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Multiwavelength Milky Way: Across the Spectrum

Seeing at different wavelengths

mosaic image of portions of separate Milky Way images

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I. An astronomical object viewed across the electromagnetic spectrum
II. Different wavelengths to study different phenomena

emission - light (electromagnetic radiation) given off by an object.

I. An astronomical object can look very different depending on what wavelength is used in its detection. The object's appearance often changes across the electromagnetic spectrum because various physical processes result in emission in different wavelength regions. Images of emission can be made by mapping brightness to colors (see "Making an Image"). For example, the optical image of the Andromeda galaxy (below left) shows glowing stars. And dark dust lanes trace out a spiral arm structure. An infrared image of the galaxy (below right) shows several concentric rings of dust rather than spiral arms. The dust is too cold (-260 degree C) to be detected in optical light.

Visible Andromeda galaxy Infrared Andromeda galaxy
The Andromeda galaxy in visible (left) and infrared (right) light
Credit: Visible - Jason Ware; Infrared - ESA/ISO/ISOPHOT and M. Haas et al.

Below are five images of the galaxy M 33. Each image is taken in a different wavelength region. In the X-ray image, we see very hot, diffuse interstellar gas, and bright point-like X-ray sources, such as X-ray binaries. The brightness in the ultraviolet image indicates star formation activity. Stars are visible in the optical image. Red supergiant stars and dust heated by massive star formation are highlighted in the near-infrared image, which looks similar to the optical image. The radio image maps out hydrogen gas in the galaxy. The red coloring is indicative of gas moving away from our line of sight. The blue colors gas moving toward us (a phenomena described by Doppler shift).

Radio image not to same scale as other images
Xray M33
UV M33
Optical M33
Infrared M33
Near Infrared
Radio M33
Credit: X-ray - ROSAT; Ultraviolet - UIT; Visible - IAC/RGO/Malin; Near IR - 2MASS; Radio - VLA/WSRT

So far, we've just looked at whole galaxies at different wavelengths, but here is a "multi-wavelength" images of an object within our solar system:

gamma ray and visible moon

The moon in visible light, and shining in gamma rays (due to cosmic rays interacting with the lunar surface).

Credit: Visible - Galileo spacecraft; Gamma Ray - D. Thompson et al./EGRET

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II. What are the typical objects studied at different wavelengths?
Radio Active galaxies (such as radio galaxies and quasars), some stars (particularly binary stars), supernovae, pulsars, and clouds of cold molecular gas are typical emitters of radio waves. The Sun and the planet Jupiter also produce radio signals. Synchrotron emission and thermal emission are two physical mechanisms behind radio emission.

Infrared If an object has a temperature above zero, it emits in the infrared. The longer wavelengths of infrared light can pass un-scattered through dust that blocks visible light. In the infrared, we see stars, including smaller and cooler stars which are hard to see in visible light. Stars do not shine as you move into the far-infrared (towards the radio region). The far-infrared emission comes from protostars, and the dust clouds that were transparent in the near-infrared. The dust is very cold, but still warm enough for thermal radiation.

Visible (optical) In the visible region of the electromagnetic spectrum, we see large and bright stars in our galaxy, nebulae, supernova remnants, planets, and galaxies.

Ultraviolet (optical) In the ultraviolet, we see hot stars in our galaxy, and quasars in other galaxies. Gas that has been heated to a million degrees can be seen in the ultraviolet.

X-rays An object shines in x-rays if it is very hot (like millions of degrees hot!). High temperatures can be reached in the presence of magnetic fields or extreme gravity, or events such as supernovae can heat surrounding material. Hot interstellar gas, neutron stars, and supernova remnants are some objects that are studied in x-rays.

Gamma rays Gamma rays are produced by changes in atomic nuclei. They are also products of collisions between cosmic rays and interstellar matter. The objects and phenomena most often studied in gamma rays include neutron stars, quasars, black holes, and gamma ray bursts.

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Related Online Resources:
Multiwavelength Messier Museum - images of selected astronomical objects at various wavelengths.

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