Chapter 15 Surveying the Stars Most massive stars

Chapter 15 Surveying the Stars

Most massive stars are 100 MSun Least massive stars are 0. 08 MSun (MSun is the mass of the Sun)

Luminosity: Amount of energy a star radiates each second Apparent brightness: Amount of starlight that reaches Earth (energy per second per square meter)

Inverse Square Law Sun is very bright, but is the Sun really a “bright” star? The amount of light received from a star decreases with distance from the star. Example: 10 ly = 100 photons 20 ly = ? photons 2 x farther = 22 or 4 times dimmer, so only 100/4 = 25 photons

Most luminous stars: 106 Lsun Least luminous stars: 10 -4 LSun (LSun is luminosity of Sun)

Apparent magnitude • measures apparent brightness • 2 nd century BC Hipparchus • 6 categories of brightness ( 1 st mag. = the brightest, 6 th mag. = the faintest) • backward scale: brighter = lower the magnitude.

How it works: • There is a 100: 1 ratio of brightness over magnitudes 1 → 6 • So each magnitude difference is 5√ 100 = 2. 5 brightness

• Apparent magnitude depends on – distance – luminosity

Absolute Magnitude Measures the true brightness of a star To find the absolute magnitude, M, of a star we need to know: -Apparent magnitude, m -Distance from us, d Absolute magnitude is given by: m – M = 5 log (d – 5)

Parallax and distance Parallax: the apparent shift in position of a nearby object against a background of more distant objects

• Apparent positions of nearest stars shift by about an arcsecond as Earth orbits Sun

How do we measure stellar temperatures? Every object emits thermal radiation with a spectrum that depends on its temperature An object of fixed size grows more luminous as its temperature rises

Properties of Thermal Radiation 1. Hotter objects emit more light per unit area at all frequencies. 2. Hotter objects emit photons with a higher average energy.

Wien’s Law: 1/T higher temperature shorter wavelengths “bluer” star.

Hottest stars are 50, 000 K Coolest stars are 3, 000 K (Sun’s surface is 5, 800 K)

Absorption lines in star’s spectrum tell us ionization level

106 K 105 K 104 K Ionized Gas (Plasma) 103 K Neutral Gas 102 K Molecules 10 K Solid Level of ionization also reveals a star’s temperature

Absorption lines in a star’s spectrum correspond to a spectral type that reveals its temperature. O B A F G K M (Hottest) (Coolest) Also each spectral class is divided into 10: Sun G 2

Why no green stars?

• Remember, the spectra of stars differ because of differences in surface temperature. • All stars are made of the same “stuff” – 75% Hydrogen – 21 -24% Helium – 4% or less of heavy elements like Neon, Oxygen, Sulfur, Iron and Chlorine.