Lecture 14: The Interstellar Medium

Chapter 18: pages 471 - 476

How do stars form?

When we look at the stars, we note the following properties:
  • Stars are often found in pairs.
  • Stars are often found in clusters.
  • Chemical composition of stars (from spectra) is fairly constant: 73% Hydrogen, 25% Helium, 2% Other Elements
  • Masses of stars range from 1/10 MSun to about 50 MSun.
  • Must be evolving, since they consume fuel and radiate
These properties suggest that stars form in large clouds composed mainly of H and He, which fragment into smaller clouds which collapse to form stars in a region of space.

The Interstellar Medium

  • We call the gas between the stars the Interstellar Medium (ISM).
  • The chemical composition of the interstellar medium is very similar to the chemical composition which we see in stars.
  • The ISM is not uniform: some regions are denser and are called interstellar clouds.
  • Regions which are denser than average can collapse due to the mutual gravitational attraction of the gas into stars.

Main Components of the Interstellar Medium

  • Hydrogen: neutral, ionized or molecular
  • Other molecules
  • Dust
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Detection of Ionized Hydrogen

  • We can detect Ionized Hydrogen when the free electron recombines with the Hydrogen nucleus.
  • The electron recombines in an excited state and then jumps down in stages to the ground state.
  • When the electron jumps down from the n=3 state to the n=2 state, it emits a red photon corresponding to the Balmer alpha transition.
  • When the electron jumps down from the n=4 state to the n=2 state, it emits a blue photon corresponding to the Balmer beta transition.
  • This process is also called fluorescence.
  • We can see fluorescent ionizied hydrogen only when hydrogen is illuminated
  • A region of ionized hydrogen glows with a pinkish red colour.
  • Astronomers call a region of ionized Hydrogen a H II region.
  • Since a H II region glows red from the emission of photons, this is also an emission nebula.
  • Nebula is Latin for cloud or mist.
Energy Levels of H The Trifid Nebula
Emission of red light from ionized H Red light from H emission in the Triffid Nebula.
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What causes the ionization?

  • Type O and B stars are very hot:
    surface temperature greater than 10,000 K.
  • Peak wavelength for 10,000 K is in the UV part of the spectrum.
  • Light radiated from these stars is very energetic and can easily ionize any neutral Hydrogen surrounding the star.
  • Suppose that a type O or B star is born inside of a cloud of neutral H.
  • Photons from the new-born star will create a sphere of ionized H (H II) surrounding the young star.
  • As the ionized Hydrogen recombines with the free electrons in a high energy state, the red Balmer-alpha light will be emitted.
Figure 20-5
An emission nebula around hot young stars
  • As time passes, the H will disperse and the star will be isolated.
  • Photo of the Rosette Nebula, showing a cluster of young stars.
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Detection of Neutral Hydrogen

  • Electrons and Protons have a property similar to spin.
  • It's possible to put one electron into the ground state of Hydrogen in two different ways:
    • Electron spin parallel to the Proton spin
    • Electron spin anti-parallel to the Proton spin
  • The ground state of Hydrogen is actually two different energy states.
  • Spins parallel is a slightly higher energy level than spins anti-parallel.
  • If H atom happens to have electron in spin parallel state, the electron will spontaneously "flip" over to the anti-parallel state and emit a photon of energy.
  • Since the energy levels are very close the energy of the photon is low, and the wavelength is large, wavelength = 21 cm (radio waves).
  • We can detect this radiation using a radio telescope.
  • Astronomers call a region of neutral Hydrogen a H I region.
Energy Levels of H Spin of particles

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  • The Interstellar Medium also has dense, cool regions where molecules can be found called Molecular Clouds.
  • Temperatures less than 100 K.
  • Molecules detected include: H2, CO, H2O, C2H5OH and other complex organic molecules.
  • Density of molecules in this room: 2 x 1025 molecules per cubic metre.
  • Density of molecules in a molecular cloud: 109 molecules per cubic metre. (Dense by astronomical standards, but not by Earth standards.)
  • Molecules have energy levels:
    • When an electron jumps from one energy level to another a UV photon can be emitted or absorbed which a UV telescope can detect.
    • When the molecule's spin changes a radio frequency photon can be emitted which a radio telescope can detect.
  • We can detect molecules by using a radio telescope tuned to the radio waves emitted by the molecules.
  • Unfortunately, molecular Hydrogen, H2, doesn't emit radio waves.
  • Usually astronomers try to detect CO instead.
  • CO emits at a wavelength of 2.6 mm.
  • Typically there are 10,000 H2 molecules for each CO molecule.

A Molecular Cloud in the Constellation Orion

Figure 20-19
Radio "photo" showing CO emission
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  • The interstellar medium is also full of dust.
  • Dust grains are solid conglomerates of many molecules and atoms including graphite, silicates, iron, ices...
  • Typical size of a dust grain is about 10-6m similar to the wavelength of visible light.

