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STARS. What is a star? Why do they shine? How old are they?. Stars shine for millions to billions of years, much longer than a human lifetime. Yet, we've been able to piece together how stars are born, shine and eventually die.
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STARS What is a star? Why do they shine? How old are they?
Stars shine for millions to billions of years, much longer than a human lifetime.Yet, we've been able to piece together how stars are born, shine and eventually die.
The brightness of a star depends on both distance and luminosity.
Properties of Stars DEFINITIONS: apparent brightness versus absolute brightness or luminosity apparent m magnitude versus absolute magnitude
Inverse Square Law Same Luminosity, Twice as far away --> 4x dimmer Measure Apparent magnitude And Distance (parallax) To stars luminosity
The relationship between apparent brightness and luminosity depends on distance: Luminosity Brightness = 4 (distance)2 We can determine a star’s luminosity if we can measure its distance and apparent brightness: Luminosity = 4 (distance)2 (brightness)
Thought Question How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/3 as bright. B. It would be only 1/6 as bright. C. It would be only 1/9 as bright. D. It would be three times brighter.
Thought Question How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/3 as bright. B. It would be only 1/6 as bright. C. It would be only 1/9 as bright. D. It would be three times brighter.
p = parallax angle 1 d (in parsecs) = p (in arcseconds) 1 d (in light-years) = 3.26 p (in arcseconds) Parallax and Distance
Most luminous stars: 106LSun Least luminous stars: 10–4LSun (LSun is luminosity of Sun)
m M = apparent magnitude, = absolute magnitude Apparent brightness of star 1 - = m m 1 / 5 ( 100 ) 1 2 Apparent brightness of star 2 Luminosity of star 1 - = M M 1 / 5 ( 100 ) 1 2 Luminosity of star 2 The Magnitude Scale
Stellar Surface Temperatures Measure the surface temperature of stars by taking a spectrum of the star and using Wien's Law.
Hottest stars: 50,000 K Coolest stars: 3000 K (Sun’s surface is 5800 K.)
In addition, the absorption lines in the stellar spectra are sensitive to temperature.
Originally classified as A,B,C,.. the classification of stellar spectra was recast into OBAFGKM by Cecilia Payne-Gaposchkin. OBAFGKM: Oh, Be a Fine Girl Kiss Me
Absorption lines in 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, Kiss Me • Only Boys Accepting Feminism Get Kissed Meaningfully
Stellar Masses Stellar masses are measured by observing binary stars, and using Kepler's 3rd Law to determine the mass of the stars from the period of their orbit. Types of Binary Stars: * Visual Binaries -- direct image shows two stars orbiting each other * Spectroscopic Binaries – two stars are too close to see as separate stars, but spectrum shows absorption lines from two stars with variable Doppler shifts. * Eclipsing Binaries -- one star disappears when it passes behind the other
Most massive stars: 100MSun Least massive stars: 0.08MSun (MSun is the mass of the Sun.)
Summary of Stellar Properties: Also there are Giants, Supergiants and white dwarfs: Same Temperature as stars in the table, but different luminosity and radii.
B star is much larger, brighter and hotter than the Sun. An example is HD93129A shown below:
The Hertzsprung-Russell Diagram • When you plot LUMINOSITY versus Temperature for stars in the sky, the result is not a scatter plot • Hertzsprung and Russell first realized this, and the diagram they made is still an important tool in astronomy for understanding stars
H-R Diagram Hertzsprung- Russell Diagram Plot Luminosity versus Surface Temperature (or equivalently, Luminosity versus spectral classification)
Main Sequence: Stars fusing hydrogen to helium Example: The Sun
Giants are more luminous than a main sequence star of the same temperature. Giants tend to be relatively cool (T < 6000 Kelvin) but luminous (L = 100 to 1000 Lsun). Supergiants are even more luminous than giants. Supergiants can have any temperature, but they are always VERY luminous, with L = 100,000 to 1,000,000 Lsun. White Dwarfs are less luminous than a main sequence star of the same temperature. They are called WHITE dwarfs because they are fairly hot; white-hot, in fact, with temperatures of T > 5000 Kelvin. The are low in luminosity, with L = 0.0001 to 0.01 Lsun.
In a sample of 1,000,000 stars from the Milky Way, • on average you'd find: • 900,000 main sequence stars • 96,000 white dwarfs • 4000 giants • 1 supergiant
So now we have a range of stellar colors and sizes. For example, Aldebaran is a red supergiant star:
Stellar Lifetimes on the Main Sequence: More Massive Stars are more luminous, and are burning hydrogen more efficiently. They therefore have shorter lifetimes on the Main Sequence before they burn up the Hydrogen in their core
After the hydrogen fuel in the core of the main sequence star is used up, There is no longer enough thermal pressure in the core to balance gravitational collapse. No more hydrostatic equilibrium What happens next? Star rearranges itself outer layers expand and cool Star becomes a red giant or supergiant Eventually more processes happen and the red giant becomes a supernova or planetary nebula, and then a white Dwarf, neutron star or black hole – more on this later
STAR CLUSTERS All the stars in a cluster are (1) at the same distance, and (2) were formed together, so are the same age. Open Clusters: Young (Less than a billion years old) found in the disk of the Milky Way typically 100's - 1000's of stars often have gas and dust Globular Clusters: Contain oldest stars in the Milky Way -- 12-13 billion years old stars in orbit around center of cluster, gravitationally bound Typically 100,000 - million stars never have gas and dust
