PES 106        Spring 2003
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General Astronomy II
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Lecture Notes:

Lives of Stars: Overview and Birth

text: Chapter 13 (sections 13.1 - 13.2)

Overview:

Why do stars change?

What in stars can change ?

  • Fuel to heat gas can change
    • run out of fuel for nuclear fusion or change fuel
    • changes temperature of hot gas
    • changes the balance between hot gas pressure and gravity
    • changes size, temperature and luminosity of star
  • Mass can change
    • stellar winds and flares
    • nuclear fusion converts mass into energy
    • other more drastic changes will also be examined
  • Composition can change
    • nuclear fusion converts H to He
    • stellar winds selectively carry away material from outer shell

A star is born: The Nursery

Interstellar matter

- gas and dust in space

"empty space" is not completely empty

gas: average of about 1 atom per cubic cm (air is about 1019 atoms/cm3)

dust: average of about 1 dust particle per million cubic meters (about 2 particles per Astrodome)

interstellar matter is about 99% gas (by mass) and 1% dust

Gas

mainly hydrogen (71%) and helium (27%)

clumps together into big clouds of gas

can have more dense regions within the clouds

OPTIONAL

detect these by

  • emission spectra (if hot and dense enough)
  • extra absorption lines in star spectra
    • very narrow because density and temperature are very low

OPTIONAL

Dust

detect dust by effects on light
  • interstellar extinction - dust absorbs and scatters light
  • interstellar reddening - dust scatters blue light better than red light
    • because of size of dust particles
    • same effect causes red sunsets and blue sky

result is that stars look dimmer and redder

we can estimate the amount of dust by comparing the Spectral Class temperature to the temperature from Color
  • polarization of light - dust causes light that passes through it to become polarized
    • dust particles are elongated
    • particles seem to be oriented in the same direction
    • blocks certain polarizations of light

Clouds:

Some clouds of gas and dust are so dense that they block the light from stars behind them.

Typical cloud properties:

size: 10 - 100 pc

mass: 100 - 1,000,000 x mass of Sun

temperature: around 10 K (COLD!!!)

In studying these clouds, we often find young stars in or near them.

In studying clusters of young stars, we often find lots of dust and gas around them.

Suggests the following model:

Birth of a Star:

1. Large Rotating Interstellar Dust/Gas cloud begins to collape

from stellar wind of nearby star or a supernova explosion

INSERT FIGURE

Why rotating? - hard to balance it so perfectly that it would not rotate

2. Rotation causes the cloud to flatten into a disk with a denser core

"pizza dough" effect

3. Core becomes denser and hotter - called a "protostar"

a million years or so after collapse

size is around 10-20 x Sun

bigger size => higher luminosity ( about 100 x Sun)

temperature is about 1500 K (so shines in the infrared)

as core continues to collapse - gravitational energy heats up the protostar

4. Nuclear fusion begins in core

now object is a "star"

INSERT FIGURE

temperature about 7 million K for fusion to start

fusion heats star further

increased hot gas pressure stops the gravitational contraction

5. Mass loss

stars lose mass at this point by several processes

INSERT FIGURE

bipolar flow

jets of gas in opposite directions help to push away gas and dust (often up and down from the disk of gas and dust)

Figure 13.6 and 13.7 show this effect. [Link to Figure 13.6] [Link to Figure 13.7]

strong stellar winds

especially in high mass stars

often accompanied by variations in luminosity

call these T Tauri stars

6. Planets may form from gas and dust in disk around the star

hard to see directly

look for wobble of stars from orbiting planet or small velocity changes in the star

we have detected over 100 planets around other stars

 

Observe these stages on HR diagram:

INSERT FIGURE

How long does it take to form a star ?

high mass stars: about 500,000 years

low mass stars: billions of years

Stellar Mass Limits

lower limit: about 0.1 x Mass of Sun
anything smaller will not be able to compress to conditions where nuclear fusion begins

"brown dwarfs" - objects that did not have enough mass to start fusion - very hard to see

upper limit: high mass = high luminosity

too high a mass star pushes surrounding material including the star's own outer layers

live for very short times

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