Transcript ref evlution of stars

Swinburne Online Education Exploring Stars and the Milky Way
Module :
Evolution of Stars
Activity:
From Gases
© Swinburne University of Technology
and Dust
Summary:
In this Activity we will study the interstellar medium:
• the properties of interstellar gas and dust
• different types of nebulae
• how they may become the birthplaces of new stars
Interstellar matter
Interstellar matter consists of gas (mostly hydrogen, but
also some other atoms, molecules and ions in molecular
clouds) and dust (little clumps of matter).
The interstellar matter is usually so very thin and cold that
most of the time it is observable as a faint haze, or not at all.
Yet it is estimated that gas and dust makes up most of the
matter in the cosmos.
If I, here on Earth,
were a piece of
interstellar matter ...
… then the nearest piece
(at the same scale)
would be on the Moon!
Tell me
about
atoms
_
Cosmic gas
+
HI
Most gas in the cosmos is
hydrogen, either atomic, molecular
or ionised.
Atomic hydrogen is electrically
neutral and is often called H I,
where the “I” is the Roman numeral
for 1.
A hydrogen atom which loses its
electron is positively charged, and
is often called H II.
We’ll look at other gases later.
Atomic hydrogen:
an atom by itself
H II
Molecular hydrogen: H2
_
+
Hydride ion: H-
_
+
Hydrogen ion: H+
Warning: Confusion!
People studying astronomy, especially if
they have done some chemistry, are often
a bit unclear about this labelling.
Astronomers label the neutral state of
hydrogen H I, and the state with one
electron removed H II.
In chemistry, on the other hand, atomic
hydrogen would be written H and hydrogen
with an electron removed would be called
H+.
The fact that it has one plus sign means
that people confuse it with H I, which is
wrong. So be careful!
Atomic hydrogen:
an atom by itself
_
+
In chemistry: H
In astronomy: H I
Hydrogen ion: H+
+
In chemistry: H+
In astronomy: H II
Visible spectra from hot gas
Some clouds of gas and dust (nebulae) can
be hot : perhaps they are collapsing under
gravity, or may be excited by ultraviolet
radiation from a nearby star.
The gas will tend to have a lot of ionised
hydrogen in it (H II) and will form an
emission nebula.
Ultraviolet in
Usually the red line of the Balmer series
dominates the spectrum, so the gas looks
reddish.
Clouds like this also radiate strongly in the
radio part of the spectrum.
Tell me
about hot
and cold
Emission spectrum out
Visible spectra from cool gas
How come I can be
seen and you can’t?
Warm gas
Gas that is cool, however,
is mostly electrically neutral
and almost all atoms will
be in the ground state all
the time.
There is no spare energy
to emit!
So clouds of gas like this
can’t be emission nebulae:
there won’t be much in the
way of Balmer and other
series emissions, and the
gas won’t be visible to the
eye.
Because I’m down
near 10 degrees K,
that is, only 10
degrees above
absolute zero!
Cool gas
Cool radio
All is not lost, however. We do have a way of detecting cold
hydrogen. The ground state of hydrogen (H I) has two levels, very
close in energy.
The proton and the electron both have “spin”, and there is a very
slight difference in energy between the two possible arrangements.
Slightly higher energy
Slightly lower energy
Tell me
about spin
The potential energy of the electrons (black dots) is about 13.6 eV …
with a difference of only about 0.000006 eV between them!
Doing the radio flip
Electrons will very, very occasionally*
flip from one state to the other by
absorbing energy or radiating energy,
and if they fall from the parallel state
to the antiparallel state then a radio
wave (21 cm) is emitted.
Measuring the intensity of radio waves
at this frequency actually gives
astronomers a lot of information about
the distribution and amount of cold
hydrogen gas in the nearby cosmos.
Higher
Lower energy
* I only get to do this
about once every
11,000,000 years!
Structure of a dust grain
Now, let’s look at dust. Although “dust” is something we can complain
about on Earth, in space it has an extremely important role to play.
