Over the
last few weeks people have been sending their questions in to the
Cosmic Comics astronomy expert.
From black holes to green stars,
colliding galaxies to space photography - nothing is beyond
Professor Paul Roche!
Check out the questions and answers
out below:
Click on the questions below or scroll
down:
A. Stars
- What is the smallest star that we know
about?
- What is the biggest star that we know
about?
- Why do stars twinkle but planets don't?
- Will there be a time when new stars stop
forming?
- Are there any green stars?
B. Galaxies
- What created the black hole in the centre of our
galaxy?
- Do galaxies move?
- Has the Great Andromeda Galaxy ever collided with
any galaxies?
- Can you see out of our galaxy from
Earth?
- What happens when two galaxies
collide?
C. Comets
and asteroids
- What is the difference between the Kuiper Belt
and the Oort Cloud?
- What are comets made of?
D. Everthing else...
- How far into the past will you see with the James
Webb telescope?
- How fast is the universe expanding?
- Do you need a special camera in space?
- Which are the most likely places in our solar
system for life to exist (aside from Earth of course)?
- If a black hole and a white hole ever collided,
what would happen? A grey hole?
- Why do people think that lifeforms on other
planets are going to be green mutant aliens or some great monster?
What if they were just like ourselves - humans?
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STARS
A1. What is
the smallest star we know about?
There is actually a lower limit to
how "low mass" an object can be before it stops being a star, and
that is around 7-8% of the mass of our Sun. Below that, the object
does not have a high enough core temperature and pressure to join
hydrogen atoms to make helium (this "fusion" is what heats the
star).
We would call one of these objects
a "brown dwarf", and even the most massive brown dwarfs might only
be slightly bigger than our planet Jupiter in physical size.
One of the smallest dwarf stars
discovered so far is AB Pictoris b which
about 147 million light years away and is twinned with a K-type
star that is slightly cooler than our sun.
So the smallest (in terms of mass
and radius) 'normal' stars are not that much bigger than gas giant
planets. But if we include dead stars into this
question, then a white dwarf star (the sort of thing that the Sun
will eventually evolve into) might only be the size of the Earth
(10-15,000 km in diameter), whilst a neutron star would be more
like 20km in diameter - and yet could still contain twice the mass
of the Sun!
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A2. What is the
biggest star we know about?
The biggest stars we observe are
called "hypergiants", and
if you placed them where the Sun is, their outer surfaces would lie
somewhere between the orbits of Jupiter and Saturn in our Solar
system - so they really are extraordinarily big!
It's very difficult to accurately
measure the size of a star, but there are several candidates for
being the "biggest". These include the "Pistol Star" and VW Canis
Majoris - but currently the biggest star observed is called V1489 Cygni, and it is thought to be almost
1,650 times the diameter of the Sun. This is a red hypergiant, and
the red colour tells us that the surface temperature of this
enormous star is much cooler that our Sun - maybe only 2, 500
degrees Celsius.
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A3. Why do stars
twinkle but planets don't?
Mars and Saturn in the night sky
The twinkling (or "scintillation")
that we see is a result of light having to pass through Earth's
turbulent atmosphere.
Our atmosphere has several layers, each with a slightly different
density and temperature. so that light passing downwards will be
bent ("refracted") by small, random amounts many times before
reaching an observer.
Stars (except for the Sun) are
extremely distant, and just appear as dots (what we call "point
sources") of light. Planets are much closer, and cover a measurable
area of sky - we say they have a "disc". The effects of the
atmosphere on a point source are far more noticeable than those on
a planetary disc, where the twinkling effects are smoothed out over
a larger area.
The light from the point source
appears to change in position and brightness - it twinkles. The
size of a planet on the sky essentially "averages out" atmospheric
effects, leaving a relatively stable, untwinkling image.
Stars close to the horizon twinkle
more than stars overhead because the light from stars lower in the
sky travels through more air and so suffers more refraction.
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A4.
Will there be a time when new stars will stop forming?
