Astronomy Benalla
Reports - 2014
Whirlpool Galaxy m51 & companion galaxy ps07 (Hubble) Black Hole Butterfly Nebula (Hubble image) Sombrero Galaxy (Hubble) Home of Astronomy Benalla Carina Nebula Pillar - ps49 (Hubble image) Most photos on this site can be zoomed by clicking the photo
Astronomy Benalla  Meeting Presentations - Wednesday 16th April 2014 Presenter: Patrick Watson THE NATURE OF STARS - COLOUR Two maint concepts were to be addressed: 1. The physics of electromagnetic radiation (EMR - radiation consisting of electromagnetic waves). 2. The perception of colour by the human eye and brain. Visible EMR - the band of basic colours  red, orange, yellow, green, blue, indigo, violet produced when white light passes through a prism (white light contains all the wavelengths of the visible electromagnetic spectrum). A brief explanation of wavelength was then presented followed by explanations for and evidence of the following: Light has properties of (behaves like) both waves and particles. Hence, light can be thought of as a stream of particles which have wavelike properties. These particles are called photons. A photon is a packet of energy. All electromagnetic radiation is emitted as photons. The shorter the wavelength the greater the energy. The concept of a BLACK BODY and also BLACK BODY RADIATION was then presented. Wilhelm Wien discovered a relationship between WAVELENGTH and TEMPERATURE “The higher the temperature of a black body, the shorter the wavelength of maximum emission.” A STAR’S COLOUR DEPENDS UPON ITS SURFACE TEMPERATURE. We then moved to the second of the two concepts - human vision and perception. The human retina contains two types of photoreceptors, rods and cones. The cones provide the eye's colour sensitivity. In normal situations, when light is bright enough to strongly stimulate the cones, rods play virtually no role in vision at all. On the other hand, in dim light, the cones are under-stimulated leaving mainly the signal from the rods. Stars as point sources: The vast distance to stars means we cannot resolve them as disks, instead we see the light from a point source. Our eyes are relatively insensitive to colour from point sources. Pupil size: At night our pupils dilate to let the dim light in. The resulting large aperture of the lens degrades image quality and produces chromatic aberration where we can see coloured haloes around objects. Dark adaptation : Our photoreceptors become more sensitive in faint light over time. Cones adapt in about 7 minutes whilst rods take half an hour to reach maximum sensitivity. Cones still remain far less sensitive than rods but as rods are much more sensitive to blue light than red light we perceive dim light as bluer than it actually is. This effect is called the Purkinje effect. One implication for visual astronomy is the tendency to underestimate the brightness of red stars. There are also effects from: INTERSTELLAR EXTINCTION: Cosmic dust absorbs and scatters light from stars. INTERSTELLAR REDDENING: Red light is less affected than blue light. Distant stars thus appear more red than they actually are. This is especially apparent via Earth’s atmosphere. TELLURIC CONTAMINATION: The molecules in the Earth's atmosphere, however, absorb photon energy and then emit their own light. In the above image, the upper disk and the lower disk have exactly the same objective colour, and are in identical gray surroundings; based on context differences, humans perceive the squares as having different reflectances, and may interpret the colours as different colour categories.  Are there green stars? There are no truly green stars. However there are a few stars that appear green to some observers. This is usually because of an optical illusion. An average star like our Sun actually peaks in the green part of the spectrum. There are more photons coming from our Sun in the green part of the spectrum, and yet it looks white. The problem is that stars like our Sun cast off photons in so many colours that it all looks white from our perspective. The Sun Additive colour mixing: adding red to green yields yellow; adding all three primary colours together yields white. In order to get a green star, you would need to have a light curve that peaks right at green, but doesn’t give off light in many other colours.
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