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Star Formation. A Star is Born. Goals. What is there between the stars? What are dust clouds? What are nebulae? How do these lead to the formation of star? Where do baby stars come from?. The Stuff Between Stars. Space isn’t empty.

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star formation

Star Formation

A Star is Born

  • What is there between the stars?
  • What are dust clouds?
  • What are nebulae?
  • How do these lead to the formation of star?
    • Where do baby stars come from?
the stuff between stars
The Stuff Between Stars
  • Space isn’t empty.
  • Interstellar Medium – The gas and dust between the stars.

All the interstellar gas and dust in a volume the size of the Earth only yields enough matter to make a pair of dice.

the distribution
The Distribution
  • Picture the dust under your bed.
    • Fairly uniform thin layer
    • Some small clumps
    • Occasional big complexes
  • Interstellar dust and gas is the same.
  • Space is dirty.
  • Dust blocks or scatters some light.
  • Result: black clouds and patterns against the background sky.
  • But what light gets through, and what light doesn’t?
absorption and scattering
Absorption and Scattering
  • Q: Why are sunsets red?
  • Light is absorbed or scattered by objects the same size or smaller than its wavelength.
  • Dust grains = wavelength of blue light
  • Dust clouds:
    • Opaque to blue light, UV, X-rays
    • Transparent to red light, IR, radio
  • A: Whenever there is a lot of dust between you and the Sun, the blue light is absorbed or scattered leaving the only the red light.
interstellar reddening
Interstellar Reddening
  • Same thing with dust clouds in space.
  • Since space is full of dust, the farther away stars are, the redder they look.
  • Enough dust and eventually all visible light is scattered or absorbed.
dust and ir
Dust and IR
  • In a dark dust cloud:
    • Even though all visible light may be gone, we can still use IR.
    • If dust is warm, IR will show its blackbody emission.
the ir universe

Orion - visible

Orion – by IRAS

The IR Universe

And allows us to see dust where we wouldn’t otherwise expect it.

interstellar gas

Haemission nebulae

Copyright - Jason Ware

Interstellar Gas
  • In Lab 2 we talked about spectral lines and how they apply to hot and cool gases.
  • Let’s look at some hot and cool gases in space.

Horsehead Nebula– copyright Arne Henden

Dust obscuring Ha emission nebula


Orion Nebula – copyright Robert Gendler

  • In order for the hydrogen to emit light, the atoms must be in the process of being excited.
  • The energy for the excitation comes from very hot stars (O and B stars) within the cloud.
cold dark clouds
Cold Dark Clouds
  • If dust clouds block light, then inside thick dust clouds there should be no light at all.
  • Without light, there is little energy.
  • With little energy, gas inside is very, very cold.
  • Inside molecules form.
gravity vs pressure
Gravity vs. Pressure
  • Stars and other interstellar material are in a perpetual battle between forces pulling in (gravity) and forces pushing out (pressure).
  • Gravity comes from the mass of the cloud or star.
  • Pressure comes from the motion of the atoms or molecules.
    • Think of hot air balloons.
    • The hotter the air, the bigger the balloon.
star formation16



Star Formation
  • Remember lecture 4:
  • Cold interstellar clouds:

No heat = no velocity = no outward pressure.

Gravity wins.

  • Gas begins to contract.
1 the interstellar cloud
1. The Interstellar Cloud
  • Cold clouds can be tens of parsecs across.
  • Thousands of times the mass of the Sun.
  • Temperatures 10 – 100 K.
  • In such a cloud:
    • Something makes a region denser than normal.
    • Force of gravity draws material to denser region.
    • Gravitational collapse begins.
contracting fragments
Contracting Fragments
  • Cloud about the size of solar system.
  • In the center:
    • Collapsing material continues to heat up.
    • Density causes heat to be retained.
  • Higher density makes center opaque.
2 protostar
2. Protostar
  • The central opaque part is called a protostar.
  • Mass increases as material rains down on it.
and the nebula
…and the Nebula?
  • Cloud around the protostar spins faster.
  • Flattens to a disk.
    • Pizza dough.
  • Dust and gas condense onto dust grains.
  • Small clumps grow bigger.
  • Bigger clumps have more mass and attract more matter.
  • Planetesimals are the building blocks of the planets.

Orion Nebula – Copyright O’Dell and Wong

3 t tauri phase
3. T Tauri Phase
  • Protostar still shrinks: 10x the Sun.
  • Still heats up: surface = 4000 K
  • Core temp = 5,000,000 K
  • Violent surface activity creates strong winds that blow material away near the protostar’s surface.
  • Clear away the dust and gas between planets.
a star is born
A Star is Born
  • Time: 40- 50 million years since the collapse started.
  • Radius: 1,000,000 km (Recall the Sun = 700,000 km)
  • Core temp: 10,000,000 K (Sun = 15,000,000 K)
    • Surface temp = 4500 K
  • Fusion begins in core.
  • Energy released creates the pressure needed to counter the contraction from gravity.
  • Contraction ends!
the main sequence
The Main Sequence
  • For the Sun:
    • While it took 40 – 50 million years to get here, the new star will spend the next 10 billion years as a main sequence star.
  • Bigger Stars:
    • Everything goes quicker.
  • Smaller Stars:
    • Everything longer.
now what
Now what?
  • The mass of the star that is formed will determine the rest of its life!
  • Recall: the more massive the star, the more pressure in the core.
  • The more pressure, the more fusion.
  • More fusion:
    • More energy produced
    • Hotter
    • Shorter life span
open clusters
Open Clusters
  • These are the new stars.
  • Small groups of young stars.
  • Slowly drifting apart.

Jewel Box – copyright MichaelBessell

homework 9
Homework #9
  • For Feb 17:
  • Read B17.3 – 17.5, B18.1 – 18.3
  • Do:
    • Ch17 : Problem 1, 4, 25
    • Ch18: Problem 1, 4, 11
    • (Math folks replace Ch18 Problem 1 with Ch 18 Problem 19)