Why are some stars blue?

While scanning our spiral neighbour, the Andromeda galaxy, 2.54 million light years away, the Hubble Space Telescope detected a group of bright blue spots near the galactic nucleus. Nothing unusual in that – blue spots are a fairly common sight indicating young, hot supergiant stars. But NASA was surprised to see their ultraviolet light in this region of space and even more surprised to discover that they were dimmer and had a range of surface temperatures that didn’t match the profile of infant stars.

In fact, these are rare ultra-blue stars – old red giants past their prime that have ejected most of their surface material to expose hot blue cores. This much ejection is unusual and likely happens because they are richer in chemical elements other than the normal helium and hydrogen, which makes it much easier for the star to slough more layers into space. Another hypothesis is that the proximity to their binary partners makes it easier for them to shed more of their mass.