galaxy

Grouping of millions or billions of stars, held together by gravity. There are billions of galaxies in the universe. There are different types, including spiral, barred spiral, and elliptical galaxies. Our own galaxy, the Milky Way, is about 100,000 light years across (a light year is the distance light travels in a year, about 9.5 trillion km/6 trillion mi; 1 trillion = 10¹²), and contains at least 100 billion stars.

The galaxies are moving away from our own in all directions. The universe is thus expanding in all directions. The evidence for this comes from examining light from the galaxies by splitting the light into a spectrum. A feature known as the red shift appears, where the light is shifted towards the red end of the spectrum due to an increase in wavelength caused by the galaxies' recession.

For a prominent galaxy in the constellation Virgo, which is about 38 million light years away, the red shift indicates the galaxy is moving away from us at 1,200 km/730 mi per second, while another galaxy in the constellation Hydra, which is about 2,000 million light years away, is moving away from us at 56,000 km/35,000 mi per second.