Atomic Theory - Testing in Space

Today, cosmologists believe that 13.7 billion years ago, there was nothing. No universe – no galaxies, no stars, no planets, no light, no space, and no time, nothing at all. Suddenly, for reasons that are still not understood, an astronomical explosion took place, and space and time began.

Figure 1. The Arecibo radio telescope on the island of Puerto Rico.

Part of the energy of this incredibly hot explosion was converted to primitive matter of quarks and electrons. The quarks quickly combined and within microseconds formed the first protons and neutrons in the universe, the elementary particles that form all of the atoms in the universe. As the new super-hot universe continued to expand, it also continued to cool, and within a few hundred seconds, it cooled from trillions of degrees to millions of degrees of temperature, which enabled some of the protons and neutrons to fuse and form the first light elements like helium, beryllium, and lithium. The fast expansion of the universe continued, and within 300,000 years, it cooled to a few thousand degrees, which allowed the electrons to combine with the protons to form the first neutral hydrogen atoms. Recently, using powerful space telescopes, astronomers have been able to map how the universe looked at that early epoch.

Figure 2. The radio spectrum of the ionized interstellar cloud S88 observed with the Arecibo radio telescope.

Within less than a billion years after the beginning, due to complex gravitational instabilities, matter separated into pieces that formed billions of galaxies. Stars were formed by the gravitational collapse of interstellar clouds that populate all galaxies. At first, when interstellar matter was abundant, the rate of star formation was higher than at the present time. Trillions and trillions of stars formed in these galaxies, many with planets surrounding them. Earth’s star, the Sun, was formed some five billion years ago in the Milky Way galaxy. Today, astronomers estimate that about one new star is born each year in our galaxy.

Figure 3. The interstellar ionized plasma cloud S88.

The Milky Way galaxy is peppered with interstellar clouds. Some are predominantly composed of cold neutral hydrogen, others are comprised of molecular species, and still others are made of a hot plasma (ionized gas) that is powered by young massive stars. Ultraviolet radiation from the stars ionizes and heats these clouds to temperatures of about 10,000 degrees. The free-moving electrons in these clouds recombine with the protons, or other heavier nuclei, and in the process, emit radiation at specific wavelengths characteristic of the atomic structures of the various elements. With radio waves, radio astronomers can detect such radiation as spectral lines, and from observed features, can deduce the physical characteristics of the interstellar hot gases, such as temperatures, densities, turbulent velocities of the gases, and cloud velocities in the line of sight.

Using the Arecibo giant radio telescope located in Puerto Rico (Figure 1), we have been investigating interstellar clouds in order to understand their physical nature and to test atomic theory. Figure 2 shows a spectrum of an ionized cloud (Figure 3 cloud named S88) in the interstellar space. The telescope was used at a wavelength of 6 cm (5,000 MHz) and obtained the spectral signatures of the various line emissions from this object. The strong hydrogen emission lines, and those of helium and carbon, as well as weak signals from heavier elements can be seen. The superb sensitivity of the Arecibo telescope makes possible the clear and unambiguous detection of such spectra to study the dynamics and chemical evolution of interstellar clouds and hence, to understand the formation and evolution of the Milky Way galaxy.

Figure 4: The Ring Nebula.

After living a long life of a few billion years, producing energy from nuclear reactions in their cores, stars explode their outer mantles, and their cores collapse into fascinating super-dense hot dwarf stars, neutron stars, or black holes, depending on their initial mass. The ejected material flies out and back into interstellar space, and eventually mixes with other diffuse clouds to recollapse and form second- or third-generation stars. The ejected envelopes of dying stars can serve as testbeds for researchers to study low-density, hot plasma and to check the predictions of atomic theories.

Using the Arecibo radio telescope we have made hundreds of observations of the material ejected from exploding stars. We have observed the initial cooler ejected material composed, in part, of molecules such as OH that emit maser spectral lines at a wavelength around 18 cm (1612 MHz). We have also observed the radio thermal emission from hotter material surrounding the dying stars and have detected the weak signals of the predicted recombination lines. These observations have been combined with optical images taken with the Hubble Space Telescope to understand these objects better. Figure 4 shows a dying star with its surrounding expanding nebula called the Ring Nebula (NGC 6720), and Figure 5 shows the Dumbbell Nebula (NGC 6853), both of which have been observed with the Hubble telescope and studied with the Arecibo radio telescope.

Figure 5: The Dumbbell Nebula.

We plan to use the exceptional sensitivity of the Arecibo radio telescope to survey the plane of the galaxy and to map all the ionized radio-emitting interstellar clouds. We shall then understand our galaxy better.