Neighbours: from the Milky Way to the cosmic web
Where Earth sits in the structure of the universe
Teaching this? Open projector mode
Our cosmic address goes from small to large:
Earth → Solar System → Milky Way → Local Group → Virgo Supercluster region → Laniakea → the cosmic web.
The structures are held together by gravity, and each sits inside a bigger one. Let us walk up the ladder.
The Milky Way is a barred spiral galaxy: a flat disc of stars, gas and dust with spiral arms and a bar of stars across its centre. It is about 100,000 light-years across (estimates for the disc run from roughly 90,000 to over 100,000 ly), but only about 1,000 ly thick. It holds somewhere around 100 to 400 billion stars, and astronomers cannot count more precisely because most of the disc is hidden from us by dust.
We live inside the disc, about 26,000 light-years from the centre, on a small arm fragment called the Orion Spur between two of the big spiral arms. That is why we see the Milky Way as a band across the night sky. Every picture of our galaxy "from outside" is an artist's impression or a model, never a photograph.
The Local Group is our home group of galaxies, about 10 million light-years across. It contains at least 80 known galaxies, but only three are large: the Milky Way, the Andromeda Galaxy (2.5 million ly away) and the Triangulum Galaxy (about 2.7 million ly). The rest are small dwarf galaxies, including the Large and Small Magellanic Clouds, which orbit the Milky Way.
Beyond it lies the Virgo Cluster, about 54 million light-years away, with well over a thousand galaxies. The Local Group sits near the outskirts of a larger region, the Virgo Supercluster.
Laniakea is the huge region (named in 2014, a Hawaiian word meaning "immense heaven") that includes the Local Group, the Virgo Cluster and about 100,000 large galaxies across about 500 million light-years. Astronomers drew its edge by mapping how galaxies flow: all the galaxies inside are drifting towards the same central region, like water in a drainage basin. Its boundary is a useful map line, not a wall.
The cosmic web. On the very largest scales, galaxies are not scattered evenly. They line up in filaments and sheets around enormous nearly empty voids, with the biggest clusters at the places where filaments meet. The whole pattern looks like a sponge or the foam of soap bubbles. Voids can be tens to hundreds of millions of light-years wide.
Quick check against a misconception: it is tempting to think the Sun is at the centre of the Milky Way, since the sky seems to be arranged around us. But we are about halfway out to the edge of the disc, on the edge of one small spiral arm. Our spot is ordinary.
Which of these is one of the three large galaxies of the Local Group?
Which list goes from smallest to largest?
On the largest scales, what does the arrangement of galaxies look like?
Where is the Sun in the Milky Way?