Download Introduction to Astrophysics, Lecture 10

Introduction to Astrophysics
Lecture 10: Star formation
Star formation
Understanding star formation is one of the most pressing
problems in contemporary astrophysics.
As well as trying to understand how our own Sun might have
formed, we want to understand the whole range of possible
stellar properties.
The Initial Mass Function (IMF)
One goal of star formation studies is to predict the Initial Mass
Function of stars, usually known as the IMF.
This tells us the number of stars forming with a given mass.
There is some dispute over
the observational form of
the IMF, with popular
examples being the
Salpeter mass function and
the Scalo mass function,
which are quite different at
the high-mass end.
Gravitational collapse
Stars are thought to be born in the gravitational collapse of large
clouds of gas and dust. The problem is complex because it is
believed stars form in clusters, rather than individually.
Gravitational collapse is possible if the gravitational
potential energy of the cloud exceeds the kinetic energy, i.e.
<v2> is the mean
square velocity
Putting M=4R3/3 gives the relation
where the last equality uses typical interstellar
densities and velocities.
Gravitational collapse
As the cloud begins to collapse, the density  increases,
which means that the minimum collapse mass decreases.
We therefore expect that the initial cloud to collapse is about
one thousand solar masses or more, and as it collapses it is able
to fragment into smaller masses, so that the original cloud
breaks up into a cluster of stars.
Computer simulation of
star cluster formation
on the UK
Astrophysical Fluids
Facility (UKAFF).
The initial cloud has a
mass of fifty solar
masses.
Credit: Matthew Bate
(Exeter)
Science from star
formation simulations
The computer simulations confirm the basic picture of
fragmentation and collapse. They indicate:
 Many stars do form in the collapse of a single dust
cloud. The collapse is very complex due to fragmentation.
 The stars interact frequently with each other after
formation, through their gravitational attraction. The
presence of stars can also induce new star formation.
 Some stars get thrown out of the gas cloud by these
interactions.
From protostars to stars
The cloud fragments into
separate collapsing regions
which become protostars. A
protostar is a hot ball of gas,
whose heat has been derived
from the gravitational collapse
of the original gas cloud.
So far it is not undergoing
nuclear burning. As it radiates
heat it collapses further.
Artist’s impression
A star like the Sun is thought to spend 50 million years as a protostar.
From protostars to stars
Eventually the density at the
centre is sufficient to ignite
the star, and nuclear burning
begins. This happens when
the temperature reaches
several million degrees.
By now any planet formation
should also have taken place.
From protostars to stars
During its early stages, our
star will have a very strong
stellar wind which will blow
away all the remaining gas.
During this phase it is known
as a T-Tauri star, after the
particular star that was the
first known example.
The T-Tauri phase is quite brief,
and after it the star becomes a
main sequence star.
Overview
 Star formation is a highly complex process, and a
detailed understanding requires computer simulations
which are just becoming feasible.
 Stars are expected to form in clusters rather than
individually.
 Current theories have not been able to predict the
Initial Mass Function of stars, which plays a crucial
role in understanding the evolution of galaxies.