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Starts With A Bang

What Would The Milky Way Look Like If You Could See All Of Its Light?

The visible light portion of the spectrum is tiny compared to the whole thing. Here’s what we’re missing.

When you look at the Milky Way in visible light, you might see billions of stars, but you miss so much more.

Multiwavelength images of M31, the Andromeda Galaxy. Quite clearly, different wavelengths reveal various details that are unseen in visible light alone. (PLANCK MISSION TEAM / NASA / ESA)

The human eye is only sensitive to a tiny fraction of the entire electromagnetic (light) spectrum.

The transmittance or opacity of the electromagnetic spectrum through the atmosphere. Note all the absorption features in gamma rays, X-rays, and the infrared, which is why they are best viewed from space. Over many wavelengths, such as in the radio, the ground is just as good, while others are simply impossible. Even though the atmosphere is mostly transparent to visible light, it still distorts incoming starlight substantially. (NASA)

Each wavelength range showcases a novel view of all that’s out there.

NASA’s Fermi Satellite has constructed the highest resolution, high-energy map of the Universe ever created. Without space-based observatories such as this one, we could never learn all that we have about the Universe. (NASA/DOE/FERMI LAT COLLABORATION)

Gamma rays: the highest-energy light originates from black holes, neutron stars, nova outbursts, high-energy antimatter-driven bubbles, and supernova remnants.

X-rays: when matter gets heated due to collisions, stellar outflows, cataclysmic events, or acceleration from neutron stars or black holes, X-rays result.

Data from NASA’s Chandra X-ray observatory reveals the central region of the Milky Way. The X-rays from Chandra (blue and violet) reveal gas heated to millions of degrees by stellar explosions and outflows from the Milky Way’s supermassive black hole. (NASA/CXC/UMASS/D. WANG ET AL.)

The strongest source of X-rays are supermassive black holes.

This mosaic of 330 images from NASA’s Swift observatory showcases the newly-formed, hot, UV-emitting stars present in the Andromeda galaxy. Unfortunately, viewing our own Milky Way from within the galactic plane is impossible in the ultraviolet, as the dust is simply too efficient at blocking ultraviolet light for those views to be useful.(NASA/SWIFT/STEFAN IMMLER (GSFC) AND ERIN GRAND (UMCP))

Ultraviolet: this light typically reveals hot, newly-formed stars, but it’s lousy for viewing our own galaxy.

There’s simply too much dust, wrecking ultraviolet light’s usefulness.

A map of star density in the Milky Way and surrounding sky, clearly showing the Milky Way, the Large and Small Magellanic Clouds (our two largest satellite galaxies), and if you look more closely, NGC 104 to the left of the SMC, NGC 6205 slightly above and to the left of the galactic core, and NGC 7078 slightly below. There are a great many galaxies to be discovered, but within about 10 degrees above and below the galactic plane, visible light cannot reveal them. (ESA/GAIA)

Visible: This is what we normally see, billions of stars with light-blocking dust.

The SDSS view in the infrared — with APOGEE — of the Milky Way galaxy as viewed towards the center. Containing some 400 billion stars, infrared wavelengths are the best for viewing as many as possible due to its transparency to light-blocking dust. (SLOAN DIGITAL SKY SURVEY)

Infrared: Finally, the previously-obscured stars are revealed.

This four-panel view shows the Milky Way’s central region in four different wavelengths of light, with the longer (submillimeter) wavelengths at top, going through the far-and-near infrared (2nd and 3rd) and ending in a visible-light view of the Milky Way. Note that the dust lanes and foreground stars obscure the center in visible light, but not so much in the infrared. (ESO/ATLASGAL CONSORTIUM/NASA/GLIMPSE CONSORTIUM/VVV SURVEY/ESA/PLANCK/D. MINNITI/S. GUISARD ACKNOWLEDGEMENT: IGNACIO TOLEDO, MARTIN KORNMESSER)

The long-wavelength nature of IR light makes it transparent to dust.

Mid-IR and far-IR light reveals cooler gas and protostars.

The first full sky map released by the Planck collaboration reveals a few extragalactic sources with the cosmic microwave background beyond it, but is dominated by the foreground microwave emissions of our own galaxy’s matter: mostly in the form of dust radiating at low but non-negligible temperatures. (PLANCK COLLABORATION / ESA, HFI AND LFI CONSORTIUM)

Microwaves: simply show heated dust.

The positions of the known fast radio bursts as of 2013, including four that were discovered that help prove the extragalactic origins of these objects. The remaining radio emissions show the locations of galactic sources like hydrogen gas and electrons. (MPIFR/C. NG; SCIENCE/D. THORNTON ET AL.)

Radio: the lowest-energy light reveals electrons and hydrogen gas.

This multiwavelength view of the Milky Way’s galactic center goes from the X-ray through the optical and into the infrared, showcasing Sagittarius A* and the intragalactic medium located some 25,000 light years away. The black hole has a mass of approximately 4 million Suns, while the Milky Way as a whole forms less than one new Sun’s worth of stars every year. Later this year, using radio data, the EHT will resolve the black hole’s event horizon. Note that even with assigned-color images like this, it is difficult to disentangle the different contributions of varying wavelengths. (X-RAY: NASA/CXC/UMASS/D. WANG ET AL.; OPTICAL: NASA/ESA/STSCI/D.WANG ET AL.; IR: NASA/JPL-CALTECH/SSC/S.STOLOVY)

With so much information, it’s better viewed in individual wavelengths.

A multiwavelength view of the Milky Way reveals the presence of many different phases and states of normal matter, far beyond the stars we’re used to seeing in visible light. The individual wavelengths showcased here are separate, rather than jumbled together, allowing us to view the information of each individual component. (NASA)

Mostly Mute Monday tells an astronomical or scientific story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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