A bubble of light as old as you
Imagine a flash of light leaving the Earth at the moment you were born. It travels at almost 300,000 kilometres every second and never slows down, so by your first birthday it is a light-year away, and by now it is as many light-years away as you are years old. The map shows that bubble around the Sun, with real nearby stars in their true order of distance (the scale is squeezed towards the edge so the closest stars fit). Stars inside the bubble have already been reached by light from your birth day. Those outside are still waiting.
It also works the other way round. When you look at a star 36 light-years away, you are seeing light that left it 36 years ago. Find the star whose distance matches your age and you have a birthday star: its light set off towards you around the time you were born.
How we calculate it
Your distance in light-years equals the time since your birth in years. To convert to kilometres we multiply by 9.4607 trillion, the number of kilometres light covers in a year. Star distances come from the Hipparcos and Gaia catalogues. Your birthday star is the star in our list whose distance is closest to your age, and the date the bubble reaches a star is your birth date plus that many years.
| Star | Distance (light-years) | Birthday star for age |
|---|---|---|
| Proxima Centauri | 4.2 | 4 |
| Sirius | 8.6 | 8 to 9 |
| Altair | 16.7 | 16 to 17 |
| Vega | 25.0 | 25 |
| Arcturus | 36.7 | 36 to 37 |
| Capella | 42.9 | 42 to 43 |
| Aldebaran | 65 | 65 |
| Regulus | 79 | 79 |
Worked example. Someone born on 12 April 1990 was 36.55 years old on 30 October 2026, so light from their birth day was 36.55 light-years away, about 346 trillion kilometres. Arcturus, 36.7 light-years away, is their birthday star: the bubble reaches it on 23 December 2026. After that comes Capella, on 5 March 2033.
Finding your star
Arcturus is easy to find in spring and summer in the northern hemisphere: follow the curve of the Big Dipper’s handle. Vega shines high overhead on summer evenings, and Capella is bright in winter. A star map app will point you to any of them, and it is a lovely thing to show someone on their birthday.
Questions people ask
What is a light-year?
It is the distance light travels in one year: about 9.46 trillion kilometres, or 5.88 trillion miles. Light moves at 299,792 kilometres per second, so it could circle the Earth more than seven times in one second.
What is a birthday star?
A star whose distance in light-years matches your age. The light from it that reaches your eyes tonight set off around the time you were born. Some planetariums and astronomy clubs use the idea to help people find a star of their own.
Are the star distances exact?
They are measured values rounded to one decimal place, mostly from the Hipparcos and Gaia space telescopes. Nearby stars are known very precisely; distant ones such as Betelgeuse have larger uncertainties.
Can I see these stars?
Most of them, yes. Sirius, Vega, Arcturus, Capella and Aldebaran are among the brightest stars in the night sky. Proxima Centauri and Barnard’s Star are too faint to see without a telescope.
Sources. Star distances from the Hipparcos and Gaia catalogues, via NASA and the International Astronomical Union; speed of light 299,792.458 km/s. Calculations run in your browser; nothing you enter is sent anywhere.