Welcome to Environmental Science!!!

The Sun

Physical Data

• Mass = 2x1030

kg (333,000 time

more massive than the Earth)

• Diameter: 7x105km (about 100

Earth radii)

• Volume: you can fit about 1.3

million earths inside the sun!

• 70% Hydrogen, 28% Helium, 2%

other stuff.

Distance to the sun

• The average earth-sun distance

is called an Astronomical Unit

(AU)

• 92.8 Million Miles

• Keppler knew the distance to

the planets in terms of the Earth-

Sun distance, but not the

distance itself (in meters)

Interior of the

Sun

• VERY hot (27 million F)

and dense (150 times

denser than water) in

the center (core)

• Density & Temp drop

rapidly toward the

outside

• 10,000 F at the surface

the Sun

• The sun is far away (93 million miles)

• And that’s why it looks so small!

• The sun is hot (10,000 F on surface)

• The sun is big (1 million Earth’s fit inside)

• Lots of gravity!

• This adds up to Temperature and pressure

being very high on the sun.

Quick Chem review

Core of the sun

• The temperature and pressure

on the sun is amazingly high.

• How high? High enough to push

hyrdogen protons together.

They fuse to form helium.

• What is this called?

Core of the sun

• Every second on the sun, 600

million tons of hydrogen is

converted into 595 million

tons of helium.

• Huh? You can’t do that!

Why not?

• CONSERVATION OF MASS!

• The law implies that matter

can neither be created nor

destroyed, although it may be

rearranged.

Core of the sun

• If 600 millions tons of hydrogen

is converted into 595 million

tons of helium… what happens

to the lost mass?

Core of the sun

• Einstein’s theory of relativity

states that under enough heat

and pressure (like in the core of

the sun) energy and mass are

interchangeable (E = M)

• Therefore the lost mass is

turned into HUGE amounts of

energy (E=mc2) Thanks Einstein.

• Again, nuclear fusion occurs in

the core of the sun.

Fusion in the sun

The Sun’s Future?

• The sun is currently crushing

hydrogen into helium in the

process of nuclear fusion.

• The sun is currently 70%

hydrogen and 28% helium.

• How will these numbers change

as time goes on?

Photosphere

• Photosphere – brightest in optical –

this is where most of the light from

the sun comes from. The spectrum

is formed here.

Photosphere

• Optical light

comes from

here

• Sunspots are

areas of

intense

magnetic

field

The End

Chromosphere

• Activity starts at sunspots and gas travels

along magnetic field lines

• If the gas loops back – prominence

• If the gas escapes to the corona -- flare

Coron

a• The top picture is in X-

ray

• The bottom two are in

optical from SOHO

• You can see material

leaving the sun

Energy transfer

• The energy created in the center of

the sun has to travel to the outside.

This happens in an orderly fashion

in the interior

• Near the outside, energy is

transferred with convection

Measurements

What we can measure What we can calculate

Distance to Venus (radar)

Apparent mag. of sun (and D)

Period of the Earth’s orbit

(and D)

Spectrum of the sun

Sunspots

Distance to the sun

Absolute mag.

(Luminosity)

Mass of Sun

Temperature, chemical

composition, rotation

Rotation

Rotation, magnetic

field

The sun as a main sequence

star

• The sun is a G2 main sequence

star with an absolute magnitude

of 4.85

• All main sequence stars change

H to He

• All spectra come from the

photospheres of the stars

• We can only detect the

chromosphere and corona of a

few stars besides the sun

Properties of Stars

Properties of Stars

• Brightness

• Luminosity (brightness and

distance)

• Magnitude scale

• Temperature (Color, spectrum)

• Composition (Spectrum)

• Velocity, rotation, magnetic field

(spectrum)

• Distance (parallax, comparison)

Triangulating the Stars

Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

Brightness and

Luminosity

• Apparent magnitude measures

the brightness of a star

• The true property of the star is

luminosity.

• Luminosity, the total power

coming from the surface of a

star, is measured using Absolute

Magnitude

Calculating Magnitude’s

• You need to know the distance

to the star in question (parallax

for the nearest stars – harder for

everything else)

• Then you can calculate the

absolute magnitude, M

Putting it together: an HR

diagram

• Luminosity is measured in Watts

or absolute magnitude

• Brightness is measured in

Watts/m2

or apparent magnitude

• Temperature is measured in

color or spectral type

(OBAFGKM)

Types of HR diagrams

Theorist’s Observer’s Color-magnitude

Here is an HR

diagram for a few

hundred randomly

selected stars

from the HD

catalog

Notice the main

sequence

Absolute Magnitude vs. Spectral Type

-7.00

-5.00

-3.00

-1.00

1.00

3.00

5.00

7.00

9.00

11.00

0 1 2 3 4 5 6 7Spectral type

Ab

so

lute

Ma

gn

itu

de

Again, on the basis of the appearance of the spectra of stars, astronomers discovered that

the density of gas and the strength of gravity at the surface of a star indicate that some

stars are much larger than other, even if their temperatures are the same. This difference

is denoted by the luminosity class of a star.

The sequence of luminosity classes is:

Luminosity Class Name assigned to class:

I or Ia Supergiants

II Bright Giants

III Giants

IV Sub-giants

V Dwarfs

VI Sub-dwarfs

The complete classification of a star is based upon the spectral type and luminosity class of a star.

Thus, it turns out that the sun is classified as a G2V star.

Our old friend Betelgeuse is an M1I star.

“Luminosity Classes” of Stars