Stars Properties of Stars
Do all stars appear the same? How are they different? Which one looks the coolest? Hottest? Are they all the same brightness? Do they all look the same size?
Luminosity: Amount of power a star radiates (energy per second = watts) Apparent brightness: Amount of starlight that reaches Earth (energy per second per square meter)
Luminosity passing through each sphere is the same. Area of sphere: 4π (radius) 2 Divide luminosity by area to get brightness.
The relationship between apparent brightness and luminosity depends on distance: Brightness = Luminosity 4π (distance) 2 We can determine a star s luminosity if we can measure its distance and apparent brightness: Luminosity = 4π (distance) 2 (Brightness)
So how far away are these stars?
Two Kinds of Brightness Apparent Magnitude: How bright the object appears to us on Earth. Absolute Magnitude: How bright a star actually is, its intrinsic brightness
The Magnitude Scale m = apparent magnitude M = absolute magnitude apparent brightness of Star 1 apparent brightness of Star 2 = (1001/5 ) m 1 m 2 luminosity of Star 1 luminosity of Star 2 = (1001/5 ) M 1 M 2
Apparent Magnitude is a number that represents the apparent Smaller/negative brightness of stars as seen on Earth numbers correspond to BRIGHTER stars and Bigger/positive numbers correspond to The larger the number the DIMMER dimmer stars the object will appear from Earth Note we use the letter m for apparent magnitude
Absolute Magnitude (M) A number that represents the actual, intrinsic brightness or luminosity of a star Compares the brightness of all the stars as if they were all the same distance from Earth (10 pc or 3.26 ly)
Compare some stars: Absolute Apparent M Sun = 4.8 m Sun = -26 M Sirius = 1.4 m Sirius = -1.46 M Betelgeuse = -5.6 m Betelgeuse = 0.50 Which star looks brightest from Earth? Which star is brightest?
By comparing the apparent (m) and absolute magnitude (M) numbers we can estimate a stars distance from Earth. When m = M, then the star is located exactly 10 pc away When m<m, then the star appears brighter than it would if it were 10 pc away so it must be closer than 10 pc When m>m, then the star appears dimmer than it would if it were 10 pc away so it must be farther than 10pc
Lecture Tutorial Apparent and Absolute Magnitudes of Stars Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take the time to understand it now! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group. If you are still stuck or are not sure what the tutorial is asking, ask me.
Stellar Temperatures Hottest stars: 50,000 K Coolest stars: 3000 K (Sun s surface is 5800 K)
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
Lecture Tutorial Luminosity, Temperature, and Size, part I Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group. If you are still stuck or are not sure what the tutorial is asking, ask me.
10 6 K 10 5 K 10 4 K 10 3 K 10 2 K Ionized Gas (Plasma) Neutral Gas Molecules Level of ionization also reveals a star s temperature. 10 K Solid
Absorption lines in a star s spectrum tell us its ionization level.
Lines in a star s spectrum correspond to a spectral type that reveals its temperature: (Hottest) O B A F G K M (Coolest)
Remembering Spectral Types (Hottest) O B A F G K M (Coolest) Oh, Be A Fine Girl/Guy, Kiss Me Only Boys Accepting Feminism Get Kissed Meaningfully
Pioneers of Stellar Classification Annie Jump Cannon and the calculators at Harvard laid the foundation of modern stellar classification.
Measuring Stellar Masses
We measure mass using gravity. Direct mass measurements are possible only for stars in binary star systems. p 2 = 4π 2 a 3 G (M 1 + M 2 ) Isaac Newton p = period a = average separation
Need two out of three observables to measure mass: 1. Orbital period (p) 2. Orbital separation (a or r = radius) 3. Orbital velocity (v) v For circular orbits, v = 2pr / p r M
Types of Binary Star Systems Visual binary Eclipsing binary Spectroscopic binary About half of all stars are in binary systems.
Most massive stars: 100M Sun Least massive stars: 0.08M Sun (M Sun is the mass of the Sun.)