Dwarf galaxies are peculiar objects. Like their name suggests, they are very small. Some dwarf galaxies contain as few as 5,000 stars, a mere speck in space compared to the Milky Way, which contains some 200 billion to 400 billion stars.
Because they are so small, they are also very difficult to see compared to the brightness of other objects in space. So far, around 30 dwarf galaxies have been identified in relatively close proximity to the Milky Way galaxy, although it's possible that there could be hundreds.
One of the earliest dwarf galaxies discovered neighbouring the Milky Way is the Sculptor Dwarf Galaxy, discovered by American astronomer Harlow Shapley in 1937. Named for its location, some 280,000 light-years from Earth in the direction of the constellation Sculptor, the galaxy is very faint. Its stars are old and scattered far apart in a spheroidal shape, which makes it hard to see.
But that which makes it hard to see is also what makes it so very interesting to astronomers. The Milky Way galaxy is what is known as a cannibal galaxy, named because it absorbed smaller galaxies, dwarf galaxies and globular clusters. Astronomers predict that in the distant future the Milky Way will in turn be cannibalised by the larger Andromeda Galaxy.
Any of these snack galaxies missed by the Milky Way still in existence today would be very old, and, as evidenced by this new photograph taken by the European Southern Observatory's MPG/ESO 2.2-metre telescope at the La Silla Observatory in the Atacama Desert in Chile, the Sculptor Dwarf Galaxy is home to some old stars indeed. This classifies it as a primordial galaxy.
To tell the age of a star, astronomers use something called spectroscopy. Different chemicals give off different "colour fingerprints," emitting radiation at specific wavelengths on the light spectrum. Old stars contain a low level of heavy elements. Astronomers can determine the age of stars by examining these colour fingerprints.
The galaxy is home to the older stars, already mentioned, and a younger population. The young stars cluster at the galaxy's core, while the older population tends to fall at its outer reaches.
The Sculptor Dwarf Galaxy contains only four percent of the carbon and other heavy elements found in the Milky Way, according to a study released in 2009 by researchers at Cornell University. This makes the Sculptor Dwarf Galaxy similar to the primordial galaxies seen at the edges of the universe.
But the Sculptor Dwarf Galaxy is a lot closer, which means the light it emits is younger. It's also a lot easier for researchers to study, allowing them to extrapolate what the ESO calls "the most accurate history of star formation ever determined for a dwarf spheroidal galaxy."
Not all dwarf galaxies are built alike. Because they are isolated from each other, they tend to evolve independently. Studying the similarities and differences in these evolutionary histories, the ESO said, will help scientists understand the formation and evolution of all galaxies.