The Universe in the Infrared What can we learn from infrared light and how do we see it? Funded by NASA’s Spitzer Science Center Images courtesy NASA/JPL - Caltech

Pilachowski / August 2005 The Universe in the Infrared Slide 2 Outline  The electromagnetic spectrum  Atmospheric windows  The colors of infrared light  Sources of infrared light  Detecting IR light  Infrared telescopes  Distances

Pilachowski / August 2005 The Universe in the Infrared Slide 3 Understanding the electromagnetic spectrum

Pilachowski / August 2005 The Universe in the Infrared Slide 4 The Electromagnetic Spectrum Infrared (IR) is a form of light (or electromagnetic radiation) IR light is found between visible light and radio waves Wavelengths extending from 1 to 200  m (microns) –A micron is one-millionth of a meter, and is abbreviated as µm

Pilachowski / August 2005 The Universe in the Infrared Slide 5 What does “electromagnetic” mean?  Properties of waves  speed (distance per second)  wavelength (length)  frequency (cycles per second) speed of light = wavelength x frequency

Pilachowski / August 2005 The Universe in the Infrared Slide 6  speed – 300,000 km per second (3 x 10 8 meters per second)  frequency – say, one billion cycles per second (10 9 cycles per second)  What is the wavelength?  What kind of light is this? 3 x 10 8 m/sec = x 10 9 /sec speed = wavelength x frequency

Pilachowski / August 2005 The Universe in the Infrared Slide 7 Terminology decimeter10 -1 meters centimeter10 -2 meters millimeter10 -3 meters micrometer10 -6 meters nanometer10 -9 meters decameter10 1 meters hectometer10 2 meters kilometer10 3 meters Visible light has wavelengths between 400 and 700 nanometers Microns and nanometers…

Pilachowski / August 2005 The Universe in the Infrared Slide 8 The Colors of Infrared Light  Astronomers refer to different types of infrared light  The precise wavelength ranges are somewhat arbitrary Near IR: 1-5  mMid IR: 5-30  mFar IR:  m

Pilachowski / August 2005 The Universe in the Infrared Slide 9 Atmospheric Windows  Some near-IR light reaches mountain- top observatories.  Clear IR windows are centered at 1.25, 1.65, 2.2, 3.5, 4.8 microns.  High-flying airplanes and balloons get above most of the atmosphere  Only space-borne infrared telescopes provide an unimpeded view of the infrared universe. Earth from  m At different wavelengths of light, the Earth’s atmosphere can be either transparent or opaque

Pilachowski / August 2005 The Universe in the Infrared Slide 10 Sources of IR Light Stars Gas Dust

Pilachowski / August 2005 The Universe in the Infrared Slide 11 All matter glows with light Cool matter glows primarily with radio or infrared light Warmer matter glows with higher energy light Matter at about 10,000 degrees centigrade glows white hot Even hotter matter glows blue hot

Pilachowski / August 2005 The Universe in the Infrared Slide 12 The glow of matter because of its temperature Blackbodies emit light at all wavelengths Cooler object peak at longer wavelengths (redder) Hotter objects peak at shorter wavelengths (bluer) The higher the temperature, the shorter the peak wavelength Very cool objects peat at radio wavelengths and very hot objects peak at ultraviolet, x-ray, or gamma-ray wavelengths

Pilachowski / August 2005 The Universe in the Infrared Slide 13 Stars as Black Bodies A very hot star will peak in the ultraviolet, but we will see it as a blue star A very cool star will peak in the infrared, but we will see it as a red star

Pilachowski / August 2005 The Universe in the Infrared Slide 14 “Black bodies” glow at ALL wavelengths The wavelength at which the black body is brightest tells us the temperature (hotter = shorter wavelength) As the temperature increases, the blackbody radiation also gets BRIGHTER Black Body Radiation Applet

Pilachowski / August 2005 The Universe in the Infrared Slide 15 Wien’s Law The sun is brightest at a wavelength of 520 nanometers. What is the temperature at the surface of the Sun? 3,000,000 / 520 = 5770 K We can determine the surface temperature from the wavelength of the peak brightness for any star

Pilachowski / August 2005 The Universe in the Infrared Slide 16 The energy emitted is directly proportional to T 4 As stars get hotter, their energy output increases quickly! A star 10 times hotter than Sun has 10,000 times more energy output Temperature Matters ! To be bright in the infrared, cool sources must be BIG

