Interstellar Space
Not as Empty as You Might Think
Dr. Andrew Fox
Space Telescope Science Institute/European Space Agency
Hubble Science Briefing
April 5 2012
What is a galaxy made of ?
Stars
Dark
Matter
Interstellar
Gas & Dust
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Presentation Outline
INTERSTELLAR MATTER
- how do we detect it?
- what forms does it take, and what’s its composition?
- how empty is interstellar space (density)?
- effects on starlight passing through it (reddening)
- importance to galaxies overall (role in galactic evolution)
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Andrew Fox, Hubble Science Briefing, April 2012
A Historical Note….
• 1626 First recorded use of the word “interstellar”, by Francis Bacon:
“The Interstellar Skie.. hath .. so much Affinity with the Starre, that
there is a Rotation of that, as well as of the Starre.”
• 1674 Suggestion that interstellar space was not empty, by Robert Boyle:
“The inter-stellar part of heaven, which several of the modern
Epicureans would have to be empty.”
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Andrew Fox, Hubble Science Briefing, April 2012
Part I: Interstellar clouds
The easiest way to see interstellar matter is to observe
the dark clouds along the Milky Way
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Andrew Fox, Hubble Science Briefing, April 2012
Part I: Interstellar clouds
The easiest way to see interstellar matter is to observe
the dark clouds along the Milky Way
Dark clouds of interstellar gas & dust
Band of light: unresolved stars
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Andrew Fox, Hubble Science Briefing, April 2012
Part I: Interstellar clouds
The easiest way to see interstellar matter is to observe
the dark clouds along the Milky Way
Dark clouds of interstellar gas & dust
Band of light: unresolved stars
• Interstellar clouds often called nebulae
• Many types of nebulae exist (emission, reflection, dark, planetary)
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Andrew Fox, Hubble Science Briefing, April 2012
Dark Clouds
Barnard 68 in Ophiuchus
Why is it dark?
An empty region of
space? Or a dense
interstellar cloud
blocking the light from
the background stars?
(the latter)
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Andrew Fox, Hubble Science Briefing, April 2012
Dark Clouds
Coal Sack (next to
the Southern Cross)
“visible” with naked
eye
Really seeing its
shadow (absence of
light from
background stars)
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Andrew Fox, Hubble Science Briefing, April 2012
Emission Nebula
Eagle Nebula (M 16)
“Pillars of Creation”
Clouds of gas and dust
being heated and sculpted
by radiation from nearby
young star cluster
Traces regions of star
formation
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Andrew Fox, Hubble Science Briefing, April 2012
Reflection Nebula
IC 349
Shows reflected light
from a nearby star, not
light emitted by the
nebula itself
As if the star is shining a
flashlight on its
surroundings
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Andrew Fox, Hubble Science Briefing, April 2012
Planetary Nebula
Eskimo Nebula
Final state of solar-mass star
(after it runs out of fuel)
Gas irradiated by hot white
dwarf star in centre
Thought to be the eventual
fate of the Sun (in another 5
billion years)
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Andrew Fox, Hubble Science Briefing, April 2012
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Andrew Fox, Hubble Science Briefing, April 2012
Supernova Remnant
Name: N63A
Final state of stars many
times more massive than the
Sun
Leftover material from
supernova explosion
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Andrew Fox, Hubble Science Briefing, April 2012
Part II: Diffuse interstellar gas
(not seen with naked eye)
Nebulae make up a tiny fraction of the volume of
interstellar space.
Diffuse gas exists between the nebulae, but you need a
spectrograph to see it…
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Andrew Fox, Hubble Science Briefing, April 2012
Spectroscopy
Modern telescopes use diffraction gratings instead of prisms to split up the light
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Andrew Fox, Hubble Science Briefing, April 2012
Spectroscopy: The Science of Rainbows
Pattern of lines in stellar spectrum indicates composition and velocity of the
star and the interstellar gas between the star and us.
Each element has its own set of spectral lines (“fingerprints”). If the star is
moving relative to the Earth, those lines will move by the Doppler effect
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Andrew Fox, Hubble Science Briefing, April 2012
Spectroscopic Binaries
Spectroscopic binary has two sets of lines (one from each star) moving
back and forth. Astronomers can measure the period and amplitude of
the shift.
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Andrew Fox, Hubble Science Briefing, April 2012
•
In 1904 German astronomer
Johannes Hartmann took a
spectrum of the spectroscopic
binary star delta Orionis (Mintaka)
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He found three sets of lines, two
moving and one staying still.
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“these sharp lines probably did not
have their origin in the [star] itself,
but in a nebulous mass lying in the
line of sight”
Telescope with
spectrograph
Diffuse Interstellar Cloud
Containing Ionized Calcium
(spectral lines stay same color)
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Binary Star
Delta Orionis
(lines become redder and bluer)
Andrew Fox, Hubble Science Briefing, April 2012
Multiple interstellar clouds can exist along a
line of sight through the Galaxy
courtesy Bart Wakker, UW-Madison
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Andrew Fox, Hubble Science Briefing, April 2012
Hydrogen atom
In a hydrogen atom, the proton and electron
normally spin in the same direction.
