Chapter 14: The Milky Way

Johannes Franz Hartmann (1865 – 1936)

Interstellar material includes gas (that is, atoms and molecules), microscopic dust grains, and possibly larger objects. Atoms of interstellar gas were discovered in 1904 when German astronomer Johannes Hartmann detected their absorption lines. While studying the spectrum of a binary star system, he accidentally discovered absorption lines caused by ionized interstellar calcium atoms that are between the Earth and this binary system. Absorption lines due to other elements, like sodium, were soon discovered. Since these lines had different Doppler shifts than the stars in whose spectra they appear, the atoms that cause the absorption must lie in the space between stars.

The composition of interstellar space.

An electron orbiting a proton with parallel spins (pictured) has higher energy than if the spins were anti-parallel.

The most important tracer of interstellar gas is a spectral line that is emitted at radio wavelengths. In 1944, the Dutch astronomer H. C. van der Hulst predicted that cold hydrogen gas would have a spectral feature at a wavelength of 21-cm, caused by a change in the spin of hydrogen atoms' electrons. A low-energy radio photon is emitted when the atom flips" between two nearly equal quantum states. Harvard astronomers detected the predicted emission in 1951 using radio equipment. The 21-cm emission line of atomic hydrogen is extremely important in astronomy for several reasons. First, the widespread detection of this line confirms that hydrogen is the main constituent of interstellar gas. Second, the long wavelength and low energy of the spectral line is a sign that the hydrogen is at a very low temperature (cold material will emit long wavelength spectral features). The 21-cm emission comes from gas as cold as 10 or 20 K. Last, the long wavelength of this radiation means that it can penetrate much greater distances through the interstellar gas and dust than ordinary, visible wavelengths of light.

 

Heliosphere and Interstellar Medium. Shows the current position of Voyager space probes in logarithmatic scale.

Star forming region NGC 3603 showing young stars surrounded by gas and dust

With a composition of silicates, metals, and ices, interstellar grains resemble the dust in our primordial Solar System. Intriguingly, reactions initiated by ultraviolet light in these materials can create complex organic molecules on grain surfaces. These microscopic particles are tiny laboratories that play an important but still poorly understood role in interstellar chemistry. Even though the tiny dust grains are rare compared to a hydrogen atom, they have a big effect on starlight.

Star forming region NGC 2264, also known as the Cone Nebula

The distribution of gas and dust in our galaxy is responsible for blocking our visible light view through the galactic disk. In other (typically spiral) galaxies this material can form opaque "dust lanes," that while beautiful are scientifically annoying because of what they prevent us from seeing. The study of interstellar materials is a still young area of research and new molecules are still being discovered, and new ways of compensating for the light the gas and dust blocks are still being mapped.