Download Observing at Other Wavelengths

Theme 6 – Observing at Other
Wavelengths
ASTR 101
Prof. Dave Hanes
Why Other Wavelengths?
To Study New Sources
In Historical Order
1. Radio radiation
2. Infrared sources
3. X-Rays
4. Gamma rays
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
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…
Reber: The Dedicated Amateur
A home-made radio telescope 32 feet in diameter (1937);
Publication in the Astrophysical Journal (1940)
Helpful Technological Developments
World War II – dishes, detectors and
electronics for radar
Jodrell Bank (UK)
one of the very first, post-war
Ideal astronomy for
the UK climate!
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.
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).
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 dots
of 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.
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…
Big Radio Telescopes
The Biggest Single Dish
Arecibo, Puerto Rico
It is 305 metres in diameter
It’s a ‘transit’ instrument: it can only observe directly overhead.
It featured (irrelevantly) in the James Bond movie ‘Goldeneye’
Even Higher Resolution
use smaller, widely-spaced telescopes in pairs
Of course you get less total signal, but you enjoy
very good resolution!
Here’s Why: Interferometry
Light is a Wave!
Here, each telescope sees a ‘crest’ – the
signals can be added constructively through
the electronic connections
It Helps to Have Many‘Baselines’
Combine the signals separately for every pair of telescopes
The Very Large Array, in New Mexico
The Ultimate: ALMA
(Atacama Large Millimeter Array)
At 5000 m altitude (in the ‘death zone’) in Atacama, Chile
Even Larger Separations
VLBI: 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.
Infrared
People Glow!
‘Night Vision’ Goggles
useful for hunters and soldiers
Two Challenging Problems
1.
2.
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.
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.
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.
Higher Than Mountaintops
SOFIA: Stratospheric Observatory for Infrared Astronomy
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
Shorter Wavelengths
Higher 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!
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!)
X-Rays: The Origins
Astronomical 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.
How to Make X-rays
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.
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?
Like Skipping Stones
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.
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.