Theme 6 – Observing at Other Wavelengths

ASTR 101Prof. Dave Hanes

Why Other Wavelengths? To Study New Sources

In Historical Order

1. Radio radiation 3. X-Rays2. Infrared sources 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)

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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 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.

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 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’

Even Higher Resolutionuse 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 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.

Infrared

People Glow!

‘Night Vision’ Gogglesuseful for hunters and soldiers

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.

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 MountaintopsSOFIA: 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 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!

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 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.

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.