The Sun and Stars

Chapters 10 and 11

Topics

• the Sun– Features– Structure– Composition– How do we know?

• Stars– brightness and luminosity– distance– temperature– mass– classification

Sun

• Photosphere– 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%)

• Chromosphere– thin layer above the photosphere– 7000 K - 15,000 K

• 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• sunspots

– 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.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

From http://sohowww.nascom.nasa.gov/

A comparison of two EIT images almost two years apart illustrates how the level of solar activity has increased significantly. The Sun attains its expected sunspot maximum in the year 2000. These images are captured using Fe IX-X 171 Å emission showing the solar corona at a temperature of about 1.3 million K. Many more sunspots, solar flares, and coronal mass ejections occur during the solar maximum. The numerous active regions and the number/size of magnetic loops in the recent image shows the increase.

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 Magnitude

Absolute Magnitude

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

6.8

5.8

4.8

3.8

2.5 X

2.5 X

2.5X2.5 = 6.25 X

2.5 X

15.6 X

2.5 X

39.1 X

2.82.5 X

100 X Difference of 5

Practice

• Rigil Kentaurus (-0.01)

• Spica (+1.0)

• Which looks brighter?

• How much more light do we receive from it?

• One level of magnitude means 2.5 times more light received!!

But...

• Spica is actually more than 1000x more luminous than Rigil Kentaurus!!

• If Spica is putting out more light, why might it appear dimmer in the sky?

Apparent brightness

• apparent brightness diminishes with distance

• inverse-square law (brightness diminishes as 1/D2)

2 feet 2 feet

?A

A 4AB/4B

Practice

Both stars have apparent magnitude +4

A B

What can you conclude about their luminosities?

4 ly 12 ly

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

12 3 4 5 6 7

8

1 2 3 4 5 6 7 8

Observer

12 3 4 5 6 7

8

1 2 3 4 5 6 7 8

Observer

12 3 4 5 6 7

8

1 2 3 4 5 6 7 8

Observer

ParallaxBut why would stars do this?

Would an object here appear to move more or less?

1 AU 1 AUJan July

p

Is p larger or smaller for a star farther away?

1 AU

TRUEPOSITION

APPARENTPOSITION

Earth (Jan)

Earth (Jul)

p

1 AU

TRUEPOSITION

APPARENTPOSITION

Earth (Jan)

Earth (Jul)

p

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

Satellite

(1989-1993)

Hipparcos

• Target: 118,000 stars

• Magnitude limit: 12.5

• Resolution: 0.001 arcsecond!!!(about 200 parsecs)

Practice

Consider two stars (X and Y). If star X is 3 parsecs away and star Y is 5 parsecs away, which has the greater parallax angle?

a)      Star X

b)      Star Y

c) Not enough information

Comparing brightness

• How can we compare brightness if stars are at varying distances?

• Calculate brightness as if the star is at a distance of 10 parsecs (33 ly).

• This is called its absolute magnitude.

What magnitude would Antares (-0.1) appear if it were at a distance of 10 parsecs? Take the currentdistance to be 100 parsecs.

Are we expecting a number greater or less than -0.1?

10 x closer = _____ x brighter100

How many magnitudes is 100 x brighter?1 mag = 2.5x

5 mag = 100x

absolute magnitude = -0.1 5- = -5.1

brighter or dimmer?

Checkpoint

• What do we know now?– 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?

• Absorption 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

O hotter than 25,000 KB 11,000 - 25,000 KA 7500 - 11,000 KF 6000 - 7500 KG 5000 - 6000 KK 3500 - 5000 KM cooler than 3500 K

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

same luminosity

very large

RED GIANTS• Cool but VERY BRIGHT!Cool but VERY BRIGHT!

• Betelgeuse: 3500 K (10% as Betelgeuse: 3500 K (10% as bright/unit area as Sun) but 100,000 bright/unit area as Sun) but 100,000 times as luminous--must have 1 million times as luminous--must have 1 million times the areatimes the area

• radius must be 1000x that of Sun!radius must be 1000x that of Sun!

same temperature

same temperature very

large

very small

Globular Cluster in Scorpius