12.1 Space.pdf - Stellar Properties Space The only information we get from distant stars are electromagnetic radiation we receive This allows us to

12.1 Space.pdf - Stellar Properties Space The only...

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Space Stellar Properties: The only information we get from distant stars are electromagnetic radiation we receive. This allows us to classify stars into various groups such as red giant, white dwarf and blue supergiant . Electromagnetic emissions from stars can tell us their temperature, chemical composition, speed of movement, approximate age and size. The Stefan-Boltzmann Law: Astronomers found a precise measure to classify the actual brightness of stars: their output power , which is known as luminosity . Luminosity is the rate at which energy of all types is radiated by an object in all directions This depends on both the object’s size and, more importantly, its temperature . Stefan-Boltzmann law, states that the output power from a black body is proportional to its surface area and the forth power of its temperature in Kelvin: 𝑳𝑳 = 𝝈𝝈𝝈𝝈𝑻𝑻 𝟒𝟒 Stefan-Boltzmann constant, σ = 5.67 x 10 -8 Wm -2 K -4 For a sphere: 𝑳𝑳 = 𝟒𝟒𝟒𝟒𝒓𝒓 𝟐𝟐 𝝈𝝈𝑻𝑻 𝟒𝟒 Assuming that the star acts like a black body emitter
Space Wien’s Law: To calculate luminosity, we needed to know the temperature of the star. Some wavelengths are given off with more intensity than others. We know that as the temperature of a black body increases , it emits more energy . At higher temperatures the curve has a more pronounced peak, and the wavelength of the peak output gets shorter as the temperature rises. Wien’s law describes the relationship between the peak output wavelength and temperature: 𝝀𝝀 𝒎𝒎𝒎𝒎𝒎𝒎 𝑻𝑻 = 𝟐𝟐 . 𝟖𝟖𝟖𝟖𝟖𝟖 × 𝟏𝟏𝟏𝟏 −𝟑𝟑 𝒎𝒎𝒎𝒎 Star Classes: Stars were put into groups according to their temperature. Stars with similar temperatures tend to share many other features: The hotter stars tend to have more mass and are more luminous . The larger gravitational pressure at the centre of a massive star makes the nuclear fusion reactions within the star run very fast . Using the hydrogen fuel in the star at an incredible rate. With large size and high temperatures, the hotter stars are very luminous, giving off a great deal of energy.
Space The Hertzsprung-Russell Diagram: The graph of luminosity against star temperature shows a general correlation. This graph can give very useful insight, but it is not known as a graph but a Hertzsprung-Russell Diagram : Most stars we observe fall on a diagonal line across the Hertzsprung- Russell diagram, which is called the Main Sequence . These stable stars will exist in this state for most of their lifetime . The correlation represents the connections between brightness and high temperature. Plot is always drawn with temperature decreasing along the X axis . Stellar Evolution: Majority of ordinary matter in the Universe is hydrogen (around 75%) or helium (around 24%) and it is from these elements that stars are initially formed. Collection of these gasses are called a protostar.

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