Stars …just kidding. In a nutshell… -born from clouds of interstellar gas -shine by nuclear...
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Stars
…just kidding
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In a nutshell…
• -born from clouds of interstellar gas
• -shine by nuclear fusion
• -shine for millions or billions of years
• -die
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• 75% of any star’s mass at birth is Hydrogen, 25% is Helium, no more than 2% is made up of elements heavier than Helium.
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• All stars are similar, but they appear different from each other for two reasons:
• -mass
• -we see stars at different ages in their lives
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• Were once classified by brightness and location in our sky. This tells us little about its true nature.
• Today, astronomers classify stars according to luminosity and surface temperature.
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Luminosity
• -
• the total amount of power it radiates into space, measured in watts
• -the sun’s luminosity is 3.8*10 e26 watts.
• -cannot be measured directly; depends on distance and true luminosity
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• Apparent brightness is the amount of light reaching us per unit area
• This obeys the inverse square law!
• (if you double the distance between us and the star, its apparent brightness decreases by a factor of ¼)
• Solar luminosity: compares a star’s luminosity to the sun’s
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Measuring apparent brightness• -We do this using a CCD (records how much
energy strikes its light-sensitive surface each second)
• -must calibrate for ground-based telescopes, since the atmosphere of Earth absorbs much of a star’s light
• -when we perceive a star’s brightness, we are only measuring the visible portion of the spectrum
• -total luminosity or total apparent brightness is used to describe the luminosity and apparent brightness IF we could detect photons across the electromagnetic spectrum
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Stellar Parallax
• -The small annual shifts in a star’s apparent position caused by the Earth’s motion around the sun.
• the distance to an object with a parallax angle of 1 arcsecond is one parsec (pc)
• 1 pc=3.26 light years=3.09 *10e13 km• d=1/p
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The Magnitude System
• -watts per square meter
• -devised by Hipparchus (190-120BC)
• -originally classified stars according to how bright they look to our eyes, since this was the only way of making observations at that time
•
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• -brightest stars: first magnitude, second brightest: second magnitude
• -These descriptions are called apparent magnitude, since it compares how different stars appear to us in the sky.
• -Star charts often use different sized dots to represent apparent magnitude.
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• Modern times
• -Has been modified: 0 is brighter than 1, -1 is brighter than 0
• -Sirius: apparent magnitude of -1, is the brightest star in the night sky
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• Absolute magnitude
• -The apparent magnitude of a star, if it were 10 parsecs away from Earth
• -The brighter the star, the smaller the magnitude!
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Stellar Surface Temperature
• -Easier to measure than luminosity, since it does not depend on distance.
• -Determined directly from its spectrum, or color
• -The surface temperature of a star determines the color that it shines
• -Red-yellow-blue (cool to hot)• -Color becomes more apparent when
stars are viewed through a telescope
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Spectral Type
• -Emission and absorption lines provide a more accurate way to determine its surface temperature.
• -Ionized elements (hot), molecules (cool)
• -Astronomers classify stars according to surface temperature by assigning a spectral type from the types of spectral lines present in a star’s spectrum.
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• O=Blue
• B=Blue-white
• A=White
• F=Yellow-white
• G=Yellow
• K=Orange
• M=Red
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Binary Systems
• Visual binary: A binary star system which we can observe with the eye.
• These stars orbit each other, and appear to change positions. These are
• Rare!• -Sometimes, we can observe a shift in position, but not
see the companion, because it is too dim to be seen. This is called an eclipsing binary.
• -If a binary system is neither visual nor eclipsing, the only way we can determine that it has a partner is from Doppler shifts in its spectral lines. This is called a spectroscopic binary.
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The Hertzprung-Russel diagram
• -Used to plot the surface temperature vs. the luminosity of stars, and classify them using these characteristics.
•• Patterns in the H-R diagram• -Most stars fall along the main sequence• -stars in the upper right are called supergiants,
they are very large and very bright• -just below the supergiants are the giants, which
are smaller in radius and lower in luminosity• -the stars in the lower left are small in radius, and
appear whit in color because of their high temperature, these are white dwarfs.
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• stars are classified by their spectral type and luminosity class that describes the region of the diagram in which the star falls.
• Class represents• I supergiants• III giants• V main sequence• *II and IV intermediate • -our sun’s classification is G2 V
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The Main Sequence
• These stars are fusing hydrogen into helium in their cores.
• Most stars call along this line, since most of their lives is spent fusing H into He.
• Stellar masses increase upward along the main sequence.
• More stars fall on the lower end of the main sequence than on the upper end, which tells us that low-mass stars are more common than high-mass stars
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• The sun is an average main-sequence star.• A star’s main sequence lifetime is determined by
its supply of hydrogen.• More massive stars live shorter lives because
they use up their hydrogen at a much faster rate than less massive stars.
• The main sequence lifetime for our sun is 10 billion years. It is currently middle-aged.
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Giants and Supergiants
• Nearing the ends of their lives because they have exhausted their core H.
• Stars grow more luminous when they begin to run out of fuel.
• These stars can be seen even when they aren’t close to us.
• Often identifiable by their reddish color.
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• These are rarer than main sequence stars because we catch most stars in the act of hydrogen burning, and relatively few in their later stages of life.
• These eventually run out of fuel completely., and become white dwarfs, which are roughly the size of Earth.
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Pulsating Variables
• Have atmospheres that alternately expand and contract, causing the star to rise and fall in luminosity.
• Most inhabit the instability strip on the H-R diagram that lies between the main sequence and the red giants.
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Star Clusters
• All stars are born from clouds of gas, and most form in groups.
• Open and globular clusters
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Open• Always found in the disk of the galaxy
• Can contain up to several thousand stars and span 30 light years.
• Pleiades: in the constellation Taurus, called the Seven Sisters.
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Globular
• Found in both the disk and the halo of our galaxy
• Can contain more than a million stars, concentrated in a ball, 60-150 light years across.
• These are both useful to astronomers because all of the stars are at the same distance from Earth, and all of the stars formed at relatively the same time.
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Cluster ages from Main Sequence Turnoff
• The precise point at which a cluster’s main sequence diverges from the standard main sequence
• Age of cluster = lifetime of stars at main sequence turnoff point
• This is the most powerful tool for evaluating the ages of star clusters.
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SUMMARY
• All stars are made primarily of hydrogen and helium.
• Much of what we know about stars comes from studying the patterns that develop when we plot stellar surface temperature and luminosity on the H-R diagram.
• Stars spend most of their lives on the main sequence, fusing hydrogen into helium.
• Much of what we know about the universe comes from the study of star clusters.