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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 14: Star Formation

Readings: Chapter 20

Key Ideas

Raw material of star formation: Giant Molecular Clouds

Formation Stages:
Cloud Collapse and Fragmentation into clumps
Protostar formation from clumps
Onset of Hydrostatic Equilibrium (Kelvin-Helmholz timescale)
Ignition of core Hydrogen burning & onset of Thermal Equilibrium

Minimum & Maximum masses of stars

The Sun is Old and in Equilibrium

If we look at the Sun today, some 4.6 Gyr after it formed, we find
  1. It is in Hydrostatic Equilibrium: its internal Pressure and Gravity are in balance, so it neither expands nor contract.

  2. It is in Thermal Equilibrium: Energy Transport balances Energy Generation, so its energy lost by its Luminosity is made up for by nuclear fusion energy generated in its core.
The big question is, how did it get this way?

Steps to Stardom...

Star birth is basically a 2-stage process:
  1. Establish Hydrostatic Equilibrium. This is the Protostar Formation phase

  2. Establish Thermal Equilibrium. This is the Pre-Main Sequence phase

The first question we have to ask is where stars are Born? We learn this by looking around the Galaxy.

The Interstellar Medium

The space between the stars is filled with Gas and dust (fine particulates). Young stars are all found in dense gas-rich regions.

This makes sense, since we expect to find new stars close to the raw materials from which they are born.

Giant Molecular Clouds (GMCs)

Interstellar clouds of molecular hydrogen gas (H2)


This is a starting density of ~1020 times smaller than that of a star. This means the collapse from cloud to star has a long ways to go.

Collapse of a GMC

A GMC is held up against its own gravity by its internal pressure. This pressure has 2 sources:

If the Gravity becomes larger than the internal Pressure, the entire cloud will start to collapse.

Possible ways to trigger such a collapse include:

Cloud Fragmentation

GMCs are clumpy:

High-density clumps are more unstable than low-density regions:

GMC fragments into dense cores
Cores have masses comparable to stars.

Building a Protostar

Cores start out low density and transparent:

Eventually, the core density rises to the point where the core becomes opaque:

Eventually, the pressures builds up until it achieves Hydrostatic Equilibrium

Protostellar core continues to grow as fresh material from the surrounding parent cloud falls onto it.

A Quick Beginning...

The protostar phase is very short-lived: only 104-5 years.

During this phase:

The combination of "Short Lived" and "Hard to See" means that very few Protostars are observed.

Protostars have Disks

As matter rains onto the Protostar, The result is that the infalling gas pancakes, forming a flattened, rotating disk around the equator of the Protostar.

Clearing out the Disk

After the protostar forms, the disk of material begins to clear away: Observations suggest that: We see dust and "debris" disks around young low-mass stars.

From Protostar to Star

Protostars shine because they are hotter than their surroundings:

Initial energy source is Gravitational Contraction (aka, the Kelvin-Helmholz Mechanism):

How long can does this last?

Kelvin-Helmholz Timescale

To understand how long a Protostar can shine by Gravitational Contraction, we need to compare two numbers The ratio is the Kelvin-Helmholz Timescale:
Kelvin-Helmholz Timescale
The Kelvin-Helmholz timescale is ~30 Myr for a 1 solar mass Protostar.


H-R Diagram of pre-Main Sequence evolution for stars of various masses:
Protostar H-R Diagram
The K-H timescale is the time to cross the diagram and land on the Main Sequence.

High-Mass Protostars

Gravitational Collapse is very fast for high-mass protostars: As the collapse proceeds, the core temperature gets hotter until it reaches ~10 Million K: The star lights up, quickly ionizing and blowing away any remaining gas and junk.

Low-Mass Protostars

Gravitational Collapse is slower for low-mass protostars: When core Temperature >10 Million K: Settles slowly onto the Main Sequence

The Zero-Age Main Sequence

As the core heats up, H fusion runs faster: Star reaches the Zero-Age Main Sequence as a full-fledged star in Hydrostatic and Thermal Equilibrium.

Minimum Mass: ~0.08 Msun

Below 0.08 Msun, the core never gets hot enough to ignite H fusion.

Object becomes a Brown Dwarf

These are the T dwarfs.

[Hubble Space Telescope images of the Brown Dwarf Gliese 229B]

Maximum Mass: 100-150 Msun

Above a mass of 100-150 Msun, the core gets so hot that

The ultimate upper mass limit is not well known.

Such stars should be (and are) extremely rare (a few per galaxy).

[Hubble Space Telescope images of a very luminous star]

What can we see?

As we look out into the Galaxy, we see stars in all phases of their life cycles:

The Pre-Main Sequence phase is longer for lower-mass stars (because of the longer K-H timescale):

The Main Sequence Phase is very long:

How long is the M-S phase? That's the subject of the next lecture...
Return to [ Unit 2 Index | Astronomy 162 Main Page ]
Updated: 2006 January 21
Copyright Richard W. Pogge, All Rights Reserved.