The Orion nebula with young stars hidden in the gas
and clouds. (c)NASA/ESA/JPL-Caltech/IRAM
New stars form in nebulae (clouds
of gas and dust) and they need hydrogen to begin the nuclear fusion
process. This process, joining hydrogen atoms together to form
helium, powers the stars during the Main Sequence where they spend
most of their lifetimes.
Hydrogen is the most
common element (around 90% of the universe's atoms are
hydrogen).
However, in order for stars to
form, there must be enough hydrogen present in an interstellar
cloud fragment for nuclear fusion to start - too little mass and
you end up with a brown dwarf, sometimes
called a "failed star".
So - not enough hydrogen for fusion
to start = no more stars!
There will come a time when there
is not enough hydrogen around for these collapsing clouds of gas to
get fusion going, but this will be a long time into the future.
As the universe is around 13.7
billion years old and still consists of 75% hydrogen by mass, we
don't need to lose any sleep over it right now! Our own star, the
Sun, has enough fuel in its central hydrogen store for another 4 or
5 billion years or so...
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A5.
Are there any green stars?
You would think that in a place as
enormous and amazing as the universe that you would be able to find
absolutely anything, right?
After all, the colour of a star
depends a lot on the temperature of it's surface. Surely if
you get the right temperature then you'll get green light...
In fact - the temperature that you
need to get green light is about 10,000 degrees Celsius.
There are lots of stars in that temperature range, including ones
that can be seen from Earth like Vega and Sirius (the
brightest start in the night sky).
So why aren't they
green?
Well, actually, they do emit green
light! The problem is that green is right in the middle of
the visible light
spectrum. A star as hot as 10,000°C on its suface will be
emitting lots of red and blue light as well, so the whole thing
blurs to white. For a star to look green to us it would have to
emit only green light, and that's impossible for a
gigantic ball of burning gas!
Contrast is important too... An
isolated "green" star would appear white, but place it next to a
red star and you would see a greenish tinge, as the eye tries to
make the "average" colour white.
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GALAXIES
B1.
What created the black hole in the centre of our
galaxy?
The answer is that we don't really
know - it's almost a "chicken and egg" situation. Which came first,
the black hole (and the galaxy forms around it) or the galaxy (and
the black hole forms within it)?
NASA's NuSTAR x-ray telescope captures an image of a
flare from the supermassive black hole at the centre of the Milky
Way (c) NASA/JPL-Caltech
Most galaxies seem to have massive
or supermassive (billions of times more massive than our Sun) black
holes at their centre, and it is still a big question in cosmology
as to where they came from initially. What we do know is that,
however they formed, they are slowly growing through the
consumption of gas, dust and occasionally whole stars (which are
torn to shreds by the intense gravity near the edge of a black
hole, forming a huge "accretion disc").
So whilst we don't quite
understand what first causes the black hole to form, we can monitor
how these systems behave, and in particular we can observe what
happens when they "over eat" - enormous outbursts of radiation
giving rise to objects such as quasars that we can observe at huge
distances across the Universe.
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B2. Do galaxies
move?
Yes, everything we see in space is
moving. When we look out at distant galaxies, they are all rushing
away from us, and the further away they are, the faster they are
moving - this is some of the evidence that makes us think
everything started in a Big Bang. But if we were on another one of
those galaxies, it would still look like all the other galaxies
were rushing away from us - it is not just that the galaxies are
moving, but also the space in between galaxies is expanding as
well.
Faulkes
Telescope Project image of the M61 Spiral Galaxy
You can imagine this effect if you
think about gluing sweets (which represent galaxies) on to a rubber
sheet (which represents space). If you stretch the rubber sheet
(expanding space), the sweets (galaxies) all move away from each
other - but every sweet (galaxy) would see the others moving away
from it, so none of them are "special", it is just what they see
from their place in space.
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B3. Has the Great Andromeda Galaxy ever collided with any
galaxies?
