The Local Interstellar Cloud: An Overview

The "Local Fluff" is the giant cloud that houses our solar system

Local fluff
Our solar journey through space is carrying us through a cluster of very low density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by IBEX is as sparse as a handful of air stretched over a column that is hundreds of light years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. NASA

As our Sun and planets travel through interstellar space, they're moving through a mixture of hydrogen and helium atoms called the "Local Interstellar Cloud" or, more colloquially, the "Local Fluff".

The Local Fluff itself, which spans about 30 light-years across, is actually part of a much bigger 300-light-year-wide cavern in space called the Local Bubble, which is very sparsely populated with atoms of hot gases. Ordinarily, the Local Fluff would be destroyed by the pressure of the heated material in the Bubble, but not the Fluff. Scientists hypothesize that it could be the cloud's magnetism that saves it from destruction. 

The solar system's trip through the Local Fluff began between 44,000 and 150,000 years ago, and it may exit in the next 20,000 years when it could enter another cloud called the G Complex. 

The Local Interstellar Cloud is incredibly thin, with less than an atom of gas per cubic centimeter. For comparison, the top of Earth's atmosphere (where it blends into interplanetary space), has 12,000,000,000,000 atoms per cublic centimeter. It is almost as hot as the surface of the Sun, but because the cloud is so attenuated in space, it cannot hold that heat. 


Astronomers have known about this cloud for several decades. They've used Hubble Space Telescope and other observatories to "probe" the cloud and light from distant stars as a sort of "candle" to view it more closely.  The light travels through the cloud is picked up by detectors on the telescopes. Astronomers then use an instrument called a spectrograph (or a spectroscope) to break the light into its component wavelengths. The end result is a graph called a spectrum, which — among other things — tells scientists what elements exist in the cloud. Tiny "dropouts" in the spectrum indicate where elements absorbed the light as it passed through. It's an indirect way of seeing what would otherwise be very difficult to detect, particularly in interstellar space. 


Astronomers have long wondered how the cavernous Local Bubble and the Local Fluff and the nearby G Complex clouds were formed. The gases in the larger Local Bubble likely came from supernova explosions in the past 20 million years or so. During these catastrophic events, massive old stars blasted their outer layers and atmospheres to space at high speeds, sending out a bubble of superheated gases.

The Fluff had a different origin. Massive hot young stars send gas out to space, particularly in their early stages. There are several associations of these stars — called OB stars — near the solar system. The closest is the Scorpius-Centaurus Association, named for the region of sky where they exist (in this case, the area covered by the constellations Scorpius and Centaurus (which contains the closest stars to Earth: Alpha, Beta, and Proxima Centauri)). It is very likely that this star formation region is, in fact, the local interstellar cloud and that the G complex next door also came from the hot young stars that are still being born in the Sco-Cen Association. 

Can the Cloud Hurt Us?

Earth and the other planets are relatively protected from the magnetic fields and radiation in the Local Interstellar Cloud by the Sun's heliosphere — the extent of the solar wind. It extends well out beyond the orbit of dwarf planet Pluto. Data from the Voyager 1 spacecraft have confirmed the existence of the Local Fluff by detecting the strong magnetic fields it contains. Another probe, called IBEX, has also studied the interaction between the solar wind and the Local Fluff, in an effort to map the region of space that acts as a boundary between the heliosphere and the Local Fluff.