The farther away we look in space, the deeper we are looking into the past.
That statement is an essential tenet of modern astronomy, born in the late 1920s when Edwin Hubble realized that the vast nebulae in the night sky are really island universes, what we today call galaxies, and furthermore that all the galaxies are moving away from each other in an expanding universe. The farther away we look in space, the deeper we are looking into the past. We see out to over 13 billion light-years, and thus we are looking more than 13 billion years back. That means the galaxies near us in space should be old, and those far away should be young. And then there is the galaxy DDO 68, otherwise known as UGC 5340, only 39 million light-years away, a hop and a skip over cosmic distances. This dwarf galaxy should be approximately as old as our own Milky Way, and yet it appears recently formed. Is it really as young as it looks?
The new image above shows the galaxy as seen by the Hubble Space Telescope, which itself has played a huge role in letting astronomers see farther into the distance, and farther back in time. The Hubble telescope has captured light that has taken billions of years to reach us; see the Hubble eXtreme Deep Field image at the bottom of this post for an example.
By studying galaxies at various distances – and therefore of various ages – astronomers have found that young galaxies are fundamentally different from older galaxies. DDO 68 looks to be relatively youthful based on its structure, appearance, and composition.
For example, it’s a characteristic of older galaxies to be populated with stars of different ages. Perhaps you’ve heard that our sun, for example, is at least a second-generation star because it contains elements such as carbon and oxygen that must have been born inside stars themselves. Newly-formed galaxies contain stars with a composition similar to the primordial matter created in the Big Bang (hydrogen, helium and a little lithium). Older galaxies are enriched with heavier elements forged inside stars, over multiple generations.
DDO 68 appears to be very low in the heavier elements. As yet, no one knows why. The Hubble telescope obtained this image while carrying out observations in order to study the properties of the galaxy’s light, and to confirm whether or not there are any older stars in DDO 68. The astronomers say they did find hints of possible older stars, but they’re not sure.
If DDO 68 does contain older stars, then it’s not as young as it looks. That will be one sort of puzzle. Why does it appear young, if it really isn’t?
If the galaxy does not contain older stars, on the other hand, and the galaxy is actually young, then it’s an even bigger puzzle. How did a young galaxy come to be near us in space?
Astronomers say they need more detailed computer modeling of the dwarf galaxy DDO 68, to be sure.
Bottom line: The nearby dwarf galaxy DDO 68 – only 39 million light-years away – looks to be relatively youthful based on its structure, appearance, and composition. But its nearness to us in space would suggest that it’s not as young as it looks. A cosmic puzzle, inside.