20.6 Interstellar Matter around the Sun

Our solar system sits inside a cosmic bubble of hot, sparse gas called the Local Hot Bubble. This region, carved out by ancient supernovas, spans about 300 light-years and contains gas heated to roughly a million degrees Kelvin. Within this bubble, we're currently passing through a denser pocket called the Local Fluff, a cooler, denser region that gives us a unique window into how our solar system interacts with the surrounding interstellar medium.

The Local Hot Bubble and Local Fluff

Structure of the Local Hot Bubble

The Local Hot Bubble (LHB) is a region of hot, low-density gas that surrounds our solar system. Think of it as a cavity blown into the denser interstellar medium.

• Spans about 300 light-years in diameter
• Gas heated to roughly 1,000,000 K
• Extremely sparse: only about 0.001 atoms per $\text{cm}^3$
• Has an irregular, non-uniform shape, so the distance to its boundary varies depending on the direction you look (shorter toward some constellations like Aquila, longer toward others like Orion)

The presence of this hot gas has been confirmed through X-ray observations from space telescopes like ROSAT and Chandra. Gas at a million degrees emits soft X-rays, which is exactly what these telescopes detect. The LHB is primarily composed of hydrogen and helium, with trace amounts of heavier elements like carbon, nitrogen, and oxygen.

Origin of the Local Hot Bubble

The LHB wasn't always here. It was carved out by multiple supernova explosions from massive stars that died roughly 10–20 million years ago. Here's how the process worked:

1. A cluster of massive stars lived and died in our galactic neighborhood.
2. When these stars exploded as supernovas, their shock waves slammed into the surrounding interstellar gas.
3. The shock waves heated and swept away that gas, creating a hot, rarefied cavity.
4. Stellar winds from nearby massive stars (such as Wolf-Rayet stars, which blow extremely powerful winds) also helped shape and maintain the bubble.

Today, the LHB's expansion is counterbalanced by pressure from the denser ambient interstellar medium pushing inward. It's reached a rough equilibrium rather than continuing to grow.

Local Fluff vs. Local Hot Bubble

The Local Fluff (also called the Local Interstellar Cloud) is a denser pocket of interstellar gas sitting inside the LHB. Our solar system is currently traveling through it.

The Local Fluff is about 100 times denser than the surrounding LHB, yet still far more sparse than the air you're breathing. It's likely a surviving remnant of a larger interstellar cloud that wasn't fully destroyed when the supernovas created the LHB. You can think of it as a cooler, denser clump that held together while the hot gas expanded around it, similar to how small dense clouds (like Bok globules) can persist inside larger, hotter regions.

Interstellar Matter and Solar System Interaction

The solar system doesn't just passively sit inside the Local Fluff. It actively interacts with it.

• The heliosphere is the bubble blown by the solar wind around our solar system. It marks the boundary where the outward-flowing solar wind collides with and is balanced by the inward pressure of interstellar gas. The Local Fluff's density and pressure directly affect the size and shape of the heliosphere.
• Cosmic rays, high-energy particles from distant sources like supernovas, constantly stream through interstellar space. The heliosphere and Earth's magnetic field deflect many of them, but some still reach Earth's atmosphere.
• Spectroscopy is the main tool astronomers use to study the Local Fluff and LHB. By looking at absorption lines in the spectra of nearby stars, astronomers can identify what elements are present in the intervening gas and measure its temperature and density.
• Interstellar dust particles within these regions absorb and scatter starlight, which is why understanding the local interstellar environment helps astronomers correct their observations of more distant objects.