The dust found between the stars mostly consists of minute grains of
silicates, and ices: that is, frozen water (H20), methane (CH4),
ammonia (NH3), carbon dioxide (CO2) and other stable combinations
of the most common atoms: H, C, N, O, Si and S.
These grains are much smaller than what we call “dust” on Earth.
Core: about 0.05
micrometres across,
made of silicates, iron,
and/or graphite.
A micrometre,
also known as a micron,
is 10-6 metres
Mantle: about 0.5
micrometres across,
made of ices of
CO2, H2O, CH4, NH3
Surface of tars and/or other molecules,
including organic molecules.
Formations of gas and dust
Although there is a thin sprinkling
of dust and gas throughout the
known cosmos, it collects into
clouds because of gravitational
attraction.
A molecular cloud is a particular
formation of this type, with a
mass 100 to 1,000,000 times that
of the Sun, a diameter of 15 to 60
parsec and a temperature of up
to 10 K.
Molecular clouds can contain
more than 60 kinds of molecules.
Nebulae
Another name for a
gathering of gas
and dust is nebula.
Nebula is an Old
High Greek word,
meaning “mist” or
“cloud”.
This is a close-up
of part of the Orion
Nebula.
As mentioned earlier, if a nebula is hot and transparent it will glow,
producing spectral lines of its own (very often in the redder part of the
spectrum): nebulae like these are called emission nebulae. The
yellowish areas of the nebula shown above are like this.
Not so bright nebulae
Other nebulae
absorb some
wavelengths of light
that pass through
them, and so will be
characterised by
absorption lines.
Some nebulae
reflect light well,
and so are called
reflection nebulae.
Light reflecting off
a nebula tends to
look blue.
Light emitted by a
hot nebula tends to
look reddish.
In some regions, the
dust is thick enough to
absorb light and looks
dark.
Light passing through nebulae
Question: Why the colour effects, then?
Answer: First of all, nebulae have a noticeable effect on the light
passing through them from stars behind them.
In particular, they tend to scatter the light from the blue end of the
spectrum, making the stars look more red than they really are.
Nebulae also make the stars behind them look dimmer.
Light reflecting off nebulae
On the other hand, someone looking at
light reflected from a nebula will see it
as being more bluish in colour.
The combined effect
Reflected light
This is why photos of nebulae are
often quite brightly coloured, as
some of the light reaching us
passed through a cloud ...
… and some of the light was
reflected off a cloud.
Transmitted light
Another example
The Helix Nebula, NGC 7293, is
a fine example of a planetary
nebula: a shell of gas exploding
outwards from the star in the
centre.
The gas is hot because it is
excited by ultra-violet radiation
from the star, so it emits light in
the visible region.
Emission and reflection
The shell of gas is both emitting and
reflecting light. (This is not unusual. You can
both look through a window and see your
own reflection in it at the same time.)
Where the hot gas is thick along our line of
sight, we see the strong red colour of the
emitted light.
The blue central area is light reflected from
the gas behind the star.
At the edges of the sphere,
the hot gas looks thickest
… but reflected
starlight looks blue
Are they really that spectacular?
Are nebulae really as
brightly-coloured as this?
The answer is,
unfortunately, no.
Photographic film, unless
specially treated, is most
sensitive at the blue end
of the spectrum.
On the other hand,
charge-coupled devices
(CCDs) used to measure
light intensity are most
sensitive at the red end of
the spectrum.
So a combination of different methods is
used to produce photos which give us the
most information in a digestible form.
Very dark
The human factor
A bit dark
If you (presumably a human being) were
to look through a telescope at the Trifid
Nebula of the last slide (trifid = three
parts), then you wouldn’t see either the
red or the blue very well.
Our eyes evolved on planet Earth to
make the best use of light from our own
Sun, which puts out most of its radiation
in the yellow-green part of the spectrum.
