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Multiwavelength Astronomy
What do different wavelength
regimes allow astronomers to “see”?
Temperature vs. peak wavelength
10-9 m
1 micron
100 microns
1 cm
50 K
0.5 K
1m
Increasing wavelength
5x106 K
5000 K
Increasing temperature
• Recall Wien’s Law: object’s temperature
determines the wavelength at which most of its
electromagnetic radiation emerges
A temperature-dependent
“hierarchy” of states of matter
• Coldest (<100 K): dense molecular gas, icecoated 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
infrared
optical
radio
A noisy “neighbor” galaxy
The “starburst” galaxy M 82
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
Optical
Infrared
X-ray
(Hubble Space Telescope)
(Gemini 8-meter telescope)
(Chandra)
New discoveries of X-rays from planetary nebulae
Chandra (left) and HST (right)
images of NGC 6543
(The Cat’s Eye Nebula)
Chandra (left) and HST (right)
images of NGC 7027