Although it is widely understood that the Earth is part of a solar system and that our solar system is part of a vast galaxy of stars known as the Milky Way, few people know anything about the local region in our galaxy. Our current knowledge is a bit like the address 'John Smith, Mountain Road, USA'. Over the past decade or two, science has been trying to fill in the gaps in order to give us a better picture of our local neighbourhood within the Milky Way.
The Local Bubble
Many of the stars that we can see with the naked eye, as well as our own Sun1, are passing through a region of space known as The Local Bubble. It is a gigantic structure some 400 light years wide. The word 'bubble' is used because it appears somewhat spherical, with a 'wall' of denser matter at its periphery, and relatively little matter inside its cavity. In fact, the interior is a near-perfect vacuum, with 0.05 atoms per cubic centimetre, much less than surrounding regions of the galaxy. Its origins are not certain by any means, but it is believed that the Local Bubble was formed by a massive star explosion, a supernova, some four million years ago, causing a tremendous shockwave that accelerated all gaseous matter into the surrounding regions and beyond. Outside the local bubble, there are similar bubbles, known as Loop 1, Loop 2 and Loop 3. Loop 1, the product of an even more recent supernova, is expanding and encroaching on the Local Bubble. The Solar System, travelling in the direction of the constellation Hercules, is currently nearing one of the 'walls' of the local bubble.
The Local Fluff
The composition of the interior of the Local Bubble is not uniform. Clouds of low-density gas float inside the Local Bubble. The solar system is currently passing through such a cloud, known as The Local Fluff, or The Local Interstellar Cloud. The cloud is about 30 light years in length, and it mainly contains hydrogen and helium gasses. A few of our closer stellar companions, including the star Altair, are also passing through this cloud. The cloud appears to emanate from a star-forming region, known as the Sco-Cen OB Association2, in the Loop 1 bubble. It is expected that our Sun will leave this cloud in the next 10,000 to 20,000 years. Much of the heavier matter (ie, heavier than hydrogen) from the cloud actually enters the solar system environment - we normally pass through a concentrated flow of interstellar gas from this cloud each November.
Star Systems within the Local Bubble
Almost all of the stars presently in the Local Bubble are just passing through, coursing randomly past each other like balls in an open-ended billiard table as they make their way around the galaxy. Some star systems, however, appear to move in tandem with each other and we call them Superclusters. There are three major superclusters within 500 light years of the solar system: The Hyades, the Pleiades, and very close to us, The Sirius Supercluster. These superclusters contain hundreds of very young stars, some as young as 70 million years. The Sun is not part of a supercluster, and it appears to be making its own way across the interstellar void. Close to the Sun at present are the bright stars Vega, Altair, Arcturus, Fomalhaut, Alpha Centauri, Sirius and Procyon.
What it Means to Us here on Earth
As our Sun travels on its 230 million-year circuit of the Milky Way, it passes through regions of very dense clouds and regions almost completely devoid of interstellar gas. Thanks to our current transit through the Local Bubble, we have a magnificent opportunity to view our galactic neighbourhood in great detail. Conversely, if we were passing through an interstellar cloud, we would see practically nothing in the sky at night3. A low density interstellar environment is also good for life on Earth, as it enables the Sun to cast its protection cover, the Heliosphere, out to great distances4 beyond the solar system, thus deflecting and repelling dangerous charged particles. However, the Heliosphere is highly sensitive to the density of interstellar gas, and it could fail almost completely if the Sun passed though a dense cloud of gas, exposing the Earth to massive amounts of harmful radiation. There is some evidence to suggest that the Earth has experienced mass extinctions caused by interstellar explosions in the past - perhaps caused because we were in a dense cloud when the effects of a supernova passed us by.
The h2g2 Tour of the Solar System
Take the h2g2 Shuttle for your whistle-stop tour of the Solar System.
- First Stop (Inward Journey Only):
1 You should never look directly at the Sun with the naked eye, as this can cause lasting blindness in seconds.
2 Named because it exists in the direction of the Scorpius and Centaurus constellations. O and B category stars are bright, young stars. An OB Association is a place where new stars are born.
3 For The Hitchhiker's Guide to the Galaxy fans, this brings to mind the experience of the inhabitants of Krikkit.
4 The Heliosphere is approximately 150AU in diameter (1AU is the average distance between the Earth and the Sun).
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