While the brightest stars dominate any astronomical image, they are far outnumbered by the fainter, lower-mass, cooler stars out there. In this region of the star cluster Terzan 5, a large number of stars are bound together in various configurations, but the large abundance of cooler, older, low-mass stars tells us that star formation mostly occurred long ago in this object. (NASA/ESA/HUBBLE/F. FERRARO)

No, Today’s Stars Are Not The Same As Yesterday’s Stars

The Universe’s idea of a ‘typical star’ has changed dramatically over time.

When you look out at the Universe today, you’re not seeing it exactly as it is at one particular instant in time: now. Because of the fact that time is relative and light isn’t instantaneously fast — it can only move at the large, but not infinite, speed of light — we’re seeing things as they were when they emitted the light that only now is arriving. For an object like our Sun, the difference is cosmically minuscule: the Sun’s light arrives after a somewhat paltry journey of only 150 million km (93 million miles), which takes just a little over 8 minutes to complete.

But for the stars, star clusters, nebula, and galaxies we see across the Universe, because of their great cosmic distances, we’re seeing them as they were a much longer time ago. The closest stars are only a few light-years away, but for the objects that are millions or even billions of light-years distant, we’re seeing them as they were a significant fraction of the Universe’s history ago. The light that we receive from the most distant galaxy discovered so far — — was emitted when the Universe was just 407 million years old: 3% of its current age.