Astro 201: Sept. 21, 2010• HW 3 and On-line Quiz 3 were due today• Turn in IR Camera write-up on Thursday• Don’t forget to sign up for Telescope Lab• First Midterm on Tuesday, Sept. 28.

• Will cover everything through Thursday’s lecture• See class web site and d2l for more info• no on-line quiz this week

• Thursday: Dr. Bechtold will finish telescopes• Today: Ken Wong on stars

STARS

What is a star? Why do they shine? How old are they?

Stars shine for millions to billions of years, much longer than a human lifetime.

Yet, we've been able to piece together how stars are born, shine and eventually die.

Why does the Sun shine?

The Sun shines because it is hot.

The Sun is hot because in its core, nuclear fusion is producing energy

The nuclear fusion that is happening in the Sun's core is the fusion of hydrogen into helium.

From models of stars and the Sun, we estimate that this fusion process will keep the Sun shining as it does today for another 4-5 billion years.

The Sun is hot

The surface of the Sun is 5800 degrees Kelvin.

The temperature inside the Sun is much hotter.

Its continuous spectrum is a blackbody with peak at a wavelength 500 nanometers (yellow) The total luminosity of the Sun is 390 trillion trillion watts.

The density of the Sun is about 40% greater than water – but the Sun is a gas.

The Sun is in HYDROSTATIC EQUILIBRIUM:

The collapse of the Sun be gravity is countered by the pressure of the gas.

The heat generated by nuclear fusion in the core of the Sun keeps the pressure high enough for the Sun to not collapse.

FUSION: 4 hydrogen nuclei combine to yield one helium nucleus

The helium nucleus weighs less than the 4 hydrogen nuclei, and the difference in mass is converted into energy, described by the famous equation:

E = m c2

Note: A very small amount of m makes a lot of E

The Sun is fusing 600 million tons of hydrogen per second into 596 million tons of helium.

• "It followed from the special theory of relativity that mass and energy are both but different manifestations of the same thing -- a somewhat unfamiliar conception for the average mind. Furthermore, the equation E is equal to m c-squared, in which energy is put equal to mass, multiplied by the square of the velocity of light, showed that very small amounts of mass may be converted into a very large amount of energy and vice versa. The mass and energy were in fact equivalent, according to the formula mentioned above. This was demonstrated by Cockcroft and Walton in 1932, experimentally."

EinsteinSpeaksE = m c2

Fission vs. Fusion

Fusion only can occur if the gas temperature (ie the speed of the atoms) is very, very high –

greater than 10 million degrees Kelvin.

In the Sun, the following chain of events makes the energy which keeps the Sun hot.

The Interior of the Sun

Energy can be transported out of the Sun via:

Conduction: atoms bump into other atomsConduction works best in solids like metals... not important in the Sun.

Convection: Large scale circulation of cells: hot blobs rise, cool blobs sink

Radiative Diffusion: Photons flow from warm, bright regions to cool, dark regions (the surface)

Important processes: Convection and Radiative Diffusion

How do we know what the interior of the Sun looks like?

1. Computer Models

How do we know what the interior of the Sun looks like?

2. Helioseismology

Global Oscillation Network Group, or GONG project, monitors the vibrations of the solar surface.

The Sun vibrates at many frequencies, like a ringing bell, or the surface of a drum.The six sites comprising the GONG Network are:

* The Big Bear Solar Observatory in California, USA. * The High Altitude Observatory at Mauna Loa in Hawaii, USA. * The Learmonth Solar Observatory inWestern Australia. * The Udaipur Solar Observatory in India. * The Observatorio del Teide in the Canary Islands. * The Cerro Tololo Interamerican Observatory in Chile.

HelioseismologyOperating since 1995, they make Doppler maps of the Sun:

How do we know what the interior of the Sun looks like?

3. Neutrino Telescopes

In addition to photons, the fusion processes inside the Sun produce tiny particles called Neutrinos.

Neutrinos move at or near the speed of light, and do not interact with matter very easily.

Deep in mines, large vats of cleaning fluid can capture neutrinos, and produce little flashes of light.

The Homestake Mine in South Dakota: (Ray Davies won Nobel Prize in Physics in 2003).

Super-Kamiokande

Neutrino Telescope in

Japan:

Radiative Diffusion: Photons Random Walk out of the Sun.

Photons take about a million years to random walk out of the Sun.

Neutrinos fly out at about the speed of light, in a few seconds.

JAVA APPLET

Properties of StarsDEFINITIONS:apparent brightness

versus absolute brightness or

luminosity

apparent m magnitude versus

absolute magnitude

Inverse Square LawSame Luminosity, Twice as far away --> 4x dimmer

Measure Apparent magnitude And Distance (parallax)

To stars luminosity

Stellar Surface TemperaturesMeasure the surface temperature of stars by taking a spectrum

of the star and using Wien's Law.

In addition, the absorption lines in the stellar spectra are sensitive to temperature.

Originally classified as A,B,C,..

the classification of stellar spectra was recast into OBAFGKM by Cecilia Payne-Gaposchkin.

OBAFGKM:Oh, Be a Fine Girl Kiss Me

Stellar Masses

Stellar masses are measured by observing binary stars, and using Kepler's 3rd Law to determine the mass of the stars from the period of their orbit.

Types of Binary Stars:

* Visual Binaries -- direct image shows two stars orbiting each other

* Spectroscopic Binaries – two stars are too close to see as separate stars, but spectrum shows absorption lines from two stars with variable Doppler shifts.

* Eclipsing Binaries -- one star disappears when it passes behind the other

Visual Binaries:

Artist’s conception of a visual binary

Eclipsing Binaries:

Spectroscopic Binaries:

Summary of Stellar Properties:

Spectral Type MASS(solar masses)

Luminosity(solar luminosities)

SurfaceTemperature(degrees K)

Radius(solar radii)

O 40 400,000 40,000 13

B 15 13,000 28,000 4.9

A 3.5 80 10,000 3.0

F 1.7 6.4 7,500 1.5

G 1.1 1.4 6,000 1.1

K 0.08 0.46 5,000 0.9

M 0.05 0.08 3,500 0.8