Pilachowski / August 2005 The Universe in the Infrared Slide 17 Temperature – The Kelvin Scale Named after Lord (William Thompson) Kelvin –19th century Scottish physicist –a one degree difference on the Kelvin (K) scale is the same as for the Celsius (or centigrade) scale The zero-point is defined to be absolute zero –the coldest possible temperature –atomic and molecular motion ceases –no negative temperatures Note: no degree symbol (°) with the Kelvin scale

Pilachowski / August 2005 The Universe in the Infrared Slide 18 Temperature and peak brightness Radio < 0.03K Microwave K Interstellar Space Infrared K Humans Visible K Sun UV x 10 6 K Hottest Stars X-ray 3x x10 8 K Neutron stars Gamma Ray > 3x10 8 K Black holes

Pilachowski / August 2005 The Universe in the Infrared Slide 19 Scattering and Extinction Dust also scatters starlight Dust clouds block visible light but are transparent to infrared light T.A.Rector (NOAO/AURA/NSF) and Hubble Heritage Team (STScI/AURA/NASA)

Pilachowski / August 2005 The Universe in the Infrared Slide 20 The Pleiades – Optical & IR A dust cloud passing near the Pleiades scatters blue starlight in this visible light image. The dust radiates in the infrared. 24  m Visible

Pilachowski / August 2005 The Universe in the Infrared Slide 21 Why infrared? Near-infrared (1-5 mm) –stars –warm gas –dust is transparent Mid-infrared (3-30 mm) –dust warmed by starlight –protoplanetary disks Far-infrared ( mm) –cold gas & dust Dust is more transparent to infrared light. We can see what’s hidden in the dust. Cold gas and dust is invisible in visible light, but glows in infrared light.

Pilachowski / August 2005 The Universe in the Infrared Slide 22 Detecting Infrared Light Single-pixel bolometers, 1960’s first semi-conductor arrays, 32x32 pixels, in early 1980’s Top left: 58 X 62 pixels, 1987 Middle left: 256 X 256 pixels, 1991 (SIRTF, IRAC) Lower left: 1024 X 1024 pixels (1 Mega Pixel), 1996 Right: 2048 X 2048 pixels (4 Mega Pixel) 2001 InSb array detectors by Raytheon (SBRC).Raytheon (SBRC). Courtesy Univ. of Rochester Astronomy

Pilachowski / August 2005 The Universe in the Infrared Slide 23 Observing at Nonvisible Wavelengths Astronomical objects radiate in wavelengths other than visible (blackbody radiation) –Stars –Hot, warm and cold gas –Dust Telescopes for each wavelength region –Require their own unique design –All collect and focus radiation and resolve details –False-color pictures to show images –Some wavelengths must be observed from space

Pilachowski / August 2005 The Universe in the Infrared Slide 24 Infrared Telescopes Space-Based Advantages –No atmospheric blurring –No atmospheric absorption –No atmospheric emission Ground-Based Advantages –Larger collecting area –Better spatial resolution –Equipment easily updated Ground-Based Considerations –Weather, humidity, and haze –Atmospheric transparency

Pilachowski / August 2005 The Universe in the Infrared Slide 25 False Color Astronomical images begin as black & white (grayscale) digital data from a single spectral region, often using wavelengths outside of the range of human vision A "true" color image or photograph recreates what our eyes would see in visible light under natural conditions To create a color image from data at other wavelengths, astronomers represent it in "false" colors Three of grayscale images from different wavelengths may be mapped to red, green, and blue and overlaid to form a color image

Pilachowski / August 2005 The Universe in the Infrared Slide 26 More false color Astronomers also “colorize” black and white images to highlight certain aspects.

Pilachowski / August 2005 The Universe in the Infrared Slide 27 Inverse Square Law If we know a star’s apparent AND absolute brightness, we can calculate its distance The inverse square law describes how the brightness of a source light (a star!) diminishes with distance For nearby stars, stellar parallaxes provide a way to measure distance brightness = 1/distance 2

Pilachowski / August 2005 The Universe in the Infrared Slide 28 Parsec: the distance to an object with a stellar parallax of one arc second The parallax of Alpha Centauri = 0.76 arcseconds A parallax of ~0.001 arc seconds is the smallest we can measure What is a Parsec??? 1 parsec = 3.26 light years A star at a distance of 1 parsec shows a parallax of 1 arc second How big is one arc second? The size of a dime at a distance of 2.3 miles!

Pilachowski / August 2005 The Universe in the Infrared Slide 29 Wrapping Up  The electromagnetic spectrum  Atmospheric windows  The colors of infrared light  Sources of infrared light  Detecting IR light  Infrared telescopes  Distances