Lesson Objectives:- Studying galaxy evolution
- Modeling galaxy formation
- Explaining differences in galaxies
- Starburst galaxies
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.