This is why when we look at colours
yellow looks the brightest to us, and we
class rich red and deep purple as dark
colours.
Brighter
Lovely!
Not so bright
Darker
Pretty dark
Very dark
Dust the Matchmaker
Although the pictures are lovely, we’ll leave
what dust looks like for now and talk instead
about what it does, as it has a very important
role in our universe.
A dust grain can act as a catalyst or
“matchmaker”, introducing atoms and
molecules of gas to each other so that they
can form larger molecules.
Atoms and small molecules may have too
weak a gravitational or electrostatic force to
attract each other, but a much larger, sticky,
tarry object and some random motion can do
the trick and bring them together.
Hello there!
Dust the Protector
I’m safe behind
the dust grain ...
Once new molecules are formed,
dust can shield them from the
harsh, high-energy ultraviolet
starlight that might otherwise break
the fragile bonds holding them
together.
So dust increases the chance of yet
more complex molecules being
formed in a molecular cloud.
… or inside it!
Ultraviolet
light
Dust the Great Mother
If enough dust and gas
gathers in a molecular
cloud, there will be
sufficient mass for
contraction to take place…
and if the cloud gets
compact and hot enough,
next thing you know, you
have got a baby star.
Baby stars are often called
Young Stellar Objects.
We’ll learn more about
them in the next Activity.
The lumps in this part of the Orion nebula are
called proplyds: they are cocoons of leftover
gas and dust surrounding baby stars.
Dust to dust
Where the heck does all
this dust come from in the
first place? It contains lots
of different elements that
can’t have been magically
created from
interstellar hydrogen.
The only way we know of
to turn hydrogen into these
heavier atoms is by
nuclear fusion, which as
we know takes place in the cores of stars, and processes which take
place during supernovae of large stars.
Dust to dust
The atoms that make up
the molecules in molecular
dust are probably formed in
the atmospheres of giant
stars.
The outer atmospheres of
giant stars can be
surprisingly cool - cool
enough for atoms to
condense onto small solid
particles, like soot forming
in a cool candle flame.
The pressure of the radiation pouring out of these old stars (and their
stellar winds) is likely to push these grains out into the interstellar medium,
where they can start to accumulate into dust clouds.
So dust is the legacy of stellar old age and death
… and also the source of star birth.
Not on our scale
The other thing to
mention again about
interstellar dust is its
size.
While dust on Earth is
usually of the order of
many microns across
and can be visible as
specks to the naked
eye, dust grains in
space are very, very
much smaller.
Earth dust
Space dust
Funny, that ..
Usually in space you
have to think larger!
This Activity has shown you something of the interstellar
medium, which is composed of hydrogen gas, other
gases and dust.
Hydrogen can be cool, and in the H I state (radio
emissions only), or warm and in the H II state (reddish).
Hydrogen, along with other gases and molecular dust
can form huge nebulae which can be emission nebulae,
reflection nebulae or just dark masses that absorb light
and don’t transmit it.
In the next Activity we will have a closer look at how
such clouds can become the birth-places of stars.
Image Credits
Hubble: Proplyds in the Orion Nebula Credit C.R. O’Dell
(Rice University), NASA
http://antwrp.gsfc.nasa.gov/apod/ap961017.html
MSSSO © M. Bessell (used with permission):
Dust in the Orion-Eridanus region
The Eagle Nebula
The Magellanic Clouds
Dust in the Orion Nebula
The Trifid Nebula
The Helix Nebula
The Crab Nebula
Hit the Esc key (escape)
to return to the Index Page
Atoms, molecules and stuff
An atom is a single unit of one substance: an element
such as hydrogen, carbon, ytterbium, mercury.
An atom is defined by the number of protons (positively
charged) in its nucleus. Hydrogen always has one, helium
2, carbon 6 and so on.
The number of neutrons (with no charge) in an atom may
vary, giving different isotopes of the same element.
