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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 ?

2 2

Stars Dark Matter Interstellar Gas & Dust

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 3

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.”


Part I: Interstellar clouds


The easiest way to see interstellar matter is to observe the dark clouds along the Milky Way

Band of light: unresolved stars

Dark clouds of interstellar gas & dust




Band of light: unresolved stars

Dark clouds of interstellar gas & dust


•  Interstellar clouds often called nebulae

•  Many types of nebulae exist (emission, reflection, dark, planetary)



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)


Dark Clouds Coal Sack (next to the Southern Cross) “visible” with naked eye Really seeing its shadow (absence of light from background stars)


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


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


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)


Supernova Remnant Name: N63A Final state of stars many times more massive than the Sun Leftover material from supernova explosion


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…

Spectroscopy


Modern telescopes use diffraction gratings instead of prisms to split up the light

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


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.

•  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)

Multiple interstellar clouds can exist along a line of sight through the Galaxy

Andrew Fox, Hubble Science Briefing, April 2012 20 courtesy Bart Wakker, UW-Madison

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


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


Galactic disk of ionized hydrogen, thickness of ~1000 parsecs Andrew Fox, Hubble Science Briefing, April 2012 23

courtesy Matt Haffner

All-sky 21 cm map of ionized hydrogen (Galactic coordinates)

How empty is the Diffuse Interstellar Medium?

Object Density (particles per cm3)

Water ~1022 (H2O molecules)

Earth’s atmosphere 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 (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

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)

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


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)


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

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 29

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)


Interstellar dust in Andromeda (M31)


Infra-red (IR) emission maps are used to trace the interstellar dust in other galaxies

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


- 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:


Questions?


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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