A Microwave Peek Into Our Stellar Neighbourhood

There might be a giant gas cloud about 400 lightyears away, and how it might have been formed has lots to say about our immediate stellar neighbourhood.

Credit: Planck/ESA

Credit: Planck/ESA

Its laboriously slow journey around the Milky Way’s centre takes the Sun and its planetary brood through a variety of stellar neighbourhoods. Because of the glacial pace of this journey, changes are never sudden, and are often realised only when the feeble wind of charged and neutral particles, light atoms and dust specks that pervades the universe seems to change direction or strength, or stops blowing altogether; remember how many times Voyager 1 left the Solar System? The constituents of the wind are altogether known as the interstellar medium (ISM).

As we speak, the Sun is crossing some sort of boundary – either between two regions with markedly different densities of the ISM or within the same region, as if slipping out of an ‘ISM storm‘. Many astrophysicists think it’s the first case, that we are crossing over from the less-dense Local Fluff to the supposedly more-dense Galactic Cloud (GC). The Sun is thought to have entered the Local Fluff, from a region of very low ISM density called the Local Bubble, at some time in the last 50,000-100,000 years. It is expected to complete the transition, and exit the turbulence, into the GC sometime in the next 7,000 to 10,000 years.

In all this time, the extended neighbourhood will have seen a lot of prominent changes – changes that we can barely anticipate, hardly worry over right now, but surely marvel at forever.

One such change is that the giant cloud of gas known as the Loop 1 Bubble will have become thinner. On June 6, scientists working on analysis data generated by the European Space Agency’s (ESA) Planck space probe, retired in 2013, released a stunning image of the microwave radiation emitted by Loop I. It is visible as a pronounced arc (also known as the north polar spur) in the northern hemisphere, above the centre and slightly to the left, and dipping sharply into the southern hemisphere. The exact size of the spur is not known. The colours indicate how the radiation from different regions is polarised, and their darkness denotes the intensity of polarisation. The somewhat-line cutting across the centre is the Milky Way, edge-on.

There are many explanations for what Loop I really is. The leading candidate is that it’s the remnant of a large cloud of gas since eroded by radiation from stars that died nearby. However, this isn’t conclusively known to be the case.

The new Planck map of the sky in microwaves with Loop I marked. Credit: Planck/ESA

The new Planck map of the sky in microwaves with Loop I marked. Credit: Planck/ESA

The stars are thought to belong to a larger belt 400-500 lightyears away, seeming to stretch in the sky between the constellations of Scorpius and Centaurus (as we’d see them) – so their name: Sco-Cen Association (‘Association’ denotes that the stars all originated out of a common cloud of gas but don’t really hang out together now). Several studies, such as this one from 1992, have shown that the stars that died within Sco-Cen in the last 15 million years could have released enough energy to leave behind shells like, and including, Loop I.

A two-dimensional map of the Sun’s extended neighbourhood would show Loop 1 to be on the other side of the Local Fluff/GC. However, we don’t know how far really Loop 1 is from us (the polarisation of microwaves coming from it doesn’t tell us about the distance the radiation might’ve travelled). Buying into the explanation that it’s the smoking gun of a bunch of dead stars in Sco-Cen would leave Loop I measuring about 700 lightyears across and place it in the vicinity of those stars, around 500 lightyears away… and in the path of the Sun, and Earth, a few million years from now. Then again, in the same time, the gas clouds making up Loop I could have dissipated.

The Local Fluff is marked as the Local Cloud. The 'map', compiled based on observations by NASA's Interstellar Boundary Explorer (IBEX) satellite, shows which way different objects in a 20-lightyear radius around the Sun. Credit: NASA Goddard

The Local Fluff is marked as the Local Cloud. The ‘map’, compiled based on observations by NASA’s Interstellar Boundary Explorer (IBEX) satellite, shows which way different objects in a 20-lightyear radius around the Sun. Credit: NASA Goddard

Why is running into such a cloud a threat? Humans are thought to have evolved at most 10 million years ago while the Solar System has been traversing the rarer pool of the Local Bubble for 5-10 million years. So, it seems quite possible that humans were never around when Earth was surrounded by a denser ISM.

The science-historian and astronomer A.J. Meadows spells out the what-next in his 2010 book, The Future of the Universe. The gist: Right now, a stream of charged particles and radiation emanating from the Sun acts as a shield against electrically charged particles from the ISM entering the Solar System. However, neutral dust particles and other gases can get as far as Jupiter even now, before material from the Sun thwarts them. But when the star runs into the clouds at the outer edges of the loop, the shield will be pushed inward, toward the Sun, by the higher density of the ISM while the neutral material may even be able to get as far as Earth. We don’t know how that influx will affect our weather, our satellites, etc.

Sounds ripe for the next Christopher Nolan production.

(Even without momentarily presenting the threat, Sco-Cen and Earth might have a connection of their own. Scientists have found an atypical abundance of iron-60 isotopes deep in Earth’s crust – specifically, at downward distances corresponding to what might’ve been exposed 2-3 million years ago. They have since realised (e.g., here and here) that such an abundance could’ve been caused by a supernova occurring around the same time, between 100 and 400 lightyears away. Iron-60 is a common terminal decay product and is produced copiously in larger supernovae, sending the metal awash into the ISM. If such a supernova did happen, it could’ve been a part of the Sco-Cen Association.)

An alternate explanation is that Loop I could have been carved by radiation from Sagittarius A*, the supermassive blackhole at the Milky Way’s centre 25,000 lightyears away.

The former explanation is favoured because:

  • The X-rays emitted by Loop I are similar to those emitted by the Cygnus Loop (a.k.a. the Veil Nebula), a filament of gases thought to be left behind by a supernova thousands of years ago, in the constellation of Cygnus some 1,500 lightyears away.
  • Loop 1 as well as some of the nearby arcs are all emitting radio-frequency waves that are linearly polarised. This means the electric fields that are oscillating to produce the radio-waves are all oscillating along a single plane, and which is lined up along the direction of the radiation’s propagation (imagine a sperm cell wiggling toward you, its flagellum only going up and down, not sideways; that’s effectively a linearly polarised wave). A well-known source of linearly polarised waves is synchrotron radiation, which is emitted when electrons travelling at close to the speed of light are bent through a magnetic field. And the shockwaves from supernovae are known to pummel electrons to such high speeds.

Explore Planck’s maps of the sky at various frequencies through this Planckoscope. A survey conducted in 1970 suggests looking at the arcs, including the north polar spur, at 38 MHz (radio-frequency). The microwave frequency ranges from 300 MHz to 300 GHz. Visualisation credit: ESA

The material and radiological pressure emanating from a dying star can also bunch up gas clouds surrounding it, folding and packing them up. The magnetic fields coursing through such a more compact arrangement, according to ESA, are more efficient at accelerating electrons. Some of that radiation is in the microwave part of the spectrum, and which Planck picked up on.

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