Astronomy and the Electromagnetic Spectrum 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….
Characterised by different frequency or wavelength (c=f) wavelength 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. • 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 Gamma rays 100 femtometres Radio waves centimetres or metres 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.
Anglo-Australian Telescope – Infrared image Infrared Radiation • Dust obscures regions of star formation. Infrared radiation can be used to see through the dust. Hubble Space Telescope – Optical image of the Orion Nebula
Active Galaxies Centaurus A
Gamma rays (10-14 m) The Sky at different wavelengths
X-rays (10-10 m) The Sky at different wavelengths
Ultraviolet (10-5010-9m) The Sky at different wavelengths
Visible light (400-70010-9 m) The Sky at different wavelengths
Infrared (10010-6 m) The Sky at different wavelengths
The Sky at different wavelengths Microwaves ( 1 cm)
Radiowaves ( 1 m) The Sky at different wavelengths
Observing EM radiation (Telescope design) • Site. • Angular resolution. • Sensitivity. • Frequency resolution.
radio X-ray visible infrared ultraviolet gamma ray Atmospheric Transmission
Angular resolution of observer observer Angular Resolution Any telescope has a limited ability to see fine detail, known as its angular resolution.
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.
The Ryle Telescope Improving Resolution with Interferometry
Milli-arcsecond resolution • Equivalent to imaging a penny at 2000km! Very Long Baseline Interferometry
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 …
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
If this were our Galaxy, our Sun would be located about here The local Universe - Galaxies Spiral Galaxy M63
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.
Interacting Galaxies HST image of colliding galaxies NGC 4038 and NGC 4039
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.
Optical + infrared image Molecular Cloud Barnard 68 A hole in the stars? Optical image
Ultraviolet (10-5010-9m) Gamma rays (10-14 m) Infrared (10010-6 m) X-rays (10-10 m) Radiowaves (More than 1 cm) Microwaves ( 1 cm) Visible light (400-70010-9 m) The Sky at different wavelengths
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.