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Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne University of Technology

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Page 1: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Module :

Activity 1:

From The Interstellar Medium

Module 6: Evolution of Stars

Swinburne Online Education Exploring Stars and the Milky Way

© Swinburne University of Technology

Page 2: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

SummaryIn this Activity we will study the interstellar medium. In particular, we will examine:• the properties of interstellar gas and dust; • different types of nebulae; and• the birthplaces of new stars.

Page 3: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

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.

Interstellar matter

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 ...

If I, here on Earth,were a piece of

interstellar matter ...

… then the nearest piece(at the same scale)

would be on the Moon!

… then the nearest piece(at the same scale)

would be on the Moon!

Tell me about atoms

Page 4: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Hydride ion: H-Hydride ion: H-

Molecular hydrogen: H2Molecular hydrogen: H2

Atomic hydrogen: an atom by itself

Atomic hydrogen: an atom by itselfCosmic gas

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.

_

_

+

Hydrogen ion: H+Hydrogen ion: H++

H IH I

H IIH II

Page 5: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Atomic hydrogen: an atom by itself

Atomic hydrogen: an atom by itselfWarning: 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! Hydrogen ion: H+Hydrogen ion: H+

+

In chemistry: H+ In astronomy: H IIIn chemistry: H+ In astronomy: H II

+

_

In chemistry: H In astronomy: H IIn chemistry: H

In astronomy: H I

Page 6: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Usually the red line of the Hydrogen Balmer series dominates the spectrum, so the gas looks reddish. Clouds like this also radiate strongly in the radio part of the spectrum.

Visible spectra from hot gas

Clouds of gas and dust (nebulae) can be hot. This may be because they are collapsing under gravity, or they may be excited by ultraviolet radiation from a nearby star. Hot gas will tend to have a lot of ionised hydrogen in it (H II) and will form an emission nebula.

Ultraviolet inUltraviolet in

Emission spectrum outEmission spectrum out

Tell me more about hot and

cold gases

Page 7: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Visible spectra from cool 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 or other series emissions, and the gas won’t be visible to the eye.

How come I can beseen and you can’t?How come I can beseen and you can’t?

Because I’m downnear 10 degrees K,

that is, only 10degrees aboveabsolute zero!

Because I’m downnear 10 degrees K,

that is, only 10degrees aboveabsolute zero!

Warm gasWarm gas Cool gasCool gas

Page 8: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Slightly higher energySlightly higher energy

Cool radioAll is not lost, however. We do have a way of detecting cold hydrogen at radio wavelengths. 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 lower energySlightly lower energy

The potential energy of the electrons (black dots) is about 13.6 eV …

with a difference of only about 0.000006 eV between them!

The potential energy of the electrons (black dots) is about 13.6 eV …

with a difference of only about 0.000006 eV between them!

Tell me more

about spin

Page 9: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Doing the radio flip

Electrons will 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.

Higher energyHigher energyLower energyLower energy

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.

* I only get to do thisabout once every11,000,000 years!

Page 10: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Now, let’s look at dust. Although dust is something we might 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.

Structure of a dust grain

Core: about 0.05 micrometres across, made of silicates, iron, and/or graphite.

Core: about 0.05 micrometres across, made of silicates, iron, and/or graphite.

Mantle: about 0.5 micrometres across, made of ices ofCO2, H2O, CH4, NH3

Mantle: about 0.5 micrometres across, made of ices ofCO2, H2O, CH4, NH3

Surface of tars and/or other molecules, including organic molecules.Surface of tars and/or other molecules, including organic molecules.

A micrometre,also known as a micron,

is 10-6 metres

A micrometre,also known as a micron,

is 10-6 metres

Page 11: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Formations of gas and dustAlthough there is a thin sprinkling of dust and gas throughout the 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 are mostly molecular hydrogen but also contain more than 60 kinds of other molecules.

NGC 6188, a region of molecular dust and hot young blue OB stars

Page 12: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

NebulaeAnother 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 regions of emission.

Page 13: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Triffid Nebula

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 reflecting off a nebula tends to look blue.

Light emitted by a hot nebula tends to look reddish.

Light emitted by a hot nebula tends to look reddish.

In some regions, the dust is thick enough to absorb light and looks dark.

In some regions, the dust is thick enough to absorb light and looks dark.

Page 14: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Light passing through nebulae

Question: What causes the colour effects of nebulae?

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.

Page 15: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

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.

Page 16: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

The combined effect

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 lightTransmitted light

Reflected lightReflected light

Page 17: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

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.

Helix Nebula

Page 18: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Helix Nebula

Emission and reflection

At the edges of the sphere, the hot gas looks thickest

At the edges of the sphere, the hot gas looks thickest

… but reflected starlight looks blue

… but reflected starlight looks blue

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.

