Astronomy and the Electromagnetic Spectrum Where you see QQQ this means ask the audience and try and involve them in answering the question. Keith Grainge

Outline Introduction to the Electromagnetic spectrum. The Universe at different wavelengths. Observing EM radiation. Cosmology.

White light spectrum A prism will split white light into it component colours This is only part of the story….

Electromagnetic Radiation Maxwell was the first to propose that light was a travelling electromagnetic wave. Predicted that the spectrum should continue beyond the visible. All these waves travel at the same speed. wavelength Characterised by different frequency or wavelength (c=f) Quantum mechanics  can think about light being carried by photons. Photon energy  frequency.

The Electromagnetic Spectrum microwave visible X-ray radio infrared ultraviolet Gamma ray Radio waves  centimetres or metres Gamma rays  100 femtometres Visible light  100 nanometres

EM radiation and Astronomy The vast majority of astronomical data comes from observations of EM radiation. Until 1950s all astronomy was done in the optical band. Astronomy now done all the way from gamma rays to radio. Observations in other wavebands open different windows on the Universe - very different phenomena visible.

Infrared Radiation Dust obscures regions of star formation. Infrared radiation can be used to see through the dust. Anglo-Australian Telescope – Infrared image Hubble Space Telescope – Optical image of the Orion Nebula

Active Galaxies Centaurus A

The Sky at different wavelengths Gamma rays (10-14 m)

The Sky at different wavelengths X-rays (10-10 m)

The Sky at different wavelengths Ultraviolet (10-5010-9 m)

The Sky at different wavelengths Visible light (400-70010-9 m)

The Sky at different wavelengths Infrared (10010-6 m)

The Sky at different wavelengths Microwaves ( 1 cm)

The Sky at different wavelengths Radiowaves ( 1 m)

Observing EM radiation (Telescope design) Site. Angular resolution. Sensitivity. Frequency resolution.

Atmospheric Transmission radio infrared visible ultraviolet X-ray gamma ray

Angular Resolution Any telescope has a limited ability to see fine detail, known as its angular resolution. Angular resolution of observer observer

Angular Resolution The resolution of a telescope depends on the size of the telescope relative to the wavelength being observed. The larger the telescope the better the resolution. The longer the wavelength the larger the telescope we need to use to achieve a given resolution.

Improving Resolution with Interferometry The Ryle Telescope

Very Long Baseline Interferometry Milli-arcsecond resolution Equivalent to imaging a penny at 2000km!

Sensitivity A telescope’s sensitivity determines its ability to detect faint (as opposed to small) objects. Depends upon collecting area.

Spectral Resolution Atoms and molecules absorb and emit at particular frequencies  line spectra. Can learn temperature, density, and chemical composition. Also velocity and distance …

The Doppler Effect

The Doppler Effect for EM radiation

Cosmological Redshift Distant objects are redshifted i.e. receding  Universe is expanding

The Cosmic Microwave Background The background is the left over radiation from the Big Bang. It has now cooled to a temperature of 2.7 K

The Cosmic Microwave Background Imprint in the background due to the motion of the Earth, about 1 part in 1,000 of the total intensity

The Cosmic Microwave Background The ripples in the background correspond to only about one part in 100,000 of the total intensity

The local Universe - The Sun A photograph of the Sun Ultraviolet image of erupting prominence

The local Universe - Galaxies Spiral Galaxy M63 If this were our Galaxy, our Sun would be located about here

The Universe on the Largest Scale - the Cambridge APM survey Over 2 million galaxies in direction of the South Galactic pole. The map covers about one tenth of the sky

Structure Formation Today the universe contains structure and is very cold (2.73K) In the beginning the universe was very hot and very, very smooth. Over 13 billion years the universe has expanded and cooled. During this time the structure we see around us today has formed under the influence of gravity. So we know what the universe looks like today. But in the beginning it was hot and smooth. Elaborate. Explain universe has expanded and cooled and is filled with radiation left over from Big Bang. We can observe universe when it was only 300,000 years old.

The formation of Structure

HST image of colliding galaxies NGC 4038 and NGC 4039 Interacting Galaxies HST image of colliding galaxies NGC 4038 and NGC 4039

Interacting Galaxies (2)

Summary We can learn about the history of the universe by observing at different wavelengths. In the beginning the universe was hot and smooth. Now it is cold and structured. Gravity is dominant on large scales and has shaped the universe.

Star formation

A hole in the stars? Optical + infrared image Molecular Cloud Barnard 68 Optical image

The Sky at different wavelengths Radiowaves (More than 1 cm) Microwaves ( 1 cm) Infrared (10010-6 m) Visible light (400-70010-9 m) X-rays (10-10 m) Ultraviolet (10-5010-9 m) Gamma rays (10-14 m)

Clusters of galaxies Abell2218 HST The size of a cluster of galaxies is about 50 times the size of our Galaxy.

Atmospheric Transmission The atmosphere is opaque over much of the EM spectrum. Ground based astronomy is only possible in the optical, the radio and the IR. Satellites needed otherwise.

The Electromagnetic Spectrum Visible light is just a small part of the EM spectrum. Runs from gamma rays to radio waves.

The Electromagnetic Spectrum Visible light is just a small part of the EM spectrum. Many examples in everyday life.