The Properties of Stars
Stars at Night When the sun sets, and the Moon is not out we quickly discover that our sky is filled with many thousands of stars.
Stars at Night Constellations, such as the Pleiades, are seen throughout the sky.
Stars at Night Upon closer examination we discover that these stars emit different levels and colours of light. Take the stars in the constellation Orion for example.
Light We are able to see these colourful stars because of the light they emit. But what is light?
Light Light is a form of energy. It travels in a straight line unless affected by extreme gravity. Like water, it travels in wavelengths.
The Electromagnetic Spectrum There are many forms of light. All of them, including visible light, are a form of electromagnetic radiation.
The Electromagnetic Spectrum All of the different forms of electromagnetic radiation (light) can be placed on a chart or spectrum.
The Electromagnetic Spectrum Stars give off all these different forms of light. Our eyes, and light collecting telescopes, can only see a very small portion of the spectrum called visible light.
Star Brightness Star brightness is dependant on three things: ProximityProximity TemperatureTemperature Size/MassSize/Mass
Star Brightness-Proximity Imagine a car was parked on a hill one kilometer from you. At the same time a friend was standing next to you with a small pocket flashlight. Each light was shone in your eyes. Which would appear brighter? Which is really brighter? Star brightness works the same way.
Star Brightness-Proximity The actual brightness of a star is dependant on how much energy it emits. Its apparent brightness is determined by this factor but also how far or close it is to Earth.
Star Brightness-Temperature Temperature can affect brightness. Which has the highest temperature: A light bulb filament.A light bulb filament. A heating element.A heating element.
Star Brightness-Temperature Different temperature stars have different brightness. The hotter the star the brighter the star.
Star Brightness-Colour Temperature also affects the colour of a star. The cooler the star the closer it is to red in colour. The hotter the star the closer it is to blue-white in colour.
Star Brightness-Colour Betelgeuse, a red giant, has a lower temperature (and therefore brightness) than our sun.
Star Brightness-Colour Rigel, a large white supergiant, is not only large but extremely hot. It is therefore extremely bright.
Hang on… Let’s not get ahead of ourselves. Turn to page 462 and …
Star Temperature-Colour As we have seen star temperature can affect brightness. Different star types also have different temperatures. They can be linked to colour.
Star Temperature-Colour Generally speaking red stars have the lowest temperatures while blue-white stars are the hottest.
Star Colour and Composition While colour can tell us about brightness and temperature it can also tell us what a star is made of. To do this they use something called a spectroscope.
Star Colour and Composition A spectroscope acts like a prism, dividing light into the different spectral colours.
Star Colour and Composition When light leaves a star it passes through the elements in the star. This leaves darkened lines on the spectrum of light as some of the elements block out that light.
Activity Turn to page 465 in your text and read the instructions with your teacher, then complete the analysis questions.
Star Size and Mass Stars come in many different sizes. Star size is measured once we know the distance to the star from Earth. (This is done using something called parallax.) Combining this knowledge with the brightness and temperature of the star astronomers can deduce the size of the star.
Star Size Our sun is the baseline. Other stars can be as small as 0.1 the radius of our sun or as large as 1000 times as large.
Star Size Arcturus is a red giant and the third brightest star in the sky.
Star Size VY Canis Majoris is the largest known star in the universe.
Star Mass For many years there was no way to calculate the mass of any star, even our sun. In order to calculate the mass of a large object (like a star) you need to see how gravity affects it. Stars are so big that they are not affected by the gravity of planetary objects.
Star Mass What was needed was another large object close to a star, such as another star.
Binary Stars A binary star is a star system where two stars orbit one another. Their orbits allow astronomers to calculate the mass by observing how the gravity of one affects the gravity of the other.
Solar Mass Star mass is expressed as solar mass. The Sun has a solar mass of 1. All the others are compared to the sun.
Putting It All Together The Hertzsprung-Russell Diagram To make sense of all this two astronomers, Enjar Hertzsprung and Henry Norris Russell devised a diagram to better explain it.
The Hertzsprung-Russell Diagram
Scientists began to notice something about the diagram. Most stars fit in the center. They were referred to as main sequence stars.
The Hertzsprung-Russell Diagram Other stars were outside this main sequence. Why? Were they special in some way? Exceptions to the rule?
Homework Please read all of section 14.1, from pages 462 to 467. Answer questions 1-6 on p.467 Vocabulary Electromagnetic SpectrumElectromagnetic Spectrum Binary starsBinary stars Solar MassSolar Mass
Homework to Hand In Page Please complete a hand drawn and coloured version of the Hertzprung-Russell Diagram. Your diagram should be neatly presented on unlined paper. Don’t forget to clearly label each axis and have the diagram coloured. With your diagram you need to include an explanation (approximately 1-2 paragraphs), in your own words of how the diagram works.