The Great Andromeda Galaxy (M31/NGC224) will collide
with our Milky Way galaxy in about 4 billion years, but in the past
it must have collided with small galaxies, just as our own galaxy
has. In fact, our Milky Way is still in the process of "consuming"
two smaller galaxies, which we call the Small and Large Magellanic
Clouds - these can be seen from southern hemisphere skies (see
below).
The Small and Large Magellenic Clouds in the southern
sky
Image credit: ESO/S. Brunier
Astronomers think that galaxies
grow by gradually consuming their smaller neighbours in a process
we call a "merger" (or sometimes more descriptively "galaxy
cannibalism"!). To become as large as it is (almost 2-3 times the
size of the Milky Way), Andromeda must have consumed many
smaller galaxies. However we can't easily see much evidence for
those past mergers today.
The Great Andromeda Galaxy
Observations suggest that Andromeda
originally formed from a merger about 8-9 billion years ago, and
since then it has had close encounters (but not collisions) with
the neighbouring (smaller) galaxies M33 (2-4 billion years ago),
M32 and M110. A large group of stars within the galaxy called the
"Andromeda Giant Stellar Stream" might be evidence of a past
merger, and there appears to have been a lot of new stars born
about 100 million years ago, which is also a good indication that a
merger took place then.
After Andromeda and the Milky Way
collide, the result will probably be a giant elliptical galaxy.
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B4. Can you
see out of our galaxy from Earth?
Yes, we can see many, many other
galaxies beyond the Milky Way (which is what we call our home
galaxy). Using large telescopes, we can see for huge distances, and
that means we can see beyond the Milky Way. We can see some nearby
galaxies, like the Large Magellanic Cloud and the Small Magellanic
Cloud, which are our closest neighbours - we are all part of a
group of about 54 galaxies called the "Local Group". But telescopes
can see much farther than this, and in the future we will have
bigger, better telescopes that will allow us to see the very first
"baby galaxies" that were born a few hundred million years after
the Big Bang.
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B5. What happens
when two galaxies collide?
Two galaxies collide (image from NASA's
Chandra X-ray Observatory)
You might think that two galaxies
colliding head-on would be an incredibly destructive event.
Imaging billions of stars rushing towards each other, ready to
crash, smash and bash!
But remember, the spaces between
stars in galaxies are massive! The nearest star to our sun is
Proxima Centauri which is over 4.2421 light years away - that's 24
trillion miles!
Think about those truly
mind-boggling gaps and you start to see that stellar collisions
will actually be rare events. In fact, galaxy "interactions"
usually lead to huge amounts of star creation, rather than
destruction.
Stars themselves are tiny compared
to the spaces between them, but the gravitational forces involved
are truly vast. When stars and galaxies come together,
gravitational shock waves result in massive increases in density in
interstellar gas clouds.
It's inside these gas clouds that
stars are born, so these events trigger intense bursts of star
formation (so-called "starburst galaxies"). Interacting galaxies
often display tidal tails - streamers of stars, gas and dust
dragged free of the parent galaxy.
The Cartwheel Galaxy -
The result of a galactic collision
As well as the immense spaces, the
timescales for collisions are huge (hundreds of millions or
billions of years), so we only ever see a snapshot of the
interaction, but many such interacting galaxies are observed.
The Cartwheel Galaxy is a particularly spectacular example of a
nearly centre-to-centre collision, resulting in the nucleus being
stretched open like an expanding ring.
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COMETS AND
ASTEROIDS
C1. What's the difference between the Kuiper Belt and the Oort
Cloud?
The Kuiper Belt is a flat disc of
comets orbiting the sun that have "fallen in" from the more distant
Oort Cloud. The disc is out beyond Neptune which means that they
take a few hundred years to complete and orbit. We think there are
some larger objects out in that region too, like the dwarf planet
Pluto - we call these Kuiper Belt Objects (not a
very imaginative name!). These objects orbit around in roughly the
same flat disc that the planets and asteroids do, around the
equator of the Sun.
