What does the term metal mean in astronomy?

Only the traditional metals like iron, gold, and copper that are solid and shiny Any element heavier than helium — including carbon, oxygen, iron, and all others on the periodic table beyond the first two elements Only the elements produced directly in the Big Bang — hydrogen, helium, and lithium Any element that can be found in solid form at room temperature

What are Population I stars?

Relatively young, metal-rich stars like our Sun, found mainly in the disk and spiral arms of the galaxy The oldest stars in the galaxy, found in globular clusters in the stellar halo The first stars ever to form, made purely of hydrogen and helium Stars with exactly one solar mass — thus named because they are identical to the Sun

What are Population II stars?

Young stars that recently formed in the spiral arms from metal-rich gas Old, metal-poor stars found in globular clusters and the stellar halo, formed from nearly pristine Big Bang gas Stars with two solar masses — the second most common type after solar-mass stars Stars found only in other galaxies, too far away to observe directly

What were Population III stars, and why have none been observed?

Stars made of antimatter that annihilated all ordinary matter stars in the early universe Stars that are currently forming in the most metal-poor dwarf galaxies orbiting the Milky Way The hypothetical first stars, made of pure hydrogen and helium — they were massive, lived briefly, and all died long ago Stars with three solar masses — the third most common category in stellar population studies

Why are Population II stars metal-poor compared to the Sun?

They formed in a region of the galaxy where heavy elements were always scarce They have burned through their heavy elements by nuclear fusion over billions of years They formed early in the galaxy's history when few previous stars had died to enrich the ISM with heavy elements Population II stars actually have more metals than the Sun — the naming convention is reversed

What is the Sagittarius Dwarf Spheroidal Galaxy?

A small galaxy that collided with the Milky Way 10 billion years ago and is now part of the central bulge A small galaxy currently being torn apart by the Milky Way's gravity, with its stars forming a stream that wraps around the Milky Way The nearest large galaxy to the Milky Way, on a collision course expected in 4 billion years A hypothetical future galaxy that will form when the Milky Way merges with Andromeda

What are globular clusters, and which stellar population do they belong to?

Young, metal-rich (Population I) open clusters found along the spiral arms Sparse collections of young stars still embedded in the nebula they formed from Compact spherical groups of 100,000 to 1 million old, metal-poor (Population II) stars, orbiting in the galactic halo Groups of neutron stars and black holes left over from multiple supernova explosions

How does the chemical enrichment of the ISM change over the history of a galaxy?

The ISM loses metals over time as heavy atoms sink into planetary cores and are removed from circulation Each generation of stars adds more heavy elements to the ISM through stellar winds and supernova explosions, so newer stars are progressively more metal-rich The ISM has always had the same chemical composition since the Big Bang Metal enrichment only happens near the galactic center — the outer disk remains metal-poor forever

How has the Milky Way grown over cosmic time?

By constantly forming new stars from primordial gas that continuously falls in from the early universe By gravitationally capturing and merging with smaller dwarf galaxies, incorporating their stars and gas over billions of years The Milky Way has always been the same size — no growth has occurred since the Big Bang By stealing gas from the Andromeda galaxy through a thin intergalactic bridge

What do the ages and metallicities of stars tell astronomers about a galaxy's history?

They reveal only the temperature and size of individual stars — nothing about the galaxy's history They reveal the galaxy's star formation history and how quickly heavy elements built up over time — a record of the galaxy's chemical evolution They reveal the location of dark matter within the galaxy They reveal only which stars were formed from material captured from other galaxies