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Multiwavelength Milky Way

Multiwavelength Milky Way

The most prominent feature of the Milky Way that we see, as we look out from our vantage point embedded in the galaxy's disk, is the vast number of stars that blend together in our view and form the band of the Milky Way in dark skies. What happens if we look out at different wavelengths, though? Is that band still the most prominent feature or does it fade away into the noise of other things our eyes can't see?

It turns out that the disk of the Milky way shines bright no matter what part of the electromagnetic spectrum we look in. But, what are we seeing when we look at these different wavelengths?

Here we share the big picture of the Milky Way. However, our galaxy is filled with stars, gas, and dust, just like other galaxies. Be sure to check out the Cosmic Objects page if you want to learn more about individual sources that we find in our own galaxy and others.

Radio

Radio emission reveals a few different things about the Milky Way depending on which part of the radio spectrum we observe. Parts of the radio continuum tell us about where electrons are being accelerated in the galaxy. Other parts tell us about where hydrogen lies in the Milky Way.

*Tell me more about spectra and what we learn from them

Radio image of the disk of the Milky Way

Radio image of the disk of the Milky Way

Intensity of the radio continuum emission from the disk of the Milky Way at 408 MHz (top) and 2.4-2.7 GHz (bottom). These radio wavelengths show astronomers where electrons are being accelerated through a variety of processes. (Credit: Haslam, et. al (1982), A&AS, 47, 1; Duncan, et. al (1995) MNRAS, 277, 36; Fuerst, et. al (1990) A&AS, 85, 691; Reich, et al. (1990), A&AS, 85, 633)

Radio continuum emission comes from electrons accelerated through one of two different processes. The 408MHz continuum, shown above, primarily shows us places in the Milky Way where electrons are accelerated by the interstellar magnetic field at nearly the speed of light. As the electrons are accelerated, they spiral around the magnetic field lines and emit radiation at radio wavelengths. In the 2.4-2.7 GHz range, some of the bright spots also show where electrons are accelerated in magnetic fields. In that part of the continuum, though, we also see light emitted by electrons accelerated by protons in the hot, ionized gases of emission nebula.

Radio image of the disk of the Milky Way

Radio image of the disk of the Milky Way

These images show the amount of atomic (top, 1.4 GHz) and molecular (bottom, 115 GHz) hydrogen from radio observations. (Credit: Burton, (1985) A&AS, 62, 365; Hartmann, "Atlas of Galactic Neutral Hydrogen," Cambridge Univ. Press, (1997, book and CD-ROM); Kerr, (1986) A&AS, 66, 373; Dame, (2001) ApJ, 547, 792)

Looking at a couple of specific wavelengths, astronomers can see places where hydrogen resides in the Milky Way. Atomic hydrogen emits a rare spectral line at 1420 MHz (or 21-cm in wavelength). Even though the line is rare, we see this line fairly prominently in the Milky Way because there is so much hydrogen. Atomic hydrogen traces places where the interstellar medium is cold or warm, which is organized into diffuse clouds of gas and dust up to hundreds of light years across.

Molecular hydrogen is difficult to detect directly, so carbon monoxide is observed as a standard tracer of molecular hydrogen. Carbon monoxide has a spectral line in the radio at 115 GHz. We find that molecular hydrogen resides is the spiral arms of the Milky Way in "molecular clouds" that are often the site of star formation.

Infrared

Infrared light does not get absorbed as easily as optical light, so infrared observations peer farther into the plane of the Milky Way than optical telescopes. Shorter wavelengths of infrared light reveal stars in the Milky Way while longer wavelengths show interstellar dust warmed by starlight.