Page 19: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

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 useful form.

Page 20: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

The human factorIf you (presumably a human being) were to look through a telescope at the Trifid Nebula of the last slide, 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.

Very dark

A bit dark

Brighter

Lovely!

Not so bright

Darker

Pretty dark

Very dark

Page 21: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Dust the MatchmakerAlthough 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!Hello there!

Page 22: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Dust the Protector

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.

Ultra-violetlight

Ultra-violetlight

I’m safe behindthe dust grain ...I’m safe behindthe dust grain ...

… or inside it!… or inside it!

Overall dust increases the chance of more complex molecules being formed in a molecular cloud.

Page 23: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Dust the Great MotherIf enough dust and gas gathers in a molecular cloud, there may be sufficient mass for contraction to take place… and if the cloud gets compact and hot enough, a baby star can form.

Baby stars are more formally called Young Stellar Objects (YSOs).

We’ll learn more about YSOs 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.

Page 24: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Dust to dustWhere does all the dust come from in the first place? It contains lots of different elements that weren’t 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. (You’ll find out about more about supernovae in the Module on Exploding Stars).

Page 25: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

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.Dust is the legacy of stellar old age and death … and also the source of star birth.

Page 26: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

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 much smaller. Funny, that ..

Usually in space youhave to think larger!

Funny, that .. Usually in space youhave to think larger!

Earth dust

Space dust

Page 27: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

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 neutral H I state, or warm and in the ionised H II state.

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 birthplaces of stars.

Summary

Page 28: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Hit the Esc key (escape) to return to the Module 6 Home Page

Page 29: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Image CreditsHubble: Proplyds in the Orion Nebulahttp://antwrp.gsfc.nasa.gov/apod/image/proplyds_hst.gif

MSSSO © M. Bessell (used with kind 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

NGC 6188

Page 30: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne
Page 31: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Atoms, molecules and stuffAn 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”Electrons in

outer “shells”

Protons and neutrons in “nucleus”

Protons and neutrons in “nucleus”

Hydrogen gas, H2

Oxygen gas, O2Water, H2O

Methane, CH4

Page 32: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Ions

Ionisation is when an atom or a molecule loses or gains an electron or so, and is no longer electrically neutral.

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.

7 electrons7 electrons

7 protons

7 protons

Nitrogen atom (neutral)

Nitrogen atom (neutral)

6 electrons6 electrons

7 protons

7 protons

Nitrogen ion (N+)

Nitrogen ion (N+)

8 electrons8 electrons

7 protons

7 protons

Nitrogen ion (N-)

Nitrogen ion (N-)

Page 33: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Return to Activity

Page 34: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Spinning charges

Negative chargeNegative charge

Positive chargePositive charge

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

Direction of spin

Direction of spin

Direction of spin

Magnetic momentMagnetic moment

Magnetic momentMagnetic moment

Any moving charge, either positive or negative, sets up a magnetic effect called a “magnetic moment”.

Spinning charges do this too.

Page 35: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

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.

The electron and proton are not comfortable when the spins are in the opposite direction and so the

moments are parallel

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

… 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

Page 36: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Return to Activity

Page 37: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

TemperatureFor 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 particlesare flying today!How are you?

A bit cold … my particlesare almost

at a standstillIs that better?

Terrific! Thanks!

Page 38: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Hotter and colder

If the average kinetic energy (KE) of the particles of something is high, we call it hot.

Just call meMr Average ...

Cool ... Hot!

But if the average kinetic energy is low, we say that the object is cold.

Energy of motionEnergy of motion

Page 39: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Smaller and larger

Now, let’s look at the energy of a gas, such as 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

Page 40: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

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 massCentre of mass Potential energydepends on distance

Potential energydepends on distance

Stop it!That tickles!

Stop it!That tickles!

Page 41: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Energy is conservedIf 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.

Not much PEbut tons of KE,

so high T

Loads of PEand not much KE,

so low T

Page 42: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

To centreof Earth

On EarthWe 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.

Moving fast: high KENear ground: low PETotal stays constant

Moving fast: high KENear ground: low PETotal stays constant

KE decreasingPE increasing

Total stays constant

KE decreasingPE increasing

Total stays constant

Slowest: KE leastHighest: PE most

Total stays constant

Slowest: KE leastHighest: PE most

Total stays constant

On the way downthe reverse

happens

On the way downthe reverse

happensSo, as the particles in a nebula move closer to the centre, they lose PE but gain KE … and get “hotter”.

Page 43: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne

Return to Activity

Page 44: Module : Activity 1: From The Interstellar Medium Module 6: Evolution of Stars Swinburne Online Education Exploring Stars and the Milky Way © Swinburne