1. Interstellar Space Not as Empty as You Might Think
Dr. Andrew Fox
Space Telescope Science Institute/European Space Agency
Hubble Science Briefing
April

2. What is a galaxy made of?
Stars
Dark Matter
Interstellar Gas & Dust 2

3. 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

4. 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

5. Part I: Interstellar clouds
The easiest way to see interstellar matter is to observe the dark clouds along the Milky Way
Andrew Fox, Hubble Science Briefing, April 2012

6. 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
Andrew Fox, Hubble Science Briefing, April 2012

7. 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)
Andrew Fox, Hubble Science Briefing, April 2012

8. 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)
Andrew Fox, Hubble Science Briefing, April 2012

9. Dark Clouds Coal Sack (next to the Southern Cross)
“visible” with naked eye
Really seeing its shadow (absence of light from background stars)
Andrew Fox, Hubble Science Briefing, April 2012

10. 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
Andrew Fox, Hubble Science Briefing, April 2012

11. 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
Andrew Fox, Hubble Science Briefing, April 2012

12. 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)
Andrew Fox, Hubble Science Briefing, April 2012

13. Andrew Fox, Hubble Science Briefing, April 2012

14. Supernova Remnant Name: N63A
Final state of stars many times more massive than the Sun
Leftover material from supernova explosion
Andrew Fox, Hubble Science Briefing, April 2012

15. 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…
Andrew Fox, Hubble Science Briefing, April 2012

16. Spectroscopy
Modern telescopes use diffraction gratings instead of prisms to split up the light
Andrew Fox, Hubble Science Briefing, April 2012

17. 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

18. 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.
Andrew Fox, Hubble Science Briefing, April 2012

19. He found three sets of lines, two moving and one staying still.
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”
Telescope with Diffuse Interstellar Cloud Binary Star
spectrograph Containing Ionized Calcium Delta Orionis
(spectral lines stay same color) (lines become redder and bluer)
Andrew Fox, Hubble Science Briefing, April 2012

20. Multiple interstellar clouds can exist along a line of sight through the Galaxy
courtesy Bart Wakker, UW-Madison
Andrew Fox, Hubble Science Briefing, April 2012

21. The 21 cm (radio) line of neutral hydrogen
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
Hydrogen atom
Radio telescope
Andrew Fox, Hubble Science Briefing, April 2012

22. 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
Andrew Fox, Hubble Science Briefing, April 2012

23. All-sky 21 cm map of ionized hydrogen (Galactic coordinates)
Galactic disk of ionized hydrogen, thickness of ~1000 parsecs
courtesy Matt Haffner
Andrew Fox, Hubble Science Briefing, April 2012

24. How empty is the Diffuse Interstellar Medium?
Object
Density (particles per cm3)
Water
~ (H2O molecules)
Earth’s atmosphere
5 x (mostly N2 & O2 molecules)
Vacuum Cleaner
~1019
Incandescent Light Bulb ~
Best vacuum ever produced on Earth
~ (cryopumped chamber)
Giant Molecular Clouds
~ (mostly molecular hydrogen)
Diffuse Interstellar Medium
~1 (mostly atomic and ionized hydrogen)
Diffuse Intergalactic Medium
~10-5
The diffuse interstellar medium is about
50 million trillion times less dense than the air we breathe
Andrew Fox, Hubble Science Briefing, April 2012

25. 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)
Andrew Fox, Hubble Science Briefing, April 2012

26. The Blue-Sky Effect Not to Scale
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
Andrew Fox, Hubble Science Briefing, April 2012

27. Interstellar Extinction
(Blue Sky Effect viewed from different angle)
Red light passes straight through
Blue light is scattered out of beam
OBSERVER
INTERSTELLAR CLOUD CONTAINING DUST
STAR
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.
Andrew Fox, Hubble Science Briefing, April 2012

28. Horsehead Nebula (Barnard 33) at different wavelengths
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
Andrew Fox, Hubble Science Briefing, April 2012

29. 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 2012

30. Andrew Fox, Hubble Science Briefing, April 2012

31. Andrew Fox, Hubble Science Briefing, April 2012

32. 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)
Andrew Fox, Hubble Science Briefing, April 2012

33. Interstellar dust in Andromeda (M31)
Infra-red (IR) emission maps are used to trace the interstellar dust in other galaxies
Andrew Fox, Hubble Science Briefing, April 2012

34. 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
Andrew Fox, Hubble Science Briefing, April 2012

35. 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
Andrew Fox, Hubble Science Briefing, April 2012

36. Questions?
Andrew Fox, Hubble Science Briefing, April 2012

37. ESA Video: Andromeda (M31) at multiple wavelengths: http://www. esa
Andrew Fox, Hubble Science Briefing, April 2012