We are very fortunate, and perhaps gifted, to inhabit a planet near a star that's not very interesting. Our Sun has been relatively monotonous for billions of years. This is a vital quality for any star to spawn and support life. But, there are many suns that do not have this characteristic. Some of those other suns regularly, some others without warning, increase their radiance by outpouring dramatic quantities of increased solar energy.
During our Sun's eleven year sunspot cycle, the amount of radiation released changes by only about one tenth of one percent. Stellar monotony is an important characteristic for the development of habitable environments- suns need to be constant over the millennia thus humankind is fortunate. Like our Sun, most stars are stable, but a significant minority experience large luminosity swings that would produce grave consequences for life that is attempting to harbor on nearby planets.
The Milky Way galaxy, considered in its entirety, has also provided the Sun and our home world a safe, sheltered galactic habitat so that our planet's long evolutionary path could lead to us. This is not meant to ignore the exotic forces within our own star system that surround and would obliterate humankind were we not protected by the safety of great distance. But one only has to look outward beyond the embrace of our galaxy's spiral arms to see other Island Universes that are much less nurturing!
Some of the external star systems with a less hospitable climates are called start burst galaxies because they contain a bewildering number of hot, bright new Suns. Like a galactic Ole Mother Hubbard, they have far more stars than most- producing them over one thousand times faster than the Milky Way, starburst galaxies are bursting with excessive energy.
How does this calamity happen?
Let's recall that stars begin as unbelievably large, thin clouds of hydrogen and dust that drift throughout the interior of galaxies. When waves of energy released by a distant supernova reaches the boundary of one of these clouds, for example, its surface is slightly squeezed inward. Something like this is all it takes for an inter-stellar cloud to loose its stability and start falling in upon itself. Over eons, the collapse accelerates as gravity within the condensing cloud becomes irresistible. Eventually, the pressure at the cloud's center will start to raise its internal temperature and when the heat becomes sufficiently hellish, a thermonuclear explosion is triggered. Incredibly, over a million years may pass before an external observer would sense the initial blast as a burst of light. The outward force of the nuclear chain reaction within the heart of the cloud is usually sufficient to stop the cloud's compression so that what began as a dark thin vapor becomes a luminous star brightening the heavens.
Inter-stellar clouds seldom form single stars. More typically, stars are created, like cookies, in batches. New star clusters with hundreds of stellar constituents is not uncommon- as the first stars ignite, their shock waves reverberate throughout the cloud and place additional pressure on their unborn siblings causing them to illuminate more rapidly, in relatively quick succession.
But, consider a circumstance where the amount of material to stimulate new star production is so vast that it forms tens of thousands of massive new suns within a short time span- and does this repeatedly. This condition can happen if a star forming region has access to a virtually inexhaustible supply of fresh hydrogen gas. Such bazaar conditions exist within galaxies that have gone starburst.
Galactic starburst behavior can originate with the merger of two galaxies or from a near-miss, close encounter of one galaxy with its neighbor. Some scientists also believe that super-massive black holes, which are common at the center of most galaxies, can create the circumstances needed for a galaxy to commence feeding upon itself without outside intervention.
About half of the spiral galaxies within observational reach of our current technology have a curious elongated bar-shaped structure- even the Milky Way is now thought to have one! These bars are enormous rivers of gas, dust and stars running through the galaxy's center connecting the spiral arms on either side. The bars affect both the motions of stars and interstellar gas throughout the galaxy. For example, astronomers believe the bar acts like an enormous feeding tube that draws gas inward from the galaxy's spiral arms thus fueling the unbridled creation of massive, hot bright stars at their centers.
The bars are generally thought to be the result of gravitational density waves that radiate out from the galaxy's center altering the orbits of the inner stars. Over time, the effect extends to stars farther from the galaxy's heart thus galactic bars may, in fact, be temporary structures and the starburst condition may, also, be just a passing phase. Interestingly, density waves also give rise to the spiral structures that grace a large proportion of galaxies. Regardless, many barred spiral galaxies are seen to exhibit abnormally active central regions such as the galaxy featured in this image of M83.
Stars in the Milky Way arise and expire in a, more or less, predictable manner. Conversely, star birth and death in a starburst galaxy resembles detonations at an stockpile of explosives! Short-lived and incredibly massive, stars formed within a starburst galaxy ultimately self-destruct in a titanic supernova blast- often these stars consume so much of their nuclear fuel that they are kindled and extinguished before making more than a single revolution around the galaxy's middle. These conflagrations raise the temperature of the surrounding gas to millions of degrees and leaves the area littered with exotic stellar carcasses like neuron stars and black holes. Many astronomers contend most galaxies endured a starburst period when the Universe was younger and galactic mergers were more commonplace. Massive winds are incited when galaxies combine and these subsequently help distribute heavy elements, like carbon, oxygen and iron throughout the Universe.
But, suppose our planet existed inside a starburst galaxy such as M83- how would the Milky Way appear from Earth if it were to go starburst?
Our planet is located about half-way from our galaxy's central region. From this vantage point, our view of the Milky Way is partially obstructed by dark gas and dust clouds that are local components of the spiral arm we occupy. Still, its possible on clear, moonless summer nights to step outside and view the bulge of stars forming our home galaxy's hub. To see this personally, you need an unobstructed southern sky view towards the constellations of Scorpio and Sagittarius. When it rises to its greatest height, the Milky Way's center forms a broad, elongated glowing bulge occupying a few percent of the sky. It's best appreciated when seen from a dark location far from city lights. Barely perceptible from urban areas, it can take your breath away when observed under ideal conditions.
If our own galaxy were to enter a starburst period, then our view of its inner most region would dramatically change- our overwhelming impression would be of a significant brightening in the Milky Way's appearance. Daylight sightings would not be uncommon. Unfortunately, our view would be rather short lived because by the time we realized our galaxy's new condition, our planet would have already been subjected by all kinds of lethal high energy radiation from the furious stellar activity occurring at galaxy's center. Sooner or later, living things on Earth would start to feel the effects and slowly, but eventually, life, at least above the microbial level and even most of that, would be extinguished.
No one wants to be near a starburst galaxy, much less within one!
We are lucky to be circling an unremarkable star in an unpretentious galaxy whose primary significance is that they foster the home world of the only intelligent life that we know, for sure, exists in the vast Universe- we, the human species!