stars The night sky is filled with stars that shine at different levels of brightness. The brightness of the stars we observe can be related to the size of the star or its distance from Earth. In order to understand this better we need to understand what a light year is. 1
To infinity, and beyond! The light year is used to measure the distance to stars or other celestial objects outside of the Solar System. The light year is a unit of distance, not time! It is the distance that light travels in one year. 1 year = 10 trillion km (9.46 x 10^12) The farther a star is from Earth, the longer it takes for the light from that star to reach Earth. The Sun is not the brightest star in space. Its luminosity (the total amount of energy produced by a star each second) is quite average. So why does the Sun appear the brightest to us on Earth? Because it is closer to us! 2
Star Colour and Temperature Astronomers use the colour of stars to determine temperature. In order of increasing temperature, stars can be red, orange, yellow, or blue. The Composition of stars Scientists use a spectrograph to analyze the light emitted by stars. Spectrographs split light energy into patterns of colour for observation. 3
Spectrograph When the light of elements go through a spectrograph only certain lines of colour will be seen. This is because each element emit light energy only at certain frequencies. These frequencies are influenced by the unique electron energy within each atom. Therefore, each atom will have its own unique spectrum To determine the elements that make up a star (star composition), spectrographs are used. They can compare the star s spectrum with the known spectra of the elements. 4
Star Masses The mass of the sun is 2 x 10^30 kg. This is referred to as one solar mass. Star masses are compared to the sun. Star masses can range between 0.1 to 120 solar masses The life cycle of stars Every star has a beginning, a middle, and an end. 5
The beginning Every star starts off inside a nebula. A massive cloud of gas and dust Gravitational forces pull in gas and dust = protostar formed The middle Nuclear fusion occurs = energy is emitted out and a glow is seen 6
The type of end depends on the mass of the star The end Red giant stars Average mass stars consume their fuel within 10 billion years. They cool, and the outer layers expand the star into a red giant. They slowly burn their fuel for up to 100 billion years and then the layers disappear and eventually they become white dwarfs. When cooled, they become black dwarfs. 7
High-mass stars consume their fuel faster and die more quickly and violently. Heavy elements form by fusion, and the star expands into a supergiant. An iron core forms that eventually collapses. The gases rush inward but are pushed back by the iron core which results in a massive explosion of the outer part of the star. This spectacular explosion is called a supernova. supernova Neutron stars (initial mass of 10-30 solar masses) A neutron star is a star so dense that only neutrons can exist at the core. The pressure is so great that electrons are squished into protons. 8
Black holes (initial mass larger than 30) The remains of the supernova explosion is so massive that gravity overwhelms all other forces, and the remains are crushed into a black hole. The gravitational force of a black hole is so strong that nothing can escape it, not even light. https://www.youtube.com/watch?v=4s7vydlgk3m 9
Page 381 in text Table 1 Hertzsprung-Russell diagram Organizational diagram that sorts stars based on their brightness, colour, size, and how long they live. P. 376 of text 10
Main sequence stars Lower, right side of the H-R diagram the stars tend to be reddish, cooler, and dim Upper left side of the H-R diagram the stars tend to be bluish, hot, and very luminous HOMEWORK Continue to work on Exam review Exam is only 6 days away! 11