theme 6 – observing at other wavelengths astr 101 prof. dave hanes
TRANSCRIPT
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Theme 6 – Observing at Other Wavelengths
ASTR 101Prof. Dave Hanes
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Why Other Wavelengths? To Study New Sources
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In Historical Order
1. Radio radiation 3. X-Rays2. Infrared sources 4. Gamma rays
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Radio Wavelengths: Very Low Energies
Used to study very cool sources (i.e. not emitting visible light)
Examples: - hydrogen gas in the galaxy (H is the most abundant element in the universe)- cool clouds containing complex molecules
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Serendipitous Discovery
Static on telephone lines: three sources- nearby lighting strikes- diffuse far-away storms- radiation from the Milky Way
The delights of academic freedom…
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Reber: The Dedicated Amateur
A home-made radio telescope 32 feet in diameter (1937);Publication in the Astrophysical Journal (1940)
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Helpful Technological Developments
World War II – dishes, detectors and electronics for radar
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Jodrell Bank (UK)one of the very first, post-war
Ideal astronomy for the UK climate!
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Why Are the Dishes So Big?
It is not to collect more light (although of course that is one consequence of their enormous size)
The principal motivation is to see (‘resolve’) finer details.
Here is why:
the angular resolution (that is, the finest angular detail detectable)
is given by:
the wavelength / the aperture (diameter) of the telescope
NOTE: the smaller the number, the better.
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Resolution
To repeat: resolution = wavelength / aperture
That statement is true no matter what the wavelength, and is a consequence of the wave nature of light. Even for a ‘point’ source, the telescope produces a round ‘blob’ of light when brought to a focus.
The wavelength of visible light is so small that even a modest-sized telescope reveals very fine details, as we can see here (stars in Orion).
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The Physics Defines the Wavelength
To study cool gas, we need to work with radio waves, with wavelengths of centimetres or even metres – at least ten thousand times the wavelength of optical light.
Imagine smearing out the dotsof light in this picture of Orion to ten thousand times their size!All detail would be lost.
But using bigger dishes gives us back some of that resolving power at radio wavelengths.
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Remember Gemini?The Gemini mirror (for studying visible light) is 8 metres in diameter.
To see the same detail at radio wavelengths, we’d need a dish 10,000 x 8m = 80 km in diameter.
That sounds impossible! But wait and see…
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Big Radio Telescopes
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The Biggest Single DishArecibo, Puerto Rico
It is 305 metres in diameterIt’s a ‘transit’ instrument: it can only observe directly overhead.It featured (irrelevantly) in the James Bond movie ‘Goldeneye’
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Even Higher Resolutionuse smaller, widely-spaced telescopes in
pairs
Of course you get less total signal, but you enjoy very good resolution!
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Here’s Why: Interferometry
Light is a Wave!Here, each telescope sees a ‘crest’ – the signals can be added constructively through the electronic connections
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It Helps to Have Many‘Baselines’
Combine the signals separately for every pair of telescopes
The Very Large Array, in New Mexico
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The Ultimate: ALMA(Atacama Large Millimeter Array)
At 5000 m altitude (in the ‘death zone’) in Atacama, Chile
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Even Larger SeparationsVLBI: Very Long Baseline Interferometry
Use two telescopes on separate continents.This gives as much detail as you would get from a single dish as large as the Earth.
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Infrared
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People Glow!
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‘Night Vision’ Gogglesuseful for hunters and soldiers
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Two Challenging Problems
1. The Earth’s atmosphere (mostly the water vapour in it) absorbs a lot of the incoming infrared radiation – only some reaches the ground. Telescopes have to be on high, dry sites.
2. The atmosphere, and the telescope itself, ‘glow’ in the infrared. (Imagine looking through a brightly-lit cityscape to try study the faint stars.) We can cool the telescope, but not the entire atmosphere.
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Orion with ‘Infrared Eyes’(warm gas, stars in formation)
Note that we see new, unexpected things – not just the same old objects in new ways.
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Higher Than MountaintopsSOFIA: Stratospheric Observatory for Infrared
Astronomy
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Better Still: Far Away from the Earth
Spitzer Space Telescope (launched in 2003)
Limited lifetimes: such missions carry coolant on board (liquid Helium) to cool the instruments and the telescope, but eventually it runs out, limiting the instrument’s sensitivity and ending the mission
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Shorter WavelengthsHigher Frequencies, Higher Energies
Let’s not forget Ultraviolet light! Given off by the Sun; much more by hot stars (hence the astronomical interest).
UV is energetic enough to (a) tan us; (b) sunburn us; and (c) cause skin cancers!
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UV and the Ozone Layer
Ozone (O3) in the stratosphere absorbs a lot of UV light – notably the energetic UV-B.
Our use of certain chemicals (like CFCs) as refrigerants and propellants in spray cans has led to a depletion of ozone, creating the ozone hole, a serious problem. (This is, however, not related to global warming!)
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X-Rays: The OriginsAstronomical X-ray sources were first found
unexpectedly in the 1960s, in rocket experiments. They come from highly energetic sources
very hot gas at ~ 1 million degrees; or
particles falling at very high speed onto dense material. The energy of the collision can lead to the emission of X-rays. This happens near neutron stars and black holes.
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How to Make X-rays
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Very Penetrating
But they don’t get through everything! In particular, not through the Earth’s atmosphere.X-ray telescopes must be put into space.
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Making an Image
An ordinary mirror reflects visible light back to make an image.
But X-rays would penetrate right into an ordinary mirror. How do we bring that very energetic light to a focus?
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Like Skipping Stones
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The Most Energetic Light: Gamma Rays
First found serendipitously in 1967 by satellites looking for gamma rays from atmospheric nuclear weapons testing by the Russians. No one had expected gamma rays from astronomical sources, but they are seen all over the sky.
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Gamma-Ray Bursts: The Most Energetic Events in the
Universe
What are they? Perhaps very energetic supernovas (the collapse and death of the most massive stars); perhaps collisions between neutron stars; etc. Not yet clear.