Gravity induces starburst events

A Burst of Star Light, continued

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Starburst galaxies near the local group

Once a galaxy exits its initial starburst period, it becomes less agitated and produces a modest number of stars each year. For example, billions of years ago our Galaxy also experienced a tumultuous episode of rapid star production before reaching its current rate of about one new star each year. Interestingly, since light takes ages to cross the vast cosmic distances, alien astronomers in a far off galaxy may now be examining telescopic images of the Milky Way as it appeared in its wild, uninhibited youth.

Too close for comfort

Gravity is one of the four fundamental forces in nature. Put simply, it causes objects to attract each other with a strength that's in direct proportion to their mass. In everyday life, this means things have weight and fall to the ground when released. In space, gravity aggregates particles, such as gas and cosmic dust molecules, then forces them to remain coalesced. Gravity's effect helps explain how the Sun, Earth and other planets were formed, why the planets orbit the Sun and how galaxies came into being.

Gravity also herds star systems into groups and clusters. Within these large scale organizations, galaxies are constantly in motion relative to each other. As a result, they sometimes come into close proximity, collide and merge. A close encounter or collision between galaxies can cause them to re-enter a new phase of furious star generation that's similar to the episode they experienced soon after their birth. To understand how this happens, let's consider how stars are gestated.

Galaxies are filled with molecular gas and microscopic dust particles that assemble into vast inter-stellar clouds gathered by the influence of gravity. These clouds can grow to enormous proportions often stretching tens or hundreds of light years in all directions. Usually, they peacefully drift between the stars for billions of years, becoming ever larger and minding their own business. Eventually, something may trigger the cloud to become slightly compressed. This instability can set off a cascade of events that results in the creation of new stars millions of years later. Sometimes this is caused by the shock waves of a distant supernova explosion nudging the cloud on one side. Other times, the cloud can be tugged by the gravity of another galaxy when it passes close by or collides with the cloud's host star system.

Generally speaking, supernova explosions are one of the most common progenitors of star formation. But, energy waves from a single supernova are too weak to cause the creation of a thousand or more stars. Conversely, when a galaxy wanders close or collides with another star system, the gravitational effect on its molecular clouds is far more enormous. Close galactic encounters or wholesale collisions send tsunami-sized shock waves through the giant galactic clouds causing them to create some of the most massive stars in the Universe; monster blue stars with more than 100 solar masses.
  • Swimming deep in the cosmic ocean, the Whale Galaxy, designated NGC 4631, is located towards the northern constellation of Canes Venatici about 25 million light years away.
  • Image credit: R. Jay GaBany

These massive stars experience relatively short lives then prematurely self-detonate, releasing more shockwave throughout the galaxy. This creates a chain reaction that cascades through the star system and within a few million years, the galaxy is forming stars at tens or hundreds of times its normal production rate. The increased rate of supernova explosions also produce expanding bubbles of multimillion degree gas and when the starburst activity is sufficiently intense, it can create a superbubble so hot and energetic it expands outside the galaxy as a superwind.

Superwinds are thought to contain heavy elements that spread throughout the space between galaxies. Eventually, this material may be captured by another star system and incorporated into its interstellar clouds. Thus, matter from one galaxy can seed the stars and planets of another distant star system.

The Whale Galaxy

Space is an ocean populated with stellar-sized inhabitants engulfed by our most profound mysteries. About 25 million light years from our location, towards the northern constellation of Canes Venatici, the 140 thousand light year diameter Whale spiral galaxy plies the cosmic sea like a leviathan seen in profile.

The visual impression of spouting water from the Whale's blowhole is actually the close passing of a small companion galaxy. Its gravational influence compressed vast clouds of gas and dust within the larger galaxy's central region thus inciting a riotous burst of hot, new stars. The radiation released from these new suns also created a superbubble of material that now surrounds the galaxy like an ever-expanding shell. Presumably, once the companion pulls further from the Whale, the star burst event will eventually quiesce.

Bar none

Some galaxies develop starbursts without a close encounter or merger. Starbursts can also occur spontaneously, prompted by internal events.
  • Starburst activity is evident in this image of NGC 253, located about 10 million light years away toward the constellation of Sculptor.
  • Image credit: R. Jay GaBany

About two-thirds of all spiral galaxies have a river of gas, dust and stars running through their inner most region that bridges the spiral arms located on either side. These are called barred spiral galaxies and even our Milky Way is now believed to possess one. The bar is thought to be caused by a density wave- one that extends laterally from the galaxy's center. At first, the wave changes the orbits of interior stars but, over time, it affects stars farther out. As more time passes, it enlarges and creates an unusual, bright bar shaped structure.

Bars cause the motions of interstellar dust clouds and a large number of stars within the galaxy to become chaotic. Collisions between the clouds become frequent and violent and this causes the clouds to move all over the galaxy's disk thus radially redistributing the gas and dust within an Island Universe. In particular, a large quantity of gas will fall along the bar and eventually reach the galaxy's central region where it can stimulate a starburst event or fuel the galaxy's central black hole. Starburst events stimulated by instabilities in their central bar can be seen in images of NGC 253 and NGC 2903.

Final thoughts

During the last century, astronomers began to suspect the Universe was filled with an all-permeating, invisible substance that pushed, tugged and held galaxies together. For much of that time, most scientists considered this was a misguided idea. However, more evidence mounted and today dark matter is a cornerstone of cosmology. While it still has not been detected by any instrument sensor, its presence can be inferred by the motions of stars and galaxies in clusters. This understanding, along with the keen eyesight of a new generation of space telescopes, has also led astronomy to better understand the role of dark matter in creating starburst galaxies that filled the early Universe.

Starburst galaxies, found closer to home, can be produced by close encounters or mergers with other star systems and spontaneously by conditions within. Science now has a more complete understanding of these tumultuous events and their role in the evolution of galaxies throughout the Universe.


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