Distances to nearby stars can be measured from their parallax

The nearest star, Alpha Centauri, is a triple star system

α Cen A and α Cen B form a double star system. In 2016, α Cen B was found to have an Earth-sized planet.	The third member α Cen C is the nearest star, Proxima Centauri.

Women astronomers, with Edward Pickering, at the Harvard College Observatory in 1913

Spectral types of stars (Oh Be A Fine Girl/Guy Kiss Me)

Hertszsprung-Russell (HR) diagram

Image Credit: ESO

HR diagram of the Pleiades open cluster

© Cosmic Perspective Fig 15.18

HR diagram of the M4 globular cluster

© Cosmic Perspective Fig 15.20

The Omega Centauri globular cluster

Zoom in

HR diagram video

HR diagram of stars with parallaxes from the Hipparcos satellite

Gaia mission (2018)

Map of 1.7 billion stars (1.3 billion with parallaxes)

HR diagram

HR diagram of main sequence stars

© Cosmic Perspective Fig 15.13

Sizes of stars

© Cosmic Perspective Fig 15.12

© Cosmic Perspective Fig 15.13

Ages of star clusters

© Cosmic Perspective Fig 15.19