Not as Empty as You Might Think Dr Andrew Fox Space Telescope Science InstituteEuropean Space Agency Hubble Science Briefing April 5 2012 What is a galaxy made of 2 2 Stars Dark Matter Interstellar Gas amp Dust ID: 321397
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Slide1
Interstellar Space
Not as Empty as You Might Think
Dr. Andrew FoxSpace Telescope Science Institute/European Space Agency Hubble Science BriefingApril 5 2012Slide2
What is a galaxy made of?
2
2
StarsDark Matter
Interstellar Gas & DustSlide3
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)
Andrew Fox, Hubble Science Briefing, April 2012
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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.”
Andrew Fox, Hubble Science Briefing, April 2012
4Slide5
Part I: Interstellar clouds
Andrew Fox, Hubble Science Briefing, April 2012
5The easiest way to see interstellar matter is to observe the dark clouds along the Milky WaySlide6
Band of light: unresolved stars
Dark clouds of interstellar gas & dust
Part I: Interstellar clouds
Andrew Fox, Hubble Science Briefing, April 2012
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The easiest way to see interstellar matter is to observe the dark clouds along the Milky WaySlide7
Band of light: unresolved stars
Dark clouds of interstellar gas & dust
Part I: Interstellar clouds
Interstellar clouds often called nebulae
Many types of nebulae exist (emission, reflection, dark, planetary)
Andrew Fox, Hubble Science Briefing, April 2012
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The easiest way to see interstellar matter is to observe the dark clouds along the Milky WaySlide8
Dark Clouds
Barnard 68 in OphiuchusWhy is it dark?
An empty region of space? Or a dense interstellar cloud blocking the light from the background stars?(the latter)Andrew Fox, Hubble Science Briefing, April 2012
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Dark Clouds
Coal Sack (next to the Southern Cross)“visible” with naked eyeReally seeing its shadow (absence of light from background stars)
Andrew Fox, Hubble Science Briefing, April 2012
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Emission Nebula
Eagle Nebula (M 16)“Pillars of Creation”Clouds of gas and dust being heated and sculpted by radiation from nearby young star clusterTraces regions of star formation
Andrew Fox, Hubble Science Briefing, April 2012
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Reflection Nebula
IC 349Shows reflected light from a nearby star, not light emitted by the nebula itselfAs if the star is shining a flashlight on its surroundings
Andrew Fox, Hubble Science Briefing, April 2012
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Planetary Nebula
Eskimo NebulaFinal state of solar-mass star (after it runs out of fuel)Gas irradiated by hot white dwarf star in centreThought to be the eventual fate of the Sun (in another 5 billion years)
Andrew Fox, Hubble Science Briefing, April 2012
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Andrew Fox, Hubble Science Briefing, April 2012
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Supernova Remnant
Name: N63A
Final state of stars many times more massive than the SunLeftover material from supernova explosionAndrew Fox, Hubble Science Briefing, April 2012
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Part II: Diffuse interstellar gas (not seen with naked eye)
Andrew Fox, Hubble Science Briefing, April 201215
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…Slide16
Spectroscopy
Andrew Fox, Hubble Science Briefing, April 201216
Modern telescopes use diffraction gratings instead of prisms to split up the light Slide17
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
Andrew Fox, Hubble Science Briefing, April 2012
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Spectroscopic Binaries
Andrew Fox, Hubble Science Briefing, April 2012
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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.Slide19
In 1904 German astronomer
Johannes Hartmann took a spectrum of the spectroscopic binary star delta Orionis (Mintaka)
He found three sets of lines, two moving and one staying still. “these sharp lines probably did not have their origin in the [star] itself, but in a nebulous mass lying in the line of sight
”
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Telescope with Diffuse Interstellar Cloud Binary Star
spectrograph Containing Ionized Calcium Delta
Orionis
(spectral lines stay same color) (lines become redder and bluer)Slide20
Multiple interstellar clouds can exist along a line of sight through the Galaxy
Andrew Fox, Hubble Science Briefing, April 2012
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courtesy Bart Wakker, UW-MadisonSlide21
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. 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
The 21 cm (radio) line of neutral hydrogen
Hydrogen atom
Radio telescope
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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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Galactic disk of ionized hydrogen, thickness of ~1000 parsecs
Andrew Fox, Hubble Science Briefing, April 201223
courtesy Matt Haffner
All-sky 21 cm map of ionized hydrogen (Galactic coordinates)Slide24
How empty is the Diffuse Interstellar Medium?
Object
Density (particles per cm3)Water
~1022 (H2O molecules)
Earth’s atmosphere
5 x 10
19
(mostly N
2
& O
2
molecules)
Vacuum Cleaner
~10
19
Incandescent Light Bulb
~10
14
-10
15
Best vacuum
ever produced on Earth
~10
5
-10
7
(
cryopumped
chamber)
Giant Molecular Clouds
~10
2
-10
6
(mostly
molecular
hydrogen)
Diffuse
Interstellar Medium
~
1
(mostly
atomic
and ionized hydrogen)
Diffuse Intergalactic Medium
~10
-5
Andrew Fox, Hubble Science Briefing, April 2012
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The diffuse interstellar medium is about
50 million trillion
times less dense than the air we breatheSlide25
Part III: Interstellar dust
Andrew Fox, Hubble Science Briefing, April 201225
“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
)Slide26
The Blue-Sky Effect
Red light passes straight through Earth’s atmosphere
Blue light is scattered toward us
Not to Scale
EARTH
ATMOSPHERE
Sun
Here the scattering is caused by molecules in the Earth’s atmosphere
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Red light passes straight through
Blue light is scattered out of beam
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.
STAR
OBSERVER
Interstellar Extinction
(Blue Sky Effect viewed from different angle)
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Interstellar dust
Andrew Fox, Hubble Science Briefing, April 201228
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 gasSlide29
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).
Andrew Fox, Hubble Science Briefing, April 201229Slide30
Andrew Fox, Hubble Science Briefing, April 2012
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Andrew Fox, Hubble Science Briefing, April 2012
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NASA Press Release
June 2011Centaurus
A (radio galaxy with active galactic nucleus)Imaged with Hubble’s Wide Field Camera 3
Numerous dust lanesStar formation in red (H-alpha emission)
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Interstellar dust in Andromeda (M31)
Andrew Fox, Hubble Science Briefing, April 2012
33Infra-red (IR) emission maps are used to trace the interstellar dust in other galaxiesSlide34
Part IV: Interstellar gas and importance to galaxy evolution
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
INTERSTELLAR GAS
Andrew Fox, Hubble Science Briefing, April 2012
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- 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
Summary: Interstellar space …..
is not completely empty. It:
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Questions?
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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 Andrew Fox, Hubble Science Briefing, April 2012
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