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A year is measured as the amount of time it takes Earth to orbit the Sun once, a Galactic year is the time it takes our sun to make one full orbit of the center of the Galaxy.

In Galactic years, how old is our Sun?

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2 Answers 2

The Galactic rotation period at the Sun's Galactocentric radius is about 230 million years (with a five percent uncertainty) and the Sun, as stated in user8's answer is 4.57 billion years old - giving $\sim 20\pm 1$ orbits.

However, the idea of a Galactic year is misleading. For instance, the Milky Way rotation curve is quite flat between 1 and 10 kpc from the centre (see picture), so the "year" also varies by a factor of 10 over this range, with the year being much shorter in the inner part of the Galaxy.

Therefore to know how many rotation periods the Sun has executed requires us to know at what Galactocentric radius it has spent its life. It is an ongoing debate as to whether the Sun has migrated to its position from outside or inside its current radius, or whether it has been where it is all along. As a result, how many "Galactic years" our Sun has experienced is still quite uncertain.

eg. see

Milky Way rotation curve

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According to the blog How old is the Sun in Galactic years? (Siegel, 2008), a Galactic year is about 223 million Earth years.

The age of the Sun, according to the Stanford Solar Centre's page How old is the Sun? is approximately 4.57 billion years.

This would put the sun at approximately 20.5 galactic years old.

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Almost old enough to buy a galactic beer! – astromax Oct 24 '13 at 2:22
In some places, it already could. – user8 Oct 24 '13 at 3:53

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