1.3. ASTROTECH – Why Astronomy is Hard

By | December 3, 2019


So why do we need all this fancy
technology? Big telescopes, big computers, space rockets… It’s all pretty expensive after all so why
do we need it? Well, astronomy is very hard to do. It involves some extreme measurement
quantities. So lets take a look at a few of those one
by one. [BLANK_AUDIO] So our first problem is that astronomical
objects tend to be extremely faint. This can be either because of power
or because of distance. After all, a candle can be pretty hard to
see if it’s 30 meters away, but also, a very powerful search light
might be hard to see if it’s 30 kilometers away. In just the same way, a small rock that’s
near to the earth may be quite hard to detect before
it hits the earth. On the other hand, the most powerful
quasar that we know can still be hard to see if it’s right at the edge of the Universe. So, let’s have a little think about what
makes things hard to measure. What determines the brightness of an
object? Let’s imagine the light spreading out from
a star. So, here is our star here. Light is spreading out from the star into an ever larger and larger spherical
surface. Okay. So the light is spread over that spherical
surface. What determines how bright it seems to us
as we catch the light, is how much light we get per
unit area. So now imagine putting a a detector of the
same size at lots of different distances. So as you go further away the fraction of the overall light that this is catching is
getting smaller. So the apparent brightness of an object is
going to depend as one over the square of the
distance. because it depends on the total surface
area of that sphere. So, if an object is 100 times further away, it’s going to be 10,000
times fainter. So that’s a problem. So if this is the sun, here, the sun is
pumping out huge amounts of energy. Something equivalent to 10,000,000,000
hydrogen bombs going off every second. It’s enormous. By the time that light reaches the Earth,
it’s diluted an awful lot. So that the light actually falling on the
surface of the earth is on average about 1.4 Kw/m^2 So that’s enough to warm you up
but a lot less. Now imagine transporting yourself to the
nearest star which is Proxima Centauri. That’s 270,000 times further away than we
are from the sun. Then the light from a star like the sun at that distance would
be a 100,000,000,000 times fainter. So you can still see that with the naked
eye. But the trouble is that many astronomical
objects, the things we want to measure, are another factor of a
100,000,000,000 times fainter than that So making astronomical measurements is
very hard. [BLANK_AUDIO] So our next problem is that we want to
deal with some pretty extreme angles, both big
ones and small ones. So sometimes we deal with big angles on the sky. We might want to map the whole of the
Milky Way. Or we might be mapping out a big region of the sky in order to measure millions and
millions of galaxies. On the other hand, most of the time in
Astronomy, what we’re obsessed with is getting the
sharpest possible pictures. So it’s all about angles – what angular size something appears to
us viewing it from here. So now imagine an object here and you are
seeing it from different distances. So seen from here It’s a pretty big angle on the
sky. On the other hand if we’re viewing the
same object from a larger distance then that makes a smaller angle on the
sky. So that’s how angles work and it goes
linearly with distance. So let’s get this into perspective. When we measure angles, mathematicians
measure angles in in radians. But astronomers, like sailors, like degrees,
minutes and seconds. So let’s put that into perspective. So if we take the whole of the circle, and
we divide it into 360 parts. Then that’s one degree. If we take one degree and divide that into 60 parts, then that makes one arc
minute. And if we take one arc minute and divide
that into 60 parts, that’s one arc second. Now, one arc minute is about what you can
resolve with the human eye and to put that into
perspective, if you imagine taking a DVD like this here
and holding it 400 meters away, that’s about one
arc minute. Now one arc second – it’s like having this DVD 24
kilometers away. So you can’t resolve that with the human eye but if you are looking through a
telescope, you can and this is about the limit of what you can see with a ground based
telescope. On the other hand, about a tenth of that is what you can get with the Hubble Space
Telescope. So this makes an enormous difference. So I am going to show you a picture of the
Milky Way. First of all, you are looking at it blurred to eyeball resolution. You
can’t see this faint with your eye but if you could
that’s what the Milky Way would look like. And then in this next picture
you see the same region of the sky, but at one arc second resolution, as you