A molecule is made of two or more atoms bonded
strongly together by their electrons (negatively
charged). The atoms may be of the same element (e.g.
in the gases H2 and O2) or different ones (e.g. the
common molecules H2O and NH3).
Electrons in
outer “shells”
Protons and
neutrons in
“nucleus”
Methane, CH4
Hydrogen gas, H2
Oxygen gas, O2
Water, H2O
Ions
Ionisation is when
an atom or a
molecule loses or
gains an electron
or so, and is no
longer electrically
neutral.
7 electrons
7 protons
Nitrogen atom
(neutral)
6 electrons
8 electrons
7 protons
7 protons
Nitrogen ion
(N+)
Nitrogen ion
(N-)
Ions are very important in astronomy. The electrical and magnetic
forces which will act on ions are many, many times stronger than
gravity (which is actually one of the weakest forces in the cosmos), and
there are lots of ions in hot gases.
Back to
Interstellar
Matter
Back to
Interstellar
Matter
Spinning charges 1
Positive charge
Magnetic
moment
Any moving charge, either positive
or negative, sets up a magnetic
effect called a “magnetic moment”.
Spinning charges do this too.
The direction of the magnetic
moment depends on whether the
particle you are spinning is
positively-charged (e.g. a proton)
or negatively charged (e.g. an
electron).
Direction
of spin
Negative charge
Magnetic
moment
Direction
of spin
Spinning charges 2
Because magnets (and magnetic moments) don’t like
pointing in the same direction, the hydrogen atoms
below “prefer” to have the magnetic moments going in
the opposite directions.
Back to
“Cool
radio”
The electron and proton are not
comfortable when the spins are in
the opposite direction and so the
moments are parallel
… but they are a bit more
comfortable, and so in a “lower”
energy state, when the spins are in
the same direction and the moments
are anti-parallel
Back to
“Cool
radio”
Temperature
For millions of years, humans (and other creatures) have been able to
sense the average kinetic energy in a whole bunch of particles.
We call it temperature.
Two objects touching each other will always tend to “compare notes”
about how much kinetic energy their particles have, and will come to
some kind of agreement (called equilibrium) about whether energy
should flow from one to the other to even things up, or not.
Boy, my particles
are
flying
today!
Is that
better?
How are you?
A bit cold …
my particles
Terrific! Thanks!
are almost
at a standstill
Hotter and colder
(energy of motion)
If the average kinetic energy (KE) of the particles of
something is high, we call it hot.
But if the average kinetic energy is low, we say that the
object is cold.
Just call me
Mr Average ...
Cool ...
Hot!
Smaller and larger
Now, let’s look at this with
respect to a gas: for
instance, a star or perhaps
a nebula.
Let’s pretend that we have a
nice tame nebula that we
can order to get larger and
smaller for us.
Greetings
Stop it!
That tickles!
Gravitational potential energy
All of the particles in the
nebula are attracted by
gravity to the centre.
The fact that the particles are
not in the centre means that
they have potential energy
(PE), and the further they are
from the centre then the
higher that energy will be.
Centre of mass
Potential energy
depends on distance
Energy is conserved
If the cloud is large, the total
potential energy must be
high.
If the cloud shrinks, the PE
goes down, so the kinetic
energy (KE) increases to
make up the difference.
This is why a molecular
cloud (or protostar) gets
hotter and hotter as it gets
smaller and smaller.
Loads of PE
and not much KE,
so low T
Not much PE
but tons of KE,
so high T
On Earth
Slowest: KE least
Highest: PE most
Total stays constant
We see something like this on
Earth whenever we throw
something into the air.
The total energy is constant: it
just changes from one form to
another, that’s all.
So, as the particles in a nebula
move closer to the centre, they
lose PE but gain KE … and
get “hotter”.
Back to
hot and
cold gas
KE decreasing
PE increasing
Total stays constant
On the way down
the reverse
happens
Moving fast: high KE
Near ground: low PE
Total stays constant
To centre
of Earth
Back to
hot and
cold gas