in space can actually absorb light from
behind it so if you look in a direction where there is lots of dust then you're
not going to see the many many stars that are actually hidden behind that dust so
what they ended up with was a map where the Sun was pretty much in the center
and a very flat structure like what was seen in the sky. into the 19th century
though telescopes have gotten much much better with the Industrial Revolution so
much so that by 1845 Lord Ross was actually able to resolve the shapes of
those fuzzy nebulae things in the sky and found that they had spiral structure
he even managed to show that in some of them you could resolve individual points
of light as if they were stars themselves giving more weight to cants
original ideas that these nebulae were actually galaxies in their own right or
these Island universes as he called them so this brings us up to the start of the
20th century and this is where things start to all kick off so in 1904 a Dutch
astronomer called Katzen found that half of the stars in the sky were moving in
one direction and the other half of stars in the sky were moving in another
direction now that was not understood at the time at all but eventually people
made the connection between those results and the ideas of right and Kant
back in 18th century the Milky Way was a structure like the solar system but it's
had a planets going round the center it was stars rotating around some center so
in 1912 henrietta we're studying a type of star in our own
Milky Way that we now call Cepheid variable stars and these stars pulse in
their brightness and what she noticed was that how fast a pulse is directly
related to how bright they are so if you can measure how bright they appear in
the sky you then know how distant they are and so this meant that we could
finally accurately calculate the distances of things in the universe and
so every single distance measurement we now make in the universe is all
calibrated on these nearby distant estimates using Henrietta Lubitz law of
pulsation time is proportional to brightness so that in 1917 Shaklee used
this to try and estimate the size of the Milky Way and he did this by using what
we call globular clusters these are clusters of about a hundred thousand
stars in these big blobs that are very very bright and very easy to spot and to
see in the sky and so because of that they also have different types of stars
in it they can help you calculate distances as well and so he managed to
estimate using gobble clusters that are also kind of insensitive to dust as well
then the Milky Way was about a hundred thousand light years wide it's a hundred
thousand light years I endure song and the Sun was about thirty thousand light
years from the center which he said within the constellation of Sagittarius
because that was where the most globular clusters were found
now that was much larger than people first thought because of the largest
distance to a star seen at least in the sun's neighborhood but that was because
they hadn't took into account the fact that when you looked at the center it's
a lot dustier and so for the first time we had a map of the Milky Way that put
the Sun not in the center in the same year though
Curtis was observing Andromeda nebula as it was known back then the things that
were thought to be part of the Milky Way and in it he observed this brief
brightening of a point source in it which he said was a nova now not a
supernova light when you have a star that explodes and dies but I know it was
a little bit fainter than that it's kind of a brief brightening of a new star and
so he said that no who is ten times fainter than anything
seen in the Milky Way before so if he used that to calculate the distance to
Andromeda you get something like 500,000 light years and that's much bigger than
the estimate that Shapley had just made of the Milky Way being about a hundred
thousand light years across and so this triggered the great debate
at the beginning of the 1920s you had Shockley on one side arguing that the
nebulae were part of the Milky Way and the Milky Way was all there was in the
universe and then Curtis on the other side that was saying actually spiral
nebulae like Andromeda were actually galaxies in their own right and the
universe was much bigger than we had ever imagined now the debate was
eventually settled in 1926 by Edwin Hubble who showed using libbets law
using the Cepheid variables that the distances to Andromeda and other spiral
nebulae in the sky were much larger than the size of the Milky Way hundreds of
thousands if not millions of light years away
now Shapley originally called hobbles result junk science until Hubble sent
him a letter with all of his observations written down in it and
Shapley apparently responded with here is the letter that has destroyed my
universe so through the 1930s Hubble's categorizing all these brand-new shapes
of galaxies that we're seeing including a lot of spiral galaxies combine that
with the fact that in 1930 Trumpler studied open star clusters slightly
different to the globular star clusters and they're a little bit smaller but
still under the same principles that they're not as sensitive to dust and he
used that to actually categorize what effect dust has on starlight and so with
that we were able to start making much more accurate maps of the Milky Way for
the first time and therefore the idea that the Milky Way might actually be a
spiral in the same that we've seen in other galaxies outside the Milky Way
started to take hold so 1940 John Orr was studying the
persistent hiss that was present on every single radio signal sent on earth
and he realized that hazal signal was coming from space and in particular from
the constellation of Sagittarius where the center of the galaxy was thought to
be so he then passed that observation on to Henry
Vanda host who then predicted that hydrogen again the most abundant element
in the universe and therefore pervading the majority of the Milky Way could
actually produce radiation with a wavelength about 21 centimeters the
microwave radio regime where we've been picking up this hiss because of two very
closely spaced electron energy levels in the atom now that's fantastic because as
I said hydrogen is incredibly abundant but also radio signals aren't sensitive
to dust so you can use them to see through the dust for the first time so
