Question

Why does the expansion of a supernova remnant slow as time passes?

Short answer

The expansion slows due to interaction with interstellar medium and energy loss.

Step-by-step solution

Understand what a supernova remnant is

A supernova remnant is the structure resulting from the explosion of a star in a supernova. This explosion ejects material into space at high velocities, creating a rapidly expanding shell of gas and dust.

Consider initial expansion phase

Immediately after the supernova event, the outer layers of the star are ejected at high velocities, typically thousands of kilometers per second. At this stage, the expansion is driven by the explosive energy of the supernova itself.

The effects of the interstellar medium

As the remnant expands, it interacts with the interstellar medium (ISM) — the gas and dust that fill the space between stars. This interaction causes the supernova remnant to lose energy and slow down due to the drag effect and momentum exchange with the ISM.

Conservation of momentum

As the supernova's high-velocity material collides with the slower-moving interstellar medium, momentum is conserved. The impact causes the expansion to slow down because the ISM absorbs some of the kinetic energy and momentum of the supernova remnant.

Energy dissipation

Over time, the supernova remnant radiates energy in the form of light, radio waves, and other types of radiation. This energy loss causes a reduction in the pressure driving the expansion, thus contributing further to the slowing down of the remnant.

Transition to Sedov-Taylor phase

After the initial explosive phase, the remnant expands in what is called the Sedov-Taylor phase, where the expansion rate follows a decelerating power-law relationship because energy is conserved in the converted thermal energy of the shock wave.

Key concepts

Interstellar Medium

The interstellar medium (ISM) is the cosmic material found between star systems in galaxies. It is composed primarily of hydrogen and helium gas, along with dust particles of varying sizes. This seemingly empty space is actually a crucial component in the lifecycle of stars. When a supernova remnant cascades outward through space, it inevitably meets the ISM.

• The ISM acts like a cosmic brake, slowing down the remnants as they pass through.
• This interaction is significant because it involves complex exchanges of energy and momentum.
• As the supernova remnant encounters the denser regions of the ISM, it begins to slow considerably due to the drag effect and energy loss.
• Even though the ISM is thin, the vast distances covered by the remnant amplify its impact on slowing down the expansion.

Conservation of Momentum

The conservation of momentum is a fundamental principle in physics that applies universally. It states that in any closed system, the total momentum remains constant, assuming no external forces act on it.
When the shell of a supernova remnant pierces through the interstellar medium, this principle plays a crucial role.

• The supernova initially propels material at extremely high velocities, which soon collide with the slower particles of the ISM.
• During these collisions, momentum is exchanged but conserved. This means the fast-moving supernova material transfers some of its energy and speed to the particles in the ISM.
• As a result, some of the remnant's momentum is mitigated by the ISM, contributing to its slowdown.
• This transferred momentum causes the ISM to move slightly, while the supernova remnant slows down significantly.

Energy Dissipation

As a supernova remnant spreads out into space, it dissipates energy in several different forms.
This process of energy loss leads to a gradual decline in the force driving its expansion.

• Much of the dissipated energy radiates as light, radio waves, and other forms of electromagnetic radiation.
• As light and other waves move away from the remnant, they carry thermal energy and pressure away from the expanding shell.
• With each emission of energy, the internal pressure of the remnant drops, leading to less force pushing it outward.
• This continuous shedding of energy contributes to the deceleration of the expansion over time.

In essence, energy dissipation weakens the remnants' drive, leading to a slowing motion.

Sedov-Taylor Phase

After the explosive initial phase of a supernova, the remnant enters the Sedov-Taylor phase. This intermediate phase marks a shift from explosive expansion to a more steady, predictable pattern.

• Named after Leonid Sedov and Sir Geoffrey Ingram Taylor, who developed the underlying models, this phase describes the deceleration of a shock wave.
• During this phase, the expansion follows a power-law relationship, which means that as time progresses, velocity reduces predictably.
• In this phase, the energy released initially by the supernova is not lost, rather it's converted into thermal energy that drives the remaining expansion.
• This energy budgeting allows scientists to derive parameters like the age, energy, and dynamics of the explosion.

Understanding the Sedov-Taylor phase helps predict how remnants will evolve and interact with their galactic environment.