Between the Stars: Gas

1
Between the StarsGas Dust in Space
2
Gas and Dust in Space

• To understand how stars form, we need to know the
  raw material from which they are made
• All the gas and dust material that lies in the
  region between stars is referred to as
  interstellar matter
• The entire collection of interstellar matter is
  called the interstellar medium
• The interstellar medium accounts for a large
  fraction of the atoms in the universe (gt50)
• and provides the raw material for new stars
• Clouds of interstellar gas or dust that are seen
  to glow with visible light or infrared radiation
  are usually called nebulae (the Latin for
  "clouds)
• Interstellar gas and dust can produce colorful
  displays when lit by the light of nearby stars

3
Interstellar Medium

• About 99 of the interstellar matter is in the
  form of gas (individual atoms or molecules)
• The most abundant elements in the interstellar
  gas are hydrogen (75) and helium (25)
• The remaining 1 of interstellar matter is in the
  form of solid interstellar dust grains
• The density of interstellar matter is very low
• It has 103 atoms per cubic centimeter (cc)
• Air has 1019 atoms per cc
• The best vacuum created on
  Earth has 107 atoms per cc
• The volume of space occupied
  by interstellar matter is huge
• Consequently, the total mass of
  interstellar matter is
  humongous

4
Interstellar Gas

• The color of a gas gives us clues about
  its temperature and composition
• The red color commonly seen in
  interstellar gas comes from
  ionized hydrogen, or
  H II
• The proton recombines with an electron
  which then moves down to the
  lowest-energy orbit by emitting a red-wavelength
  photon
• H I refers to a neutral
  hydrogen, and Fe
  III
  a doubly ionized iron

type of region temperature (K)
HI cold clouds 100
HI warm clouds 5000
hot gas 500,000
HII regions 10,000
giant molecular clouds 10
5
H II Regions

• These regions have temperatures near 104 K,
  heated by nearby stars
• The ultraviolet light from hot O and B stars
  ionizes the surrounding hydrogen gas
• The free electrons recombine with protons,
  forming excited H atoms
• Excited states emit light
• The red glow is characteristic
  of hydrogen (the red Balmer
  line)

6
H II Regions Dusty Nebulae in Sagittarius
Constellation

• The red glow that dominates this image is
  produced by the red Balmer line of hydrogen
• This indicates that there are hot stars nearby
  that ionize these clouds of gas

7
Absorption Lines

• Most of the interstellar medium is cold and hence
  not ionized
• Mostly hydrogen and helium
• Other atoms and molecules are also seen Ca, Na,
  CN, CH, H2, CO
• The cool gas between the Earth and the stars will
  cause an absorption spectrum

8
Neutral-Hydrogen Clouds

• Vast clouds of neutral-hydrogen (H I) gas are
  cold and, therefore, do not emit strong (visible)
  radiation
• The first evidence for absorption by interstellar
  clouds in H I regions came from the analysis of
  spectroscopic binary stars
• binaries doppler shift moves
  spectral lines
• some lines don't move
• reason absorption lines
  in gas between binary
  pair and Earth

9
The Hydrogen 21-cm Line

• Hydrogen proton (p) plus electron (e)
• Both p and e have spin "up" or "down"
• Ground spin-state p up, e down
• Excited spin-state p up, e up
• The electron can move between the spin states by
  emitting or absorbing a photon
• The photon has a wavelength of 21 cm, a radio wave

10
21-cm Line From Cold H-I Regions

• The spin flip in hydrogen was predicted to
  produce 21-cm-long radio waves
• The prediction was confirmed by observation in
  1951 using radio telescopes
• This indicates that neutral-hydrogen clouds must
  be cold, having temperatures of about 100 K
• Most of cold hydrogen is confined to a very flat
  layer (less than 300-LY thick) that extends
  throughout the disk of the Milky Way Galaxy

11
Ultra-Hot Interstellar Gas

• Astronomers were surprised to discover hot
  interstellar gas, even though there was no
  visible source of heat nearby
• The hot temperatures are about 1 million degrees
  K!
• We now understand that the gas is heated by
  supernovae, the explosions of massive stars
• This topic will be discussed in Ch. 22

12
Cosmic Dust

• There are dark regions on the sky that are
  seemingly empty of stars
• But they are not voids, but clouds of dark dust
• The dust betrays its presence by
• blocking the light from distant stars
• reflecting the light from nearby stars
• making distant stars look redder and fainter than
  they really are
• Each dust particle has a rocky core that is
  either sootlike (carbon-rich) or sandlike
  (containing silicates) and a mantle made of icy
  material

13
Blue Sky Red Sunset

• Blue light is scattered more
  easily than red
• because red wavelengths are
  longer than blue
• The blue colors in sunlight are
  scattered repeatedly by
  molecules in
  the air, and this
  makes our sky look blue
• Seen directly, the Sun looks
  yellowish, as the light
  from it is
  missing some of its blue
• At sunrise or sunset, the Sun
  appears redder than at noon
  because the
  light from it
  travels a longer path through
  the air than at
  noon and
  hence is missing more of its
  blue

14
Scattering of Light by Cosmic Dust

• Interstellar dust particles are
  very small, about the same
  size as the
  wavelength of
  visible light
• The particles scatter blue light
  more efficiently than
  red light,
  thereby making distant stars
  appear redder
  and giving
  
  clouds of dust
  near stars a
  bluish hue

15
Reflection Nebulae

• Some dense clouds of dust are close to luminous
  stars and scatter enough starlight to become
  visible
• Such a cloud is called a reflection nebula
  because the light that we see from it is
  starlight reflected off grains of dust
• Since dust grains are
  tiny, they scatter light
  with blue
  wavelengths
  better than light with
  red wavelengths
• As a result, a reflection
  nebula usually appears
  bluer than its
  illuminating
  star

A reflection nebula (NGC 1999), illuminated by a
star, which is visible just to the left of center
16
Trifid Nebula in Sagittarius Constellation

• It is about 3000 LY from the Sun, and about 50 LY
  in diameter
• The reddish H-II region is surrounded by a blue
  reflection nebula

17
The Dust Filaments in the Trifid Nebula are due
to debris from supernovae
18
Dust Glows in the Infrared
infrared
visible
19
Visible and Infrared Images of
Horsehead Nebula in Orion
20
Dust Pillar
very bright star blowing dust off of a star near
the pillar's tip
21
(No Transcript)
22
Cosmic Rays

• These are particles that travel through
  interstellar space at a typical speed of 90 the
  speed of light
• The most abundant elements in cosmic rays are the
  nuclei of hydrogen and helium
• Positrons (anti-electrons) are also found
• Many cosmic rays are probably produced in
  supernova explosions