# Star

• Self-luminous, gaseous objects that fuse elements
• Obtain energy from nuclear fusion
• Considered a star when it can sustain itself through nuclear reactions (making its own energy)
• How long it takes a star to form depends on its mass
• Twinkle due to the atmosphere
• The brightest stars are not the ones close to us
• They are the ones emitting a large amount of energy
• There are ~6000 stars visible to the unaided eye
• A star is said to be true if it becomes hot enough to fuse protons to form helium

## Measuring the Characteristics of Stars

Characteristic Technique
Surface temperature Determine the colour (very rough)
Measure the spectrum and get the spectral type
Chemical composition Determine which lines are present in the spectrum
Luminosity Measure the apparent brightness and compensate for distance
Radial velocity Measure the Doppler shift in the spectrum
Rotation Measure the width of spectral lines
Mass Measure the period and radial velocity curves of spectroscopic binary stars
Diameter Measure the way a star's light is blocked by the Moon
Measure the light curves and Doppler shifts for eclipsing binary stars

## Measuring Distance from Earth

1. Parallax
2. Using brightness measurements
• Look at the star, determine it's color spectrum (good indication of it's actual brightness)
• Compare the actual brightness with the apparent brightness, they can determine the distance
• It's apparent brightness will be dimmer
3. Using an H-R diagram
1. Observe distant star's apparent magnitude and spectral type
2. Spectrum gives luminosity class
3. Spectral class and luminosity class gives a brightness independent of distance
4. Distance-magnitude relationship gives you distance

## Mass

• Knowing the mass of a star helps us estimate how long it will shine and what its ultimate fate will be
• The smallest mass that a true star can have is about $\frac{1}{2}$ that of the Sun
• Massive stars live for a short amount of time (die faster)
• They are brighter
• The brighter they are, the faster they burn
• More mass --> hotter core and denser core
• Makes fusion process happen faster

## Colour and Temperature

Star Colour Approximate Temperature
Blue 25,000 K
White 10,000 K
Yellow 6000 K
Orange 4000 K
Red 3000 K