ALMA Science

The Atacama Large Millimeter/submillimeter Array is the precision multitool for mapping the once hidden, detailed activities of the Cosmos.

ALMA is a premier telescope for studying the first stars and galaxies that emerged from the cosmic "dark ages" billions of years ago. We find them at great cosmic distances, with most of their light stretched out to millimeter and submillimeter wavelengths by the expansion of the Universe.

In the more nearby Universe, ALMA provides an unprecedented ability to study the processes of star and planet formation. Unimpeded by the dust that obscures visible-light observations, ALMA reveals the details of young, still-forming stars, and shows young planets still in the process of developing.

Using the Universe as a giant chemistry laboratory, ALMA allows scientists to learn in detail about the complex molecules of the giant clouds of gas and dust that spawn stars and planetary systems.

Many other astronomical specialties benefit from the new capabilities of ALMA, such as:
  • Mapping gas and dust in the Milky Way and other galaxies.
  • Investigating ordinary stars
  • Analyzing gas from an erupting volcano on Jupiter's moon, Io.
  • Studying the origin of the solar wind.

Here are parallel pictures of the Horsehead Nebula in the optical / radio. The Horsehead Nebula at different wavelengths: In the optical, dust obscures star-forming activity. In the infrared, the hot, thin layer of dust around the cloud glows. At radio wavelengths, both dust and molecules glow, providing a wealth of information on regions that are otherwise invisible in the optical range.

ALMA Deep Field

Most of the galaxies that are detected in sensitive ALMA images have large redshifts, meaning that they are very very far away from us. This is illustrated in the top row that shows the number of low redshift (z<1.5) and high redshift (z>1.5) galaxies expected from a simulated deep ALMA observation. Although the high redshift galaxies are more distant, much more of the dominant emission from warm dust is redshifted into the ALMA frequency bands.

The bottom row shows that with an optical image, such as the Hubble Deep Field, most of the detections are of galaxies with z<1.5. In stark contrast to the optical image, 80 percent of ALMA detected galaxies will lie at high redshifts.

ALMA Deep Field
Top images from Wootten & Gallimore (2000, ASP Conf. Ser. Vol. 240, pg. 54). Bottom images from K. Lanzetta, K. Moore, A. Fernandez-Soto, and A. Yahil (SUNY). © 1997 Kenneth M. Lanzetta.

Star and Planet Formation

Star formation is the tracer of structure and history in galaxies. Stars form where there is enough gas and dust to make them, so they show us the clumpiness of a galaxy. Big stars explode and leave a buildup of the heavy elements responsible for the creation of the planetary environments in which life in the Universe has become possible.

We know that star formation involves gravitational collapse, but the flow of gas that forms a new star had yet to be found before ALMA came online.

Further, ALMA’s excellent mapping precision allows astronomers to study the characteristics of parent molecular clouds from which stars form. Its sensitivity, angular and velocity resolution, and high frequency performance allows the study of smaller structures, including protostellar fragments, outflows, and disks.

Detecting Extrasolar Planets with ALMA

In order to answer the most basic questions about planetary systems, such as their origin, their evolution, and how common they are in the Universe, scientists need to find and study many more planets around other people’s suns. However, detecting planets circling other stars light-years away is a particularly difficult task.

As soon as it came online, ALMA began providing valuable information about these so-called “extrasolar” planetary systems at all stages of their evolution.

NRAO Fomalhaut highres smallMillimeter/submillimeter-wave telescope arrays such as ALMA can see more detail than current optical or infrared telescopes. The longer waves it detects are not scattered or reflected by interplanetary dust, either in the extrasolar system or our own Solar System. Another important advantage is that, at millimeter and submillimeter wavelengths, the star is not glaring and overwhelming our view of its potential planets as it does in shorter wavelengths. While the star is still brighter than a planet, the difference in brightness between the two is far less in millimeter radiation.

ALMA can see planetary systems in the earliest stages of their formation. It will also be able to detect many more young, low-mass stellar systems and determine if they have the disks from which planetary systems are formed. In addition, ALMA can examine the properties of these disks in detail, including their size, temperature, dust density, and chemistry.

Aging Stars and Dust

Winds of charged gas and particles from aging, cooler stars are the “starstuff” from which Earth and we were formed. The grains shine in the far infrared wavelengths through to ALMA’s millimeter wavelengths. ALMA will see the dusty zone around all giant stars within a few hundred light-years away from Earth.
ALMA has made observation of the shells of gas and dust coughed off by these aging stars, giving us details about the final years of their evolution into white dwarfs and planetary nebulae. Measurements of the shell masses of a large number of planetary nebulae, their brightnesses and movement will help astronomers better understand the recipe needed to make them.
The diameters of these bloated aged stars can be so huge that if you popped one in place of our Sun, it would take up the entire inner Solar System out to Jupiter. ALMA can image these stars even well beyond the distance to the Galactic center. Measurements of distances to a large numbers of these objects will map the dust producing factories in the Galaxy.


The first molecule discovered in space was helium in 1868 in an optical absorption spectrum taken of the Sun. In 1963, radio telescopes began picking out molecules in space, starting with the hydroxyl radical.

Orion widespectra smallInterstellar gas and dust are concentrated into large regions known as molecular clouds, the birthplaces of new stars, including our Sun, and their planets. This is an amazingly long process, because even the most dense of these clouds is nearly a vacuum by laboratory standards -- atoms rarely collide in them. Their low temperatures (10-50 K) mean that few of these rare collisions can even lead to chemical reactions. It is incredible to think that such a vastly empty cloud created everything we’ve ever known on Earth.

Molecular hydrogen, the most abundant gas molecule in space, is formed when two hydrogen atoms stick to the surface of a dust grain and diffuse until they merge into a molecule. When these molecules collide with other molecules, they get knocked into spins. The spin agitates the molecules’ electrons, which emit specific wavelengths of radiation, typically in the millimeter or submillimeter wavelength range. If the molecules are hit hard enough for the bonds between their atoms to bend, then the radiation given off by their wobbling is at infrared levels.

After molecular hydrogen the most abundant molecule we find is carbon monoxide (CO), which astronomers use to map out interstellar clouds in nearby as well as in distant galaxies. More than 180 different kinds of molecules have been found in space, ranging in size from a joined pair of atoms like molecular hydrogen to molecules made from thirteen atoms bonded together.

Most of the molecules are organic (carbon-containing), including ones similar to Earth-like molecules but still others that are a strange assortment of species that could never be stable on Earth. There is also evidence for much larger molecules such as polycyclic aromatic hydrocarbons, which resemble the soot from automobile emission.

The Sun

ALMA can observe a wide variety of phenomena on the Sun:

  • The structure of the quiet solar atmosphere.
  • Coronal holes (where vast solar winds originate because of diverging magnetic fields)
  • Solar active regions
  • Active and quiescent filaments, and
  • Energetic phenomena like filament eruptions and flares.

One of the great mysteries of the Sun is why it has a solar corona, a huge atmosphere of superhot plasma. At the height of the photosphere (the visible surface of the Sun), the temperature is ~5880K. The temperature then decreases with height for several hundred kilometers. But then something amazing occurs: at greater heights, the temperature increases, gradually at first, and then suddenly to ~3 million degrees! ALMA will probe the "temperature minimum" region of the Sun’s lower atmosphere to learn how that structure is maintained.

At ALMA’s submillimeter wavelengths, it should be possible to detect hydrogen and certain ions in the lower atmosphere. These will tell us about the temperature, density, magnetic field strength, and motions in the low solar atmosphere, layers of the atmosphere that are inaccessible by other means.

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