Infrared image of the disk of the Milky Way

Infrared image of the disk of the Milky Way

Infrared image of the disk of the Milky Way

Infrared views of the plane of the Milky Way. The top image shows a composite of mid- and far-infrared observed by IRAS (3,000-25,000 GHz). The middle image is mid-infrared observed by the MSX satellite (28,000-44,000 GHz). The bottom image shows near-infrared as observed by COBE (86,000-240,000 GHz). (Credit: Wheelock (1994) IRAS Sky Survey Atlas Explanatory Supplement, JPL Publication 94-11; Price (2001) AJ, 121, 2819; Hauser (1995) COBE Diffuse Infrared Background Experiment Explanatory Supplement, Version 2.0, COBE Ref. Pub. No. 95-A (Greenbelt, MD: NASA/GSFC))

Infrared light can help us find young stars that are embedded in their parental molecular clouds – these clouds obscure our view in visible light, but are nearly transparent to infrared light. These are visible in the images above as small bright spots.

When light from stars encounters interstellar dust, it warms it, making it shine in infrared light. By observing longer infrared wavelengths, we can trace clouds of interstellar dust. This is seen in the images above by diffuse emission throughout the plane of the Milky Way.

Optical

Optical observations of the Milky Way are probably the most familiar. One challenge, though, is that optical light is absorbed quickly by interstellar gas and dust, so we can't see as far as we can in some other wavelengths.

Optical image of the disk of the Milky Way

Optical view (0.4 - 0.6 micron) of the plane of the Milky Way. (Image courtesy of Mellinger, A., Milky Way Panorama)

Due to the strong obscuring effect of interstellar dust, the optical light shown above is primarily from stars within a few thousand light-years of the Sun, nearby on the scale of the Milky Way. We can also see bright red regions produced by glowing gas. The dark patches are due to absorbing clouds of gas and dust, which can be see in the molecular hydrogen and infrared maps as emission regions.

Ultraviolet

The ultraviolet band is where we see the stars that heat up the interstellar medium and star forming regions that are present in the maps of the other wavebands.

Ultraviolet image of the disk of the Milky Way

Ultraviolet view of the plane of the Milky Way from GALEX data. The black bands are places where these is no data. (Credit: D. Schiminovich (Columbia), M. Seibert (OCIW) and GALEX science team, led by Prof. C. Martin at Caltech)

Young, hot stars emit light in ultraviolet wavelengths which, in turn, heats the surrounding hydrogen gas. Visible in the image above are stellar clusters, wisps of emission from supernova remnants, and pronounced dusty absorption features surrounding star-forming regions and molecular clouds in the Milky Way's disk.

X-ray

In the Milky Way, we see X-rays from hot gas, binary star systems, young stars and stellar clusters, supernova remnants, and matter falling into our galaxy's central black hole.

X-ray image of the disk of the Milky Way

Soft X-ray view (0.25, 0.75, and 1.5 keV) of the plane of the Milky Way from ROSAT observations. (Credit: Snowden (1997) ApJ, 485, 125)

Soft (lower energy) X-ray emission is detected from hot, shocked gas. At the lower energies especially, the interstellar medium strongly absorbs X-rays, and we see cold clouds of interstellar gas as shadows against background X-ray emission. Color variations indicate variations of absorption or of the temperatures of the emitting regions.

Gamma ray

Most of the gamma-ray objects we detect originate from outside the Milky Way. However, we do see gamma-ray background emission from collisions of cosmic rays with hydrogen nuclei in interstellar clouds and emission associated with a few bright compact objects like pulsars.

Gamma-ray image of the disk of the Milky Way

Gamma-ray view (500 GeV - 2 TeV) of the plane of the Milky Way from 6 years of Fermi data. (Credit: NASA/DOE/Fermi LAT Collaboration)

The above shows gamma-rays observed by the Fermi Gamma-ray Space Telescope with energies between 500 GeV and 2 TeV. Most of the gamma rays shown are from collisions between cosmic rays and hydrogen nuclei. However, a few bright sources can be seen as well. These pulsar wind nebulae and supernova remnants.

In addition to seeing a gamma-rays from the disk of the Milky Way, the Fermi Gamma-ray Space Telescope also discovered huge lobes of gamma-ray emission above and below the plane of the Milky Way. These lobes of gamma-ray emitting gas extend more than 25,000 light years above and below the plane of the Milky Way and astronomers are still working to figure out their nature and origin.

*Learn more about the gamma-ray lobes discovered by Fermi

*See the multiwavelength Milky Way galaxy on a poster

Updated: February 2016

 

A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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