Lesson 55: Galaxy Evolution
In this lesson, we will look at how scientists study the evolution of galaxies,
and theoretical models used to study the earliest stages of galaxy formation.
We will then discuss reasons for why different types of galaxies formed, and
learn about starburst galaxies, where as many as one hundred new stars form
every year.
Astronomers can study the evolution of galaxies by looking at galaxies at
different distances.
When we look at an object such as a galaxy that is far away, we are seeing that
object as it was a long time ago, because it took a long time for the light
from that galaxy to reach us. Remember that when astronomers talk about
distances of far away galaxies, they are actually talking about their lookback
times, so a galaxy that is 13 billion light years away has a lookback time of
13 billion years.
Since most galaxies were born early in
the universe's history, we can look at galaxies at different distances to see
galaxies of different ages. For example, assuming the universe is 14 billion
years old, studying a galaxy with a lookback time of 13 billion years allows us
to observe a galaxy that is less than a billion years old. If we look at a much
closer galaxy with a lookback time of 1 billion years, then it may be over 13
billion years old.
Current technology does not allow us to study galaxies in
their earliest stages of formation since the youngest galaxies we can see are
around a billion years old. Scientists have had to use theoretical models based
on observational evidence to look at the earliest stages of formation.
The most successful models of galaxy formation indicate that hydrogen and
helium gas, along with dark matter, did not fill the universe perfectly
uniformly. There were areas with slightly enhanced density that gradually
contracted into protogalactic clouds, and as we studied previously, stars began
to form in these clouds as the gas cooled. Early, massive stars lived and died
over relatively short periods, seeding the cloud with its first heavy elements.
Over time, gas settled into a rotating disk as the protogalactic-cloud system
merged to form a galaxy.
The basic model seems to
explain how spiral galaxies formed, but does not explain why elliptical
galaxies formed without gas-rich disks. Some scientists believe that the type
of galaxy that formed depended on the characteristics of the
protogalactic-cloud system in which it formed. Specifically, elliptical
galaxies may have formed in clouds that had slower rotation or higher density.
If the protogalactic-cloud system had high angular momentum, it would have
rotated quickly as it collapsed, leading to the formation of a disk. However,
if the original system had little or no angular momentum, then it would have
formed no disk, resulting in an elliptical galaxy.
The density of the system may also have been a factor. Clouds with a greater
density of matter may have formed stars so quickly and used up all of the gas
before it could settle into a disk, while lower-density clouds would have
formed stars more slowly, allowing plenty of time for some of the gas to form
into a spinning disk.
In addition to looking at birth conditions, scientists also believe that
elliptical galaxies may have formed as a result of collisions and mergers
between spiral galaxies. This would explain why elliptical galaxies are more
common in galaxy clusters, where many galaxies are close together, while spiral
galaxies are more common outside of clusters.
Some galaxies, known as starburst galaxies, are
producing stars at a very fast rate -- as many as one hundred stars per year.
At this rate, these galaxies would consume all of their available gas in 100
million years. Starburst galaxies, which make up a small percentage of
galaxies, may represent a short-term stage in a galaxy's life before it goes
back to being an ordinary spiral, elliptical or irregular galaxy.
The formation of so many stars at once means eventually, a lot of the more
massive stars die at the same time. Their combined supernovae can erupt
outward, creating a galactic wind that pushes most of the gas out of the
galaxy. This can result in a shutdown of star formation for billions of years
until the ejected gas cools and falls back into the galaxy. This process may
act to regulate the star-gas-star cycle in galaxies.