Transcript d 2
The Sun – Details • • • • • • • Radiation Zone and Convection Zone Chromosphere Photosphere Corona Sunspots Solar Cycle Flares & Prominences Sunspots • Dark, cooler regions of photosphere first observed by Galileo • About the size of the Earth • Usually occur in pairs • Frequency of occurrence varies with time; maximum about every 11 years • Associated with the Sun’s magnetic field Sunspots and Magnetism • Magnetic field lines are stretched by the Sun’s rotation • Pairs may be caused by kinks in the magnetic field The Solar Cycle Understanding Stars • “Understanding” in the scientific sense means coming up with a model that describes how they “work”: – Collecting data (Identify the stars) – Analyzing data (Classify the stars) – Building a theory (Explain the classes and their differences) – Making predictions – Testing predictions by more observations Identifying Stars - Star Names • Some have names that go back to ancient times (e.g. Castor and Pollux, Greek mythology) • Some were named by Arab astronomers (e.g. Aldebaran, Algol, etc.) • Since the 17th century we use a scheme that lists stars by constellation – in order of their apparent brightness – labeled alphabetically in Greek alphabet – Alpha Centauri is the brightest star in constellation Centaurus • Some dim stars have names according to their place in a catalogue (e.g. Ross 154) Classification by Star Properties • What properties can we measure? – – – – – – – distance velocity temperature size luminosity chemical composition mass Distances to the Stars • Parallax can be used out to about 100 light years • The parsec: – Distance in parsecs = 1/parallax (in arc seconds) – Thus a star with a measured parallax of 1” is 1 parsec away – 1 pc is about 3.3 light years • The nearest star (Proxima Centauri) is about 1.3 pc or 4.3 lyr away – Solar system is less than 1/1000 lyr Homework: Parallax • Given p in arcseconds (”), use d=1/p to calculate the distance which will be in units “parsecs” • By definition, d=1pc if p=1”, so convert d to A.U. by using trigonometry • To calculate p for star with d given in lightyears, use d=1/p but convert ly to pc. • Remember: 1 degree = 3600” • Note: p is half the angle the star moves in half a year Our Stellar Neighborhood Scale Model • If the Sun = a golf ball, then – – – – – Earth = a grain of sand The Earth orbits the Sun at a distance of one meter Proxima Centauri lies 270 kilometers (170 miles) away Barnard’s Star lies 370 kilometers (230 miles) away Less than 100 stars lie within 1000 kilometers (600 miles) • The Universe is almost empty! • Hipparcos satellite measured distances to nearly 1 million stars in the range of 330 ly • almost all of the stars in our Galaxy are more distant Reminder: Three Things Light Tells Us • Temperature – from black body spectrum • Chemical composition – from spectral lines • Radial velocity – from Doppler shift Luminosity and Brightness • Luminosity L is the total power (energy per unit time) radiated by the star, actual brightness of star, cf. 100 W lightbulb • Apparent brightness B is how bright it appears from Earth – Determined by the amount of light per unit area reaching Earth – B L / d2 • Just by looking, we cannot tell if a star is close and dim or far away and bright Brightness: simplified • 100 W light bulb will look 9 times dimmer from 3m away than from 1m away. • A 25W light bulb will look four times dimmer than a 100W light bulb if at the same distance! • If they appear equally bright, we can conclude that the 100W lightbulb is twice as far away! Same with stars… • Sirius (white) will look 9 times dimmer from 3 lightyears away than from 1 lightyear away. • Vega (also white) is as bright as Sirius, but appears to be 9 times dimmer. • Vega must be three times farther away • (Sirius 9 ly, Vega 27 ly) Distance Determination Method • Understand how bright an object is (L) • Observe how bright an object appears (B) • Calculate how far the object is away: B L / d2 So L/B d2 or d √L/B Homework: Luminosity and Distance • Distance and brightness can be used to find the luminosity: L d2 B • So luminosity and brightness can be used to find Distance of two stars 1 and 2: d21 / d22 = L1 / L2 (since B1 = B2 ) i.e. d1 = (L1 / L2)1/2 d2 The Magnitude Scale • A measure of the apparent brightness • Logarithmic scale • Notation: 1m.4 (smaller brighter) • Originally six groupings – 1st magnitude the brightest – 6th magnitude is 100x dimmer • So a difference of 5mag is a difference of brightness of 100 • Factor 2.512=1001/5 for each mag. Absolute Magnitude • The absolute magnitude is the apparent magnitude a star would have at a distance of 10 pc. • Notation example: 2M.8 • It is a measure of a star’s actual or intrinsic brightness called luminosity • Example: Sirius: 1M.4, Sun 4M.8 – Sirius is intrinsically brighter than the Sun Finding the absolute Magnitude • To figure out absolute magnitude, we need to know the distance to the star • Then do the following Gedankenexperiment: – In your mind, put the star from its actual position to a position 10 pc away – If a star is actually closer than 10pc, its absolute magnitude will be a bigger number, i.e. it is intrinsically dimmer than it appears – If a star is farther than 10pc, its absolute magnitude will be a smaller number, i.e. it is intrinsically brighter than it appears Measuring the Sizes of Stars • Direct measurement is possible for a few dozen relatively close, large stars – Angular size of the disk and known distance can be used to deduce diameter Indirect Measurement of Sizes • Distance and brightness can be used to find the luminosity: L d2 B (1) • The laws of black body radiation also tell us that amount of energy given off depends on star size and temperature: L R2 T4 (2) • We can compare two values of absolute luminosity L to get the size Sizes of Stars • Dwarfs – Comparable in size, or smaller than, the Sun • Giants – Up to 100 times the size of the Sun • Supergiants – Up to 1000 times the size of the Sun • Note: Temperature changes!