Occasionally the electron flips to spin the
other direction. Happens only about once
every 100 million years for each atom.
Radio telescope
When the electron flips it emits a radio wave
with a frequency of 1420 MHz and a
wavelength of 21 cm (was predicted in 1944
by Dutch astronomer Hendrik van de Hulst)
21 cm emission from interstellar space first
detected in 1951
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Andrew Fox, Hubble Science Briefing, April 2012
All-sky 21 cm map of neutral hydrogen (Galactic coordinates)
Galactic disk of neutral hydrogen, thickness of several hundred parsecs
→ The Milky Way is full of diffuse interstellar gas radiating radio waves
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Andrew Fox, Hubble Science Briefing, April 2012
All-sky 21 cm map of ionized hydrogen (Galactic coordinates)
Galactic disk of ionized hydrogen, thickness of ~1000 parsecs
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courtesy Matt Haffner
Andrew Fox, Hubble Science Briefing, April 2012
How empty is the Diffuse Interstellar Medium?
Object
Density (particles per cm3)
Water
Earth’s atmosphere
~1022
(H2O molecules)
5 x 1019
(mostly N2 & O2 molecules)
Vacuum Cleaner
~1019
Incandescent Light Bulb
~1014-1015
Best vacuum ever produced on Earth
~105-107 (cryopumped chamber)
Giant Molecular Clouds
~102-106 (mostly molecular hydrogen)
Diffuse Interstellar Medium
~1
Diffuse Intergalactic Medium
~10-5
(mostly atomic and ionized hydrogen)
The diffuse interstellar medium is about
50 million trillion times less dense than the air we breathe
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Andrew Fox, Hubble Science Briefing, April 2012
Part III: Interstellar dust
• “Dust” means small solid
particles (silicates and carbonate
chemicals), rather than gaseous
atoms or molecules
• Dust makes up only about 1% of
the mass of interstellar matter
(the rest is gas)
• Dust causes interstellar
extinction (scattering of starlight
out of the beam)
• Dust changes the colour of
starlight passing through it
(interstellar reddening)
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Andrew Fox, Hubble Science Briefing, April 2012
The Blue-Sky Effect
Blue light is scattered
toward us
Red light passes
straight through
Earth’s atmosphere
EARTH
Sun
ATMOSPHERE
Not to Scale
Here the scattering is caused by molecules in the Earth’s atmosphere
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Andrew Fox, Hubble Science Briefing, April 2012
Interstellar Extinction
(Blue Sky Effect viewed from different angle)
Red light passes
straight through
Blue light is scattered
out of beam
STAR
OBSERVER
INTERSTELLAR CLOUD
CONTAINING DUST
• Here the scattering is caused by interstellar dust grains
• The more interstellar gas along the sight line, the more reddening occurs
• Distant stars appear redder than nearby ones
• Astronomers have to correct (de-redden) a stellar spectrum to account for
this and to derive the star’s true color.
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Andrew Fox, Hubble Science Briefing, April 2012
Interstellar dust
• As well as scattering visible light, dust emits infra-red and microwave radiation
Horsehead Nebula (Barnard 33) at different wavelengths
• Interstellar clouds are often opaque to optical (visible) light but transparent to
infrared and radio light
• These wavelengths open new windows to studying interstellar gas
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Andrew Fox, Hubble Science Briefing, April 2012
Planck is a microwave satellite designed to measure the leftover radiation from the Big Bang.
To Planck, interstellar dust is a foreground source of contamination (noise).
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Andrew Fox, Hubble Science Briefing, April 2012
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Andrew Fox, Hubble Science Briefing, April 2012
NASA Press Release
June 2011
• Centaurus A
(radio galaxy with
active galactic nucleus)
• Imaged with
Hubble’s Wide Field
Camera 3
• Numerous dust lanes
• Star formation in red
(H-alpha emission)
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Andrew Fox, Hubble Science Briefing, April 2012
Interstellar dust in Andromeda (M31)
Infra-red (IR) emission maps are used to trace the interstellar dust in other galaxies
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Part IV: Interstellar gas and importance to galaxy evolution
INTERSTELLAR
GAS
Interstellar clouds are the start and end points of a star’s life.
Dying stars release heavy elements back into interstellar space, which becomes
richer and richer in heavy elements over time (its metallicity goes up)
All the heavy elements in the Earth were made in stars, then spent time in
interstellar space before the Solar System formed
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Andrew Fox, Hubble Science Briefing, April 2012
Summary: Interstellar space …..
is not completely empty. It:
- contains many different types of nebulae
- contains diffuse gas and dust
- can be studied with spectroscopy at many
wavelengths
- changes color of starlight passing through it
- plays a key part in the life cycle of galaxies
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Andrew Fox, Hubble Science Briefing, April 2012
Questions?
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Andrew Fox, Hubble Science Briefing, April 2012
ESA Video: Andromeda (M31) at multiple wavelengths:
http://www.esa.int/esa-mmg/mmg.pl?type=V&single=y&mission=Herschel&start=1&size=b
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Andrew Fox, Hubble Science Briefing, April 2012