# The Hertzsprung Russell (HR) Diagram

Source: Richard Powell CC-BY SA 2.5

# Coloured dots of light

• The HR diagram shows us that more luminous stars are bluer; less luminous stars are redder.
• The vast majority of stars lie on the main sequence.
• The vast majority of main sequence stars are at the red end.

# Kelvin temperature scale

• Named after Lord Kelvin of Glasgow University
• 0 K is (approximately) -273°C
• 0 K is absolute zero; there's no lower temperature
• Kelvin to Celsius: subtract 273
• Celsius to Kelvin: add 273

# Typical temperatures

Example Temperature
Cosmic background 3 K
Pluto 44 K
Mars 210 K
Earth 288 K
Venus 737 K
Proxima Centauri 3000 K
The Sun 5800 K
Rigel 11,000 K
Solar corona 2,000,000 K

# The electromagnetic spectrum

Source: P. Wiena CC-BY SA 3.0

# Temperature and colour

Fundamental physics tells us that objects change colour as they heat up, e.g.

• less than about 1000 K - infrared emission
• 1000K to 4000 K - red to orange
• 4000 - 7000 K - orange to yellow/white
• 7000 K and above - yellow/white to blue/white, some ultra violet

# Spectrum and temperature

Source: Darth Kule Public Domain

# Wavelength

• The colour of light can be quantified by its wavelength
• 1 nm is 1 nanometre or one billionth of a metre
• 1 nm = 1×10-9 m = 0.000 000 001 m
• Also used is 1 μm or 1 micrometre or millionth of a metre
• 1 μm = 1×10-6 m = 0.000 001 m
• The Sun's spectrum peaks at about 500 nm or 0.5 μm

# Wien's law

Wien's law states that temperature in K is inversely proportional to peak wavelength.

For example:

• The Sun's temperature is 5800 K and its spectrum peaks at 500 nm, which is visible.
• A star at double the temperature - 11,600 K - has a spectrum that peaks at 250 nm, which is ultraviolet.
• A star at half the temperature - 2900 K - has a peak at 1000 nm, which is infrared.

But, this is just peak emission - the Sun also emits in the infrared and Rigel (11,000 K) emits in the visible too.

# Emission examples

Example Temperature Emission
Cosmic background 3 K Microwaves
Pluto 44 K Infrared
Mars 210 K Infrared
Earth 288 K Infrared
Venus 737 K Infrared
Proxima Centauri 3000 K Infrared/visible
The Sun 5800 K Visible
Rigel 11,000 K Ultraviolet/visible
Solar corona 2,000,000 K X-rays