Multiwavelength Astronomy

What do different wavelength regimes allow astronomers to “see”?

Temperature vs. peak wavelength

Ultrav

iolet

(UV)

X Ray

s

Visib

le Lig

ht

Infra

red (I

R)

Microwav

es

Radio

wav

es

1 micron 1 m1 cm10-9 m 100 micronsIncreasing wavelength

Increasing temperature

• Recall Wien’s Law: object’s temperature determines the wavelength at which most of its electromagnetic radiation emerges

5000 K 50 K5x106 K 0.5 K

A temperature-dependent “hierarchy” of states of matter

• Coldest (<100 K): dense molecular gas, ice-coated dust• “Warm” (100-1,000 K): warm dust & molecules• Hotter: (1,000 - 10,000 K): atomic gas (molecular bonds break down)• Hotter still: (>10,000): ionized gas (electrons separated from nuclei;

plasma)

Radio/microwave radiation• Generally, probe of “coldest” matter: dense gas & dust

– Afterglow of “Big Bang” (2.7 degrees K)

• Probe of molecular gas– long list of molecules that have been detected in interstellar space via their radio radiation

• carbon monoxide, water, hydrogen cyanide, ammonia, alcohol…

• Very penetrating– most matter is transparent to radio waves

Mid- to Far-infrared radiation• Probe of “dust grains”

– huge variety known, from giant molecules to grains of glass

• Most of the known dust in the universe shines in the mid- to far-IR– Dust forms around dying stars– Dust congeals into planetary systems now forming around

young, recently formed stars– Dust surrounds the massive centers of many galaxies

• Planets emit most strongly in the mid- to far-IR• Very penetrating

M17 star cluster: Optical Photograph + Far Infrared

Near-infrared radiation

• Probe of “hot” dust and molecular gas• Somewhat penetrating

– 2 micron light penetrates matter 10 times easier than visible light

• Probe of stars that are cool and/or surrounded by dust clouds– this includes stars just formed and stars that are “kicking off”

Visible Near-Infrared

Hot molecules and dust

Image mosaic of the NGC 6334 star formation region obtained with SPIREX/Abu at the South Pole

Visible light• Stars dominate the visible-light universe

– Starlight can be detected directly (the stars themselves) or can be seen in light reflected off dust grains located near stars

– Stars represent a primary constituent of galaxies, so distant galaxies are usually first detected in visible or near-IR light

• Gas ionized by UV from hot stars (and heated to about 10,000 K) also emits brightly in the visible– case in point: the Great Nebula in Orion

• Easily blocked by dust clouds

Our Nearest (Galactic) Neighbor in visible light: a twin to the Milky Way?

Andromeda Galaxy, Optical

Ultraviolet light

• Probe of the hottest stars and ionized gas• Matter spiraling into a massive object (a collapsed

star or the center of a massive galaxy) emits strongly in the UV as it gets heated to >10000 K

• Easily blocked by atomic gas and by dust clouds

X-rays• Probe of cosmic “collisions” that produce plasma at

temperatures in excess of 1,000,000 K– Example: gas ejected at high speed from a rapidly dying star

hits gas that was ejected more slowly by the same star => gas heated to X-ray-emitting temperatures

– Most stars, especially young stars, have a tenuous outer atmosphere (corona) hot enough to produce X-rays

– Many compact, massive objects thought to be black holes display X-ray emission

• Highly penetrating; dust is almost transparent to X-rays

X-rays trace explosive events

Supernova remnant Cassiopeia A

The many faces of the supernova remnant Casseopeia A

X-ray

optical

infrared

radio

The “starburst” galaxy M 82

A noisy “neighbor” galaxy

It takes images at a variety of wavelengths to find every newborn star

Central Orion Nebula region: left, X-ray; right, infrared

Stars like the Sun don’t exactly go quietly into the night

The planetary nebula BD +30 3639

Infrared(Gemini 8-meter telescope)

Optical(Hubble Space Telescope)

X-ray(Chandra)

New discoveries of X-rays from planetary nebulae

Chandra (left) and HST (right) images of NGC 7027

Chandra (left) and HST (right) images of NGC 6543

(The Cat’s Eye Nebula)