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Between the Stars: Gas

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... dust that are seen to glow with visible light or infrared ... The red glow that dominates this image is produced by the red Balmer line of hydrogen ... – PowerPoint PPT presentation

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Title: 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
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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
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