can get with a big telescope. And you can see it makes an enormous difference to whether you can actually see
what’s going on. So here’s another picture now. This is a very distant galaxy, in this
case, as you would see it with a ground based
telescope. at one arc second resolution. And here it is again with the Hubble Space
Telescope at 1/10th arc second resolution. And, again, it transforms what you can
understand about that object. So, angles are crucial. [BLANK_AUDIO] Our next problem is that detecting
different kinds of light needs completely different
technologies. So, let me explain why this matters in
Astronomy. It’s because astronomical objects cover a
huge range of temperature. so, for example, here in this room right
now, it’s about 20 degrees Celsius. But of course, as Physicists, we prefer to
measure things above absolute zero. Absolute zero is at minus 273 Celsius. So, it’s about 293 degrees Kelvin, i.e. above
absolute zero, here in this room. So, that’s fairly typical for a
planet around a star. Now, on the other hand, the coldest molecular clouds in the interstellar medium can be just 20
degrees above absolute zero. The surface of the sun is 6,000 degrees. An accretion disc around a black hole can
easily be 10,000,000 degrees. So the light changes when objects are at
different temperatures. If you take an object and heat it up two
things happen. The first is that you just get much more
light in total and the second thing is that the light shifts to
shorter wave lengths as you get hotter. And you get completely different kinds of
radiation. So let me spell that out and tell you why
it matters. So, if we have something at about 20
degrees Kelvin – K for Kelvin – like that intersteller cloud, that’s going
to make microwaves. And to detect that we need radio dishes,
radio receivers, et cetera. On the other hand, something at about 300
degrees K, like the surface of the typical planet, that makes infrared radiation. So
to detect that we need infrared detectors. Okay, so what about the surface of the
sun, something that’s 6000 degrees K – that makes
regular visible light. So to detect that these days we use the
CCD cameras just like the camera you have in
your phone. On the other hand, something at 10,000,000
degrees K. Well, that gives you x-rays. To detect x-rays, you need an x-ray
telescope and x-ray detectors. But also, you need to go into space. [BLANK_AUDIO] Astronomy is famously the science of big
numbers – – we talk about “astronomical numbers”. So for example the Milky Way has got
something like a 100,000,000,000 stars in it, maybe as
many as 400,000,000,000. We’re still not quite sure. Strangely enough the number of galaxies in
the observable universe is pretty similar – about 100,000,000,000 objects. Now, we can’t possibly measure and catalogue
every single one, all those stars and all those galaxies individually, but we do
make databases in modern astronomy that are pretty
scarily big. So for example, this picture you are
looking at here. This is a map of the Milky Way in the
infrared… this database has about a billion
objects in it. Now, what we do is to run our computer
over the image, recognising each one of those
dots as a star. We measure a variety of properties for
each one of those stars. So we turn this image into a giant table
as you’re seeing here. And that table has one row per star, and
each of the columns is a different item of
information about that star. And sometimes in modern Astronomy, we want
to trawl through a huge table like this
trying to find exactly the information we want
about exactly one particular star or a handful
of stars. That processing takes pretty big
computers, and that’s a challenge we’re going to take up in week
five. [BLANK_AUDIO] So, most of the universe changes rather
slowly. The universe as a whole has been evolving
over billions of years, and even the massive styles which burn their
fuel very quickly take a few million years to burn
through the fuel. So most of the universite is relatively stately by human terms but some things happen
fast. A supernova explosion can rise up within a
few days and decay over a few months. A gamma ray burst can happen in a few milliseconds. A rock heading towards the earth that may
be dangerous can change its position
radically in a few hours. So some things do happen fast, and those
are becoming very important in modern
astronomy, those variable events. Dealing with those poses a lot of challenges – technical challenges, but also organisational challenges and social
challenges. How do we share out our telescope time to deal
with that sort of situation? How do we respond rapidly to alerts which
come at us over the internet? All those are very interesting technical
challenges.

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