Key Areas covered The temperature of stellar objects

Key Areas covered • The temperature of stellar objects is related to the distribution of emitted radiation over a wide range of wavelengths. • The wavelength of this distribution is shorter for hotter objects than cooler objects. • Qualitative relationship between radiation per unit surface area and temperature of a star. • Cosmic microwave background radiation as evidence for the big bang and subsequent expansion of the universe.

What we will do today • State how temperature of stellar objects is related to the wavelengths of the radiation they emit • Describe how their wavelengths vary for hot and cold objects • State the relationship between radiation per unit surface area and temperature of a star • Describe the Big Bang Theory • Give evidence to support the Big Bang Theory

Big Bang Theory

What happens to the colour of objects as they are heated? • When an object is heated it does not initially glow, but radiates large amounts of energy as infrared radiation. We can feel this if we place our hand near, but not touching, a hot object. • As an object becomes hotter it starts to glow a dull red, followed by bright red, then orange, yellow and finally white (white hot). At extremely high temperatures it becomes a bright blue-white colour.

Practical application - Pottery • Early potters used the fact that materials glow different colours at different temperatures to determine the temperature inside their kilns. • In 1792 the famous porcelain maker Josiah Wedgewood noted that all bodies become red at the same temperature. Colour Temp (o. C) 550 Dark red Cherry 750 red Orange 900 Temp (K) 823 1023 1173 Yellow 1000 1273 White 1200 1373

Light and temperature • We can see that the temperature of an object affects the light it gives off. • This means that the temperature of an object is linked to both the frequency and wavelength of the light it emits. • A graph of intensity versus wavelength has a characteristic shape and can be shown in a “Planck distribution. ”

Planck distribution • Is also known as a black-body spectrum and has three main features: 1. The basic shape is more or less the same 2. As the temperature of the object increases, the peak intensity wavelength decreases (so frequency increases) 3. As the temperature of the object increases, the intensity and energy increases • • Summarised as follows: T ↑ λ ↓ therefore f ↑ intensity ↑ energy ↑

2014 Revised Higher B

Lesson starter – Olber’s paradox • If the night sky is full of stars why is it dark? • Why is the sky dark at night? [VIDEO]

Thinker - what have we learned so far? • • Redshift – when objects move away from us their wavelength is increased, we now know that galaxies are moving away from us Hubble’s law – can help us work out the speed and distance of stellar objects, we now know galaxies that are further away from us are speeding up Expanding universe – scientific evidence shows us that the universe is expanding and this expansion is accelerating QUESTION: What does all the evidence above tell us about the beginning of the universe?

How did the universe begin? • There was a sudden appearance of energy which consequently became matter and is now everything around us. • There were two theories regarding its beginning: 1. The Steady State Universe – where the universe had always been and would always continue to be in existence. 2. The Created Universe – where at some time in the past the universe was created

What is the Big Bang Theory? The Big Bang

What is the Big Bang Theory? • • • The Big Bang took place around 13. 8 billion years ago. The universe was originally very hot and very dense concentrated in a tiny point known as a singularity (smaller than an atom). It caused our universe to expand suddenly from the singularity bringing time and space into existence. Following the Big Bang, temperatures rapidly cooled and tiny particles of matter began to form. The first atoms to form were hydrogen and helium. This matter created stars, galaxies and planets.

What evidence is there for the Big Bang? • There a number of factors that provide evidence for the Big Bang:

1. The expanding universe • • • Light from the stars in the galaxy is enough to light our sky at night. The only explanation that it is not is that the stars are moving away from us. An expanding universe must have started out infinitely small and agrees with the Big Bang Theory

2. Cosmic Microwave Background Radiation • • This radiation can be detected on Earth coming from all directions in space. This radiation is constant throughout the universe. As the universe expands the wavelength of the radiation emitted increases (less frequency) down to the microwave region. This radiation provides a constant temperature throughout the universe (approximately 3 K).

3. Abundance of light elements • • The universe has an abundance of light elements such as Hydrogen and Helium. As the universe grew these elements fused together to create the other known elements.

2014 Revised Higher D

2012 Revised Higher – Qu: 22 (1/2)

2012 Revised Higher – Qu: 22 (2/2)

Solution

Solution

2013 Revised Higher – Qu: 25

Solution (a)

Possible solution (b) • Light travels at approximately 3 x 108 ms-1 • One light year is the distance travelled by light in one year – 3 x 108 x 365 days x 24 hours x 60 mins x 60 seconds = 9. 46 x 1015 m. • As stars in our galaxy can be well over hundreds and thousands of light years away the time it takes their light to reach us is massive. • For example our nearest star (apart from the Sun) is over 4 light years away, this means that the light we see from this star was sent over 4 years ago. • Effectively, we are looking back in time.

Open-ended question 2014 Revised Higher

Possible answer • Yes the Universe is expanding. We know this because of: 1. Stars / galaxies are moving away from us; 2. Cosmic microwave background radiation; and 3. The abundance of light elements • It is expanding in all directions with the space between each galaxy increasing as they move away from each other • “as fast as it can go, at the speed of light” – this lyric is wrong as it suggests a constant speed however the expansion is actually accelerating. • “ 12 million miles a minute” - using 1610 metres per mile this converts to 1. 932 x 1010 metres a minute which is 3. 22 x 108 m/s and this is slightly more than the accepted value for the speed of light. • “the fastest speed there is” – this is true as nothing can travel faster than the speed of the light

Open-Ended Watch the Video for the song ‘Big Bang Theory’ by the Bare Naked Ladies. Use your knowledge of physics to comment on the accuracy of the Physics in this song