The Universe's age - the time from the Big Bang until now - is about 14 billion years.
The age of the Universe has been determined by figuring out the age of the oldest stars in the Universe and by calculating how long it has taken the Universe to expand to its current size.
Oldest Stars in the Universe
Knowing the age of the oldest stars in the Universe doesn't tell us the Universe's exact age, but it does provide us with a minimum age for the Universe.
Globular Clusters
Globular clusters are very dense collections of a large number of stars - usually about a million stars.
All of the stars in a globular cluster were formed at about the same time.
The older a globular cluster is, the less mass its hydrogen-burning stars will have.
The masses of the oldest globular clusters show that they are about 12 billion years old.
White Dwarf Stars
White dwarf stars are very small, dense stars - about a million times as dense as water. They are about as massive as the Sun but with a radius only the size of that of the Earth.
A white dwarf is the last stage in the evolution of a star that is not extremely massive.
White dwarfs come from red giant stars.
A red giant is a large, glowing giant star in the later stages of evolution.
When a red giant reaches the end of its life, it sheds its outer layers, ejecting an envelope of gas and plasma known as a planetary nebula.
The star's hot core remains. This core becomes a white dwarf.
A white dwarf's glow comes from the heat left behind by the red giant.
As time goes by, the white dwarf cools down.
The older a white dwarf is, the cooler and fainter it will be.
By studying faint weight dwarfs, astrophysicists have been able to estimate the age of the oldest white dwarfs.
These stars are at least 10 billion years old.
Expansion of the Universe
The age of the Universe can be calculating using the Hubble Constant, which is a measure of the rate at which the Universe is expanding, and then extrapolating backwards to the Big Bang.
There have been different estimates of the Hubble constant.
Depending on which estimate is used, age of the Universe has been calculated to be between 10 billion and 20 billion years.
However, since some of the oldest stars in the Universe are more than 10 billion or 12 billion years old, there is a problem with some of the lower estimates.
Cosmic Microwave Background Radiation
By studying the temperature of the Cosmic Microwave Background Radiation, scientists have been able to come up with a more accurate estimate of the rate at which the Universe is expanding.
Right after the Big Bang, the Universe was made up of a hydrogen plasma that was filled with radiation.
This plasma was so hot that atoms were unable to form.
Eventually, the Universe cooled down enough for atoms to develop.
However, the atoms couldn't absorb the radiation and it lingered in the Universe.
In 1964, Arno Penzias and Robert Wilson detected this Cosmic Microwave Background Radiation though a radio antenna.
Today, a NASA spacecraft known as the Wilkinson Microwave Anisotropy Probe (MWAP) measures differences in the temperature of the Cosmic Microwave Background Radiation across the sky.
Using these measurements, astrophysicists have calculated that the universe is 13.73 billion years old, with a margin of error of 1% (0.12 billion years).
This is the most precise estimate of the age of the Universe that has ever been made.