Oldest "Clocks"

Pushing the limits of its powerful vision, NASA's Hubble Space Telescope has uncovered the oldest burned-out stars in our Milky Way Galaxy. These extremely old, dim stars provide a completely independent reading of the universe's age without relying on measurements of the universe's expansion. The ancient white dwarf stars, as seen by Hubble, turn out to be 12 to 13 billion years old. Because earlier Hubble observations show that the first stars formed less than 1 billion years after the universe's birth in the big bang, finding the oldest stars puts astronomers well within arm's reach of calculating the absolute age of the universe.

Q & A: Understanding the Discovery

1. 1. Why are white dwarfs used to determine the age of the cosmos?

2. The white dwarfs viewed by Hubble are the oldest white dwarfs residing in the globular cluster M4. Globular clusters contain hundreds of thousands of stars and are among the oldest clusters of stars in the universe. Previous Hubble telescope observations showed that stars first began forming about 1 billion years after the big bang. If that's true, then the oldest stars in globular clusters were among the first grouping of stars.

   The oldest white dwarfs in the globular cluster M4 are so faint that astronomers used the Hubble telescope's sharp "eyes" to find them. Astronomers also selected the closest globular cluster to Earth. M4 is 5, 600 light-years away.

   Conceptually, the age-dating observation is as elegantly simple as estimating how long ago a campfire was burning by measuring the temperature of the smoldering coals. In this Hubble observation, the "coals" are white dwarf stars, the burned-out remnants of the earliest stars that formed in our galaxy.

   Hot, dense spheres of carbon "ash" left behind by the long-dead star's nuclear furnace, white dwarfs cool down at a predictable rate - the older the dwarf, the cooler it is, making it a perfect "clock" that has been ticking for almost as long as the universe has existed.

   It wasn't easy to find these objects, even with Hubble's sharp "eyes." White dwarfs are steadily cooling off. As they cool, they get fainter. To pinpoint these increasingly fainter objects, the telescope took many snapshots of the stars in M4. In fact, it took nearly eight days of exposure time over a 67-day period for Hubble to observe M4. By observing the cluster intermittently over a long period of time, the telescope was able to take pictures of the coolest—and oldest—dwarfs. These stars are extremely feeble: they are less than one-billionth the brightness of the faintest stars that can be seen by the naked eye.

3. 2. Why do astronomers need the Hubble telescope to find white dwarfs? And how did the orbiting observatory find them?

   Astronomers believe the universe began in one cataclysmic explosion called the big bang. But they're still searching for clues on how our universe has evolved since then. Knowing the universe's age could help astronomers in that quest, especially in understanding the evolution of our galaxy and solar system.

   Though previous Hubble research sets the age of the universe at 13 to 14 billion years based on the rate of expansion of space, the universe's birthday is such a fundamental and profound value that astronomers have long sought other age-dating techniques to cross-check their conclusions.

   This approach has been recognized as more reliable than age-dating the oldest stars that are still burning by nuclear fusion, which relies on complex models and calculations about how a star burns its nuclear fuel and ages. White dwarfs are easier to age-date because they are simply cooling, but the trick has always been to find the dimmest and hence longest-running "clocks."

4. 3. Why is it important to know the universe's age?