aster

The root aster comes from Greek astēr / astron and its Latin cousins astrum and sīdus — all meaning 'star.' To the ancients, the night sky was not decoration but information: a calendar, a compass, and an oracle all at once. That double life of the stars — as objects you study and as omens you read — is why this one root branched into two very different families.

The science branch treats stars as things to observe and measure. With Greek nomos ('arrangement, law') you get astronomy, literally 'the ordering of the stars' — mapping where each one sits. With logos ('account, study') you get astrology, 'the account of the stars' — once the same enterprise, later split off as the fortune-telling cousin. The people who do these are the astronomer and the astrologer.

The shape branch uses 'star' as a picture. A little star is an asterisk (Greek asteriskos, 'little star') — the symbol. Something star-shaped is an asteroid (aster + -oid* 'resembling') — the name early observers gave the tiny points of light that looked like stars but moved like planets.

The travel branch imagines space as a sea. An astronaut is a 'star sailor' (astron + nautēs 'sailor'), and astronautics is the science of that voyage. The Latin variant stella shows up in interstellar ('between the stars').

Then come the two great surprises. Disaster is dis- (bad, ill) + astrum (star): an 'ill-starred' event. To medieval minds, calamity meant the stars had turned against you, and the word kept the disaster long after people stopped blaming the heavens. Even more unexpected is consider: Latin con- ('thoroughly') + sīdus / sīder- ('star, constellation') gave considerāre — literally 'to observe the stars carefully,' what an augur did before any big decision. To consider something was to study it as intently as a stargazer reads the sky. The astronomy faded; the careful weighing stayed, and built a whole family: consideration, considerable, considerate, reconsider.

Finally, English keeps its own Germanic word for the same object: star (and starry, stardom) is a cognate, descended from the same ancient Indo-European root by a separate northern path. So when you call a celebrity a 'star,' you are using the homegrown sibling of astron.

The pattern to hold onto: when the word is about studying or sailing among stars, the spelling is astr-/astro-; when it's about a star-shaped thing, it's aster-/asteroid; when fate is involved, look for the hidden -astr- (disaster) or -sider- (consider).