The Sun and Stars

Topics the Sun Stars Features Structure Composition How do we know? brightness and luminosity distance temperature mass classification

Sun Photosphere Chromoshpere Corona the bright disc we see as “the sun” the “surface”of the Sun causes the absorption lines in the Sun’s spectrum (called Fraunhoffer lines) temperature 5800 K mostly hydrogen (94%) and helium (5.9%) Chromoshpere thin layer above the photosphere 7000K – 15,000K Corona halo of high energy gas, very hot (2 MK) emits radiation (thus we can see emission lines), mostly x-rays

Sun

Activity on the Sun Sun Spots prominences Solar Flares cyclic cooler region of the photosphere related to the Sun’s magnetic field prominences Solar Flares solar storm emits radiation and particles high temperature (>5 MK) affect radio communications on Earth

Sunspot cycle 11 year average cycle Time between maxima can be as long as 15 years or as short as 7 years. Solar activity may affect Earth’s climate, but the mechanisms are unknown.

General Theory of Relativity Newton’s theory of gravitation does not explain everything bending of light near massive objects (light is seemingly attracted to mass?) precession of the perihelion of Mercury (now it’s known that the perihelions of other planets precess as well) Local space-time is curved by the presence of mass light (and everything else) travels in a curved space-time. objects left to themselves travel in straight lines a straight-line on a curved surface is a geodesic, or great circle Early evidence that Einstein was right was the observation that light from a star was bent as it passed near the Sun (this could only be seen during a total solar eclipse of course)

Are other stars like our Sun? Let’s measure Apparent Brightness the amount of radiation we receive per second Luminosity the amount of radiation emitted by the star per second distance temperature composition

Brightness and Luminosity Brightness: How bright a star appears. Luminosity: How much light the star is actually giving off. Apparent Brightness Absolute Brightness

Apparent magnitude scale Introduced by Hipparchus(160-127 B.C.) Vega (0), Venus (-4), Sun (-26.8), Moon (-12.6), Faintest objects observed with HST (+30) What does it measure? measures ratios of actual amount of light energy received receive 2.5x more energy from a mag. 1 star than a mag. 2 star difference of 5 magnitudes is 100x difference in received energy

How does it work? 7.8 39.1 X 15.6 X 100 X 2.5 X Difference of 5 6.8 2.5 X 5.8 2.5 X 4.8 2.5 X 3.8 2.5 X 2.8

Apparent brightness apparent brightness diminishes with distance Inverse-Square Law (brightness diminishes as 1/D2)

What can you conclude about their luminosities? Practice Both stars have apparent magnitude +4 A B 4 ly 12 ly What can you conclude about their luminosities? How do we find distance?

Parallax Parallax - the apparent change in position of an object due to the change in position of the observer. January June

4 5 3 6 2 7 1 8 Observer 1 2 3 4 5 6 7 8

4 5 3 6 2 7 1 8 Observer 1 2 3 4 5 6 7 8

Parallax But why would stars do this? 4 5 3 6 2 7 1 8 Would an object here appear to move more or less? Parallax But why would stars do this? Observer 1 2 3 4 5 6 7 8

Is p larger or smaller for a star farther away? 1 AU 1 AU Jan July

Earth (Jan) APPARENT POSITION 1 AU p TRUE POSITION Earth (Jul)

Earth (Jan) APPARENT POSITION 1 AU p TRUE POSITION Earth (Jul)

TRIANGULATION the smaller the parallax angle the greater the distance one arc second=1/3600 of 1 degree distance from Earth to a star where p=1 arc second is called the parsec a parsec=3.26 ly

Parallax Results: Closest star is Proxima Centauri, which is located at 1.3 parsecs. Method good to about 250 parsecs! Gives knowledge of fraction of one percent of stars in Milky Way About 1/100 of diameter of galaxy!!

Hipparcos Satellite HIgh Precision PARallax COllecting (1989-1993)

Hipparcos (about 200 parsecs) Target: 118,000 stars Magnitude limit: 12.5 Resolution: 0.001 arcsecond!!! (about 200 parsecs)

Checkpoint What do we know now? What else do we want to measure? apparent brightness is different than luminosity and depends on distance to the star for the nearest stars, we can use parallax to determine distance we describe apparent brightness with the apparent magnitude scale and luminosity with the absolute magnitude scale What else do we want to measure?

Temperature temperature can be directly measured for a blackbody by plotting the brightness vs. wavelength (i.e. a blackbody curve). temperature affects the color of the star peak wavelength depends on temperature

Blackbody curves

Temperature and brightness As T increases, the wavelength for peak brightness decreases. As T increases, the brightness increases.

How does temperature affect aborption spectra? Absorptions lines tell us about the composition of the stars. Stars were originally grouped according to similar absorption spectra Later we realized that the intensity (i.e. darkness) of certain H lines were indicative of temperatures

O B A F G K M

Classification of stars by brightness Each type is further divided into 10 subtypes (0-9)

H-R diagram A graph of stars’ luminosity or absolute magnitude since they are related) vs. temperature (or spectral type since they are related) short for Hertzsprung-Russell diagram

same luminosity

very large same luminosity

RED GIANTS Cool but VERY BRIGHT! Betelgeuse: 3500 K (10% as bright/unit area as Sun) but 100,000 times as luminous--must have 1 million times the area radius must be 1000x that of Sun!

same temperature

same temperature very large very small

Globular Cluster in Scorpius