Scattering of Light by Dust

  • Long wavelength waves can easily diffract around a small barrier. (Example: water waves move around a grain of sand in a lake.)
  • Short wavelength waves are stopped by a large barrier. (Example: water waves blocked by an island.)
  • All visible wavelength light tends to get scattered by dust grains, but blue light is scattered more by the dust than the red light.
  • the blue light, but the red light will remain. The light will look redder than if no dust existed.
  • When we look directly at the light source through a dust cloud and see that it looks redder than expected, we call this interstellar reddening.

Blue Light is reflected by dust while the red light is transmitted.

Figure 20-5a
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Reflection Nebula

  • The blue light gets reflected by the dust, so an observer in another direction will see blue light.
  • A reflection nebula is a thin cloud of dust which shines by the reflection of light from a star.
  • The Pleiades star cluster is surrounded by a reflection nebula.
Figure 20-18a
The Pleiades Star Cluster
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Interstellar Reddening

  • Interstellar Reddening can be seen in the photo below.
  • Both red nebulae are emission nebulae glowing due to Hydrogen fluorescence.
  • The nebula on the right looks pinker than the nebula on the left.
  • The nebula on the right looks "pink" because there is more blue light than the left nebula.
  • The left nebula is 7200 pc away from us.
  • The right nebula is 2400 pc away from us.
  • Light emitted from the left nebula has to travel through more dust than light emitted by the right nebula.
  • Since light from the left nebula has travelled through more dust, more blue light has been lost and the colour has been reddened.
Figure 20-5b
Interstellar Reddening of emission nebulae
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Reflection and Reddening by the Earth's Atmosphere

  • The Earth's atmosphere is composed of Oxygen and Nitrogen molecules which scatter light.
  • The molecules are much smaller than dust grains, but they also prefer to scatter blue light over red light.
  • When you look at the sky in a direction different from where the Sun is, you see light reflected from a molecule.
  • Since blue light is reflected more often than red, the light from the sky looks blue.
  • When you look at the Sunset, you are looking directly at the Sun through the atmosphere.
  • Blue light has been reflected out of the direction we are looking in, so we see reddened light.
Box 5-4a Box 5-4b
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Infrared Emission by Dust

  • Dust is typically quite cool. Typical temperatures are around 100 K.
  • If dust emits light as a blackbody, its peak wavelength will be at 29 x 10-6 m, which is infrared light.
  • An infrared telescope can detect the emission from dust clouds.
  • The IR picture below (left) shows infrared emission from dust clouds.
  • Using visible light, we would only detect the stars in the diagram on the right.
Figure 20-6a Figure 20-6b

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Dark Nebulae

  • If a cool mixture of molecules and dust is much denser than a reflection nebula, it will instead completely block out light.
  • A very dense cloud of molecules and dust which is opaque is called a dark nebula.
  • We can only detect a dark nebula if it lies in front of stars or an emission nebula.
A dark nebula: Figure 20-3

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The Interstellar Medium (True colour portrait)

  • Red = ionized hydrogen
  • Black = cool dense molecular clouds
  • Blue = reflection nebulae
Figure 12.4

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The Region near Alnitak in Orion shows all type of nebulae

  • Alnitak is the left-most belt star in Orion.
  • This region shows examples of all the types of nebulae discussed so far!
Figure 20-2 Figure 20-1

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The Local Interstellar Cloud (Artist's Diagram)

  • This diagram shows the location of gas clouds within 10 light-years of the Sun.
  • The purple colour represents a denser region called the local interstellar cloud.
  • The Sun will exit the local interstellar cloud in about 10,000 years.
  • The local interstellar cloud is moving away from a group of young O and B stars called the Scorpius-Centaurus Association.

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The Sun's Neighbourhood (1500 light-years across)

  • This diagram shows the location of the region near the Sun within 1500 light-years.
  • The thin purple strips include the local interstellar cloud.
  • The darkest regions have the lowest density of any type of gas.
  • The Sun has been travelling for the last several million years through the Local Bubble a low density region of the interstellar medium.
  • The orange regions are cold, dense molecular clouds
  • The Gum Nebula (green) is a complex region of ionized hydrogen.

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The Heliosphere

  • The Sun's magnetic field and the Solar Wind produce a region about 100 AU in radius around the Sun called the heliosphere.
  • Most charged particles from the interstellar medium are deflected from this region, protecting the Solar system.
  • The size of the heliopause is defined by a balance between the force of the Solar wind and the gas pressure of the interstellar medium.
  • In about 50,000 years we may enter a denser region of the interstellar region, which may push the heliosphere inwards.
  • If the heliosphere were to shrink to a size smaller than the Earth's orbit, charged particles could harm life on Earth.


Next lecture: Star Formation
Read Chapter 18, pages 477-490