An illustration of the Kuiper Belt
and Oort Cloud in relation to our solar system (c)
NASA
The Oort Cloud is not a flattened
disc like the Kuiper Belt, but instead it is a huge sphere (or
cloud) of maybe a trillion (a thousand billion) comets that
surrounds the entire solar system. This is the edge of our solar
system, and it might even overlap with a similar cloud that we
assume surrounds the nearest stars to us - so we might even be
swapping comets with nearby stars!
Interestingly, no-one has ever seen
the Oort Cloud - the comets are too far away, and much too faint,
to be seen - we believe it is there because of the paths of some
"long period" comets, that seem to fall in towards the Sun from all
sorts of directions. The Kuiper Belt comets would approach the Sun
from directions roughly lined up with it's equator, as that is
where they orbit, so the fact that we see some comets coming from
above and below the poles of the Sun suggests they must be orbiting
around in directions very different from the rest of the solar
system objects we see.
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C2. What
are comets made of?
Artist's impression of the Rosetta "comet chaser" spacecraft which
will make history
next year by orbiting a comet and sending a lander to it's surface
(c) ESA 2001
Comets are often described as
"dirty snowballs", but we now think they might be more like "snowy
dirtballs" - the solid bit of a comet, called the nucleus, is
made of water ice mixed with "dry ice" (solid carbon dioxide), and
seems to be coated with a very black material that is rich in
carbon. The ice is very dirty, with lots of rocky material mixed
into it, which is where we get the "snowy dirtball" name!
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EVERYTHING ELSE...
D1. How far into the past will you see with the James Webb
telescope?
As we look out into space, we look
back in time, because the light we are now seeing has been
travelling for a long time through the Universe.
The James Webb Space Telescope will
be the largest space telescope to ever be built and launches in
2018. Using it's array of instruments it will be able to see
infrared light which was emitted by galaxies about 300 hundred
million years after the Universe was created in the Big Bang (which
scientists think was 13.8 billion years ago).
This full-scale model of the James West Space Telescope is as
big as
a tennis court and as tall as a four-story building (c) NASA/Chris
Gunn
This means it will show us some of
the first galaxies that existed ("baby galaxies" - although these
babies are 300 million years old..!)
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D2. How fast is the
universe expanding?
When we look at distant galaxies,
we see that they all seem to be rushing away from us - and the
further away they are, the faster they are moving. So we can't say
exactly how fast any particular galaxy is going to be moving, we
just have to measure each one separately. This was first noticed by
the astronomer Edwin Hubble (after whom the Hubble Space Telescope
is named) in 1929.
We can estimate roughly how fast a
galaxy should be moving, using something called "Hubble's Law" -
this is an equation that allows scientists to estimate just how
fast a galaxy should be moving, if we know how far away it is. To
do this, we need to know a number called the "Hubble Constant", and
for a long time this was very hard to measure - but now we think we
know the value of this number quite accurately, so we are more
confident that we can estimate how fast the universe is
expanding.
A snapshot of the oldest light in our Universe (from 380,000
years ago) showing regions of slightly different densities (seeds
of all future structure including the stars and galaxies of today).
(c) ESA/Planck
Collaboration
Note: The value of the Hubble
Constant was very recently determined (21st March, by
measurements from the Planck satellite) to be 67 kilometres per
second per Megaparsec - that means that for every 1 million parsecs
away a galaxy is, the speed will increase by 67 kilometres per
second. A million parsecs is about 3,260 million light years.
Using this value for the Hubble
Constant, the age of the universe is now thought to be 13.81
billion years (plus or minus 0.05 billion years).
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D3.
Do you need a special camera once you are in space?
The cameras that are used in space
are specially designed to survive in the difficult conditions they
will have to work in. When the camera is in darkness, it will be
extremely cold, but when it is in sunlight, it will be very hot - a
normal camera would not be able to survive this freezing and
roasting, so the cameras used on spacecraft are protected to allow
them to work in space.
Hasselblad
500EL/M, similar to the
cameras used in the Apollo Program
We also have special cameras that
allow us to see different types of light that are not usually seen
from the surface of the Earth (because the atmosphere does not let
them through), such as infrared and ultraviolet light from stars
and galaxies.
