Interstellar Gas

3. Gas Absorbing Light In addition to dust grains, there is low-density gas between the stars. This gives extra absorption lines in the spectrum of a star. But how do we distinguish the lines caused by interstellar gas from those caused by the star’s own atmosphere?

Stellar Spectra: a Reminder

What is Our Goal? We want to determine the total amount and composition of the gas between the stars. (In the end, we find that the abundances are like that for the material in the stars!)

How to Detect It You may detect a common feature in the spectra of a lot of different stars in some direction. This suggests the presence of an interstellar cloud of gas.

A Clear Diagnostic The absorption lines in a star’s spectrum are wide because of the star’s heat. (Individual atoms are rushing to and fro at various speeds, with varied small Doppler shifts.) The I/S lines are narrow because the gas between the stars is so cold.

Notice the Very Narrow Lines

Some Real Spectra These spectra tell us that there are free-floating Calcium atoms in the ISM Other common atomic species are found as well

4. Gas Emitting Light Sometimes very conspicuous, like celestial neon lights! Sometimes very conspicuous, like celestial neon lights!

“Fluorescence” The gas absorbs energy and emits visible light.

The Source of Energy There are two ways of inputting energy into a cloud of gas. First, collisions: 1. Material rushing out (perhaps from an exploding star) collides with the gas and rips off electrons from the atoms. Again, when they later recombine, visible light is emitted.

Very Familiar Very Familiar This is what happens in a fluorescent lamp or a neon tube. This is what happens in a fluorescent lamp or a neon tube. Electrons rushing through the lamp collide with the low- density gas within it. Electrons rushing through the lamp collide with the low- density gas within it.

Alternatively: Ultraviolet Light 2. A very hot star in the heart of the gas cloud gives off lots of ultraviolet light. This rips off electrons, ‘ionizing’ the gas. When the electrons subsequently recombine with the atoms, light is given off.

‘Repackaged’ Energy The visible glow of the Orion nebula is really just ultraviolet starlight that’s been‘repackaged.’ The energy originates in the hot star.

How About Other Wavelengths? In general, interstellar gas is quite cool and emits no visible light (except in clouds surrounding hot stars). So we need to study it at other (longer) wavelengths, using lower-energy photons.

Radio Astronomy! Holland,1944

Van de Hulst A famous predictionre-enacted

The Underlying Physics

The Unlikelihood Such ‘spontaneous’ transitions are very unlikely. If unperturbed, a randomly chosen hydrogen atom would sit in interstellar space for 10 7 years – yes, that’s 10 million years! -- before undergoing this process. Doesn’t this mean that there’d be very few photons produced?

The Saving Grace The galaxy contains an enormous amount of hydrogen gas, multiplying the chances. Moreover, occasional ‘bumps’ between atoms can spark the transition. So at any given instant, there are huge numbers of these 21-cm photons being emitted. Our radio telescopes pick this up very strongly. This is one of the most important ways in which we study the universe!

One Great Use: Making Maps Imagineyourselfhere:

Under These Conditions

Wanting to Construct This

The Second Saving Grace The long wavelength (21 cm) means that the photons pass unimpeded through the gas and dust in the Milky Way. (To them, interstellar space is nearly transparent!) So we can map out the entire structure of our galaxy!

Where Is the Hydrogen Gas? The bright features indicate where the photons come from. We see spiral structure, in a galaxy that is about 100,000 light years across!

Last But Not Least: Interstellar Molecules An important distinction: simple molecules contain at most a few hundred atoms! simple molecules contain at most a few hundred atoms! the dust grains, although very tiny in human terms, each contain trillions of atoms: much bigger! (We have already considered those.) the dust grains, although very tiny in human terms, each contain trillions of atoms: much bigger! (We have already considered those.)

They Weren’t Expected! In molecules, the bonds between atoms are feeble. Any simple molecule will be quickly ripped apart by energetic photons moving through the emptiness of space. Consequently, fifty years ago, no astronomers expected to find evidence of any complex molecules ‘between the stars.’

How Might We Detect Them? In molecules, atoms are held together by electical forces felt between electons and nuclei. But a molecule is not static: it ‘jiggles’ around (vibrates) and ‘tumbles’ (rotates). How Might We Detect Them? In molecules, atoms are held together by electical forces felt between electons and nuclei. But a molecule is not static: it ‘jiggles’ around (vibrates) and ‘tumbles’ (rotates). Vibrating Molecules (Cartoons) Vibrating Molecules (Cartoons)

How Do They Emit Light? A rapidly-rotating molecule (visualize a drum major’s baton) may change from a state of rapid rotation to one of slower rotation, losing energy. The lost energy shows up in the form of a photon. (The same holds true for energy lost when a molecule slows down in its vibration.)

These are Quantized Transitions Remember: many rotation rates are possible, but not all. (As in a food blender: only certain speeds can be selected!) In a molecule, slowing from one allowed speed to another gives off light of a fixed, determined energy (wavelength).

An Example: Hydrogen Chloride (gives off light of the frequencies shown, not any other!)

A Surprising Discovery! Decades ago, the first very simple molecules were found in the Interstellar Medium: OH, NH 3, H 2 O, CO, etc. Later, many very complex molecules were discovered, including PAHs (polyaromatic hydrocarbons), amino acids, etc.

For Example

…a Long and Growing List!

How Can They Survive? The molecules are deep within GMCs (giant molecular clouds), up to 10 5 – 10 6 x the mass of the sun. These clouds are quite cool. In the deep interior, molecules can form, and are shielded from potentially harmful collisions with photons streaming through space.

Importance for Life? Molecules of amino acids, the building blocks of proteins, have been detected in such locations. Is this related to the later emergence of life in the universe? (Probably not in any direct way, since most of the gas later condenses into stars which are hot enough to rip the molecules apart!)

Final Conclusions The abundance of the elements in interstellar space reflects the distribution of material in the cosmos as a whole. (No big surprise.) The heavier elements and the grains are the products of a recurrent cycle of star formation, life, and death, with recycling of material.