Starburst galaxy

Some galaxies make stars very fast.

This is called a starburst. It happens when a galaxy has much gas.

Often, two galaxies bump into each other.

This crash pushes the gas together. The squeezed gas turns into many new stars.

These stars are very bright and hot. They burn through their fuel quickly.

Because they use gas so fast, the burst does not last long. It is just a short phase.

After the gas is gone, the galaxy stops making stars. It becomes a quiet place.

Most galaxies make stars at a steady pace. But some galaxies make them very quickly. We call these starburst galaxies. In a starburst, the rate of making stars is much higher than normal. For example, our Milky Way makes about 3 suns worth of stars every year. A starburst galaxy can make 100 or more!

What starts a starburst? Often, it is a close encounter with another galaxy. Two galaxies might bump into each other or pull on each other. This is called a merger. These movements squeeze gas and dust together. The squeezed gas then turns into many new stars. This can also happen if a galaxy has a stellar bar. A bar is a long shape of stars in the center. It can funnel gas toward the middle.

Starbursts do not last forever. They use up their gas very fast. Because they use fuel so quickly, the burst is just a short phase. Once the gas is gone, the galaxy stops making stars. We call these quiet galaxies post-starburst galaxies.

A starburst galaxy is a place where new stars are being born at a very high rate. Most galaxies make stars at a steady, slow pace. For example, our Milky Way galaxy makes about 3 suns worth of stars every year. A starburst galaxy is much more active. It can make 100 or more suns worth of stars in a single year! Because they use up their gas so quickly, these bursts are just a short phase. They do not last for the whole life of the galaxy. This makes them very exciting for scientists to study.

How does a galaxy start making stars so fast? It usually happens because of a big change in the galaxy. Most starbursts are caused by mergers, which is when two galaxies bump into each other. Other times, a close encounter with a neighbor galaxy can pull on the gas. This pull can cause gas and dust to lose their momentum. The material then falls toward the center of the galaxy. As the gas gets squeezed together, it rapidly forms new stars. A stellar bar can also help by funneling gas toward the middle.

Scientists have studied many different kinds of these busy galaxies. One famous example is Messier 82, which is about 12 million light-years away. It is a prototype for starbursts and is in the constellation Ursa Major. Another example is the Antennae Galaxies, also known as NGC 4038 and NGC 4039. These two galaxies are in the middle of a merger.

There are also very dusty ones called Luminous Infrared Galaxies, or LIRGs. These galaxies are so full of dust that they glow in infrared light. Some are even more extreme, like the Ultra-luminous Infrared Galaxies known as ULIRGs.

There are many specific details that help us group these galaxies. For instance, Blue Compact Galaxies are often small and have very little metal. They look blue because they have many hot, young stars. Some galaxies, called Green Pea galaxies, are small and look like very old starbursts.

In very large starbursts, massive stars are born. These stars burn bright and fast. They often end their lives in huge explosions called supernovae. These explosions leave behind remnants that interact with the space around them.

Understanding starbursts helps us learn about the history of the whole universe. Many of the most distant galaxies we see are starburst galaxies. Because they are so far away, we cannot see their details clearly. However, by looking at nearby starbursts, we can guess what happened long ago.

We can see how the early universe worked when it was much younger. It is like looking at a map of the past. This helps us see how galaxies grow and change over billions of years.

A starburst galaxy is a system undergoing an exceptionally high rate of star formation. This rate is much higher than the long-term average for that galaxy. It is also much higher than the star formation rates seen in most other galaxies. For example, the Milky Way forms about 3 solar masses of stars every year. In contrast, a starburst galaxy can form 100 solar masses or more annually. Because they consume fuel so quickly, starbursts are only a brief phase in a galaxy's evolution. They use up their molecular gas reservoirs much faster than normal galaxies. While a typical galaxy might use its gas over 1 to 2 billion years, a starburst can deplete its material in just 0.1 billion years.

To identify a starburst, astronomers look at three specific, interrelated factors. First, they measure the star-formation rate (SFR), which is how fast gas converts into stars. Second, they look at the available quantity of gas available for this process. Third, they compare the timescale of gas consumption to the age or rotation period of the galaxy. Scientists also use a "birthrate parameter," labeled as "b." This is the ratio of the current SFR to the past-averaged SFR. Generally, a galaxy is considered a starburst if this parameter is 3 or higher.

Most starbursts are triggered by processes that cause massive density surges. The most common cause is a merger, where two galaxies collide or encounter one another. During these encounters, tidal gravitational forces or direct collisions between gas clouds occur. These forces cause dust and gas to lose angular momentum. As a result, the material falls inward toward the galactic center. This compressed gas then rapidly forms new stars. Sometimes, galaxies do not merge but still interact. These interactions can cause a "bar instability." This instability funnels gas toward the nucleus to ignite star formation.

Astronomers classify starburst galaxies into several distinct types based on their characteristics. Blue compact galaxies (BCGs) are often low-mass and contain very little metal. They appear blue in optical and ultraviolet light because of their many hot, young stars. There are also small, compact Green Pea galaxies (GPs) that resemble primordial starbursts. Another type is the Blueberry galaxy (BB), which is a dwarf starburst. While BBs have a low star-formation rate, their specific star-formation rate is high, similar to Green Peas.

Some starbursts are defined by how much dust they contain. Luminous infrared galaxies (LIRGs) are extremely dusty objects with high luminosities. The ultraviolet radiation from young stars is absorbed by dust and re-radiated as infrared light. Ultra-luminous infrared galaxies (ULIRGs) are even more extreme versions of these. Many ULIRGs appear to be double-cored systems, which supports the idea that they are powered by major mergers.

Even more massive are the Hyperluminous Infrared galaxies (HyLIRGs). At very high redshifts, these are often called Submillimeter galaxies (SMGs) because their light is observed at sub-millimeter wavelengths.

The environment inside a starburst is incredibly intense. The massive amounts of gas lead to the creation of many massive stars. These young, hot stars ionize the surrounding hydrogen gas, creating H II regions. These stars often belong to groups known as OB associations. Because they are so massive, these stars burn through their fuel quickly. They often end their lives in violent supernova explosions.

The resulting supernova remnants interact with the surrounding interstellar medium. This process can even create naturally occurring masers.

Studying these galaxies helps us understand the history of the entire universe. Many of the most distant galaxies seen in deep fields are starbursts. While we cannot see their details clearly due to distance, nearby examples like Messier 82 serve as prototypes. By studying local starbursts, we can model what was happening in the early universe. This provides a window into how galaxies formed and evolved over cosmic time. Eventually, these galaxies may become "Post Starburst" galaxies. This happens when they exhaust their fuel or when stellar winds and magnetic fields drive gas outflows.