You can find out more about
infrared imaging by clicking on the picture below:
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D4. Which are the most likely places in our solar system for
life to exist (aside from Earth of course)?
The search for places where life
might exist is usually driven by the search for liquid water - on
Earth, where there's life, there's water. So we need to look at
places that might still have liquid water, and outside of the Earth
there are not too many of those.
Many scientists think that life
might once have existed on Mars, when it was warmer and wetter than
it is today - if it did, it would very simple life-forms, like
microbes, not the sort of aliens you see in films! There are some
parts of Mars today where life might still exist, below the surface
and sheltered from the intense cold and the radiation that the
surface is exposed to - but we need to send scientists who can
drill deep below the ground before we can really study that in
detail.
SETI (Search of Extraterrestrial Intelligence) Institute
scientists research life
in extreme environments, life on Mars, exoplanets and more (c) SETI
Some other places where we might
look would be beneath the icy surfaces of some of the moons of
Jupiter and Saturn. Jupiter has a moon called Europa that we think
has a liquid water ocean hidden beneath a crust of ice that might
only be a few hundred metres think in some places - and that ocean
could be 200km deep.
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D5. If a black hole and a white hole ever collided, what would
happen? A grey hole?
Wow! This is a real
head-scratcher...
There really isn't any research on
this topic, and the biggest problem is that the concept of a "white
hole" is theoretical - a mathematical trick which couldn't exist in
the universe without breaking the laws of physics as we understand
them.
The idea is that, if a white hole
was real, it would be the opposite of a black hole.
Instead of sucking in energy and light it would spew it out into
the universe. Whereas a black hole is impossible to get out of, a
white hole would be impossible to get into...
In the end though, the idea of a
white hole comes out of certain mathematical equations. Black holes
on the other hand have been observed and studied for several
decades now, so we are confident that they exist (or at least, as
confident as we can be without actually seeing one!).
An entirely made-up picture of a black and white hole.
Don't include this in an essay! It is quite
pretty though...
So, if a white hole
did exist, and collided with a black hole
- what would happen?
As you might be able to guess,
there really isn't an answer to this one (or if there is somewhere
in the world of theoretical physics we probably wouldn't be able to
work out what the sums meant).
A question we can answer is what
might happen if two black holes collided. In
theory an enormous amount of energy would be released. In fact, we
think that we might actually see this happen as events called Gamma-ray Bursts.
These GRBs have been observed for
over 40 years, sowe think we have good evidence that two black
holes can collide and create a bigger black hole. These events will
be extremely rare (because getting two black holes close together
will be very rare) but the universe is very big, so we see a few
events that might be black hole-black hole collisions every
year.
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D6. Why do people think that lifeforms on other planets are
going to be green mutant aliens or some great big monster? What if
they were just like ourselves....humans?
For most people, the view of what
alien life might look like is mainly from television and movies and
so is more often the ideas of an author or a special effects person
rather than a scientist.
As many TV shows and films show
aliens as aggressive invaders, we might be forgiven for thinking
that this is how science thinks alien life will be. However,
many scientists believe that the most likely "aliens" that we will
discover will actually be very simple lifeforms, like bacteria.
In particular, there are things
called "extremophiles" that have been found to live in extreme
environments on Earth, like inside volcanoes, poisonous lakes or in
the depths of the oceans - lifeforms like these, which can survive
in places that would kill a human (too hot or too cold; full of
toxic chemicals; massively high or low pressures etc.), would be
able to survive on places like Mars (which has no air, is freezing
cold and has almost no atmospheric pressure).
SETI (Search of Extraterrestrial Intelligence) Institute
scientists research life
in extreme environments, life on Mars, exoplanets and more (c) SETI
The chances of life just like us
(i.e. humans) existing out there would probably be extremely small,
as we have evolved to fit in with our particular environment over
billions of years.
Many science fiction shows have
aliens with the same basic physical set-up as us: 2 arms,
legs, eyes, etc. This is probably because it is easy to do this for
the make-up and special effects people, rather than because we
think our body shape is likely to be very common in the
universe!
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