by 1951 there's a mission at 21 centimeters from hydrogen in the sky had
actually been detected for the first time by urine and Percel again strongest
in the direction of Sagittarius but what it allowed was for Christian and Hindman
in 1952 to actually map the motion of this hydrogen gas in the Milky Way for
the first time revealing the spiral structure and also that the Milky Way
probably had a bar structure as well in the very center so once you had that map
of spiral arms in the gas he then knew where to look for the spiral structure
in the Stars and so Morgan in 1953 managed to find evidence of a spiral
structure in the positions of stars in the Milky Way as well so now the spiral
structure have been confirmed work in the 60s turned towards that more
interesting result that suggested that the Milky Way could have one of these
bars structures so there's a lot of work by people like Duvall Claus Peters and
Hamersley all investigating this bar especially if they could find the stars
in the bar as well and so there was a lot of debate about where that bar was
how long the bar is as well which actually kind of still continues to this
day so because of this through the 1970s
people were still looking for a way to see the stars behind all of this
interstellar dust yes they'd used the hydrogen emission in the radio to see
the gas structure behind the dust but really what I they wanted to see was the
Stars now ideally you'd do that with infrared light infrared light is not
affected by dust but unfortunately it's also absorbed by the Earth's atmosphere
so you can't use a telescope from the ground to do that no George no no George
design in 1976 so they came up with the next best thing they were going to use
the hot young OB stars that were incredibly incredibly bright the
brightest stars we knew of especially bright in blue and UV wavelengths to
trace the spiral structure because they weren't as sensitive to dust as the
cooler redder stars were and so using that they were able to find evidence for
four spiral arms in the Milky Way the Perseus arm the Sagittarius Carina
arm the Scouten crooks and Taurus arm and the Norma arm also at the end of the
70s viewer Reuben was studying the rotation of stars in galaxies and
actually found that the speed of their rotation couldn't be accounting for how
much stuff you could see in galaxies and actually there had to be this huge
extended halo of matter that we couldn't see so this was the first evidence for
dark matter that galaxies are embedded in these Dark Matter halos and the same
should also hold true for the Milky Way and that's a completely other story
about dark matter but if you are interested in dark matter either wrote a
whole chapter to it in my new book that's coming out in September but you
can pre-order it now and the link is in the description box below so shameless
book promotion aside what this meant was that through the 1980s in the 1990s a
lot of the focus shifted towards measuring the total mass of the Milky
Way and whether that mask the accounted for by the amount of stars that we can
see in 1994 Hubble Space Telescope observations by Bakkal and collaborators
showed that faint stars were nowhere near enough to make up the amount of
Mato we needed to explain the mass calculated by Zaretsky and collaborators
in 1989 1.3 trillion times the mass of the sun of matter in the Milky Way
so into the 2000s we finally managed to launch an infrared Space Telescope so
that we could see past the dust for the first time and it meant that in 2005 the
first conclusive proof that the entire Milky Way was a spiral galaxy finally
came through and that came from Benjamin and collaborators who by looking at 13
million stars in the infrared managed to show that the Milky Way had three arms
and confirmed the fact that the Milky Way had that bar structure again so the
rest of that decade and into the 2010 the number of spiral arms in the Milky
Way was heavily debated because there was lots of different conflicting
results some showing the Milky Way had to three and up to four spiral arms
until eventually a lot of measurements were combined across different
wavelengths from the radio in the infrared all the way through to the
ultraviolet it eventually showed actually the young hot stars that were
very sensitive to the UV were tracing to spiral arms and then the older stars
that were more sensitive to the infrared we're tracing another two spiral arms
giving four in total at least that's four main arms now there are a lot of
branches and Spurs that come up those four main spiral arms very similar to
what we see in the galaxy Messier 101 in fact the Sun is thought to be on the
Orion arm which is one of these little Spurs and branches so we can't put a
definitive number on how many there are in total but it means the general
structure is agreed on there's a bar in the center there's four main spiral arms
all of that is embedded in a very very low-density stellar halo we've got very
few stars scattered over a large space and then that again is then embedded a
much larger Dark Matter halo as well but this picture could very easily change as
new evidence comes to light such is the nature of science as we've
just heard as theories evolve and change as new results pour in now as we look
toward the next decade the future of this field definitely rests with the
gaia spacecraft gaia is gonna make what we call a six dimensional map of the
Milky Way so you'll have the 3d positions in x y&z of a billion stars
along with the velocities in those dimensions as well so the velocity in
the x y&z direction so that for the very first
will not just be able to get them out of the positions but also a map of the
actual movements of all of those stars in the Milky Way as well so the first of
these maps is going to be released in 2022 and I for one I'm so excited to see
the evolution of the Milky Way go from this static structure that we see in the
night sky to having its dynamical nature revealed in all of its glory captain
captain captain captain cat time captain fuck all the time is all mine all mine I
got the time is all mine John Triplette always named cracks me up every time
just because I'm gonna have to explain this to American viewers on it
Trump is flanked in Britain for to pass wind only if it was a Figaro in this
science history jobs you'd be like Galileo Galileo Figaro I can't get the
high why do I even try hey all right dinnertime I'm so well great
you