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Astrophysics E2 Stellar Radiation and Stellar Types
Stellar Radiation and Stellar Types Star Formation Space contains particles of gas (mainly hydrogen) and dust, called the Inter Stellar Medium (ISM). Over billions of years a cloud of these particles may form due to gravitational attraction. The potential energy lost is converted to kinetic energy, resulting in higher temperatures. If the total mass is > 0.08 M⊙ then temperatures suitable for fusion will be created.
A star is a big ball of gas, with fusion going on at its center, held together by gravity! The mass of the star will determine the pressure in its core. i.e. more pressure due to gravity, so higher temperature, more radiation pressure and fusion and thus greater luminosity!
Luminosity and Apparent Brightness Luminosity:This is the energy emitted per second by (or total power of) a star, measured in Watts. Apparent brightness:This is the amount of energy per second per unit area arriving on Earth’s surface from a star (or power per unit area). Sometimes referred to as its intensity. Measured in Wm-2.
Apparent Brightness As the light travels further from the Star the photons become ‘spread out’ more. Therefore the further the light travels from the star, the less photons per unit area. So the further the star is from Earth, the less energy per second reaches one square meter of Earth’s surface. i.e. Decreases apparent brightness.
d b = L 4 π d2 Apparent brightness = Luminosity Area of sphere Sun Earth
Q. The Sun is a distance d=1.5 x 1011 m from the Earth. Estimate how much energy falls on a surface of 1m2 in a year (L= 3.90x1026 W).
Luminosity The Stefan-Boltzman law: So... Or... Power per unit area of star = σ T4 Luminosity = surface area x σ x T4 L = A σ T4 • L = 4π r2σ T4 σ (sigma) is the Stefan–Boltzman constant = 5.6 x 10-8 Wm-2K-4
Wien’s Law All objects above zero Kelvin emit a spectrum of electro-magnetic radiation (even objects we cannot see with the naked eye). The wavelength of radiation which is emitted with maximum intensity (‘brightness’) is called λmax (the peak wavelength):
From the graph we can see that the hotter the star, the shorter the peak wavelength. • Wien’s Law states... • so... λmaxα 1 / T • ( The constant = 2.90 x 10-3mK ) • so... • Measuring λmax allows you to determine a stars T ‘ λmax is inversely proportional to the stars Kelvin Temperature ’ λmax =constant / T • λmax =2.90 x 10-3T
The Hertzsprung-Russell Diagram Q. How could this diagram be used to find the distance from Earth of a main sequence star? (Hint: we could first measure b and λmax from earth.)